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










                                                                      . .........






                                                                     ..........
                     Ccoastal Shoreline Defense Structures

                                                                      By Thomas Barnard


                        Objectives
                             The purpose of this self-taught education unit is to         that sea level rise is accelerating due to global climate
                        acquaint the reader with the various types of shoreline           change. This factor has not been completely docu-
                        defense structures which are employed in the Chesa-               mented however, primarily due to the difficulty in-
                        peake Bay region and the U.S. in general. There are               volved with modelling weather and climate features on
                        more erosion control measures than those explained                a global scale and determining long term trends in the
                        here but these are the most generally used and accepted           absence of adequate data.
                        within the marine community. Each structure will be                    Erosion or shoreline retreat may be caused on a lo-
                        defined and its use along the shoreline will be de-               cal scale by differences in erodibility of soils, water cur-
 QQ                     scribed. General design and location considerations               rents, boat wakes etc. Any of these local features, sea
                        will be discussed as well as the general definitions and          level rise or a combination of any and all may be the
                        terminology necessary for each type of structure.                 reason a structure is employed along a particular reach
                             Following completion of this study uniy     he reader        of shoreline. It may also'be present for purely land-
                        will be gencraily acquainted with:                                Sc'ape and/or aesthetic reasons.
                             I .Shoreline erosion and its causes;                              The remainder of this educational unit will charac
                             2. The types of structures most often em-                    terize and describe the following shoreline erosion de-
                                ployed to address shoreline erosion;                      fense structure types:
                             3. General design considerations for each                         1 .bulkheads
                                shoreline structure;                                           2. riprap
                             4. Specific definitions and ten-ninology for                      3. marsh toe protection
                                each structure.                                                4. breakwaters
                        Introduction                                                           5. groins and jetties
                                                                                               6. vegetative control
                             At the present time the Chesapeake Bay and most
                        areas of the east and gulf coasts are experiencing va   .ry-           Bulkheads
                        ing degrees -of relative sea level rise. This is defined as            A bulkhead (Figure 1) is a vertical wall generally
                        the net change in water elevation due to the combined             aligned parallel to the shoreline and designed to retain
                        influence of local land movement (subsidence) and the             granular backfill material (soil, sand) and to prevent
                        absolute change in water level. The primary result of             wave-induced erosion. Bulkheads are usually con-
                        sea level rise is shoreline retreat (erosion) along with          structed of chemically t'reated wood with galvanized fix-
                        other secondary shoreline changes.                                tures. Bulkheads are also constructed of concrete,
                             Chesapeake Bay sea level rise is approximately               asbestos plates, steel, aluminum and most recently from
                        one foot per century at present. There is some evidence           recycled plastics. Asbestos is no longer used and the








                   2


                                                      hardwarIe     waters              (JU fill
                                                                                    San                          44.
                                                                                    L!
                                                                                                tieback rod

                                                                                                                                 hardware
                                                        H



                                     MHW - - - - - - - -                                                     deadman
                                     MLW-

                                                                          filter cloth

                                                                        sheet pile                      anchor pile

                                                                                        0



                                                                                                                            0
                                                                                                                           10




                                NOTES                                       fender pile

                     H     Height of sheet pile above MLW.                                  a. All hardware (bolts, nuts, washers, etc.) should
                     D - Depth of sheet pile below MLW. (D should                              be galvanized.
                          be equal to or greater than H.)                                   b. Large end of all piles-should go into ground.
                     L- Represents the difference in length between the                     c. All wood should be pressure-treated to a minimum
                          sheet and fender piles (minimum of 2 to 4 feet.)                     of 1.5 IbS/ft3 Of CCA or have a minimum creosote
                     MLW - mean low water                                                      level of 12 IbS/ft3.
                              mean high water                                               d. In general, the length of the tieback rod should be
                                                                                               equal to or greater than the length of the sheet pile.
                                                                                            e. Filter cloth should extend to at least iMLW elevation.
                                                                                            1. Fill should be free of debris and a gcpd-quality,
                                                                                               sandy soil.                                                    I

                   Figure 1. Bulkhead cross section. (From Department of Conservation and Recreation, Shoreline Programs section.)

