Physical processes and geologic hazards on the Oregon coast

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

Physical Processes & Geologic Hazards
On The Oregon Coast

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

I. . . . . . . . . .

QE Oregon Coastal Zone Management Association, Inc.
581
-K65
1979

This report was prepared as part of a larger document addressing various
beach and dune planning and management considerations and techniques.
Other segments of the document and additional materials are:

I. BACKGROUND ON BEACH AND DUNE PLANNING:
Background of the Study
An Introduction to Beach and Dune Physical and Biological Processes
Beach and Dune Planning and Management on the Oregon Coast: A
.Su=ary of the State-of-the-Arts
II. BEACH AND DUNE IDENTIFICATION:
A System of Classifying and Identifying Oregonfs Coastal Beaches
and Dunes

III. PHYSICAL AND BIOLOGICAL CONSIDERATIONS:
Physical Processes and Geologic Hazards on the Oregon Coast
Critical Species and Habitats of Oregon's Coastal Beaches and
Dunes

IV. MANAGEMENT CONSIDERATIONS:
Dune Groundwater Planning and Management Considerations for the
Oregon Coast
Off-road Vehicle Planning and Management on the Oregon Coast
Sand Removal Planning and Management Considerations for the
Oregon Coast
Oregon's Coastal Beaches and Dunes: Uses, Impacts and Management
Considerations

Dune Stabilization and Restoration: Methods and Criteria
V. IMPLEMENTATION TECHNIQUES:
Beach and Dune Implementation Techniques: Findings-of-Fact
Beach and Dune Implementation Techniques: Site Investigation
Reports
Beach and Dune Implementation Techniques: Model Ordinances*
VI. ANNOTATED BIBLIOGRAPHY:
Beach and Dune Planning and Management: An Annotated Bibliography
VII. EDUCATIONAL MATERIALS:
Slide show: managing Oregon's Beaches and Dunes
Brochure: Planning and Managing Oregon's Coastal Beaches and Dunes

*Prepared under separate contract between Oregon Department.of Land Conserva-
tion and Development and the Bureau of Governmental Research, Etigene,

Cover design by Arlys Bernard, Newport, Oregon. Photos depict
accretion at Clatsop Plains and erosion at Bay Ocean, Oregon.

Property Of CSC Library

PHYSICAL PROCESSES AND GEOLOGIC HAZARDS

ON THE OREGON COAST

V - S . DEPARTMENT OF COMMERCE NOAA
COASTAL SERVICES CENTER
2234 SOUTH HOBSON AVENUE-
CHARLESTON , SC 29405-2413

by
Paul D. Komar, PhD
Professor
School of Oceanography
Oregon State University

Kathy Bridges Fitzpatrick
Editor and Project Administrator

Oregon Coastal Zone Management Association, Inc.
313 S. W. 2nd Street, Suite C P. 0. Box 1033
Newport, Oregon - 97365

May, 1979

Qt-

Funding for this study was provided by the Office of Coastal Zone
Management, National oceanic and Atmospheric Administration, under
40 Section 306 of the Coastal Zone Management Act through the Oregon
partment of Land Conservation and Development

cy-
'2!5 L@-)

PREFACE

The following report presents the results of an overview of beach
and dune processes and erosion on the Oregon Coast. The study was
conducted by Dr. Paul D. Komar, Associate Professor at Oregon State
University, Corvallis under contract with the Oregon Coastal Zone
Management Association, Inc. and with assistance from OCZMA's Beaches
and Dunes Study Team composed of Carl Lindberg, Project Leader,
Christianna Crook, Project Associate, Arlys Bernard, Project Secretary,
Wilbur Ternyik, Project Coordinator and Kathy Fitzpatrick, Project
Administrator. This report constitutes one element of an overall
analysis of planning for and managing coastal beaches and dunes as
required by Oregon's Beaches and Dunes Goal.

OCZMA extends special appreciation to Dr. Komar for the professional
and timely manner in which this report was conducted. Additionally,
OCZMA acknowledges the valuable review and comment made by the Beaches
and Dunes Steering Committee composed of:

R. A. Corthell, U.S. Soil Conservation Service
Steve Stevens, U.S. Army Corps of Engineers
Sam Allison, Oregon Department of Water Resources
Peter Bond and John Phillips, Oregon Department of Transportation,
Parks and Recreation Division
Bob Cortwright, Oregon Department of Land Conservation and Development
Jim Lauman, Oregon Department of Fish and Wildlife
Anne Squire, Oregon Land Conservation and Development Commission
Jim Stembridge, Oregon Department of Soil and Water Conservation
Steve Felkins, Port of Coos Bay
Rainmar Bartl, Clatsop-Tillamook Intergovernmental Council
Gary Darnielle, Lane Council of Governments
Cathy McCone, Coos-Curry Council of Governments
Marilyn Adkins, City of Florence Planning Department
Phil Bredesen, Lane County Planning Department
Steve Goeckritz, Tillamook County Planning Department
Oscar Granger, Lincoln County Planning Department
Curt Schneider, Clatsop County Planning Department

