[From the U.S. Government Printing Office, www.gpo.gov]





















































 7

















               QC
               944
                H3
               E*88
               1993





 7                       ESMA'nONANALysis
 7                           AND MAPPING OF
 7                 COASTAL OVERWASH ONKAuNi
                      DURING THE LANDFALL OF
 7                           HuRRIcANE INim


                                                     @-v























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 7

                             C.H. Fletcher, I.T. Stewart,
                            T.A. Schroeder, G.M. Barnes,
                            B.M. Richmond, HA. Krock,
                                    and D. Neill
 7






                                                               U.S. DEPARTMENT OF COMMERCE NOAA
                                                               COASTAL SERVICES CENTER
                                                               2234 SOUTH HOBSON AVENUE
                                                               CHARLESTON, SC 29405-2413
                          ESTIMATION ANALYSIS
                            AND MAPPING OF
                       COASTAL OVERWASH ON KAUA'I                                            I
                        DURING THE LANDFALL OF
                            HURRICANE INIKI



                                     C.H. Fletcher, I.T. Stewart
                       Geology and Geophysics, SOEST, University of Hawai'i,
                                        Honolulu, HI 96822
                                    T.A. Schroeder, G.M. Barnes
                   Meteorology, SOEST, University of Hawai'i, Honolulu, HI 96822
                                         B.M. Richmond,
                  US Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025
                                        H.-J. Krock, D. Neill
                Ocean Engineering, SOEST, University of Hawai'i, Honolulu, HI 96822


                                            Prepared for:

                             Hawaii Coastal Zone Management Program
                                       Office of State Planning
                                       Office of the Governor

                                 National Coastal Geology Program
                                   United States Geological Survey
                                      Department of the Interior

                                    National Science Foundation


                                             October, 1993

               A publication of the Hawaii Office of State Planning pursuant to National Oceanic and
                          Atmospheric Administration Award No. NA36OZ0000-01.
                                                                                   
                                                                                  
                                                                      Property of CSC Library      
































    



























































                   COVER: The marine overwash of Hurricane Iniki on the southern coast of Kauai left a line
                   of debris behind this condominimum complex near Kukuiula Harbor. The lower floors,
                   and the foundation, were damaged by marine floodwaters. The upper floors and roof were
                     stroyed by wind. Beach erosion exposed soff layers that caused coastal turbidity for
                   several weeks following the storm.





                                                                            Coastal Overwash During Hurricane Iniki


                      COASTAL, OVERWASH ON KAUA'i DURING HURRICANE INIKI

                                                         TABLE OF CONTENTS
                    List of Figures       .............................................................................................3
                    Executive Summary            ....................................................................................4
                    Introduction       ..................................................................................................6
                    Storm History        ..............................................................................................9
                         Tuesday, September 8           ............................................................................9
                         Wednesday, September 9             ....................................................................... I I
                         Thursday, September 10            ......................................................................... 12
                         Friday, September 11          ............................................................................. 13
                    Marine Conditions            ....................................................................................... 16
                         Initial Set-up     .....o....................o ................................................................ 19
                         Pressure Set-up         ....................................................................................... 19
                             Estimation of RMW and Vemax                ......................................................... 20
                         Wave Set-up       .......................o ................................................................... 26
                         Wind and Bottom Stress Set-up               ............................................................. 28
                         Wave Run-up         ....................................................................o .................... 29
                         Discussion     ..............................................................................o ............... 30
                    Mapping the Overwash             ........................................................o ...................... 33
                         Methodology         .......................................................................................... 33
                         Overwash Conditions          .............................................................    34
                         Kekaha     ...............................................o ................................................... 36
                         Waimea       .............o.................................................................................... 40
                         Hanapepe       ................................................................................................ 42
                         Kukuiula to Keoniloa Bay            ...................................................................... 44
                         Kapaa     .....................................o.....................o ......................................... 50
                         Other Areas       ............................................................................................ 52
                             Hanalei Bay         ....................................................................................... 52
                             Na Pali Coast       .......................o ............................................................. 53
                             Barking Sands       ........................................................o .......................... 53
                             Pakala Village      ..............................................................................o .... 53
                             Poipu to Nawiliwili         ........................................................................... 53
                             Nawiliwili to Wailua          ......................................................................... 54
                             Kapaa to Princeville         .......................................................................... 55
                    Environmental Response             ............................................................................. 56
                         Beach Erosion        .....................................................................o .................. 57
                         Poipu Offshore Survey           ........................................................................... 59
                             Coral Impacts       .................................................................................... 59
                             Debris Build-up        ................................................................................. 62
                             Field Observations         ............................................................................ 62
                             Recommendations..           ........................................................................... 63
                    References       ................................................................................................... 64


                                                                       2.





                                                                           Coastal Overwash During Hurricane Iniki



                                            COASTAL OVERWASH ON KAUA'I
                                                  DURING HURRICANE INnu


                                                            LIST OF FIGURES
                    Figure 1. Overwashed and wind-blown sand                     ..............................................5
                    Figure 2. An 80-unit condominium complex                     ..............................................7
                    Figure 3. The Bull Shed Restaurant               ............................................................8
                    Figure 4. Houses in the Kukuiula Harbor area                   ............................................ 10
                    Figure 5. Path of Hurricane Iniki            ................................................................. 11
                    Figure 6. Map of Kaua'i          ............................................................................... 14
                    Figure 7. Tide gauge records            ....................................................................... 18
                    Figure 8. Radial profiles of wind speed               ...................................................... 21
                    Figure 9. NOAA Hurricane Research Division                     ........................................... 22
                    Figure 10. Overwash on an open field, Waipouli                     ....................................... 32
                    Figure 11. 'Me overwash debris line               ........................................................... 35
                    Figure 12. Overwash map of Kekaha                  .......................................................... 37
                    Figure 13. The end of the Kekaha revetment                   .............................................. 38
                    Figure 14. Overwash was channeled down coastal avenues                          ....................... 39
                    Figure 15. Overwash map of Waimea                  ........................I ................................. 41
                    Figure 16. Overwash damage to a chain-link fence                     .................................... 42
                    Figure 17. Overwash map of Hanapepe                   ...................................................... 43
                    Figure 18. Overwash debris line near Kukuiula Bay                      .................................. 45
                    Figure 19. Overwash map between Kukuiula Bay and Keoniloa Bay                               ........ 46
                    Figure 20. Coastal damage between Kukuiula Bay and Keoniloa Bay                              ....... 48
                    Figure 21. Coastal damage at Poipu Beach Park                    ......................................... 49
                    Figure 22. Overwash map of Kapaa coast                   ................................................... 51
                    Figure 23. An overwash fan             ........................................................................ 54
                    Figure 24. The Na Pali Coast suffered little damage                    .................................. 55
                    Figure 25. Erosion at Waipouli beach. Accretion at Mahaulepu                          ................ 58













                                                                      3.





                                                         Coastal Overwash Duzing Hunicane Iniki






                           -EXECUTIVE SUMMARY-



                      Hurricane Iniki was a minimal Category 4 subtropical cyclone that
               crossed Kaua'i September 11, 1992. The eye made landfall at Kaumakani at
               approximately 3:20 pra and departed the north shore by 4:00 pm. The loss of
               life was miraculously minimized. Economic losses from high winds and
               marine overwash rank the storm as the fifth costliest insured catastrophe in
               U.S. history and the worst recorded storm in Hawaiian history.
                      This is a study of the marine overwash of Kaua'i during Iniki. The
               goals of the study are to provide a numerically estimated description of the
               meteorologic and oceanographic environment that contributed to the
               overwash, to map the overwash line at significant points around the island,
               and to describe impacts to the offshore environment of the south shore. The
               research was supported by the Hawaii Coastal Zone Management Program
               pursuant to N.O.A.A. Award No. NA360ZOO22-01, the National Coastal
               Geology Program of the U.S. Geological Survey, and the National Science
               Foundation.
                      Analysis of the initial set-up and astronomical tide, the pressure set-
               up, wind and wave set-up, and wave run-up provides a description of
               overwash components. These are compared to observed overwash elevations
               at two areas on the southern Kaua'i coast, near Kukuiula Bay, and on the
               Poipu-Koloa coast. Our estimations match the field observations.


               Location       Initial   S       Sww     ASw     Ruv     Total     Obs d
               Kukuiula       1.8ft   1.3?ft    5.3 ft  0.2 ft  9.3 ft  -18 ft    17.2 7
               Koloa-Po ipu   1.8 ft  1.21 ft   5.3 ft  0.2 ft  14 ft   22.5 ft  19.7-27.8 ft





                                                     4.






                                                        Coastal Overwash During Hurricane Iniki


                      Overwash maps are provided and discussed for five localities:
               Kekaha, Waimea, Hanapepe, Kukuiula to Keoniloa Bay, and Wailua to
               Kapaa. A continuous overwash line along the south and east coasts of Kaua'i
               is archived in. the State of Hawai'i Geographic Information System. GIS
               basernaps for the five regions displayed here depict the FEMA FIRM V-
               Zone and A-Zone, as well as the 1986 GIS shoreline, and Iniki overwash.
                      Diver surveys between Kukuiula to Makauena Point determined that
               no significant structural-debris accumulation is found in the marine
               environment. Comparisons of coral-distribution data spanning 20 yr indicate
               a one-third reduction in total coral coverage along the 30 ft depth contour.
               Along the 70 ft contour, total coverage has not changed, but previously
               dominant encrusting genera have been replaced by genus Pocillorpora.










                                                         0(41

















               lFigure 1. Overwashed and wind-blown sand on the coast road at.Kekaha.


                                                     5.






                                                            Coastal Overwash During Hurricane Iniki






                                      -INTRODUCTION-



                       Shortly after 3:00 PM (HST) on September 11, 1992, Hurricane Iniki
                 made landfall on the south shore of the island of Kaua'i. The eye passed the
                 Kaua'i coast near Kaumakani (-5 km west of Port Allen) with devastating
                 consequences. Sustained winds of about 130 mph, gusting to 160 mph, and
                 extensive coastal flooding commonly exceeding 10 ft in elevation, destroyed
                 or damaged 14,350 houses. Of these, 1,421 were destroyed, and another
                 5,152 suffered major damage. Statewide, an additional 607 houses sustained
                 damage or were destroyed. Six deaths are attributed to the storm, and over
                 one thousand injuries reported. A year later, the Property Claims Services
                 Division of the American Insurance Services Group, and the Insurance
                 Information Institute, ranked the $1.6 billion (1990 dollars) in losses from
                 Iniki as the fifth most costly insured catastrophe in U.S. history.
                       The principal goal of this report is to provide maps of the overwash
                 pattern on Kaua'i in and around the population centers of Kekaha, Waimea,
                 Hanapepe, Kukuiula to Poipu, and Kapaa. We also describe environmental
                 and meteorological factors that influenced overwash impacts in the coastal
                 zone of Kaua'i. Principal attention is given to the south shore of the island, in
                 the right forward quadrant of the storm as it made landfall. It was here that
                 overwash was most intense. Using field investigations, marine surveys, and
                 aerial photograph analysis, the overwash zone was mapped around the
                 island, and digitally recorded in the Office of State Planning Geographic
                 Information System. We also report on weather and marine conditions
                 during the overwash and discuss impacts to the offshore coral community.
                       Separate reports on various aspects of the Iniki tragedy are emerging
                 from various concerned economic and scientific sectors of our society. Many


                                                        6.






                                                                Coastal Overwash During Hurricane Iniki




















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                  Figure 2. An 80 unit condominium complex, lower floors destroyed by
                  overwash, upper floors destroyed by wind. Note the overwash debris line in
                                                                                                     @ 7-fw










                  the background. East of Kukuiula Bay, Kaua'i (see cover).



                                                            7.






                                                          Coastal Overwash During Hurricane Iniki


                of these have overlapping coverage, and, we have found, to some degree
                report conflicting details. To the extent possible here, we utilize the most
                reliable and reasonable of these in our o w*n. descriptions. Our approach is to
                estimate, through field data and simple numerical calculations, the
                meteorological and oceanographic processes that both mitigated and
                exacerbated the marine overwash of Kaua'i during Hurricane Iniki.
                       At this writing, one year later, Kaua'i still struggles to recover her
                lifes tyle and economy. It our sincere hope that the contributions in this
                report provide some lessons that may help mitigate future levels of tragedy.