                         . . . ............
                                                                                            new recycled products are presently being tested in a
                   Wetlands Program                           December 1993                 variety of situations.
                   College of William and Mary                                                   In general, a bulkhead is constructed of round-pil-
                   School of Marine Science                  4@@Pr%ï¿½                        ings which are driven (by pile drivers) grjetted (by
                   Virginia Institute of Marine Science                                     water pressure) into the bottom. Vertical tongue-and-
                   Gloucester Point, VA 23062
                                                                                            groove sheeting forms,the "wall" andgoes into the bot-
                                                                                                                                          :1
                   Dr. Carl Hershner, Program Director                                      torn. in the same manner. The structure should be
                   Published by: VIMS Publication Center                                    driven or jetted to a depth at least equal -to the height of
                   "A publication of the Virginia Department of Environmental               the structure above     the mud line. Whalers run horizon-
                   Quality's Coastal Resources Managqment Program pursuant to               @ally between the pilings and brace the sheeting. Tie-
                   National Oceanic and Atmospheric Administration Award                    backs, usually galvanized steel rods, pass through the
                   No. NA27OZO312-O I.'
                                                                                            pilings and the wall adjacent to the bulkhead and are
                                        'This paper is funded in part by a grantl
                                        cooperative agreement from the National             tied to anchor piles (wooden vertical posts, commonly
                                        Oceanic and Atmospheric Administration.             called deadmen) which help to anchor the wall. Screw
                                        The views expressed herein are those of the
                                                                                            anchors may be employ@d in specific situations where
                             @01        author and do not necessarily reflect the           it is not feasible to use deadmen and tiebacks. Return
                                        views of NOAA or any sub-agencies. "
                                                                                            walls are employed at the ends of the bulkhead in situ-
                                                                                                 _dl





                                                                      ile@ 19973
                                                                       OWO_
                                                                             P%ï¿½

















                                                  Printed on recycled paper.









                                                                                                                                                      3


                        ations where it does not tie into an existing structure           berthed immediately adjacent to the upland. The ex-
                        protecting adjacent property. Return walls run back to            pected life span of a properly designed and installed
                        the land, approximately perpendicular to the wall, and            treated wood bulkhead is twenty to twenty-five years.
                        prevent the wall from being outflanked as erosion con-            Untreated wood is subject to destruction relatively
                        tinues to occur on adjacent unprotected property. The             quickly in marine (salt) waters due to wood-boring or-
                        landward side of the entire structure is generally faced          ganisms.
                        with geotechnical material (filter cloth), a woven syn-                Riprap
                        thetic textile -which when placed between the wall and
                        the backfill material, prevents the washout of soil while              A riprap revetment (Figure 2) is a sloping structure
                        allowing water to pass. This minimizes the build-up of            consisting of layers of stone or other material placed
                        a hydraulic head landward of the structure.                       along an eroding b  'ank. The structure is generally com-
                            Bulkheads should be aligned landward of all wet-              posed of smaller "core stone" placed on filter cloth and
                                                                                          this is covered with at least 2 layers of larger stone, in-
                        lands and should be properly engineered for the marine
                        environment. Riprap, placed along the seaward toe of              cluding an "armor" layer of the largest stone needed for
                        the bulkhead, may be necessary to prevent scour in                the particular wave conditions at the site. Riprap can
                        front of the structure due to wave energy. A major                be concrete rubble, granite stone or other material used
                        problem of bulkheads is that they reflect most waves              to build the revetment.
                        striking them. In many cases this results in passing the               Riprap is used to prevent erosion in the same man-
                        erosion problem along to the next strip of unprotected            ner as a vertical bulkhead but has the added advantage
                        shoreline. Bulkheads provide minimal habitat for ma-              of being able to dissipate most wave energy rather than
                        rine organisms and may adversely affect organisms       -liv-     merely reflect it as with vertical structures.
                        ing in adjacent bottom sediments.                                      In designing a riprap revetment for a particular
                             Bulkheads can be employed along any typeof                   site, considerations include the slope of the revetment
                        shoreline and in any land use situation but are most              (usually 2 horizontal: I vertical or 2H: IV) and the size
                        often used in residential situations where the waterway           of the armor stone. Each factor affects the other with
                        is narrow and in industrial areas -where ships need to be         the goal to create a structure which is stable for the par-