TABLE OF CONTENTS

Chapter Page

Preface .................................. .............. i
List of Tables and Figures ................. ............ iv
I. Introduction ............................................ 1
Ii. Coastal Processes and Land Forms ........................ I

A. Beaches
B. Sources of Beach Sands
C. Dunes
D. Climate
E. Wave Conditions
F. Beach Cycles
G. Nearshore Currents and Sand Transport
H. Tides
I. Tsunami
J. Sea-level Changes
III. Erosion Due to Jetty Construction ........................ 22
IV. Sand Spit and Foredune Erosion (Siletz Spit) ............ 30
V. Other Areas of Foredune Erosion or Potential Erosion .... 37
A. Nestucca Spit Erosion
B. Netarts Spit
C. Nehalem Spit
D. Alsea Spit
E. Seaside - The Necanicum River Inlet
F. Cannon Beach (Breakers Point)
Vi. Sea Cliff Erosion ....................... ............... 48

A. Processes of Erosion
B. Rates of Sea Cliff Erosion
C. Methods of Sea Cliff Protection
VII. The Coastal Dune Sheets ................................. 60
A. Active Dune Types
B. Vegetation Effects
C. Older Vegetated Dunes
VIII. References Cited ........................................ 69

Table LIST OF TABLES Page
1. The budget of littoral sediments ........................ 5

2. Ranges of maximum backshore erosion rates ............... 58

LIST OF FIGURES
Figure Page
1. A portion of the north Oregon coast illustrating how
it consists of a series of pocket beaches separated
by pronounced rocky headlands .......................... 2

2. The effects of beach sand grain size on the profile,
Gleneden Beach being much coarser and thus having a
steeper slope than does the beach to the south of
Devil's Punchbowl, Otter Rock .......................... 3

3. The beach at Neakahnie Beach with a steep cobble
storm ridge of large rocks derived from the nearby
basalt headland, backing an otherwise sandy beach ...... 4

4. Areas of sand accumulation in Yaquina Bay indicating
that the river sands deposit before reaching the
ocean and that marine sands are transported through
the inlet and also deposited in the bay ................ 6

5. An approximate budget of beach sands for the stretch
of coast fronting Lincoln City, south past Siletz
Spit to Lincoln Beach .................................. 7

6. The active dune field of the Coos Bay dune sheet
which stretches for some 55 miles along the
mid-Oregon coast ....................................... 9

7. An example of the seasonality of river discharge
with large winter discharges and negligible summer
discharges, following the seasonality of rainfall ...... 10

8. Storm system on December 23, 1972 with winds blowing
across most of the Pacific and directed toward
Siletz Spit ............................................ 11

9. Significant wave breaker heights and periods measured
at Newport during July 1972 through June 1973 .......... 12

Figure Page
10. Schematic illustration of the beach profiles produced
by storms versus gentle swell waves ..................... 14

11. Profile changes measured at Gleneden Beach from
August 1976 to July 1977 showing the winter erosion
of the exposed portion of the beach followed by
deposition as the spring and summer months of lower
waves return ............................................ 14

12. A rip current flowing outward across the beach
hollowing out an embayment into the beach and
eventually into the foredunes causing property
losses .................................................. 16

13. Tidal elevations as measured in Yaquina Bay ............. 17

14. Maximum heights of tsunami waves recorded at tide
stations or by observations along the Washington-
Oregon coast ...................... ..................... 19

15. Destruction at Seaside from the March 1964 tsunami ...... 20

16. Schematic of water level changes on the Oregon
coast as compared to the East coast and the coast
of Alaska ............................................... 21

17. Patterns of beach deposition and erosion resulting
from construction of the north jetty at the entrance
to Tillamook Bay ........................................ 23

18. Erosion on Bayocean Spit leading to the loss of the
natatorium with an in-door swimming pool ................ 24

19. Schematic of shoreline changes (deposition and
erosion) produced by jetty construction in areas
experiencing a net littoral drift versus an area
such as the Oregon coast where there is a zero net
littoral drift .......................................... 25

20. Compilation of shoreline changes resulting from
jetty construction at the mouth of the Siuslaw River .... 26

21. Schematic of the filling of the shoreline embayment
created between the newly constructed jetty and the
pre-jetty shoreline ..................................... 27

Figure Page
22. Compilation of shoreline changes resulting from
jetty'construction and then later extension at
Yaquina Bay ............................................. 29

23. Destruction of house under construction on Siletz
Spit due to the rapid wave erosion of the foredunes
upon which the house was being built .................... 31

24. House left on a promentory of riprap on Siletz
Spit as adjacent unprotected empty lots continued
to erode ................................................ 32