                                                         J
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                                                ago,











               [FiVure 3. The Bull Shed Restaurant, Kapaa. Overwash reached >2 m.
                             @t L-1






                                                       Coastal Overwash During Hurricane Iniki






                                 -STORm HiSTORY-



                     Iniki was first identified as Tropical Depression (TD) 18-E
               approximately 2690 km southwest of Baja California over the warm waters
               of the eastern Pacific near , 120N 135OW on September 5, 1992.
               Meteorologists at the National Weather Service in Silver Spring, Maryland,
               and at the Central Pacific Hurricane Center in Honolulu, speculate that the
               storm may have originated in August as a mid-tropospheric tropical wave
               off the coast of Africa. An area of disturbed weather was tracked by the
               National Hurricane Center across Central America for several days when it
               moved into the Pacific on August 28. TD 18-E crossed 140OW and moved
               into the Central Pacific on the morning of September 6 heading west-
               northwest at 7 m/sec with maximum sustained winds of 15 m/sec. Initially
               expected to dissipate, TD 18-E did weaken under unfavorable wind shear,
               but by mid-day on September 7 the system became imbedded in a deep
               easterly flow along the southern edge of the seasonal subtropical high-
               pressure ridge extending from 40ON to 450N latitude between 130OW and
               170OW longitude. By 5 pm that day, TI) 18-E was upgraded to Tropical
               Storm Iniki moving west at 4.5 m/sec with maximum winds of 18 m/sec.
                     Tuesday, September 8 - Iniki slowly intensified as it moved west
               with maximum sustained winds of 20 m/sec. Forecast to head west-
               northwest, Iniki continued west at 7 to 7.5 m/sec throughout the day as
               winds strengthened to over 30 m/sec with gusts to 38 m/sec by 5 pm. At 11
               pm, Tropical Storm Iniki was upgraded to Hurricane Iniki at a position of
               13.20N 152.10W moving still west with maximum sustained winds of 34
               m1sec, gusting to 40 m/sec. With a forward translation speed of 7 m1sec,
               Iniki's central pressure slipped to 992 mb as the hurricane assumed a new


                                                   9.






                                                        Coastal. Overwash During Hurricane Iniki



















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                                                  4W                                   .Vqk



                                                                   Ail



               FIGURE 4. Houses in the Kukuiula Bay area, on the south coast of Kaua'i,
               were subjected to gusts reaching 64 m/sec and overwash reaching elevations
               4.5 to 5.2 m. Houses were swept from their foundations and carried inland as
               much as 100 m by the inundation. Those buildings that remained in place
               were battered by boulders and debris carried in the raging sea, and flooded
               through their lower levels. Simultaneously, the wind ravaged the uppermost
               floors. Demolished structures along the waterline, such as seawalls , poured
               concrete lanai's, and paving stone acted as battering rams when carried by
                 e overwash. Several homes sited on bluffs near. Koloa landing were
               damaged @y @reaking waves reportek exceeding 8 m in height.


                                                    10.





                                                                             Coastal Overwash During Hurricane Iniki





                                    9-13-92 5:00am
                                             65/80 mph
                                      Downgraded to
                                  tropical depression                                  P        T H           0 F,



                                 9-12-92 5:00prn                                     National Weather Service
                               75/90 mph 990 mb                                             Pacific Region
             300-        Extra-tropical in 17 'hours

                                                                           P a c i f i c
                         (9-11-92) 11:00prn                                                        0 c e a n
                        115/145mph 960mb
                            Storm weakening
                       0                               (9-11-92) 5:00pm 130/160mph 945mb
                                                       Conditions on Kaua'i improve
                        (9-11-92) 3:00pm
                     130/160mph 945mb               Kaua'i
                         Landfall imminent              @%O'ahu
             200 - (9-11-92) 11:00 am                            Maui  Hawai'i
                      145/175mph 940mb
                        Maximum strength                        0@
                                                        10-92) 1 1:00prn
                                                        0/155mph 947mb
                             (9-10-92) 5:00am          auai on alert           (9-8-92) 11:00prn
                          115/145mph 960mb                                     75/90mph 992mb
                      Turns sharply northward                                  Upgraded to hurricane       (9-7-92) 5:00pm -
                                                                               Iniki= "piercing wind"      40mph 1002mb
                                         (9-9-92)  5:00prn                                                 Tropical storm
                                    100/120mph 980mb
                                 Rapidly gaining strength
                         High surf advisory for all islands

              30'l          I                    I                  I                   I                   I
               N                             160OW                                   1500                                    1400



                     Figure 5. Path of Hurricane Iniki, and significant episodes in the evolution
                     of the storm (National Weather Service).
                     heading to the west-northwest by 5 am, the next day.
                             Wednesday, September 9 - At 6 am, high surf advisories for the
                     south-facing shores of the Big Island were issued. Predictions called for
                     waves between 1.2 and 2.5 rn high. When the swell eventually arrived, 11






                                                           Coastal Overwash During Hurricane Iniki


                hours later, it measured 2.5 to 3.5 m. By I I am, sustained winds had
                strengthened to 40 m/sec and gusts exceeded 49 m/sec. Expected to
                strengthen still further, the system continued tracking west-northwest and by
                late afternoon was approximately 484 km south of South Point, Hawail
                hai was now heading west-northwest at 6.3 m/sec with maximum sustained
                winds of 45 m/sec, and gusts to 54 m/sec.
                       At 5 pm, a high-surf advisory was issued for the south-facing shores
                of all Hawaiian islands. By 11 pm on September 9, a USAF reconnaissance
                plane found a "small intense hurricane with gale winds out 240 km in the
                north semicircle." At this point Iniki was gusting to 56 m/sec, and central
                pressure had dropped to 980 mb.
                       Thursday, September 10 - Iniki increased in intensity (sustained 52
                m/sec, gust 65 m/sec, 951 mb) but did not change from its heading of a day
                earlier. This situation was similar to previous hurricane systems that had
                ventured near Hawai'i. Had Iniki continued on its path toward the western
                edge of the Central Pacific subtropical high pressure ridge at 145oW, it
                would have never approached the State any closer than its earlier position
                south of the Big Island. However, further west, a large upper low pressure
                system and cold trough had moved southward along and just east of the
                dateline. A low pressure area associated with this movement developed near
                30ON just east of the trough. T'his was destined to turn Iniki to a more
                northerly track. The position of the storm at the time it turned and the path it
                followed would determine the level of threat to the islands of 0'ahu and
                Kaua'i.
                       During the morning hours of September 10, Iniki, located 675 km
                south-southwest of Honolulu, ominously slowed its forward motion to 5
                m/sec and maintained its earlier intensity. Throughout the afternoon,
                however, conditions changed as the storm grew in strength and moved on a
                new heading to the north-northwest. By 5 pm on September 10, Iniki turned
                to the northwest. Having continued to grow in intensity with top winds of 56
                m/sec, gusting to 69 m/sec, the center was now located near 160N and
                160oW, 645 km south of the city of Lihue on KauaT
                       A hurricane watch was issued at 5 pm for islands from Kaua'i to
                French Frigate Shoals. Reconnaissance plane and satellite pictures indicated
                a strengthening of the storm, and forecasters predicted the storm would
                follow a northwest track at about 4 m/sec. A National Weather Service news


                                                      12.






                                                                  Coastal Overwash During Hurricane Iniki


                  release at 5 prn forecast a slow weakening of the storm as it turned on a more
                  northerly track, eventually passing 320 km west of Kaua'i within 48 hours.
                  Surf was reported subsiding on the Big Island, and building to 2.5 to 3.5 m
                  elsewhere. The south shore of Kaua'i was warned of sustained, rough surf
                  that would damage shoreline property by Friday. By 8:30 pm the aler                't was
                  elevated. A Hurricane Warning was issued for Kaua'i and Ni'ihau, a Tropical
                  Storm Warning for 0'ahu, and a Tropical Storm Watch for Maui Co.


                         "Iniki has turned on a more northerly track. Eye forecast to move close to
                         Kaua'i and Ni'ihau. Position 16.80N and 159.50W with max. winds 125
                         mph and gusts to 155 mph. Iniki moving NNW 14 mph."
                                          National Weather Service Release, 8:30 pm HST Thu. Sept. 10, 1992


                         Within two and a half hours the alert status was elevated again. This
                  time 0'ahu was upgraded to a Hurricane Warning.


                                        "Hurricane Warning issued for 0'ahu ... Upgraded from
                         Tropical Storm Warning. Hurricane warning continued for Kaua'i and
                         Ni'ihau. Hurricane Watch continued for islands west of Kaua'i to French
                         Frigate Shoals. Position 17.50N, 160.OOW moving north 15 mph with max
                         winds 125 mph and gusts to 155 mph. Tropical Storm Watch remains in
                         effect for Maui Co."

                                         National Weather Service Release, 11:00 prn HST Thu. SepL 10, 1992


                         Friday, September 11 - Throughout the night of Sept. 10, and into
                  the early morning of Friday, Sept. 11 Iniki accelerated and strengthened as it
                  continued to turn to the north. All warnings and watches remained
                  unchanged as Iniki marched north at 6 m/sec with maximum sustained winds
                  of 65 m/sec, and gusts to 78 m/sec. By dawn on Sept. 11, residents of 0'ahu
                  and Kaua'i, who had been awakened earlier by several punctuated blasts on
                  the public siren system, were securing their homes and listening to
                  continuous media coverage on the progress and predictions of Hurricane
                  Iniki. "Life-threatening" surf of 9 ni or more was predicted for Kaua'i and
                  Ni'ihau by the evening. Building surf and waves of 3 to 6 m were forecast
                  for the south and west-facing shores of 0'ahu. Torrential rains and major
                  flooding were expected on Ni'ihau and Kaua'i by nightfall.


                                                             13.





                                                                    Coastal Overwash Dtifing Hunicane Iniki




                                                                               Departure
                                                                            -4:00 prn HST
                                                      Haena             Princeville


                                                                            Kilauea


                      Makaha Ridge               Kokee

                                                                                          Kapaa
                                                                                                       Map 5
                                                                                   Wailua River


                    Barking
                     Sands
                                                                                     Lihue


                              Kekaha            Waimea
                                                                                                Nawiliwili
                           Map 1                            Kalaheo                              Harbor
                                   Map
                                            Kaumakani.j
                           Landfall              Hanapepe port                Poipu
                      -3:20 prn HST                        Allen Kuku    la   oloa Makahuena
                                                         t ap 3             LandingI  Point
                                                                        Map 4
                                               10


                                                                            0                      lorni


                                                                            0             10km


                  FFigure 6. Mal) of Kaua'i, and path of Hurricane Iniki.
                           By late morning, 11 am, Iniki had reached maximum recorded
                   strength due west of Maui and 210 krn south-southwest of Lihue.
                   Reconnaissance aircraft sent back reports of sustained maximum winds of 65
                   m/sec gusting to 78 m/sec, and a central pressure of 938 mb, the lowest ever
                   recorded in a Central Pacific Hurricane. The eye diameter was only about 33
                   kin, a small tight center with intense energy. On Kaua'i, the first waves were
                   reported crossing the coastal highway at Kekaha and Poipu. Winds had built


                                                               14.






                                                          Coastal Overwash During Hurricane Iniki


                to 18 to 22 m/sec between Kekaha and Kalaheo. On O'ahu, the Civil Defense
                ordered that all persons residing within "300 ft" of all shores evacuate to
                higher ground. Flash flooding was forecast for 0'ahu, Kaua'i, and Ni'ihau.
                      Iniki moved northward toward Kaua'i throughout the day. A series of
                slight wobbles in the movement of the eye led to short-term speculation. that
                the storm had turned to the east and was on a path for the Kaua'i Channel,
                between the Islands of O'ahu and KauaT These reports came through the
                media at about the same time that power was being lost to much of O'ahu,
                and the Honolulu waterfront was being overwashed in places to an elevation
                of nearly 2 m. As power failed, many residents of O'ahu spent the afternoon
                fearing that Iniki would pass close enough to cause major damage to the
                entire island. However, by 3 prn Iniki was 60 km southwest of Lihue and
                moving north, poised to strike the coast of Kaua'i.
                      As Iniki made landfall west of Port Allen on Kaua'i shortly after 3 pm,
                the eye was only 18.6 km wide. A small eye diameter normally indicates that
                strongest winds are located near the eyewall (Trapp, 1993). Unfortunately'.
                no usable wind data is available from near the eye. The NOS tide station at
                Port Allen did record a low pressure of 960 mb at 3 pm, and 968 mb at 4 pm.
                Extrapolation of the hourly pressure record suggests a minimum pressure of
                about 952 mb at Port Allen (NWS, 1993). Twenty kilometers to the east, at
                Makahuena Pt., an anemometer recorded sustained winds at 36.3 m/sec
                before it failed. A peak gust of 64 m/sec was later extracted from the digital
                data recorder at Makahuena. At Lihue, 32 km east of the eye as it passed
                over the island, sustained winds of 43.5 m/sec (3:52 pm) and a peak gust of
                57.8 m/sec: (3:02 pm) were recorded.
                      By 5 pm, having left the island along a path between Haena and
                Wainiha, Iniki was already 80 km north of Kaua'i, and the Hurricane
                Warning for O'ahu was downgraded to a Tropical Storm Warning. In
                confusion, some 'officials concluded that this new warning heralded the
                approach of a second storm. All warnings for the State were either cancelled
                or downgraded by 11 pm as Iniki sped on to the north. High Surf Advisories
                were maintained the following day for south and west-facing shores of all
                islands. September 12 dawned with clear skies as citizens of Kaua'i faced the
                task of recovering from the worst recorded catastrophe in the State's history.