                                                                                      2



                                                                                                                    splash apron



                                                                                                            armor   rock


                          MHW    -----------------                                           core stone
                          MLW    ---- --
                                   D                                            filter cloth                      NOTES

                                                                                           a.   Core stone is anything smaller than armor rock.
                                                                                                If the recommended weight of the armor rock is
                                                        buried toe/apron                        less than or equal to 75 pounds, core stone is
                                                                                                not necessary.
                                                                                            b.  Depth of buried toe/apron (D) below MLW is
                                                                                                generally equal to the anticipated wave height.


                        Figure 2. Representative cross section - riprap revetment. (From Department of Conservation and Recreation,
                        Shoreline Programs section.)








                4


                ticular wave height expected at the site. Stability refers        structure. Riprap, due to its overall weight, is limited to
                to movement of the stone or other material. 'If waves             shorelines with sediment types which can support its
                are able to move the revetment material around, it is not         overall structure.
                stable.                                                                From An environmental perspective, riprap is fa-
                     Additionally, the seaward toe or bottom edge of              vored over vertical structures made of wood or con-
                the revetment should be buried below the sediment sur-            crete. Riprap provides habitat where organisms can
                face to minimize wave scour around the structure. Fil-            hide, feed, rest, attach and grow. Its long life span mini-
                ter cloth should be placed under and up the landward              mizes future disruptions to the shoreline environment.
                face of the wall so that it is between the Wall and the           Like any sediment impermeable structure however, it
                backfill. Note that a very wide sheet or several overlap-         blocks the resupply of sediments to the shoreline from
                ping sheets of standard width cloth may be needed to              wave-fastland interactions. This can result in beach nar-
                run under and behind the revetment. The filter cloth              rowing, steepening and/or drowning immediately sea-
                serves the same purpose as with the bulkhead but with             ward of the structure and on the adjacent shoreline.
                riprap it also serves as a structural base helping to dis-
                                                                                       Marsh Toe Protection
                tribute the weight of the structure more evenly.                       Marsh toe protection (Figure 3) is a specialized
                     Properly designed and constructed riprap revet-              form of riprap revetment designed to attenuate erosion
                ments have many advantages, not the least of which is             taking place on the fa Ice of a wetland scarp. .The riprap
                an unlimited life span in the env  'ironment. Adding              in this case is low profile, meaning the revetment is less
                some stone when differential settling occurs may be
                needed from ti  me to time. Riprap also can be molded             than a foot higher than the marsh surface and there is
                to the curves of the natural shoreline contours, many             no backfill involved. The low profile structure protects
                                                                                  the marsh face from further erosion but allows tidal in-
                times helping to reduce total length and cost of the













                                    MHW     ----------------------------------


                                                                    2
                                                                                                            marsh peat surface




                                    MLW      ------



                             :D%
                                                             armor    rock                            NOTE

                                                   f fif er cloth
                                                                                   Depth of buried toe/apron (D) below MLW is
                                            buried foe/apron                       generally equal to the anticipated wave height.