25. Erosion during the winter of 1977-78 along the
narrowest portion of Siletz Spit, nearly leading
to its breaching ........................................ 32

26. Embayments cut out of the beach and into the
foredunes on Siletz Spit leading to property
losses during December 1972 and January 1973,
produced by seaward flowing rip currents ................ 33

27. Drift logs washed into an embayment cut by a rip
current on Siletz Spit, now actively trapping
wind-blown sands and beginning to reform the
foredunes ............................................... 35

28. Large piles of riprap employed on Siletz Spit
to protect the homes built on the foredunes ............. 36

29. Erosion of riprap on Siletz Spit by a series of
storms, exposing the dune sands to wave attack .......... 36

30. Nestucca Spit, showing the areas of foredune
erosion and breaching during February 1978 .............. 37

31. Homes to the south of Cape Kiwanda protected by
riprap placed due to the erosion of the foredunes
upon which they were being constructed .................. 38

32. The breach in Nestucca Spit produced by a
combination of unusually high storm waves and
high Spring tides in early February 1978 ................ 39

33. The wood piling bulkhead built on Netarts Spit
to stop wave attack of the dunes ........................ 40

34. Degradation of the dunes on Netarts Spit due to
visitors cutting a path from the beach to the
state park .............................................. 41

Figure Page
35. Long-term progressive erosion in Manzanita now
nearing some of the homes ....................... ...... 42

36. Homes built on Nehalem Spit in an area of active
foredunes susceptible both to ocean wave attack and
wind erosion ............................................ 43

37. Bay-shore erosion at Gearhart, caused by the flow of
the Neawanna Creek against the property ................. 4.5

38. Bay-shore erosion on Siletz Spit where the Siletz
River strikes the backside of the spit .................. 46

39. Foredunes at Breakers Point, Cannon Beach, backed
by older, well-vegetated dunes into which waves at
some time cut a near-vertical scarp ..................... 47

40. Typical sea cliffs of the Oregon coast formed by
erosion of marine terraces .............................. 49

41. The extent of tallus accumulation at the base of the
sea cliff can give some indication of the frequency
or recentness of wave attack ............................ 50

42. Sea cliff erosion at Taft during the winter of
1977-78 ................................................. 51

43. Small landslides are an important process to
sea cliff erosion, especially where the cliff is
composed of terrace sandstones .......................... 52

44. Large landslides in the Jumpoff Joe area of
Newport ................................................. 53

45. The sea cliff retreat in the Jumpoff Joe area
of Newport as documented by Stembridge (1975) from
aerial photographs ...................................... 54

46. The compilation of landslide occurrences on the
Oregon coast from newspaper reports, showing their
development during the-winter months at times of
high precipitation and wave action ...................... 55

47. Graffitti carved into a sea cliff at Lincoln City,
having a significant effect on the long-term cliff
retreat rate ............................................ 56

vii

Figure Page
48. A variety of sea cliff protection approaches
have been employed on the Oregon coast, mainly
involving log sea walls, concrete sea walls and
riprap ................................................. 59

49. The two active dune types found on the Oregon
coast, the transverse-ridge pattern and the
oblique-ridge pattern, both now largely confined
to the Coos Bay dune sheet ............................. 61

50. Cooper (1958) has shown that the transverse-
ridge dunes do not align exactly perpendicular to
the wind direction, instead forming an angle of
about 11 to 23 degrees, the dune facing (and
migrating) more landward ............................... 62

51. A precipitation ridge of the Coos Bay dune sheet
migrating slowly landward, burying trees in its
path ................................................... 64

52. An example of the effects of the introduction of
European beachgrass to the Oregon coast at Coos
Bay, diminishing the extent of active dunes sands
and encouraging the formation of foredunes and
deflation plains ....................................... 65

viii

I. INTRODUCTION

The coast of Oregon is made up of stretches of sandy beaches
separated by rocky headlands jutting out into the sea (Figure 1). The
major headlands such as Cape Blanco, Arago, Perpetua, Foulweather, Cas-
cade Head, Lookout, Meares, Falcon and Tillamook Head are composed of
hard basalt, resistant to wave attack. The stretches of beach vary in
length from small pocket beaches nestled amongst the rocky headlands
to the 50-mile long beach extending from Heceta Head south to Cape
Arago near Coos Bay. The beaches are backed in part by sea cliffs cut
into lithified sedimentary rocks, sandstones and mudstones, in all cases
much less resistant than the basaltic headlands. In some areas the
beaches are backed by foredunes consisting of loose sand; such foredune
areas show little resistance to wave attack even when well vegetated.
Sand spits, such as Coos, Siletz, Nestucca, Netarts, Bayocean and
Nehalem, are almost all loose sand and have thus shown the greatest
amounts of erosion when attacked by waves. Because sand spits are also
particularly attractive building sites with views both of the ocean and
bay, the most dramatic examples of erosion destruction have occurred
there.