                                                     15.






                                                         Coastal Overwash During Hurricane Iniki







                              MARINE CONDITIONS -



                      Coastal overwash results from the movement of a hurricane across the
               ocean surface when it encounters land. Flooding of the coast is the product
               of a number of meteorological and oceanographic factors related.to:


                  -Initial set-up of the water from either an early arriving long wave, or
                      local sea-level anomalies, plus phasing of the astronomical tide
                  -Pressure set-up from decreased atmospheric weight on the water column
                  -Wind set-up from the shear of the winds across the water surface
                  -Wave set-up effects inside the breaker zone
                  -Wave run-up controlled by bathymetry, topography and surface
                      roughness
                  -Shape of the coastline, and bathymetric morphology


                      Other factors adding to overwash, such as rainfall and the Coriolis
               effect are usually considered negligible on open ocean coasts lacking run-off
               collection basins such as estuaries and lagoons, or shoreline features
               perpendicular to the mass movement of deflected water.
                      Typically the region in the right front quadrant of a moving storm
               experiences the greatest wind shear because the speed of the storm adds to
               the actual wind velocity. When a storm crosses a shoreline, or moves
               adjacent, the combined effects of storm surge and high waves can cause
               flooding, called overwash. Overwash can lead to coastal erosion, loss of soil
               fertility and standing-water quality by salinization, damage to buildings and
               transportation networks, interruption of power and communication grids,


                                                     16.






                                                         Coastal Overwash During Hurricane Iniki


                bodily injury and loss of life, and crop destruction. Numerous secondary
                effects may result from overwash, including landsliding, excessive coastal
                turbidity with deleterious effects on the coastal ecosystem, and even elevated
                seismic risk in seismically active regions associated with the dramatic
                pressure changes. The decrease in the weight of air in a hurricane can.be as
                much as 2-3 million tonnes per kM2of land over a matter of hours (Bryant,
                1991). Overwash on the order of 8-10 m in height can lead to an increase of
                pressure on the earth's surface of 5-7 million tonnes per kM2. Areas such as
                the south shore of the Big Island may be under enhanced seismic risk during
                landfall of a hurricane.
                      In the following estimation analysis we attempt to unravel the various
                components that additively comprise the rise in sea-level that led to flooding
                of the Kaua'i shore in the region between Kukuiula and Poipu. More detailed
                descriptions are provided in Agrawal (1993) and, based on his analysis,
                Bretschneider et al. (1993). We also refer the reader to the excellent
                modeling of Sea Engineering and Bretschneider (1986). Their worst case
                scenario predicts the overwash experienced on this coast during Iniki. A
                report currently being generated by the U.S. Army Corps of Engineers also
                tackles this problem (Yamamoto and Sullivan, 1993). These references
                provide the reader with more detailed descriptions of the numerical
                approach.
                      Our assumptions simplify our findings. We use a maximum deep
                water wave height, maximum wind speed, and we reduce the complicated
                coastal zone experiencing overwash to a uniform, rough slope extending
                approximately from the wave break point to the upland 6 m contour. Our
                analysis is only applied in two locations, east of Kukuiula Harbor, and
                between Koloa Landing and Poipu Beach Park. It is not our purpose to
                produce a new numerical model of overwash, rather it is our goal to
                understand the relative components of overwash. In so     'much as our final
                estimations of overwash are close to observed values, our analysis has
                achieved its goal.








                                                     17.






                                                                                         Coastal Overwash During Hurricane Iniki


                                        PORT ALLEN - HURRICANE INIKI
                                                       AS RECORDED AT NOS TIDE STATION
                                                                                                                               1020
                                                                         OR]) M GH. FID] 1: 6@. FT                    A'L
    f"                                 6                                                                                       IM0
    L                                                 C
                                       5
    L
                                       4                                                                                       990
                                0                                                                                                     0

                                       3                                                                                       980




                                       2                                                                                       970




                                                                                                                               960




                                       0                                                                                       950
                                                   aaw (pw mw i@ i@ m@ mw                       mw uw i&w 1@ um c@
                                                               TBM (UTC) 11-12 SEFIF 1992


                                               NAWILIWILI - HURRICANE INIKI
                                                             AS RECORDED AT NOS TIDE STATION
                                       7





                                             R         EU     TWE: 5.8FMFJ T

                                       5



                                       4


                                 0     3


                                       2                                                                A





                                       0
                                          lz,m L
                                               @m 019 L'rW IM M9 MW M.00 D&P, MOD ZM =59 MM OLM MM ffiM 000 ftW mw am 07:9 Gom Um I" 11M
                                                                   TIME IN UTC (11-12 SEPT 1992)
                                             AQUATRAK SENSOR                          - BACK-LJP PRESSURE SENSOR
                                  AQUAMARSENSOR REACRFDBL4NZMVG ZONEATABOUY5.0 FEET




                        Figure 7. Tide gauge records from Port ARen and Nawildwili duflng In
                                          I         xe  @AA










































                                                                                   18.





                                                                Co:astal Overwash During Hurricane Iniki

                                              INITIAL SET-UP

                         If a decrease in atmospheric pressure should suddenly be applied to
                 the ocean surface, a wave will be generated with a celerity that is depth
                 dependent. Over the open sea the velocity of propagation of such long waves
                 is 180 to 270 m/sec and thus the storm cannot keep pace. This long wave
                 arrives at the coast as a "forerunner", and may raise the still-water level as
                 much as 0.5 rn or more. Sea Engineering and Bretschneider (1986) argue
                 that the forerunner should not be treated as an explicit term because it is
                 accounted for in the wave set-up and pressure set-up. In Agrawal (1993) it is
                 treated with the astronomical tide. Values for initial set-up on the Texas
                 coast have been found to be 0.6-0.75 m (Marinos and Woodard, 1968).
                 Initial set-up values on the Atlantic coast are typically lower than either Gulf
                 coast or Pacific values.
                        Because the forerunner is poorly understood, and cannot be accurately
                 predicted, we will express it imbedded within the astronomical tide
                 component of the sea-level. Based on the available tidal hydrograph data
                 (Fig. 4), Bretschneider et al. (1993) estimate a value of 0.55 m above mllw
                 (local Hawaiian datum) for the combined tide and forerunner at Port Allen at
                 the landfall of Hurricane Iniki. In the hurricane vulnerability study of Poipu,
                 Kaua'i, Sea Engineering and Bretschneider (1986) use a value of 0.52 m (1.7
                 ft) for their scenario hurricanes because of the frequency of occurrence of
                 this tide level. Landfall of the eye of Hurricane Iniki unfortunately coincided
                 with the peak diurnal astronomical tide for the day, thus the resulting
                 overwash was enhanced by the phased. tide and surge. As Iniki crossed the
                 coast of Kaua'i, then, the initial water-level was at or near a maximum, and
                 damage was exacerbated.


                                            PRESSURE SET-UP

                        The water-level increase, or set-up (Sp), due to a pressure variation
                 (Ap) on the surface of the ocean is a function of the pressure drop from the
                 periphery to a point within the hurricane (Ap/p,,g), where p,, is the density
                 of seawater (1025 kg/M3). Myers (1954) developed an empirical relation for
                 the surface pressure distribution based on an analysis of historical data.


                                                           19.






                                                        Coastal Overwash During Hurricane Iniki



                                         Pa-Pr @ [(pa-p,) 100]( 1-e -Rmw/r)

               Where Pa is the ambient pressure (assumed to be 1013.3 mb), p, is the
               pressure at any distance r from the center, and pc is the central pressure of
               the storm (945 mb). RMW is the radius of maximum sustained wind, usually
               somewhere in the eyewall. Thus, the set-up becomes,

                                         SP   [(Pa-Pr)(l00)APswg)1                    (2)

                                             [(Pa-Pc)(l00)APswg)1 (I- e -RMwIr)       (3)

               Using (2), the set-up under the center of Iniki, where Pr= 945 mb, becomes
               0.68 m (2.2 ft). However, to calculate the pressure set-up outside the center
               of the storm requires an estimate of the distance of maximum sustained
               winds, RMW, from the center.


                            Estimation of RMW and Vemax

                     The clear eye radius is approximately 9.3 krn (reported diameter, 10
               nautical miles at sea prior to landfall; Trapp, 1993). Maximum winds, Vema,
               will be located an unknown distance beyond the eye, within the wall at the
               right frontal quadrant of the storm.
                     An approximation Of Vema,, and its location, RMW, begins with a
               theoretical consideration of the cyclostrophic. structure of a symmetrical
               vortex. The ratio of the square of maximum velocity to the radius of
               maximum winds (Vgma.,2/RMW) is estimated by the inverse of atmospheric
               density (1/1.1 kg/m3) and the pressure gradient across some representative
               distance (Ap/Ar). Dropsonde data near Kaumakani records a central pressure
               of 945 mb, while 5 krn away at Port Allen, the extrapolated pressure record
               suggests a minimum of 952 mb (NWS, 1993). If we choose a distance of 12
               km for the radius of maximum winds, an estimate of the sustained wind
               around the vortex center is given by




                                                    20.






   rpm
                                                                            Coastal Overwash During Hurricane Iniki


                                               (VOmax2)/(RMW) = G/Paun) (AP/Ar)                                    (4)
                                               VOmax =               1) (952-945) (100)) 12000] 0.5                (5)
                                                                              5000
   r                                                   = 40.98 m/sec

   PR                              80                                                                       +200
   U
                                         MOO GMT)                                            (0640 GMT)'
                                   70 -                                                                  -+too
   U
   r                               60 -                                                                  - 0
                                                                                                                3>
                                                                                                                C:)
                                'j; 50 -                                                                 - -1100 C-
                                                                                                                C:


                                                                                                                M

                                Lw'j 4 0 -                                                                  -200

                                                                                                                3>
                                                                                                                r

                                R- 30 -                                                                     -300


                                   20 -   HURRICANE ANITA                      Vorl= CONST               - -400
                                          SEPT. 2,  1977
                                          ALTITUDE   30Q M                              X=.5             - -500
                                   to-.              REL. WINDS                ........ X z.6
                                                     D VALUE
                                    01                        1     -   I         I         1        1     1-600
                                          150      too       50         0        50       too       1150
                                                            RADIAL   DISTANCE   (KM)


                     Figure 8. Radial profiles of wind speed (m/sec) and pressure in Hurricane
                     Anita. Note increased velocity to right of eye. Also shown are plots of the
                     Modified Rankine Vortex model, V@ r W for x--0.5 and 0.6 (Anthes, 1982).
                             This is the stationary velocity. Winds in the right frontal quadrant
                     would have an additional translation speed (Fig. 8). There is some dispute
                     about the actual speed of Iniki as in crossed the south coast of Kaua'i.
                     According to the Iniki Disaster Survey Rep                ort (NWS, 1992) the storm had a
                     forward speed of "30 mph" (about 13.4 m/sec). However, in its 3 pni HST


                                                                      21.





                                                       Coastal Overwash During Hurricane Iniki


               news release, prior to landfall, the    National Weather Service citing
               reconnaissance aircraft'data reported that "Hurricane Iniki was speeding
               up(,) moving north at 21 mph" (about 9.4 m/sec). Thus, the effective
               maximum sustained velocity, Ve,,., was in the range of 50.4 m/sec to 54-38
               rn/sec (52.4ï¿½2 rn/sec, for an average translation speed of 11.4 m/sec).