                Figure 3. Representative cross section      riprap wedgefor eroding marshfringe. (From Department of Conservation
                and Recreation, Shoreline Programs section)









                                                                                                                                                           5


                          undation (hydrology) to continue over and through the              ened" shorelines being those protected by vertical
                          stone revetment. This maintains the viability of the wet-          seawalls and riprap revetments. The soft approach gen-
                          land and allows most ecological functions to continue              erally refers to the use of vegetation (see below) or a
                          within the system. These functions include intercom-               structure, which though it modifies the behavior of the
                          munity interactions such as feeding by fish and birds at           near shore zone, does not fix it permanently in place.
                          different tide stages, the recycling of nutrients on the           The shoreline responds to natural physical changes
                          marsh, and the trapping of sediments in the water col-             such as storms, sea level rise, etc., but still retains a
                          umn by the wetland.                                                measure of protection from erosion due to the structure.
                              For design-considerations see the preceding section                Fixed breakwaters are generally constructed so that
                          on riprap.                                                         their top elevation is one to three feet above mean high
                              Breakwaters                                                    water. Since most erosion is s   'poradic, occurring during
                              A breakwater (Figure 4) is an offshore structure               storms, the breakwater must be high enough to allow
                          aligned parallel to the shoreline. The purpose of the              for the storm surge. Storm surge is high water which is
                                                                                             generally a product of low pressure systems such as
                          structure is to intercept and dissipate wave energy be-            "nor'easter's" and hurricanes generated or passing over
                          fore it reaches the shoreline and initiates erosion. Be-           large bodies of water.
                          cause the waves are "tripped" by the breakwater, the                   Fixed breakwaters are usually constructed of stone
                          area between the breakwater and the shore becomes a                or concrete rubble. They may also utilize gabion bas-
                          relatively quiescent, low energy zone. Sand and other              kets which are heavy gauge wire baskets filled with
                          sediments tend to settle out in this quiet area forming            stone or Isometimes other heavy materials. Gabion bas-
                          sediment deposits. These deposits may then be colo-                kets are wired together and,filled. They can be stacked
                          nized by marsh grasses such as saltmarsh cordgrass,                and strung end  Ito end to form a continuous structure.
                          Spartina alterniflora and associated fauna such as
                                                                                             The advantage of the baskets is that they allow smaller
                          worms, snails, crabs and shrimp to name a few.                     (cheaper) materials to be used but once placed in the
                              Breakwaters are generally considered to be a                   baskets a total mass is achieved which resists move-
                          softer" approach to shoreline protection with "hard-               ment during storms.



                                                                                            T
                                                                             2       -------------                       T
                                                                                                                         H

                                        MHW  ------------------                                                            -----------      MHW
                                                                                                                          ------------     MLW
                                        MLW   -----------
                                                                                                                                      -------------



                             P- A'                                                                                                                   A`

                                                      filter cloth                                               NOTES


                                                                                                 T = Minimum top width based on rock size
                                                                                                 H = Minimum height above MHW
                                                                                                 A = Minimum apron width



                          Figure 4. Representative cross section - riprap breakwater. (From Department of Conservation and Recreation,
                          Shoreline Programs section.)