This report will examine particular erosion problems associated
with the various sites on the Oregon coast and what can be done from a
coastal planning viewpoint to minimize future problems. Sand spits and
foredune erosion have presented the greatest erosion problems and there-
fore have been most extensively studied. The problems associated with
dwelling construction in active foredune areas will be considered,
followed by an examination of the longer-term erosion of the sedimentary
sea cliffs, the erosion of which is important to communities such as
Brookings, Bandon, Waldport, Newport, Lincoln City, Cannon Beach and
many others.

Erosion of the Oregon coast cannot be understood properly without
reference to the physical processes causing that erosion: the ocean
waves, tides, nearshore currents, tsunami and winds. For that reason
this report will begin with a discussion of these factors and what is
known about physical processes on the Oregon coast. At the same time
the sources of sand to the beaches and dunes and their' general morphology
will be examined.

II. COASTAL PROCESSES AND LAND FORMS

A. Beaches

Like most other continental beaches, the beaches of Oregon are com-
posed mainly of quartz and feldspar sand grains derived originally from
the weathering of granitic-type rocks. But within the beach sands are
lesser amounts of dark heavy minerals such as hornblende, magnetite,
augite, garnet and epidote, having shades of green, pink and black.

Cape
Folcon

Ne /,,r RW

Nehalem Bay
and Spit

Tillamook Bay
Bayocean Spit Kilc is

Cape Meares s r

Netarts Bay
and Spit

Cape Lookout

5
kilometers

Sand Lake

Figure 1. A portion of the north Oregon coast illustrating
how it consists of a series of pocket beaches separated by
pronounced rocky headlands.

At times these heavy minerals can become locally concentrated so that
the beach sand appears greenish-black rather than having the tan color
of the quartz and feldspar grains. In certain south Oregon beaches
there are 'black sands' containing grains of gold, platinum and chromite,
as well as the usual quartz, magnetite, etc. (Twenhofel, 1946). The
gold and platinum attracted the attention of prospectors as early as 1852,
and little now remains of those minerals in the black sands. The stra-
tegic mineral chromite was mined during World War II; although uneconomical
to mine now, some chromite remains as a resource.

The beach sands generally have median grain diameters.in the range
0.2 to 0.5 mm (fine to medium sand)(Wentworth, 1922), depending upon
location. The overall grain size of the beach sand has important effects
on the morphology of the beach and its response to erosion. In general,
the coarser the beach sand the steeper its offshore slope (Komar, 1976,
p. 303-8). Thus the beaches on Siletz Spit and at Gleneden Beach to the
south, with a median grain size of about 0.4 mm, are much steeper
(average slope - 3.1 degrees) than the more common finer grained
beaches (0.2 to 0.3 mm) with average slopes of about 1.7 degrees
(Figure 2). As shown by the study of Aquiler and Komar (1978) at two
such beaches, a coarse-sand beach also has a higher rate of erosion
when attacked by stonn waves and a greater amount of total erosion.
This in part explains, for example, why Siletz Spit in particular has
suffered much erosion.

Bot h Profiles 2 April 1977
GLENEDEN BEACH 10X Vertical Exaggeration
median grain size - 0. 35 mm

high - tide level
- - - - - - - - - - - DEVIL'S PUNCHBOWL BEACH
median groin size - 0.23mm

I meter

low-tide level
----------

L L L I I I
0 20 40 60 80 100 120 140 160 180 200
DISTANCE IN METERS

Figure 2. The effects of beach sand grain size on the profile,
Gleneden Beach being much coarser and thus having a steeper slope than
does the beach to the south of Devil's Punchbowl, Otter Rock.

Small pocket beaches in headland areas usually consist of basalt
pebbles and cobbles (4 to 250 mm), the wave energy being too great for
sand to remain on the beach. Continuing the trend of increasing
beach slope with increasing grain size, these cobble beaches reach
slopes of 5 to 25 degrees. Basaltic cobbles and pebbles are also
found as a steep storm ridge along the flanks of headlands, backing
the otherwise sandy beach (Figure 3). Such cobble ridges form a
natural protective barrier from wave attack, important to such areas
as Neahkahnie Beach south of Cape Falcon (Figure 3).

I M 4AO"

Figure 3. The beach at Neakahnie Beach with a
steep cobble storm ridge of large rocks derived from the
nearby basalt headland, backing an otherwise sandy beach.
The cobble ridge offers protection to the coastal property,

B. Sources of Beach Sands

Management of beaches and coasts requires a knowledge of the
natural sources and losses of beach sands. For example, if the
principal source is sand brought to the coastal zone by rivers, then
dammi,ng of the ri'vers would cut off much of that source, resulting

in the long-term diminishing of the size of the beach and an increase in
coastal erosion.