                   NOAA Hurricane Research Division Surface Wind Analysis
                             Hurricane Iniki 12 Sept, 1992 0006 UTC
                   based on data between 1 10OUTC 11 Sept.-0600UTC 12 Sept.
                  empirical adjustment of Air    Force recon winds from 10,000 ft.
                   ON
                             30
                                                                 for over-
              2 3. 5  -P'reli     hary
                                                          -expos u re


                3. 0



                                                       -17



                    0-
                                                           . . ....      -70 -









                1   0



             :::20  5


                            161.0 160.5       160.0    159.5 159.0      158.5
                     Maximum 1, min sustained surface winds       10 kt interval

                                                                      ............ ..... ........

               Figure 9. NOAA Hurricane Research Div. surface winds at landfall using
               empirically adjusted reconnaissance aircraft data (Powell, pers. comm.).



                                                  22.






                                                          Coastal Overwash Duzing Hunicane Iniki


                       An independent estimation of maximum winds can be performed by
                several means. Reconnaissance aircraft reported observations of maximum
                sustained flight-level winds of 65.5 m/sec (127 knots, 146 mph) south of
                Kaua'i. Powell (1987), in an analysis of the wind structure of Hurricane
                Alicia (1983) at landfall, suggests a conversion ratio of 0.78 for estimating
                surface winds from reconnaissance flight-level data. This suggests a value of
                51.1 m/sec (99 knots, 114 mph) for maximum sustained winds at or near
                landfall. Figure 9 shows the preliminary surface wind analysis using
                empirically adjusted reconnaissance aircraft data (Powell, pers. comm.). This
                is a close match to the estimate of'Vem,,., given above.
                       An alternate method utilizes the Atlantic Empirical Equation, which
                relates the difference of ambient and observed central pressure to the
                velocity of maximum wind (Vemax=14 [1013.3 mb - 945 mb]0.5) based on
                historical observations in the North Atlantic basin. This provides a value of
                59.6 m/sec (115.7 knots, 133 mph).
                       Our estimation of 52.4 ï¿½2 m/sec for Vom. is based upon a number of
                stated assumptions, each of which may legitimately be challenged.
                Nonetheless, the value is a reasonable one, and each of the assumptions, by
                implication, are therefore within the range of possibility. In addition, our
                purpose in estimating Vomax is towards the goal of increasing our
                understanding of the factors influencing marine overwash, and not motivated
                by pure meteorological curiosity alone.
                       A number of variables in this evaluation can be modified to allow for
                a range of Von,,, estimations. Tle extrapolation of the pressure trace at Port
                Allen to a value of 952 mb should not be accepted without qualification. It is
                not a directly measured value, nor is it theoretically derived. In fact, the
                assumptions upon which it is based were not reported. It is, rather, a "best
                guess" of what a continuous record might have provided. In addition,
                equation (3) extrapolates the pressure slope between the center and Port
                Allen as representative of the entire pressure field on the right side. Is this
                appropriate? Unfortunately, no other data to constrain the pressure field is
                available within the RMW. Likewise, the choice of 12 kin for the radius of
                maximum winds is a "best guess", arrived at iteratively, and should not be
                accepted uncritically. The Atlantic Empirical Equation suggests a Vomax of
                59.6 m/sec. Hindcasting with this value using (3) suggests that the radius of
                maximum winds is greater, on the order of 15 km for a translation speed of


                                                      23.






                                                          Coastal Overwash During Hurricane Iniki


                13.4 m/sec. This is also an acceptable result. So the derivation provided here
                is not a unique solution. Lastly, as we have stated, the speed of the storm is
                under question. The NWS (1993) estimates that Rai crossed Kaua'i between
                3:20 and 4:00 pm on Sept. 11, 1992. Covering a distance of 22.2 miles in 40
  r             minutes equates to about 30 mph, giving some credence to the higher
                number. Nonetheless, we choose to take an average of the two reported
  r             values.
                       An additional test for our assumptions is provided by the data
                available from Makahuena Point. A maximum gust value of 124 knots was
                extracted from the digital recorder there. Powell's (1987) analysis of the
                landfall of Hurricane Alicia provides a useful gust factor of 1.64, suggesting
                that maximum sustained winds at Makahuena Pt. were about 75 knots (39
                m/sec; 87 mph). By calibrating the Modified Rankine Vortex Model
                (Anthes, 1982) with Vemax, we estimate the maximum sustained winds.


                             Ve r (.5-.6) = Iniki Constant = (40.98) (12,000)0.55       (6)
                                                        = 7179.95 m2/sec
                                                     Vo = 7179.95/ r (.55)              (7)


                       Makahuena Pt. is a distance (r) of about 20 krn from the center.
                Equation (7) predicts a Ve of 30.9 m/sec at this location, which becomes
                42.3ï¿½2 m/sec for a translation speed of 11.4ï¿½2 m/sec. Thus, estimates of Vo
                at Makahuena Pt. using Powell's gust factor (39 m/sec) and the calibrated
                vortex model (42.3ï¿½2 m/sec) correlate to within 8%, a reasonable fit.



  PM


  U








                                                      24.






                                                           Coastal Overwash During Hurricane Iniki


                       Let us return to a consideration of the hydrostatic pressure set-up (SP)
                at points away from the center of hai. Using a value of 12,000 m for RMW,
                with Myers' (1954) pressure distribution relation, we can estimate the
                pressure set-up at two representative locations where extensive overwash
                occurred in the region from Kukuiula Harbor to Poipu.


                       Location                   Observed Overwash                1: (km)
                       (1) Kukuiula              5.2 m (17.2 ft)                  16.5
                       (2) Koloa-Poipu           - 6-8.5 m (19.7-27.8 ft)          19

                                           SP = 1(pa-PAl00)/(pswg)] (1- e -Rmw1r)        (3)
                                           S1 = [0.68] (0.52) = 0.35 m (1.2 ft)
                                           S3 = [0.681 (0.47) = 0.32 m (1.1 ft)


                       This is a time-dependent result however, and it is incomplete until we
                include a factor for the depth- and time-relative resonance of the hydrostatic
                set-up by a long-wave generated with the moving storm. These waves reach
                greatest amplitude when the speed of the storm equals the speed of a shallow
                water wave [(gd)0-5], and when the storm has travelled at the required speed
                for sufficient time for the wave to completely develop. A factor for the
                influence of this wave is used to correct the pressure set-up calculated by (3).
                The critical water depths for hurricane speeds of 10, 20, and 30 knots are 3,
                11, and 26 m respectively (Sorensen, 1978). Ewing, et al. (1954) report on a
                squall moving over the appropriate depth of southern Lake Michigan for 0.5
                hr. The hydrostatic set-up (0.03 m) was amplified by water column
                resonance and a 1-2 m surge was observed in the path of the squall-
                generated wave at Michigan City, Indiana.
                       Based on a laboratory study, Abraham (1964) suggests the long-wave
                surge can approach twice the value given by (3) if a storm moves over water
                of between 0.75-1.25 times the critical depth for a period of 1 hr or more,
                and triple that value for a depth range of 0.9-1.1 times the critical depth for
                the same period. hai moved at speeds between 8.9-17.6 m/sec (15-30 mph)
                in the period prior to landfall. The critical resonance depth can be calculated
                from [dc, = V2/g]. Thus 8.1 m < dcr > 31.6 m, and 0.75 dcrmin = 6 m, while
                1.25 damax = 39.5 m. According to Abraham (1964) if the hurricane remains



                                                      25.






                                                         Coastal Overwash During Hurricane Iniki


                in the depth range 6-39.5 in for an hour then resonance amplification
                becomes a factor of 2. The bathymetry of the southern Kaua'i coast is steep,
                and these two depths are an average of 2-2.5 km apart. At an average
                translation speed of 11.4 m/sec, Iniki would only have remained in this depth
                interval for 3-4 minutes. Since the resonance factor of 2 requires Iniki to
                have travelled this depth range for an hour, the resonance factor must be
                significantly less. But because Iniki was moving, the value must be greater
                than 1. We will use a value of 1. 1 for the resonance factor.
                      Agrawal (1993) defines a resonance factor, R, that is dependent upon
                wave celerity and storm translation speed, [(gd)0-5]2/([(gd)0.5]2 -V?). He
                chooses a value of R=1.2, but his criteria are not made clear.
                      Our values of resonance enhanced, pressure set-up are:

                      Location                  Hydrostatic        Combined (Sp)
                      (1) Kukuiula              1.2(l.1)              1.32 ft
                      (2) Poipu                 1.1 (1.1)             1.21 ft


                                           WAVE SET-UP


                      Waves generated by Iniki winds resulted in mass transport into the
                shorezone that elevated the water level, called wave set-up. In small-
                amplitude wave theory, water-particle motion in the water column is
                described with closed orbits, and therefore no net transport of water mass
                occurs as waves advance to the coast. In reality, measurements of water level
                inshore of the point where waves break reveals that sea levels increase with
                larger waves. Tbus, some mass transfer of water into the coastal zone must
                occur as a function of wave characteristics. In real waves, there is a small
                forward mass transport as water particles advance slightly with each orbit.
                The velocity of transport increases as wave steepness and wave celerity
                (speed) increase, and as the relative water depth decreases. As mass
                accumulates in the breaker zone, currents are generated by the return flow
                which may move quasi-uniformly along the bottom, or as rip currents spaced
                periodically along the coast, and as longshore currents.moving down a
                pressure gradient towards areas of lower set-up or offshore flow.


                                                     26.






                                                          Coastal Overwash During Hurricane Iniki


                       Laboratory and theoretical studies (USACE, 1984) suggest that wave
                set-up (S,,) can be estimated by
                                    S"'= 0.19 [1-2.82 (Hb/gT2)0.5] Hb                    (8)

                Where Hb is the breaker height in the surf zone, T is wave period. Typically
                wave set-up will amount to about 15% of the breaker height. In this analysis
                we neglect the influence of wave-set-down. Breaking wave heights of about
                9 m were reported in the region of the Poipu coast at the height of the
                hurricane. Calculations using solitary wave theory in Bretschneider et al.
                (1993) suggest that wave heights prior to breaking at Kewalo Basin, O'ahu
                were in the range 7-8.9 m with periods of 14 sec. Offshore Buoy 51002,
                located about 280 miles south-southest of Honolulu, recorded a significant
                wave height of 6 m, with a most probable maximum deep-water height of
                nearly 11 m. (NOS Buoy Data). However, Buoy 51002 is located somewhat
                off the path of Iniki and we can conservatively estimate deep-water wave
                heights (Hd) of about 12.2 m (40 ft) for positions along the hurricane track.
                Agrawal (1993) on the same basis assumes that Hd=45 ft. Equation (9)
                derives the relationship between breaker height and deep-water height,

                                    Hb=Hd/3.3(Hd/Ld)0-33                          (9)

                A deep-water wave length (Ld) can be calculated using (Ld=[g/2n]T2).
                Assuming a wave period of 14 sec, Ld= 306 m (1004 ft). Equation (9)
                predicts a breaking wave height of 10.7 rn (35 ft). Thus, (8) provides an
                estimate of the wave set-up, S,,w, of about 1.6 m (5.3 ft). Pressure set-up and
                wave set-up are additive:


                       Location                  Initial      SP           SWW
                       (1) Kukuiula              1.8 ft       1.32 ft      5.3 ft.
                       (2) Koloa-Poipu           1.8 ft       1.21 ft      5.3 ft.







                                                      27.







                                                             CoastaL Overwash During Hurricane Iniki


                              WIND AND BOTTOM STRESS SET-UP


                        A stress (ts) is generated as the wind blows across the water surface
                that is a function of air density (Pa), wind velocity (U m/sec), and drag (Cd),
 
                                          Ts=CdPaU2                                       (10)

                 Values Of Cd typically vary from 1.5x10-3 for light winds to 2.4x10-3 for
                 strong winds. Equation (10) is often written relative to water density (p),

                                          Ts= k pU2                                       (11)

                 where k =(1.19xl0-3)Cd. It has been shown (Van Dorn, 1953) that,
   
                                     k=1.21x10-6 + 2.25x10-6 (1-[7.2/U])2
  
                        Using our estimation of 52.4ï¿½2 m/sec for Vomax, k becomes 3x10-6.
                 The surface wind stress leads to development of bottom currents that
                 generate a bottom stress (Tb). Saville (1952) found that Ts/Tb=(3.3x 10-6)pU2.
                 Assuming that k =3.0X10-6, he suggested that Tb/Tb=0.1.This can be used to
                 determine a combined surface and bottom stress coefficient K, where
                 (Ts+Tb=KpU2). Thus, K becomes 3.3x10-6. Across a segment of seafloor 
                 normal to the coast of length AX, the depth changes from d to d+^Sw, where
                 Sw is the set-up due to wind and bottom stresses acting over the length ^X.
                 Because the water surface slope (^Sw/^X)is negligible,

                                     Ts^X+Tb^X+0.5gpd2-0.5gp(d+^Sw)2=0

                                     ^SW=d[([(2KU2^X)/gd2]+1)0.5-1]                       (12)

                 Equation (12) is applied sequential.ly along the bottom profile offshore of the
                 area of interest. Flat sections of bottom of constant depth, d, and length ^X
                 are used to calculate ^S,. The cumulative sum of preceeding set-up values
                 are added to (d) along the profile in the shoreward direction. The highest
                 ^Sw then becomes the wind and bottom stress component.