                6


                     Many fixed breakwaters are designed with open                     Groins
                spaces between sections of the structure. These are
                termed gapped breakwaters (Fi     .gure 5). They have gen-             Shoreline erosion is a problem that has been
                erally been found to form the most stable shorel     ines         around ever since man began settling adjacent to the
                                                                                  water and groins are one of the oldest types of struc-
                when properly designed and constructed. The ideal                 tures used to deal with the problem. Groins are struc-
                situation occurs When a tongue of sand, termed a tom-
                                                                                  tures placed perpendicular to the shoreline which
                bolo, ties each segment of breakwater to the land and             extend into the water a prescribed distance based on the
                this area is further stabilized by vegetation. This, of
                                                                                  geomorphology of the shoreline (Figure 6).
                course, requires an adequate supply of sand in the near-               The purpose of the groin is to trap sand moving,
                shore system to make up the tombolo. Sand may also                along the beach with the longshore currents. When
                be brought in from upland sources, spread and vege-               groins are Iworking properly, sand accumulates on the
                tated.
                     Breakwaters can be floating; that is constructed of          updrift side of the structure acting to widen and raise
                                                                                  the elevation of the beach. Incoming waves then dissi-
                tires, logs, fabricated containers, baffles or other float@       pate their energy on the accumulated sand and only at-
                ing materials. Floating breakwaters depend on their               tack the fastland during storm events which produce
                width,,not height, to dampen waves as they try to move
                                                                                  higher than normal water levels. Groins are generally
                through the structure. Problems with floating structures          ineffective in preventing shoreline erosion, but exhibit
                range from the attachment of fouling organisms causing            their most success if there is enough sand moving in the
                the structure to sink, to the failure of anchors and tie          nearshore zone that they trap and raise the elevation of
                materials during storms.                                          the backshore enough that vegetation can establish and
                 .   As with- revetments, breakwaters should be used              further stabilize the shbreline.             I
                with filter cloth and should have armor stone designed                 As recognized a   .bove, groins are dependent on long-
                to resist movement when under wave attack during ex-              shore drift and in general do not achieve their intended
                pected storm severity for the local area. The design              function where sand is not being transported along the
                wave that the structure is meant to attenuate is based on         beach. Depending on wind direction, sand may be
                geomorphology of the basin, fetch, water depth, expo-             trap .ped on either side of a groin at any given time.
                sure and other factors.                                           Sand may also move onshore and offshore. The net di-
                     Environmentally, gapped breakwaters are preferred            rectiori of movement determines the updrift side of the
                for the-same rea 'sons as described under the preceding           groin where the largest amount of sediment is depos-
                riprap discussion but have the further advantage of al-
                lowing the shoreline to
                flex with changing condi-
                tions. Bulkheads and
                revetments are imperme-                                     Breakwater                 Tombolo
                able, immovable barriers
                which do not change with
                changing parameters such
                as sea level. Because a

                                                                    77
                wetland or beach, located
                seaward of one of these
                .structures, cannot migrate
                landward as sea level
                                                                                 MHW
                rises, they will eventually
                be covered with water
                and disappear.
                                               Figure 5. Breakwater system. (From Shoreline Development BMP's, VA11RC 1993.)









                                                                                                                                                  7


                      ited. In the vast majority of cases, groins deprive the          same design as the groin which is attached to the down-
                      immediate downdrift shoreline of sand causing a loss of          drift side and extends out from the groin parallel to the
                      beach or "notching," which is accelerated erosion. This          shoreline. The spur may be placed anywhere along the
                      reaction to the groiff generally disappears a short dis-         groin between mean low water and the channelward
                      tance downdrift of the structure.                                end, depending on specific shoreline geomorphology.
                           Groins are primarily constructed of two materials           Spurs generally cause the accumulation of sand land-
                      although others have been tried with highly variable             ward of their location, helping to minimize downdrift
                      success. Timber tongue and groove sheeting with pil-             effects of the groin.
                      ings for stability are used along the majority of shore-              Provided that there is sufficient sand in the near-
                      lines. These are either driven or jetted into the bottom,        shore zone for groins to be effective, the most impor-
                      with driven structures being the most durable. The sec-          tant additional structural and design considerations are
                      ond most popular material is riprap. It is generally             two:
                      placed on a bed of geotechnical material (filter cloth)               I . Groins must be securely fastened to the up-
                      and is constructed in a manner to be free standing and                    lands so that sand cannot get around them
                      thus is trapezoidal in cross-section.                                     at their shoreward ends. Many groins fail
                           In general, groins should be designed to mimic the                   when erosion occurs so quickly that the
                      beach and should be higher at their landward end in or-                   landward end of the groin is exposed before
                      der to build the elevation of the backshore. Length is
                                                                                                the structure has enough time to build up
                      not critical in most cases, and commonly need only be                     sand and stabilize the beach. If rapid ero-
                      extended ten to twenty feet beyond mean low water.                        sion is a concern, riprap may be placed ad-
                      Groins which are too long interfere with the normal                   -   jacent to the point where the groin meets
                      movement of sand in the nearshore area and may de-                        the fastland.
                      prive downdrift beaches of sand needed to maintain
                      equilibrium with erosion forces. Low-profile is the rec-              2.  GroIins must be solid structures which only
                      ommended design for groins of either timber or stone.                     allow passage of sand overtop of them or
                      Low-profile requires the channelward end of the groin
                                                                                                around their channelward ends. Groins con-
                      have an elevation no greater than
                      that of mean low water (Figure 7).
                      This allows sand to begin bypass-                 UPDRIFT                            DOWNDRIFT
                      ing the groin more quickly once the                                       TERMINAL GROIN
                      groin cell has filled, lessening the
                      period of interrupted longshore                                                   DIFFRACTION POINT
                      sand movement and minimizing to                                                                  @WAVE CRESTS
                      a degree, the adverse effects of the
                      groin to downdrift shorelines. If                           is                      SPUR
                                                                                                               _.,@DIFFRACTI 0 N
                      groins are considered desirable in a                                                                 POINT
                      given situation, downdrift sand dep-
                      rivation can be minimized by fill-                   FASTLAND