Such problems are best approached through a consideration of the
budget of sediments (Bowen and Inman, 1966; Komar, 1976, Chapter 9).
Such a budget involves assessing the sedimentary source contributions
(credits) and losses (debits) and equating these to the net gain or
loss (balance of sediments) for a given beach. The balance between
gains and losses is reflected in local beach erosion or deposition.
Table I summarizes the usual possible sources and losses of beach sands.

Table 1. The budget of littoral sediments

Credit Debit Balance

Longshore transport Longshore transport Beach deposition
into area out of area or erosion
River transport Wind transport out
Sea cliff erosion Offshore transport
Onshore transport Deposition in submarine canyons
Biogenous deposition Solution and abrasion
Hydrogenous deposition Mining
Wind transport onto beach
Beach nourishment

Unfortunately, the sources and losses of sands to the Oregon
beaches are generally only poorly known and usually cannot be quanti-
tatively assessed. On most coasts, rivers are the principal sources,
but this does not appear to be true for the majority of Oregon beaches.
Many of our rivers pass through sizeable estuaries before reaching the
ocean. The river sands are deposited in the estuaries rather than
reaching the ocean beaches (Kulm and Byrne, 1966). This can be seen
in Figure 4 which shows the areas of sand accumulation in Yaquina Bay
and the sources of those sands. In that example the river sands do
not reach the ocean beaches, and in fact beach sand is transported
into the bay through the inlet so that the estuary represents a loss
of beach sands in the budget of sediments. Although more study is
needed of other bays and estuaries to determine whether they have
similar sand depositional patterns, wide bays such as Coos, Siletz and
Tillamook are probably all sinks of river sands so that those rivers
do not provide sand to the beaches. Narrow river estuaries such as the
Rogue, Umpqua and Siuslaw may be able to transport sand out onto the
beaches. However, dredging activities in those estuaries may also

remove them as sources of river sands to the beaches. Minor streams
(without estuaries) do provide sands to the beaches, but in most cases
they are quantitatively minor.

NEWPORT
. ... .... . . REALMS
....... ...
-.= ..........
... ........
M."A. of
..........
POINT
DEPOSITION
... .........
............. .
... ........
o

.........

MARINE- FLUVIATILE
......... .
.. . . ............
......... . . . . .
11 111 E FLUVIATILE
COOUILL
1860
POINT
..........
SHORELINE

III YAQUINA
44' 36' 44' 36'

ONEA TA

124-00-

Figure 4. Areas of sand accumulation in Yaquina Bay
indicating that the river sands deposit before reaching the ocean
and that marine sands are transported through the inlet and also
deposited in the bay (from Kulm and Byrne, 1966).

Of major importance to most Oregon beaches is the sand derived
from sea cliff erosion. This is especially true where Pleistocene
terrace sands form part of the sea cliffs, Removal of this source by
the placement of riprap or sea walls (comparable to building a dam on
a river) will lead to the long-term decrease in the size of the beach
and an increase in coastal erosion.

At present the principal losses of beach sand are by winds blowing
the sand inland to form dunes or by losses to the offshore deeper waters.
The offshore losses are long-term; as the sand is abraded while on the
beach, progressively decreasing in grain size, it may become sufficiently
fine to be carried far enough offshore during a storm that it is unable
to return to the beach.

Along most of the Oregon coast the sources and natural losses of
beach sands are quantitatively small. For this reason, removal of
beach sand by sand and gravel companies or others may have a major
impact on the beach, this unnatural loss being a major factor in the
total budget of sediments. An example of this was the impact of the
removal of sand from the beach at Gleneden Beach south of Siletz Spit.
An approximate budget of sand for the area is shown schematically in
Figure 5. The principal source of sand to this beach has been from
sea cliff erosion, estimated to contribute 16,000 cubic meters per year
(that volume represents only sand coarse enough to remain on the beach,
finer sediments being lost offshore). A study of the mineralogy and
grain size of the sands shows that sand brought to the coast by the
Siletz River does not contribute to the beach (Rea, 1975). The Salmon
River and other small coastal streams contribute a minor amount. Prior
to sand mining on the beach (1965 to 1971), the beach appears to have

Cascade
Head

1.00010
3,000 m3/year

cliff Erosion
Y- io cm /yr
16,000 m3/year

Siletz
Bay 7,000 to
9,000 M3/ year
On Offshore
(small) Sand Mining
11965-711
12,000 m3/year

Government
Point BUDGET OF
SEDIMENTS
Siletz Spit Area
(schematic)

Figure 5. An approximate budget of beach sands
for the stretch of coast fronting Lincoln City, south
past Siletz Spit to Lincoln Beach. Shown are estimates
of the sources and losses of sand from the beach,
including that due to sand minina.

neither increased nor decreased in overall width over the years,
indicating the natural losses of sand approximately balanced the
gains from the sources; these losses must have been to the offshore,
to the dunes on,Siletz Spit, and some beach sand movement into Siletz
Bay. The sand mining during the years 1965-71 removed an average of
12,000 cubic meters per year, an amount nearly the same as that
contributed to the beach by sea cliff erosion. Presumably the natural
losses remained the same, so that the sand mining represented a net
loss of beach sands and a decrease in its total volume. Such a
decrease would result in the progressive lessening of the beaches'
ability to protect the coastal properties from erosion. As
discussed in Section IV, the sand mining at Gleneden Beach was not the
primary contributory factor in the recent erosion of Siletz Spit, but
most certainly was an aggravating factor in causing increased amounts
of erosion.