                                                        28.
 





                                                                    Coastal Overwash During Hurricane Iniki


                          Agrawal (1993) provides data on the bathymetry offshore of the
                   Koloa-Poipu region:


                                          Koloa-Poipu Region
  U                                       d (m)          AX (in)         AS,,
                                          5.5            361             0.06 in
  PR                                      18.3           626             0.03 in
  U                                       91.4           2022            0.02 rn
                                          183            2359            0.01 M
  U                                                                      ASwmax 0.06 m (0.2 ft)

  r
  U                In this way the combined wind stress and bottom stress set-up under Vema,,=
                   52.4 m/sec becomes about 0.2 ft for the Poipu coast. We win assume the
  r                same value for the Kukuiula coast.

                   Location                       Initial        SP             SWW             ASW
  U                (1) Kukuiula                   1.8ft          1.32 ft        5.3 ft          0.2 ft
  r                (2) Koloa-Poipu                1.8 ft         1.21 ft        5.3 ft          0.2 ft
  r                                               WAVERUN-UP

  U                       The extent of wave run-up on a slope is a function of permeability or
                   roughness, bathymetric and onshore slope, coastline configuration, breaking
                   wave height and wave period. In this estimation analysis, we neglect
                   coastline configuration by assuming the bathymetry consists of straight and
  r                parallel contours, a common assumption. Although it is not possible to
                   theoretically predict run-up because of the large number of variables, it is
                   possible to estimate run-up from a few simple parameters following the
                   guidelines of some laboratory studies (USACE, 1984).
                          The following conditions apply: Hd= 12.2 in, T=14 sec, and the slope
                   (tanO) from the break point to the 6 in onshore contour at Kukuiula is
                   approximately 0.02, and at Koloa-Poipu approximately 0.03. This approach
                   also neglects the influence of the water column between the break point and
                   the shoreline, and treats the run-up as a simple function of slope and


                                                               29.






                                                          Coastal Overwash During Hurricane Iniki


                permeability between the break point and the upland. Equation (13) is a
                modified form of Hunt's (1959) empirical formula relating onshore slope,
                breaker height and wave period to significant vertical wave run-up
                [Ruv=T(gHb)0-5tan0]. Horizontal excursion can be calculated with
  r             Ruh=Tr(gHb)0-5, where r is a roughness factor.
                      We have estimated a breaker height (Hb) Of 11 m. Agrawal (1993)
                calculates a probability parameter (1.5), using wave-energy analysis applied
  r             to a modified -Hunt formula. This factor converts from significant wave run-
                 p to maximum run-up, with a 2% exceedance, under given hydraulic
                conditions. Agrawal (1993) and Sea Engineering and Bretschneider (1986)
  r             u
  PR            assume a friction value of r=0.85 to estimate the effects of slope
  U             permeability and roughness which is a value used to describe grassy slopes.
                However, the shoreline at both areas of interest is composed of basalt
  r             boulders, rocky outcrops, seawalls, and various structures. Accordingly we
                will use r--0.65.
  r                                Ruv= (1.5)(0.65) T[Hb g]0.5 tanO                    (13)

                Thus, for Kukuiula Ru=2.8 m (9.3 ft), and for the steeper, Koloa-Poipu coast
                Ru=4.25 m (14 ft).


                                             DiscusSION


                Location       Initial Sp        Sww     ASw     Ruv     Total    Observed
                Kukuiula       1.8ft   1.32 ft   5.3 ft 0.2 ft 9.3 ft -18 ft       17.2 ft
                Koloa-Poipu    1.8 ft 1.21 ft    5.3 ft 0.2 ft 14 ft     -22.5 ft 19.7-27.8 ft


                      Our estimations provide    a picture of the relative contribution of the
                various components comprising the overwash of the south shore of Kauai.
                Clearly, from the point of view of damage in the coastal zone, the most
                important aspects of the overwash are wave induced set-up and wave run-up.
                Natural factors that mitigate the wave hazard are a gentle offshore slope,
                natural roughness of the coastal zone, and the presence of offshore
                topographic barriers such as a barrier reef and submerged headlands and


                                                     30.






                                                          Coastal Overwash During Hurricane Iniki


                rocky outcrops. The wave hazard is exacerbated by a steep offshore slope, a
                featureless bathymetry, initial water set-up, and the pressure and wind stress
                set-up. Features that mitigate the wave environment under incident
                hurricane-force winds should be considered in coastal zone management
                decisions.
                      The shape of the coastline is an important factor. Any sector of the
                shoreline that is V-shaped, including submarine topographic channels, will
                lead to lateral compression of the storm-surge wave, increasing the wave
                height. The size of such a coastal basin can also lead to resonance if it has a
                shape that matches the period of any entering wave.
                      The V-shaped shoreline at Koloa Landing is probably responsible for
                the highest level of overwash resulting from Hurricane Iniki. Located
                immediately to the east of Nahumaalo Point, overwash into this narrow
                embayment, and up the boat ramp and the adjacent Waikomo Stream Valley
                reached an elevation of over 22 ft. Immediately east of the boat ramp an
                overwash debris line behind a house measured over 26 ft above mean sea
                level. Offshore of Koloa Landing, to the immediate west and east, are two
                submerged rocky platforms, and between them a low-lying coralline-algal
                surface with a relative relief of about 15 ft. This submerged valley, leading
                directly to the Koloa Landing area must have channeled and heightened the
                storm surge and allowed deeper penetration shoreward of the relevant wave
                characteristics.
                      It is clear that careful evaluation of offshore morphology and slope
                should be a consideration in coastal management decisions with regard to
                development planning and zoning on shorelines with high hazard ratings.















                                                      31.






                                                    Coastal Overwash During Hurricane Iniki


  L













                                                  A

















                                                               -Ir




















  L
                                                A








              Figure 10. Top, overwash on an open field at Waipouli near Kapaa. Bottom,
              200m north of the top photo, roadways and houses suffered severe damage.
               Overwash causes little concern where development is adequately set-back
              jLrom the shore.
                                                            04,
                                                         IAMO


















                                                 32.





                                                           Coastal Overwash During Hurricane Iniki






                          MAPPING THE OVERWASH


                                           METHODOLOGY


                       On Tuesday, September 15, 1992 a research team composed of
                personnel from the University of Hawai'i, and the U.S. Geological Survey
                Center for Coastal Studies, conducted an aerial reconnaissance of the coastal
                zone of Kaua'i to assess the level of marine inundation and damage to the
                littoral environment. Videotapes of the entire coast were acquired to assist in
                later mapping exercises at an average elevation of 500 ft and a flight speed
                of 80 knots. Three days of ground-based surveying followed to identify the
                extent, and characteristics of overwash at Kekaha, Waimea, Hanapepe,
                Kukuiula to Kawelikoa Point, Wailua to Kapaa, Princeville, and Hanalei.
                Personnel from the University of Hawai'i conducted another three weeks of
                ground-based investigations at the same locations that autumn.
                        In February, 1993 a team from the University of Hawai'i conducted a
                series of marine surveys between Kukuiula Bay and Makahuena Point to
                determine the level of damage to the coral community, and the extent of
                debris coverage in the depth range 90 ft to the shore. The results of this study
                are described in a later section of this report, and in Krock and Neill (1993).
                       Aerial photographs at an enlarged scale of 1:1200, and high altitude
                NASA infra-red aerial photography provided coverage of all coastal
                population centers on Kaua'i. These were used in conjunction with field
                observations, and with survey data from the U.S. Army Corps of Engineers
                (Pacific Ocean Division), to construct maps of the continuous overwash line
                from west of Kekaha along the south and east shore of Kaua'i, to north of



                                                       33.





                                                          Coastal Overwash During Hurricane Iniki


                Kapaa. The overwash line was drafted from field data and aerial
                photographic interpretation onto digital basemaps supplied by the Office of
                State Planning. The overwash line was digitized on the Universal
                Transmercator Zone 4 Coordinate System and entered into the State of
                Hawai'i, Office of State Planning Geographic Information System under the
                file heading INIKLOWASH.
                       In the following sections we present and discuss maps showing the
                digitized hai overwash line, the 1986 shoreline, the FEMA Flood Hazard
                Zone V ("subject to inundation by the 100-year flood with the additional
                hazards associated with storm waves"; FEMA, 1987), and the FEMA Flood
                Hazard Zone A ("subject to inudation by the 100-year flood"; FEMA, 1987).
                These maps are State GIS products. Because of intervening shoreline
                movement between determination of the FIRM lines, and the overwash line,
                and differences in the FEMA shoreline and the State shoreline, there is some
                significant positioning inaccuracy in these maps. There are instances (see
                Waimea) where the FEMA FIRM lines wander slightly offshore. These
                obvious errors have been retained in the maps as an example of the inherent
                inaccuracies in this type of mapping technique in a dynamic environment.
                We estimate a positioning accuracy of +50 m.



                                   OVERWASH CONDITIONS


                      A total storm-surge height of 1.8 m above mllw is recorded at near
                peak high tide at Port Allen, Kaua'i located 5 km southeast of eye landfall at
                Kaumakani. At Nawiliwili Harbor, 30 km northeast of Kaumakani, a storm-
                surge height of 1.7 m (ml1w) is recorded. Moored wave buoys located 355
                kin southwest of Honolulu and 452 km south-southeast of Honolulu,
                recorded significant wave heights of 5.5 m and 6.0 m (10.97 m most
                probable maximum wave height), resp. Winds at Barking Sands Naval
                Facility, 22 km left of the eye, reached a maximum sustained speed of 31
                m/sec (69 mph) with gusts to 45 m/sec (100 mph). At Lihue, 32 km right of
                the eye, sustained winds of 43.5 m/sec (97 mph) with gusts to 57.8 m/sec
                (129 mph) were recorded. At Makahuena Point, 20 km right of landfall, a


                                                     34.






                                                       Coastal Overwash During Hurricane Iniki














                                                                                       A
















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                                           iji.i

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                                                             Or,





  7,           Figure 11. The overwash debris line east of Kukuiula Bay, south shore of
               Kaual Overwash reached elevations between 4 and 5 rn (mllw) along this
               coast. The debris line is composed of irrigation piping, housing debris, and
  L
             00'"'*














              ,[various structural elements from the coastal developments near the water. A


                                                   35.






                                                           Coastal Overwash During Hurricane Iniki


                maximum gust value of 124 knots was extracted from the digital recorder,
                and we have estimated that maximum sustained winds there were between
                39-42 m/sec (87-94 mph).
                       Overwash excursion and elevation are primarily a function of relative
                position and orientation to the storm core, and secondarily a function of   local
                shoreline geometry, topography and bathymetry, and marine conditions.
                Maximum overwash excursions (>200 m inland) and elevations (-8.2 m
                msl) are not coincident, apparently this is the influence of secondary forcing
                parameters related to bathymetry and topography.
                       As we have discussed, Iniki traveled north-northeast across Kauai,
                exiting east of Haena on the north shore. The south coast experienced the
                highest overwash, and broad patterns of overwash elevation and inland
                excursion are relative to Kaumakani.