                                                                                                                                      MHW
                                                                                                ............
                      ing the groin cell artificially from
                      an upland sand source.
                                                                                                                                       EA
                                                                                                                                      B CH
                           Another method which can be                                          ......                                BLUFF
                      used in specific situations to mini-                                                                            FASTLAND
                      mize downdrift erosion due to
                      groins is the attachment of a spur                      RELATIVE LITTORAL TRANSPORT
                      (Figure 6). The Spur is generally a
                      short (12-15 feet) structure of the      Figure 6 Groinfield with spur. (From Anderson, Hardaway, and Gunn, 1983.)








                    8,


                             structed of used tires, well casings or other                    helps to build and maintain the intertidal and subtidal
                             porous materials are ineffective because the                     zone, minimizing the severity of erosion in many cases.
                             sand passes through them. Old groins with                             Many types of fringe marshes can be very effective
                             loose or missing timbers fail for the same                       as shoreline stabilizers, with a width of eight feet or
                             reason.                                                          greater width on the shoreline generally being highly ef-
                         jetties                                                              fective. Not all situations are suitable for vegetative
                                                                                              control however. Research indicates that shorelines
                         Jetties are structures very similar to groins in de-                 with' less than a mile of fetch generally can be stabi-
                    sign and construction. The difference between the two                     lized with wetland vegetation. Freshwater marshes do
                    is that whereas groins are used on and attached to any                    not generally have the thick*root and rhizome systems
                    suitable reach of shdreline@    'Jetties generally define and,            that brackish and salt marshes have, so they must be sig-
                    protect inlets and/or harbor entrance channels from                       nificantly wider to have@ the same wave baffling effec-
                    shoaling by preventing sand from accumulating in the
                                                                                              tiveness that their saltwater cousins have.
                    channel or moving across the channel with longshore                            Marshes can be established on suitable shorelines
                    currents. Jetties may also function to dampen waves
                                                                                              using either transplants from established wetlands or
                    moving across an inlet or entrance channel,, making                       nursery grown stock. In either case so-me knowledge of
                    navigation easier and safer.
                                                                                              wetland plants is necessary along with. information on
                         Vegetative Control                                                   planting elevations (the primary factor in successful es-
                         Although not a structural option, per se, using vege-                tablishment), fertilization requirements        'and plant spac-
                    tation to control shoreline erosion can be very effective                 ing. This information is available from the Virginia
                    in the right circumstances'and has the added benefit of                   Institute of Marine Science, the Department of Conser-
                    providing highly beneficial habitat to marine and fresh-                  vation and Recreation and a number of publications
                    water systems. Vegetative control may be used by it-                      available from libr@fies and bookstores.
                    self or in concert with conventional structures such as                        Many waterfront property owners reject the idea of
                    gapped breakwaters or offshore sills.                                     controlling erosion with marsh plantings as infeasible
                         When vegetation, usually some type of wetland or                     because the wetlands cannot be counted on to inhibit
                    submerged aquatic vegetation (SAV),           Iestablishes on a           erosion on a long term basis. Many fringe marshes,
                    shoreline, its root and-rhizome system serves to stabi-                   once established however, may last at least as long as
                    lize the existing substrate in place and the subaerial                    the design life of the average wood bulkhead (i.e. 20
                    shoots baffle water movement causing sediment parti-                      yeais). When one considers the lowe17 cost, reduced ad-
                    cles to be deposited along the shoreline. This action                     verse impact and positive environmental contribution
                                                                                              of this option, it should be considered a viable alterna-
                                                                                                                              tive to structural erosion con-
                                                                                                                              trol. It bears repeating how-
                                                                                                                              ever, that vegetative control
                                                                                                MHW                           should only be considered for
                                                                                                                              shorelines with less than a
                                                                                                              MLW             mile of fetch. A good indica-
                                                                                 ............................ ..................... -@*
                                                                                                           ...... .................. tor of potential success is
                      Planking
                     Protection                                                                                               whether wetlands exist in
                                                                                                                              other sections of the same
                                                                                                                              reach of shoreline.