C. Dunes

Sand dunes, active or vegetated, occupy approximately 140 of
Oregon's 310 miles of coast, or about 45 percent (Cooper, 1958; Lund,
1973). The largest area of active dunes with little or no vegetation
extends for a distance of 55 miles between Coos Bay on the south to
Heceta Head on the north (Figure 6). This strip averages about 2 miles
in width, reaching a maximum width of 3 miles at Florence. A major
portion of this active dune sheet lies in the Oregon Dunes Recreational
Area, a division of the Siuslaw National Forest. A second area of
important active dunes is present to the immediate north of Sand Lake
on the northern coast.

There are extensive areas of older, well-vegetated dunes, commonly
covered with forests of large trees and dense brush. The dune origin of
the underlying sands is not always readily apparent; dune sands are best
identified by the cross-stratification of sands exposed in roadcuts or
other excavations. These vegetated dunes were formed during the
Pleistocene Epoch, and are now perched on marine terraces well above
sea level. They are common along the Oregon coast with the exception of
Curry County south of Cape Blanco.

There are many problems concerning the management of these coastal
dune areas. Particularly susceptible to erosion problems are the
foredunes which immediately back many beaches. Such foredunes are usually
active or only conditionally stabilized by dune grasses. Therefore the
sands of the foredunes are often rapidly moved about by the strong
coastal winds. In addition, because the foredunes are adjacent to
beaches they may be eroded rapidly by waves,

ft"U
'Rk
MY

xv,

NC:e
.__4'4AW'

Figure 6. The active dune field of the Coos Bay dune sheet which
stretches for some 55 miles along the mid-Oregon coast (photo courtesy
of U.S. Army Corps of Engineers, Portland District).

D. Climate

Climate exerts a major influence on Oregon coastal beaches and
dunes. In large part, rains govern the surface moisture in the dunes
which in turn partly control whether the sands can be moved by winds.
The direction and strength of the wind then determines the direction and
rate of the resulting sand transport and dune migrations. The coastal
winds are also important in wave generation and in the creation of
nearshore currents along the beaches.

The climate of the Oregon coast is highly seasonable, more so than
most other midlatitude coasts. During the winter months, storm systems
move inland from the north Pacific bringing rains and a predominance
1, WM"t",

of strong winds out of the south to southwest. The summer months are

dry with milder winds, mainly from the north to northwest (Cooper, 1958).

Water runoffs in the coastal rivers and streams closely follow the
seasonal variations in rainfall, discharges in the winter months being
30 to 50 times greater than during the summer (Figure 7). The Columbia
River is the one exception to this discharge pattern in that it has two
periods of maximum discharges, one during the winter due to the rains,
and a second in May and June due to snow melt in the Cascade Mountains.

200 1 1 1 1 1 1 1 1 1 1 1 - 7000

SIUSLAW RIVER
Mean Monthly Discharge_ 6000

C:
0 C-)
150- ,.,.Mapleton gage
>
-5000

rri

E -4000 7

ia 100
@3
-3000-0
Uj
0
CC
<
a:
-2000
50

Austo gage 'c' 1000

cr I I I I I I 1 4 -
OCT NOV DEC JAN FEB MAR APR MAY JUNE JULY AUG SEPT

Figure 7. An example of the seasonality of
river discharge with large winter discharges and
negligible summer discharges, following the season-
ality of rainfall.

E. Wave Conditions

Wave conditions along the coast of Oregon also follow a seasonal
pattern in response to the parallel changes in weather patterns and
wind speeds. This is to be expected as it is the wind that generates
the waves; the greater the wind speed the higher the waves produced
(Komar, 1976, Chapt. 4). Other factors in wave generation are the
duration of the winds and the extent of water area (fetch) over which
the winds blow. Not all waves reaching the Oregon coast come from

local storm systems adjacent to the coast, An example is shown in
Figure 8 from a storm that occurred over the north Pacific during
20-25 December 1972, a storm which generated 23-foot high breaking waves
along the coast, resulting in much erosion, especially on Siletz Spit.
It is seen that the winds that generated these exceptionally high waves
blew across a major portion of the Pacific but were not particularly
strong at the Oregon coast itself.