                                                 KEKAHA


                       Ten kilometers left of the eyewall, in Kekaha, maximum debris line
                and still-water level heights averaged 3.4 m (msl). Damage from overwash,
                while relatively light with respect to other areas of the island, was related to
                high velocity overwash flooding at the first and second line of houses in the
                vicinity of Oomano Point. Shore-normal streets acted as conduits for
                channelizing overwash, but these flows generally remained confined to the
                paved roads and did not cause significant damage among dwellings away
                from the water front. Although the revetment along Kaumualii Highway
                (coastal road) was overtopped by the flooding, apparently the wall was a
                significant factor in mitigating overwash damage to much of Kekaha west of
                Oomano Point. The revetment, built to halt shoreline retreat and chronic
                erosion resulting from the updrift (east) construction of Kikiaola Harbor,
                effectively reduced both the volume and velocity of the overwash in the
                western and central portions of Kekaha.
                      The history of shoreline erosion was an exacerbating factor in Kekaha,
                as in many other overwash areas on Kaua'i, and identifies chronic erosion as
                one common preconditioning agent that tends to maximize flood damage.


                                                      36.






                                                                                                                                                                                                                                                                     Coastal Overwash During Hurricane Iniki








                                                                                                                                                                                                                                                                                                             E!
                                                                                                                                                                                                                                                                                                             CU


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                                                                                                                                                                                                                                                                                                                         17

















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                                                                                                                                                                                                                                                                                               ...... .. .
                                                                                                                                                                                                                                                                . . .. ... ......



                                                                                                                                                                                                                                                                                                                                                                                            .... ..... .


                                                                    =Fiure 12. Overwash map of Kekaha

                                                                                                                                                                                                                                                37.






                                                        Coastal Overwash During Hurricane Iniki


                      Abundant aeolian transport occurred in central and western Kekaha.
  r",          Beach sands there are coarse-grained carbonate, whereas in east Kekaha,
  IL           they are fine-grained volcaniclastics with abundant heavy minerals. Wind
               transport increased the amount of sand deposition occurring on the highway
               and immediately inland, requiring special efforts to clear roads for vehicular
               traffic.
                      Overwash excursion exceeded 200 rn at Oomano Point. The overwash
  L            line takes a significant shift landward, moving from approximately one half
               of a city block landward of the highway to a full block and a half landward,
               exactly where the protection afforded by the revetment ends.
                      Figure 5 demonstrates that throughout the region overwash exceeded
               the FEMA V-Zone prediction of 100-year flooding with hazardous wave
               action. The overwash line falls within the 100-year flood zone, and had all
  7            buildings been elevated to the suggested AE and/or VE elevations, the level
               of damage would have been significantly reduced. As noted in the FEMA
  7


                                                 0%








                                                ilk-



                                              oar-
  L










               Figure 13. The end of the Kekaha revetinent, and the erosional offset in the
               shoreline caused by Kikiaola Harbor in the distance.


                                                    38.




  7
                                                          Coastal Overwash.During Hurricane Iniki








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                Figu  e  14. Overwash    was channeled down coastal avenues, although
  7             velocity and total water volume was mitigated by the Kekaha revetment.
  L_            Building Performance Assessment Report (1992), homes that were elevated,
                even as little as 0.6-1 m above grade, often were spared flooding and
  7             sustained little flood damage. As also noted in that report, the level of flood
                damage prevented, reinforces the importance of properly elevating a
                structure to suggested elevations within flood-hazard areas.
                       The coast between Oomano Point and Kikiaola Harbor has undergone
                retreat at a rates of 0.15-0.6 m/yr (MOESE, 1992). Construction of the
                harbor in 1959 interrupted the net westward littoral drift and when erosion
                threatened the coast highway, the Kekaha revetment was built. Erosion
                continues west of the harbor, threatening the onshore stability of the west
                jetty which was flanked by overwash during Iniki. Flooding skirted, and
                partially eroded a dune system behind the west jetty and penetrated a local
                cemetary. Damage was minimal.
                       Between the harbor and the neighboring coastal town of Waimea, the
                shoreline is largely undeveloped and no significant structural damage was


                                                      39.






                                                           Coastal Overwash Dufing Hunicane InW


                sustained other than to some lands under agricultural use, and the native
                vegetation. Overwash fans, extending landward 50 to 150 m, are noted in the
                aerial photography demonstrating that a large volume of sediment was
                removed from the beach and deposited beyond the active littoral zone. Much
                of this may be returned to the beach and littoral dunes by the tradewinds.
                Possible salinization of the worked soil may have occurred. The
                geomorphology of the backshore region is uniform, and the overwash line
                shows no significant features other than as reflected by variations in run-up
                and wave energy. Overwash elevations are approximately 1.8-2.4 m (msl).



                                                 WAIMEA


                       In Waimea, maximum        overwash heights averaged 2.6 m (msl).
                Landward excursion of the flooding was        generally confined to the back
                beach and immediate areas (10-30 m). Marine flooding damaged the lower
                level of the first row of houses on the west bank of the Waimea River mouth.
                       The highest measured overwash in Waimea occurred near the river,
                -2.5-2.8 in. Debris lines located below the top of the embankment revetment
                at the river mouth suggest that only minor overtopping occurred. Thus,
                flooding onto the low-lying coastal plain adjacent to the river was
                predominately marine, and did not contain any notable contribution from
                riverine discharge. It has been noted elsewhere that rainfall during Hurricane
                Iniki was generally below expected levels, and little actual river flooding
                occurred on Kaua'i. Precipitation at Lihue was approximately 2.3 cm, and 3
                cm at Princeville (Trapp, 1993). 'Me notable increase in marine excursion
                near the river was the result of the generally lower elevation around the
                mouth, rather than actual fluvial flooding. The east bank of the river is a high
                basalt exposure several meters in elevation and marine overwash was
                confined to a gravel beach located at the base of the headland.







                                                      40.





                                                         Coastal Overwash During Hurricane Iniki




















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               FFiaure 15. Overwash map of Waimea.


                                                     41.






                                                  Coastal Overwash During Hurricane Iniki




                                        HANAPEPE


                   Hanapepe, 4.5 km right of landfall at Kaumakani, experienced
              maximum overwash heights exceeding 2.8 m (msl). The most extensively
              flooded area was on the low-lying fluvial floodplain east of the mouth of the
              Hanapepe River. Residents report Iniki overwash penetrated several hundred
              meters inland along this lowland, to the edge of the coastal highway (Rt. 50)
              where it crosses the river. The bridge there is a critical transportation link for





























                   e 16. Overwash damage to a chain-link fence, Hanapepe Coastal


                                               42.




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                                                                                                                                                                                                                                    I. Ittzoll
                                                                                                                                                                                                                                ....... . . ....
                                                                                                                                                                                                                             EMA04000






                                                             Coastal Overwash During Hurricane Iniki


                 communities in west Kaua'i and a determination should be made as to
                 whether the bridge was at any point threatened by the overwash. This region
                 falls within the FEMA 100-yr flood Zone A, but beyond their determination
                 for the marine hazard Zone V.
                       Extensive damage to the coastal park in Hanapepe Bay occurred. A
                 revetment was one source of large boulders that were carried in suspension
                 by the overwash and destroyed park facilities including a bathing house and
                 tennis court fencing. Soil erosion, evidenced by scarping of terrigenous
                 deposits behind the revetment, led to high turbidity levels in the Bay.



                               KUKUIULA To KEONILOA BAY


                       At Kukuiula, 16-17 km right of the eye at landfall, maximum
                 overwash around the Bay, and in the fields to the east, measured between
                 3.8-4.9 in (msl). Because of the low relief, the gentle onshore gradient, and
                 the absence of offshore barriers, this sector experienced a high degree of
                 inland penetration by marine waters. The overwash passed through a line of
                 coastal dwellings, across a road and drainage ditch and carried debris deep
                 into the adjacent agricultural fields. Aerial photos of the resulting debris line,
                 in places over 250 in inland, have been widely published (Fig. 11). These
                 provide telling testimony to the power and destructive potential of overwash
                 in the region.
                       This same reach of coast was devastated by Hurricane Iwa in 1982,
                 and was the subject of numerical modeling under contract to the U.S. Army
                 Corps of Engineers in 1986 (Sea Engineering and Bretschneider, 1986). That
                 work reports on scenario models that predict the overwash expected during
                 an Iniki-type storm. One lesson learned by this experience is that numerical
                 models of hurricane overwash have been developed to a high level of
                 reliability, and should be incorporated in future planning along the Hawaiian
                 coastline. Another lesson learned through the combined overwash under Iwa
                 and Iniki, is that the coastal developments around and east of Kukuiula Bay
                 are vulnerable to marine flooding and massive damage. Despite the obvious
                 consequences, rebuilding activity proceeds unabated at this writing along


                                                       44.






                                                         Coastal Overwash During Hurricane Iniki










                                                       Apt#
                    7


































               Figure 18. Overwash debris line near Kukuiula Bay.



                                                     45.





                                                                                                                                                                                                                                                                                           Coastal Overwash During Hurricane Iniki











                                                                                                                                    ...... .....


















                                                                                                                                                                                                                                                                                         ............














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                                                                         =Fiure 19. Overwash map between Kukuiula Bay and Keonfloa Bay-

                                                                                                                                                                                                                                                                   46.






                                                          Coastal Overwash During Hurricane Iniki


                this twice-devatated coast.
                      The overwash line runs well inland of coastal developments between
                Kukuiula Bay and Nahumaalo Point. Single family dwellings, multi-unit
                condominium structures, and resort hotels were all subjected to high-velocity
                marine flow. A double debris line in places (Fig. 18), suggests that the
                overwash occurred in waves and that structures were subjected to multiple
                episodes of flooding in a short space of time. Dry stream-beds and former
                natural drainage valleys contain evidence that marine penetration of inland
                regions was heightened by locally lower elevations. Several low-lying
                regions, some over 100 rn from the coast, contained extensive ponded sea
                water and were clogged with housing and pavement debris. Local pocket
                beaches along this coast experienced severe erosion, and the resulting
                exposure of underlying soil led to high levels of coastal turbidity that
                persisted for over a month after the hurricane. Beach accretion has been slow
                to develop and at this writing, a year later, many sections of beach remain in
                an eroded state and have not fully recovered from the effects of Iniki.
                Although we cannot determine if there was pre-existing ecosytern
                degradation, the prolonged turbidity in the Kukuiula to Poipu coastal zone
                may be responsible for some shallow-water coral mortality and algal growth.
                      Because of the intensity and extent of overwash in this sector a
                number of structures sustained damage from high-velocity flooding,
                breaking wave forces, foundation scour and undermining, and battering by
                suspended debris in the water column. There are field reports of hydraulic
                buckeling of cement slab-type foundations, and examples of heightened
                damage from large boulders and paving stone literally battering down walls.
                At Poipu Beach Park coconut trees bear impact scars to a height of 1.8 in
                above ground level. Smaller structures were frequently swept off their
                foundations and either floated inland, or experienced destruction of the
                lower floors while upper floors and roofs survived to be deposited some
                distance inland. Residents at Nahumaalo Point report that wave spray against
                the headland overtopped three-story structures, and had sufficient velocity to
                break windows and damage roofing materials.
                      The highest elevation of overwash evidence, a debris line behind a
                small dwelling, was discovered by surveyors at the U.S. Army Corps of
                Engineers. Tlis was in the area immediately east of Koloa Landing. As
                previously mentioned, the configuration of the coast and bathymetry there


                                                     47.






                                              Coastal Overwash During Hurricane Inild












                          7










                                 -'S




















             Figure 20. Coastal damage between Kukuiula Bay and Keoniloa Bay
             included beach erosion, coastal turbidity, structural damage from high-
             velocity overwash, and salinization of terrestrial envirom-nents in the coastal
             zone. Note overwash ponding, and the debris line behind this hotel.



                                           48.






                                                        Coastal Overwash During Hurricane Iniki


                apparently led to maximum run-up and breaking wave heights, and
                amplification of stonn surge components. Debris on. the road at the head of
                the boat ramp at Koloa measured 7 m in elevation. While to the east the
                maximum debris line was measured at over 8 m. Nearby, at an elevation of
                nearly 6 m was an overwash debris line consisting of stone blocks weighing
                several hundred pounds, and large household appliances.
                      Toward the Poipu coast the overwash line meanders inland behind
                townhouse and condominium developments, and the Sheraton Resort Hotel,
                keeping in the elevation range between 3.5 and about 5 m. Large ponds of
                brackish water mark upland areas where return flow collected in local basins
                (Fig. 20). In the parking lot behind the Waiohai Resort, automobiles were
                actually stacked atop one another by the overwash. The much of the
                subdivision landward of Poipu Beach Park and Brenecke's Beach, a popular
                recreation site, was overwashed. A still-water mark in a house on Pane Rd.
                measured 5.6 m. The small beach in front of the seawall at Brenecke's,
                originally destroyed by Hurricane Iwa and having only returned in recent
                years, was completely eroded by Iniki. One year later it has not returned.
                F------












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                                                            UFA

                                                                                     6W@




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               =Fiiare 21. Coastal damage at Poipu Beach Park.
                                                  7----

























                                                    49.