                  Figure 7. Low profile groin. (From Shoreline Development BMP-'s, VMRC, 1993.)
                        =ng
                           tion







                                                                                                                                            9


                      Glossary.
                      Anchor piles       These are anchors, usually vertical piles driven into the ground, on the landward side of
                                         the bulkhead, to which the bulkhead is tied by tiebacks or tie-rods (commonly called de.admen).

                      Armor              This refers to the larger stone used as the outer layers of a revetment which is directly exposed to

                                         waves.


                      Breakwater         A breakwater is an offshore structure which is aligned parallel to the shoreline. A fixed breakwa-
                                         ter refers to one generally constructed of stone or gabion baskets (wire baskets or mattresses
                                         which are filled with stone), placed on the bottom. Floating breakwaters should be firmly an-
                                         chored and may be constructed of tires, logs, specially fabricated boxes and baffles, or other float-
                                         ing materials.

                      Buried toe         This is the practice of trenching in the seaward toe of a riprap structure to help prevent scour and
                                         shifting of the structure.


                      Core               The core is the smaller stone used as the base of the revetment which is not directly exposed to

                                         waves.


                      Fetch              Fetch is the distance that wind blows over water prior to its reaching a shoreline. Generally it is
                                         used as an estimate of potential wave energy or stress the shoreline may expect.


                      Filter cloth       Filter cloth is the synthetic textile placed between sheeting and backfill which prevents soil loss
                                         but is water-permeable.


                      Groin              This is a structure that is perpendicular to the shoreline and extends into the water. Theyfunction
                                         in trapping sand moving in the along-shore currents.


                      Jetting            Jetting is a method of sinking structures in substrate where high pressure water "washes" the struc-
                                         ture down and the hole refills with sediment as the pressurized water is cut off.


                      Jetty              As with groins, jetties are linear structures placed perpendicular to the. shoreline and cross the inter-
                                         tidal zone to deeper water. They function to intercept sand moving along the shoreline and protect
                                         channels and inlets from shoaling and wave energy.


                      Low-profile        This is a recommended design for either timber or stone groins, in which the elevation of the chan-
                                         nelward end of the groin is no greater than that of mean low water. This allows the sand to bypass
                                         the groin more quickly once the groin cell is filled, lessening the interruption of sediment move-
                                         ment to downdrift shorelines.


                      Marsh toe
                      protection         This is a low-profile rock structure placed channelward of a marsh, usually being placed directly
                                         -against an eroding scarp.









              10


              Return walls     These are walls located at each end of the bulkhead and shoreline, approximately perpendicular to
                               the bulkhead and shoreline, whicii tie the bulkhead into the upland and prevent the bulkhead from
                               being flanked.


              Revetment        A revetrnent is a sloped structure consisting of multiple layers of stone or other material placed
                               along a bank.


              Riprap           R.iprap is the stone used to build a revetment. Frequently, the structure itself is called riprap.