23 December 1972 1800Z
17e E H

70*"

Ise

6WR
64
68
170'. 30 &0 76

94
100
10,
100 !PD_ 5WN
ro

'0 30
124 45
16WW 0 30 SO-
45
H \55 40-N

140'W 30-, --- oew 120*W

Fiqure 8. Storm system on December 23, 1972
with winds blowing across most of the Pacific and
directed toward Siletz Spit. This storm qenerated
unusually high waves along the Oregon coast which
caused major erosion at Siletz Spit and elsewhere.

Waves reach Oregon from storms in the far South Pacific as well,
although they are not nearly as large as those generated by storms in
the North Pacific. Komar and McKinney (1977) analyzed storms such as
that of Figure 8 and their role in generating waves and beach erosion
on the Oregon Coast.

ocean wave conditions have been measured daily at Newport by a
seismic recording system that detects microseisms produced by the waves.
This yields a measure of the highest one-third of the waves, as well as
the periodicity of the wave motions. This system has been in operation
since November 1971, measuring waves four times daily. This wave data
set is the longest and most complete available for wave conditions on
the Oregon coast, and has been summarized by Komar, et al. (1976b) and
Creech (1977).

Figure 9 gives an example of the annual changes in wave heights
and periods, this example extending from July 1972 through June 1973.
The solid lines give the average wave breaker heights and periods for

13 1 1 1 1 -_ -
10 WAVE PERIOD

10-
0
a:
W
a.

E3-

7 1
JULY AUG@ SEPT OCT NOV DEa JAN. FEB. MAR, APR. MAY JUNE

7 1 1 1 1 1 1 1 1 1 1
BREAKER HEIGHTS

E'
4

3 A
%

0
JULY AUG. SEPT 19'72 OCT NOV DEC. JAN. FEB. MAR, 1973 APR. MAY JUNE

Figure 9. Significant wave breaker heights and periods measured at
Newport during July 1972 through June 1973. Each datum point gives the
average for one-third month. The dashed lines give the maximum and
minimum breaker heights during those one-third month intervals. Note
the arrival of large storm waves during the last part of December 1972,
caused by the storm of Figure 8.

each one-third month. Also given are the maximum and minimum wave
breaker heights that occurred during those one-third month intervals
(the dashed lines). It is seen that much larger breaking waves prevail
during the winter months, reaching an average of about 15 feet.
However, individual storms produce maximum daily waves with significant
heights of some 23 feet. Such storm waves occurred in the last one-third
of December 1972, (Figure 9), associated with the storm system of
Figure 8. Breakers of 23-foot height are truly exceptional. The
seismometer system has measured such high waves only three times since
its installation in 1971--in December 1972, October 1977 and February
1978. Each instance was marked by severe beach erosion along the coast.
Thus, not unexpectedly, unusually high waves are largely responsible
for the episodes of coastal erosion.

F. Beach Cycles

Beaches respond to sea sonally changing wave conditions as schemat-
ically illustrated in Figure 10 and for the beach at Gleneden Beach in
Figure 11. During the summer months of low waves, sand moves onshore
forming a wide berm--the nearly flat exposed portion of the beach
(Figure 10). During the stormy winter months of high waves, sand is
eroded from the berm and moves offshore, depositing there in offshore
bars. Such seasonal cycles of the beach profiles occur on most beaches,
and have been documented by Aguilar and Komar (1978) on two Oregon
beaches (Figure 11). Komar (1977) reviews the Oregon beach profiles
obtained during 1945-6 by the U.S. Navy using an amphibious DUKW
(pronounced "duck"). Only with such an amphibious vehicle have
profiles been obtained over the deep outer bars of Oregon beaches.

Although the cycle between the two beach profile types of Figure
10 is approximately seasonal, they are really a response to high storm
waves versus low regular swell waves such as occur most commonly during
the summer. But if low waves should occur during the winter, sand will
move onshore and the berm widen so that the two types of profiles are
not always strictly seasonal.

The principal importance of this cycle of beach profiles is that the
swell (summer) profile with its berm helps protect the sea cliffs and
foredunes from wave attack and erosion. In contrast, the absence of a
berm in the storm (winter) profile allows the waves to swash up against
the coastal property, producing erosion. This is one of the principal
reasons that wave erosion of coastal properties on the Oregon coast is
limited mainly to the winter months.

It is seen that an important role of the beach is that it acts as
a buffer between the ocean waves and the coastal property, causing the
waves to break offshore and dissipate most or all of their energy
before reaching the coastal property. With a wide berm the waves are
not able to reach the coastal property at all. Removal of sand from
the beach by sand mining, as mentioned earlier, will reduce the total
volume of beach material and hence the winter waves have less sand to

swell (summer) profile
storm profile shoreline Sea
swell profile shoreline
berm Cliff
mean water level

trough bar
bar
storm (winter) profile

Figure 10. Schematic illustration of the beach profiles produced by
storms versus gentle swell waves. On the Oregon coast these profile
changes are approximately seasonal due to our storms occurring principally
during the winter months.