                                                         Coastal Overwash During Hurricane Iniki


                The seawall at this locality was damaged by Iniki, and this is an excellent
                opportunity to remove it completely in order to encourage beach recovery
                and long-term stability.
                       On Makahuena Point, a headland with a coastal elevation exceeding 6
                m (msl), the development is sufficiently set-back, and the protection
                afforded by the cliffs sufficiently effective, that little overwash damage
                occurred. That portion of the headland composed of carbonate sandstone
                (aeolianite) did experience some hydraulic undermining from wave action.
                This was also noted across Keoniloa Bay at Makawehi Point, another
                limestone headland. While no slumping was observed on basaltic coasts,
                there was overhang collapse and slumping along many sectors of limestone
                shore, demonstrating that a hazard is associated with development on these
                sites. '
                       The beach at Keoniloa Bay was temporarily eroded, and beachrock
                exposed. Accretion, and the construction of a partially buried, armored
                overwash barrier, has since taken place. Overwash in the region of the Grand
                Hyatt Hotel exceeded 4 m, but damage was min     imized by the wide set-back
                there. Unconsolidated coastal dunes in this area were cut and channelized to
                a depth of over 2 rn where overwash apparently forced breaching of the
                backshore area. To the north, Mahaulepu Beach was overwashed to an
                elevation of 3 to 4 m, but nonetheless experienced massive lateral beach
                accretion exceeding 10 m because of a large sand supply available from the
                adjacent dune system there.



                                                 KAPAA


                       The highest overwash on the east coast of Kauai, in and near the town
                of Kapaa, was over 3 m, but averaged approximately 2.5-2.8 m. Because the
                Kapaa coast is oriented north-south, the winds of Iniki did not blow directly
                onshore for sustained periods and the additional translation speed of the
                storm did not exacerbate the overwash. The coastal orientation was oblique
                to most of the surge components. Also, waves generated by Iniki had to
                refract around the south coast of the island before striking the Kapaa


                                                     50.






                                                        Coastal Overwash During Hurricane Iniki


               shoreline. As a result, breaking wave heights were in the range 3-5 m. Kapaa
               generally did not experience extreme overwash. Nonetheless, there were
               pockets of extensive damage in the first row of houses on coasts with a
               history of chronic erosion. One of these, the Waipouli neighborhood,
               suffered overwash damage as a result of crowded shoreline development,
               low-grade housing lacking the minimum elevation suggested by FEMA
               flood maps, and a condition of pre-existing erosion.








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                                             n
                                                Rd
                                              a



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                                                                    h    on
                                                                      Wan


              =Fjure 22. Overwash map of Kapaa coast.


                                                   51.






                                                         Coastal Overwash During Hurricane Iniki


                      Frontage properties in Papaloa, Waipouli, and Kapaa Town suffered
               damage from overwash, flooding, road washouts, and bank failure under
               wave assault. Coastal erosion was prevalent and beachrock exposure was a
               frequent tell-tale that entire beaches had been eroded. 71is was especially
               true where shorelines were armored, or soil and rock cliffs marke        'd the
               landward extent of the beaches. The region between Waikaea Canal, which
               traps littoral sediment from the south, and Moikeha Canal, which traps
               littoral sediment from the north, has experienced chronic erosion for over a
               decade. Major portions of this coast have lost their beaches and thus had no
               natural buffer to absorb wave energy and dissipate run-up. Fortunately much
               of the development in this area is set-back behind Kapaa Beach Park, and
               while overwash reached significant elevations (3.4 m) and landward
               excursion was on the order of 25 to 50 m, structural damage was limited.
               South of Waikaea Canal lies yet another jettied canal at Waipouli. This, like
               the others, is unfortunately responsible for effectively trapping the littoral
               transfer of sediment. A condition of sediment starvation along nearly 2.5 km
               of coast is responsible for exacerbating what probably would have been a
               significantly less damaging overwash environment. Removal, or at the least
               truncation and lowering of these jetties would not only improve coastal
               sediment flux and lead to reduced beach erosion, possibly beach accretion,
               but more robust sediment storage along the coast would provide some
               mitigation against future overwash damage.



                                           OTHERAREAS


               HANALEI BAY: Overwash averaged 1-1.5 m and breaking wave heights
               were negligable because first winds (as storm approached) blew westerly
               and offshore, and second winds (as storm retreated) reversed to the east,
               while remaining offshore. Wind damage to Hanalei Town was extensive,
               and housing debris was noted on the seafloor at offshore sites in Hanalei
               Bay. No river flooding occurred, and the only coastal impact from the storm
               was post-event turbidity from deforested drainage areas. No observable reef
               damage resulted from the storm.


                                                     52.






                                                           Coastal Overwash During Hurricane Iniki


                NA PALI COAST: Although the media initially reported that the Na Pali
                Coast, on the north shore of Kaua'i had "geologically aged thousands of
                years", in fact there was little environmental damage to the coastal zone. Our
                reconnaissance of this coast, by helicopter, revealed little evidence of beach
                erosion, no slumping of the headlands, and no evidence of alteration to the
                unique coastal architecture of caves and arches.
                       Turbidity of coastal waters was noted adjacent to streams draining
                uplands that had experienced deforestation and uprooting in runoff areas.
                But this was minor in comparison to the extensive turbidity of the Kukuiula-
                Poipu coast. We suspect that the relatively light rainfall associated with Iniki
                was an impotant reason that environmental damage was reduced in the Na
                Pali coastal zone. Another mitigating factor was the direction of storm
                movement and predominate winds that placed this sector in the lee of
                maximum sustained winds.


                BARKING SANDS: Visits to Barking Sands recreational area and the
                surrounding dune fields revealed no environmental     damage. Comparisons to
                pre-Iniki aerial photography showed no significant beach erosion, and some
                isolated cases of new dune formation.


                PAKALA VELLAGE: Pakala Village coast was overwashed to an elevation of
                over 3 m and the first line of houses experienced flooding. There was also
                flooding in the cane field to the immediate east of town behind a small
                coastal bluff as evidenced by a line of dead plants. A history of erosion, and
                poorly engineered shoreline protection structures exacerbated the impact of
                the storm. No data is available regarding historical erosion rates at the site.
                The beach has disappeared along much of the fronting coast at Pakala, and
                trees are toppling into the water. Apparently erosion has been chronic and
                rapid.


                POIPU To NAWILIWELI Local beach erosion occured at Gilliens Beach, a
                local popular recreational site. As mentioned earlier, accretion was also
                noted in the area where beaches are backed by large dune fields. Cultural
                artifacts were reported exposed by the erosion at Gilliens, and strewn by the
                waves. Maximum overwash measured over 4.5 m in elevation. Carbonate




                                                      53.






                                                 Coastal Overwash During Hurricane Inild


             aeolian headlands and cliffs were hydraulically excavated to the point of
             overhang collapse at several localities.

             NAWELBVILI TO WAELUA: Nawiliwili Harbor sustained damage that was
             noted from aerial reconnaissance only. A large sailboat was washed
             broadside against the rock jetty, and several vessels sustained damage from
             wave surge in the harbor. Of those vessels that were hauled out, collapsed
             blocks apparently resulted in damage as they fell against the pavement.



                                            V!




























             Figure 23. An overwash fan and dead sugar cane near Pakala Village.



                                             54.






                                                       Coastal Overwash During Hurricane Iniki


               Local beach erosion was noted along the ocean coast. Several incidents of
               coastal turbidity were observed, and the Wailua Golf Course revetment
               failed at a site that has suffered chronic erosion adjacent to a break in the
               fringing reef.

               KAPAA TO PRINCEVILI : The natural coastal environments along this sector
               apparently sustained little environmental damage. There was widespread and
               massive defoliation of the upland forests that led to fears of stream mouth
               debris build-up, a hazard that can produce flash flooding and further
               environmental damage. Many of the beaches in this sector are boulder ramps
               and suffered little under the high waves. Occasional overwash was noted
               from the air, to elevations of approximately 1 rn or less. Rarely was the
               back-beach envirorunent overwashed.





















              =Fiure 24. The Na Pali Coast suffered little coastal zone damage.


                                                   55.






                                                              Coastal Overwash During Hurricane Iniki






                       -ENVIRONMENTAL RESPONSE-


                        Environmental response to the overwash varied. Beaches suffered the
                 greatest erosion on stabilized shorelines where steep, energy-reflective faces
                 interfered with overwash. In unstabilized regions both erosion and accretion
                 occurred. Beach erosion exposed ferruginous soil beds that leached silt into
                 coastal waters, and created sustained turbidity following the storm. Surveys
                 of coral response in the region of greatest overwash (Kukuiula to Poipu)
                 revealed slight mechanical damage to the sparse Porites sp. and Pocillopora
                 sp. community in the nearshore region, and negligible damage in deeper
                 offshore waters. Heightened algal growth, and coral mortality in waters
                 shallower than 5 m may be a response to post-storm turbidity.
                        A short list of environmental effects in the coastal zone includes:
                 defoliation of standing forests leading to mortality and debris build-up; soil
                 erosion from highland regions and coastal settings; overwash mortality to
                 coastal vegetation and agriculture; local soil and water salinization; impact
                 damage and uprooting of trees under intense overwash; beach erosion and
                 accretion; coastal dune channelization and erosion, some dune formation and
                 migration; coastal soil exposure by beach erosion and leaching of soil into
                 coastal waters causing turbidity and siltation; some coral abrasion; possible
                 coral mortality by turbidity in the shallow zone between Kukuiula and
                 Makahuena Point; structural debris in the nearshore zone; boulder-strewn
                 fields across the coastal upland under intense overwash; undercutting of
                 carbonate headlands; debris build-up by overwash.
                        In the remaining sections of this study, we report briefly on beach
                 erosion, Poipu coral coverage, and offshore debris build-up.




                                                          56.






                                                             Coastal Overwash Dufing Hw-ricane Iniki


                                            BEACHEROSION

                        While many beaches sustained little erosion during Iniki, and cases of
                 accretion were noted, the more common response of impacted beaches was
                 to erode. We have made a qualitative assessment of beach erosion by
                 measuring beach width on aerial photographs of the coast between Kukuiula
                 Bay to Makahuena Point, and between the Wailua River mouth to north
                 Kapaa Beach Park. Black and white photographs taken 7/28/87 at a scale of
                 1 "=500', were compared to a set of color photos at the same scale taken on
                 9/18/92. No photogrammetric, or tidal corrections have been applied in this
                 assessment. Our criteria for erosion were simply the recognition of minor
                 subaerial beach narrowing (ca. <15 m change over the period 1987-1992), or
                 significant subaerial narrowing or disappearance (ca. > 15 rn change over the
                 period 1987-1992). We cannot deten-nine when the narrowing may have
                 occurred, but we feel there is some likelyhood that Iniki played a role on
                 those beaches where significant narrowing was noted over the 5 year period.

                 Kukuiula-Makahuena Point                 Wailua River-Kapaa Beach Park
                 Kukuiula Bay beach (min.)                Wailua River State Park beach (sig.)
                 East of Ekaha Point (sig.)               Wailua Bay beach (sig.)
                 Hoai Bay (min.)                          Kapaa Beach Park north end (sig.)
                 Kaheka beach (sig.)                      South of Waipouli (min.)
                 Kihouna Point, E&W (sig.)                Waipouli coast (sig.)
                 Keoniloa Bay beach, middle               Waipouli Beach Park (sig.)
                        portion (sig.)                    North of Moikeha Canal (sig.)
                 Poipu Beach Park (sig.)                  North of Waikea Canal (sig.)
                 Brennecke's beach (sig.)                 South of Moikeha Canal (sig.)
                 Total length: 855+100 m                  Total length: 3045+100 m

                        The total measured length of significantly-eroded beach between
                 Kukuiula Bay and Makahuena Point is 855+100 m. Between the Wailua
                 River and north Kapaa Beach Park it is 3045+100 m. These lengths include
                 beaches that have either disappeared or significantly narrowed over the
                 period 7/28/87 to 9/18/92.




                                                         57.






                                                              Coastal Overwash During Hurricane Iniki

















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                1IFigure 25. Top, erosion at Waipouli beach. Bottom, accretion at Mahaulepuj
                      'IT














































                                                         58.