              Screw anchors    Screw anchors refer to another anchoring method that consists of rods that screw into the upland.


              Sill             Sill is a continuous low-profile breakwater structure.


              Spur             Spurs are attached to the downdrift side of the groin and oriented perpendicular to the groin, and
                               parallel to the shoreline. The spur may be aligned anywhere between MLW and the channelward
                               end of the groin. The purpose is to prevent characteristic erosion of.sand immediately downdrift
                               of the groin.


              Tiebacks         These are rods used to connect the bulkhead to the land anchor pile or deadmen (usually the hori-
                               zontal piles connected to the anchor pile).


              Tombolo          This is the name given to the build-up of sand landward of gapped breakwaters.


              Up- & down
              drift            Updrift and downdrift refer to longshore drift, or the movement of sediment along the shore. Sedi-
                               ment may move   in both directions along a particular shoreline., The net direction of movement de-
                               tennines the net accumulation.of sediment by a groin. Groins necessarily deprive downdrift
                               shorelines, of their sand supply worsening any existing erosi(?n problems.


              Vegetative
              control          Vegetative control is the use'of wetlands vegetation to deter erosion, either alone or in concert
                               with an offshore breakwater or sill. Vegetation may be planted or allowed to colonize naturally.


              Whaler           Whaler refers to a structural member of a wood bulkhead or groin which runs horizontally be-
                               tween pilings and braces the sheeting.







                               Coastal Shoreline Defense Structures
                                                         Exam Questions


                  1. Define the following:
                             1. Shoreline defense structure


                             2. Sea level rise


                             3. Riprap


                             4. Deadman



                             5. Armor stone



                             6. Breakwater


                             7. Geotechnic material


                             8. Longshore drift


                             9. Jetty


                             10. Downdrift




                  2. What is the primary cause of shoreline erosion in the Chesapeake Bay?



                      What other factors can cause shoreline erosion on a localized level?




                       Explain.





                  3. Most bulkheads are constructed out of                       but                       and
                       may also be used.


                  4. Describe in general how a bulkhead is installed and describe its component parts and their purposes.



                  5. Describe the behavior of a wave as it strikes a bulkhead vs. striking riprap.


                                                                                                                College of William and Mary
                                                                                                                School of Marine Science
                                                                                                                Virginia Institute of Marine Science
                                                                                                                Gloucester Point, Virginia 23062









           2


           6. Explain the general structure and components of a riprap revetment.




           7. Explain the "soft" and "hard" approaches to shoreline erosion control.




           8. Discuss three different types of breakwaters.



                    2.



                    3.



           9. What are the most important design factors to consider when constructing a groin?




           10. What are the general adverse effects of a groin and how can this be minimized?




           11. What are the differences between groins and jetties?




           12. What is vegetative control of erosion?



                When is it likely to work and not work?



                Explain your answer.




           13. Given the examples of shoreline erosion discussed in this unit, which would you choose if you had an erosion
                problem on your property located on a small creek with little-boat use?


                Explain, (You can ignore cost.)









                     Literature Cited

                     Anderson, G.L., C.S. Hardaway, and J.R. Gunn. 1983.
                          Beach response to spurs and groins. In Coastal
                          Structures 1983. Ed. J.R. Weggel. Amer. Soc. of
                          Civil Engineers. New York, New York. pp 727-
                          739.

                     Virginia Marine Resources Commission. 1993. Shore-
                          line Development BMP's. 54 pp'.


                     Suggested Reading List
                     Hardaway, Scott and Gary Anderson. 1980. Shoreline
                          erosion in Virginia. Educational Series No. 31.
                          VIMS Sea Grant Advisory Service.
                     U.S. Army Corps of Engineers. 198 1. Low cost shore
                          protection ... a property owner's guide. Norfolk, Vir-
                          ginia. 159 pp.
                     U.S. Army Corps of Engineers. 1990. Chesapeake Bay
                          shoreline erosion study. Feasibility report. Balti-
                          more and Norfolk Districts. I I I pp.





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