                                                          Coastal Overwash During Hurricane Iniki



                                  POIPU OFFSHORE SURVEY


                Coral Impacts - Divers surveyed coral coverage at 16 stations along the 30
                ft and 70 ft depth contours in the region between Makahuena Point and
                Kukuiula Bay (Krock and Neill, 1993). These sites were chosen to duplicate
                stations used in a similar survey conducted in 1972 as part of a water quality
                management study for the county government (Sunn, Low, Tom & Hara,
                Inc., 1973). Sites were spaced approximately at 0.5 mile intervals along each
                contour. Sampling procedure consisted of photographing 2 to 3
                representative bottom locations at each station with a reference scale and
                station identification number. The percentage of coral coverage in the
                photographs was determined by planimeter. The coral were grouped into two
                classes: 1.) those that present little cross-sectional profile to the nearshore
                hydrodynamic environment, consisting principally of the encrusting genera
                Porites and Montipora; 2.) and those with a larger cross-sectional profile,
                head coral, principally the genus Pocillapora.
                       Dr. Ralph Bowers, the biologist responsible for the 1972 survey,
                revisited the region following Hurricane Iwa and reported extensive damage
                to the coral community along both depth contours. However, his
                observations were not quantitative, and do not serve as a basis for numerical
                comparison (Krock and Neill, 1992).
                       Data in the following tables is from Krock and Neill (1992), and serve
                to illustrate several points. Over the period of observation there has been
                approximately a one-third reduction in total coral coverage on the 30 ft
                contour. This is supported by diver observations of abundant broken
                Pocillapora , and abraded and broken encrusting corals at this depth. The
                greatest damage was observed where unconsolidated boulder pavements are
                present. The 30 ft contour was the approximate depth of initial wave
                breaking during Iniki, and it is likely that this high-energy environment
                resulted in the entrainment, and saltation of many of the clasts.
                      The data suggests that there has been little overall change in coral
                coverage at the 70 ft contour. The previously dominant encrusting coral at
                this depth has, however been replaced by higher profile Pocillapora. The
                1972 survey showed an encrusting coral and head coral mean coverage of


                                                      59.






                                                                          Coastal Overwash During Hurricane Iniki


                     about 4% and 0.5% respectively. By 1993, the relative coverage had
                     changed to approximately 0.9% and 5% for the encrusting and head coral
                     respectively. Although damage was noted at this depth following Hurricane
                     Iwa (Bowers, pers. comm.), divers in the 1993 survey found very little
                     evidence of damage despite the presence of unconsolidated boulder
                     pavements.
                             If high levels of damage were sustained at 70 ft during Hurricane Iwa,
                     it is likely that the current abundance of Pocillapora represents
                     recolonization of the substrate. Although tropical storms had passed nearby,
                     prior to Iwa, Kauai had not sustained a direct hurricane hit since Dot in
                     1959. The intervening 22 yr may have been enough to allow sustained coral
                     colonization that was especiaRy vulnerable to damage. Sport diver reports
                     from 0'ahu suggest that Hurricane Iwa caused more damage than Iniki.
                     Possibly this is a result of the relative length of quiet periods between major
                     storms. It is unclear if the benthic environment during Iwa was significantly
                     more energetic than during Iniki, but there is no hydrodynamic argument to
                     support this conclusion.




                                                                  Table L
                                         Percent coral coverage for nearshore Kauai stations

                     Station     % Encrusting Coral                 % Head Coral                   Approx. Depth
                     Number      1993            (1972)             1993             (1972)                     (ft)
                     81          1.8             (3.2)              2.7              (6.2)                      30
                     82          sand            (1.0)              sand             (0.4)                      70
                     83          2.5             (1.2)              4.9              (1.3)                      30
                     84          3.0             (11.0)             3.6              (0.0)                      70
                     85          5.0             (4.7)              3.4              (4.3)                      30
                     86          0.2             (1.7)              0.0              (0.0)                      70
                     87          1.2             (7.3)              4.1              (4.3)                      30
                     88          sand            (sand)             sand             (sand)                     70
                     89          0.7             (6.0)              10.1             (8.0)                      30
                     90          0.1             (5.0)              6.5              (0.3)                      70
                     91          sand            (1.5)              sand             (7.0)                      30
                     92          0.0             (sand)             7.2              (sand)                     70
                     93          0.3             (1.0)              0.0              (8.0)                      30
                     94          0.1             (2.0)              7.0              (0.8)                      70
                     Note: 1972 Data is in Parenthesis







                                                                    60.






                                                                        Coastal Overwash During Hurricane Iniki


                            Alternatively, the abundance of damaging clasts at the 70 ft level may
                    have declined in the interim. Coral stress levels prior to Hurricane Iwa must
                    also be considered in evaluating the survey results. It is possible that the
                    benthic substrate in 1982 consisted of a large number of available clasts that
                    accumulated over the relatively long period of quiesence since earlier
                    storms. Increased coral damage during Iwa may have occurred as a result
                    and the responsible materials shifted to an offshore location. There is a ledge
                    very near the. 70 ft level (at approximately 80 ft) that would serve as an
                    effective barrier against onshore return of these clasts, under fairweather

                    waves.





                                                                Table 11.
                                                      Patchiness of coral growth
                                                   Percent of area with hard substrate
                                              Arithmetric mean and variance over mean

                    Station       Stations      % Encrusting Coral                        % Head Coral
                    Number/       w/ Hard     1993 Mean      Var/Mean                1993 Mean       Var/Mean
                    Depth         Subst       1972 Mean      Var/Mean                1972 Mean       Var/Mean
                    89-83         6           1.92           1.51                    4.2             2.22
                    30 ft         (7)         (3.22)         (1.56)                  (6.57)          (0.96)
                    82-94         5           0.86           1.61                    4.86            1.56
                    70 ft         (5)         (3.96)         (3.67)                  (0.49)          (0.35)]
                    Note: 1972 Data is in Parenthesis




                                                               Table Ill.
                                                   Estimate of gross coral coverage
                                                   Percent of area with hard substrate
                                           Arithmetric mean and 95% confidence interval

                    Station       Stations      % Encrusting Coral                         % Head Coral
                    Number/       w/ Hard     1993 Mean      Conf. Interv            1993 Mean       Conf. Interv
                    Depth         Subst       1972 Mean      Conf. Interv            1972 Mean       Conf. Interv

                    89-83         6           1.92           0.14<m<3.70             4.2         1.00<m<7.40
                    30ft          (7)         (3.22)         (1.35<<4.58)            (6.57)      (3.94<<8.97)
                    82-94         5           0.86           0.00<m<2.31             4.86        1.44<m<8.28
                    70 ft         (5)         (3.96)         (1.06<<8.46)            (0.49)      (.001<<1.63)

                    Note: 1972 Data is in Parenthesis







                                                                   61.





                                                          CoasW Overwash Dining Hurficane InM


                Debris Build-up - Surveys of nearshore waters in the region revealed only
                limited distribution of hurricane debris. Collecting principally in channel
                areas, items such as lanai furniture, acoustic ceiling tiles, plastic pipes and
                sheeting, guttering, occasional small items from kitchens, and plastic siding
                were noted on an individual basis. Reports from the local sport diving
                community identified specific clean-up activities had been conducted on a
                volunteer basis following Iniki. Local divers reported having removed
                several large items such as bicycles, a refrigerator, micro-wave ovens, bed
                springs assemblies, and some household furnishings. The same individuals
                reported that debris was no longer a concern in this region, or in any other
                area they had heard of.

                Field Observations - related to the diving survey are as follows:
                a. There is no reef between Makahuena Point and Spouting Hom.
                b. There is instead a sandy or varying volcanic bedrock seafloor consisting
                of: outcropping lava flows that form benches and overhangs; bouldery basalt
                pavements with diameters ranging from 0.5m to 2m; polygonal basalt
                pavements, usually in association with boulder fields, that are of low relief
                and smooth texture; sandy seafloor with uniformly fine to medium sand that
                is well-sorted and of mixed carbonate-volcanogenic mineralogy with a
                minor silt/organic component.
                c. The sandy bottom had no coral growth.
                d. The volcanic hardbottom had sporatic coral development with
                Pocillapora as the dominate genus and Porites sub-dominate, both genera
                grow in this area as separate and usually isolated patches approx. 5 cm to 30
                cm in diameter.
                e. Because it presents a prominate vertical profile, Pocillapora sp. displayed
                varying degrees of abrasion, breakage, and scour generally increasing in
                frequency in the shallow depths, and increasing where a bouldery bottom
                existed, several instances of total abrasion were noted where an that was left
                was a white scar, a former foothold, on a basalt substrate.
                f. Porites sp. displayed a lesser degree of damage due to mechanical action.
                g. Debris was absent to sparse in all sectors of the survey, there was a clear
                increase in the amount of debris in the shallow water zone, there were no
                areas of debris build-up, debris components were isolated and widespread
                consisting of pvc piping of varying length and diameter, aluminum gutter,


                                                      62.





                                                           Coastal Overwash During Hurricane Iniki


                kitchen storage racks, ceiling acoustic particle board, lightweight lanai
                funishings, some textile material, occasional plastic sheeting, and other
                unidentifiable components, a clean-up effort by the local population has
                apparently been successful in removing the majority of large and abundant
                debris.
                h. A common debris element was fishing line wrapped around Pocillapora
                sp., which did appear to cause damage to the coral, this would seem to be
                unrelated to the hurricane.
                i. There was notable algal growth in the nearshore zone on every exposed
                surface save living coral, a significant percentage (1-5%?) of the Porites sp.
                and Pocillapora sp. poulation were dead and covered with algae, some
                (much?) of this may be due to month-long water column turbidity following
                the hurricane, turbidity developed because beach erosion exposed large
                amounts soil to wave action, silt eroded into the water for several weeks
                following the storm, the dramatic decrease in photic penetration may have
                impacted the viabilitiy of nearshore corals and resulted in the algal growth.
                j. Algal growth in the nearshore (0 ft to 20 ft) was the dominate biological
                component of the substrate.
                k. There are bathymetric highs in the coastal zone that appear to be remnant
                bedrock outcrops from shoreline retreat, these often have crests that are intertidal
                and that extend 100m or more offshore, it is likely that these bathymetric highs,
                and the accompanying adjacent lows (total relief of approx. 20 ft) influenced the
                elevation and excursion distance of the hurricane overwash by modifying the
                wave height at the shoreline, by increasing the immediate roughness coefficient
                and decreasing run-up, and by modifying the energy level and set-up.


                Recomendations -
                1.) No further clean-up is necessary.
                2.) Continued monitoring of coral coverage and algal growth at 3-yr intervals
                would provide anexcellent data-set on this ecosystem in the eventuality of
                another storm.
                3.) The algal growth should be monitored and surveyed on a 6-month interval,
                sources of nutrification should be identified and monitored.






                                                       63.





                                                Coastal Overwash During Hurricane InW


                                     References
             Abraham, G. (1964) "Hurricane Storm Surge Considered as a Resonance
               Phenomenon", Waterloopkundig Laboratorium, Delft, Netherlands
               pp.585-602.
             Agrawal, J. D. (1993) "Calculation of Storm Surge for Hurricane Iniki,"
               Department of Ocean Engineering, University of Hawaii at Manoa,
               Honolulu, pp.81.
             Anthes, R. A. (1982) "Tropical Cyclones, Their Evolution, Structure and
               Effects", American Meteorological Society, Boston, MA, pp. 208.
             Bretschneider, C. L., J. D. Agrawal, N. C. Cannella, and K. A. Wohlmut
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             Bryant, E. A. (1991) "Natural Hazards", Cambridge University Press,
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             Ewing, M. F., F. Press and W.L. Donn (1954) "An Explanation of the Lake
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                                            64.






                                                        Coastal Overwash During Hurricane Iniki


                  Beach Processes on Kauai," Proceedings of the American Society of Civil
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               U.S. Army Corps of Engineers (1984) "Shore Protection Manual" Vol. I &
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               Van Dom, W. G. (1953), "Wind Stress on an Artificial Pond", Journal of
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               Yamamoto, S. H., and S. P. Sullivan (1993) "Measurement and Hindcasting
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                  Dec 1-3, 1993.







                                                    65.

















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                               State of Hawai'i Office of State Planning
                                 Coastal Zone Management Program

                                       U.S. Geological Survey
                                 National Coastal Geology Program


                                    National Science Foundation
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