BEACH PROFILES -
GLENEDEN BEACH
(1) 2T AUO@T WS
0 2 (2) 7 OCTOBER 19M -
CS [email protected]. 1976
14) W NMEMSER 1976
:51 15 CE EMBER 1976
6
9
)ANUARY 1977
-4 (7)2F MUARY le@
(8 17 FEBRUARY 1977
z 4 - ?0)6ilARCH 197T
:9) 15 . RC. 1917
(11)2APRIL 1977
I0.1
>
Lu
-1

1 .10 1 "a
60 1.
0 AO DISTANCE IN METERS .2o .40

I I II - --111 .............
BEACH PROFILES
GLENEDEN BEACH
(10) 16 MarCh 1977
-2- (11) 2 Aprit 1977
(12) 7 May t977
(131 3 June 1977
-3 13 (14) 23 July 1977

-4 - tI
12 [-ge-4]

Lu
1 14 -g.-4]
Lu -6-

---------- --------- ---------
12
13 [-q.-4j
-60 -L -L 10
210 40 60 80 W 12 160 1W 200
DISTANCE IN METERS

Figure 11. Profile changes measured at Gleneden Beach from August
1976 to July 1977 showing the winter erosion of the exposed portion of
the beach followed by deposition as the spring and summer months of
lower waves return. The profiles do not extend far enough offshore to
show the offshore bars (from Aguilar and Komar , 1978).

shift offshore before attacking coastal property. Sand mining reduces the
beach's ability to act as a buffer between the land and the erosive
ocean waves.

G. Nearshore Currents and Sand Transport

Waves reaching the coast generate currents in the nearshore zone
that are important to sand movements on the beach and to beach erosion.
These currents are independent of the normal ocean currents that prevail
further offshore, being negligible on the beaches (except for the tidal
currents which are important to beach processes in some cases).

When waves break at an angle to the shoreline they generate a cur-
rent that flows parallel to the shoreline (Komar, 1976, Chapt. 7). This
current, together with the waves, produces a transport of sand along the
beach known as littoral drift. On Oregon beaches the waVes tend to
arrive from the southwest during the winter months and from the north-
west during the summer (corresponding to the changes in wind directions).
As a result, there appears to be a seasonal reversal in the direction of
littoral drift; north in the winter, south during the summer. The
difference between the two, the net littoral drift, appears to be
nearly zero, at least when averaged over a number of years.

That the net littoral drift is essentially zero is demonstrated by
the absence of continuous accumulations of sand on one side of jetties
or rocky headlands, with erosion on what would be the downdrift side.
Instead, sand tends to accumulate and/or erode symmetrically around
newly constructed jetties. The major rocky headlands appear to protrude
sufficiently far out into the sea that the sands forming the beaches
cannot pass around them. Thus, they would also block any net littoral
drift if it did exist. Instead, the beaches of Oregon are essentially
pocket beaches, isolated from one another by the headlands, with zero
net littoral drift prevailing in each pocket. For this reason, when
one develops a budget of littoral sand for a particular beach the
analysis should include the entire length of the pocket beach between
two major headlands (as was done in Figure 5). Only the -beach of the
Clatsop Plain north of Seaside does not fit into this concept of being
a pocket beach, this exception being due to the presence of the
Columbia River.

Most of the time the waves approaching the Oregon beaches are nearly
parallel to the shoreline trend. Under such circumstances the nearshore
currents form a cell circulation, the most prominant part of which are
the rip currents (Figure 12), narrow currents flowing offshore away
from the beach. The rip currents are fed by longshore currents flowing
roughly parallel to shore, but for only a short stretch of beach, unlike
the longshore currents which are generated by waves breaking at an angle
to the shoreline. The cell circulation pattern illustrated in Figure
12 rearranges the sand on the beach, the feeder longshore currents
following troughs in the beach profile shoreward of the offshore
bars, and the rip currents bisecting the bars. Of special importance
to beach erosion, the rip currents often carry sand offshore, hollowing
out embayments into the beach as illustrated in Figure 12. At times
these embayments can extend across the entire width of the beach and begin
to erode into foredunes or sea cliffs. Such patterns have been important
to erosion on Siletz Spit.

OCEAN

CL
b eaking waves

dune edge

El El 0 El M El E-1 El
I I
SPIT endangered
homes

Figure 12. A rip current flowing outward across
the beach hollowing out an embayment into the beach
and eventually into the foredunes causing property
losses.

H. Tides

Tides on the Oregon coast are moderate with a maximum spring tide
range of about 13 feet and an average range of about 6 feet. There are
two highs and two lows each day, with the two highs and two lows usually of
markedly different levels (Figure 13).
@
,e
u'n @ee,
d

TIDAL ELEVATIONS ON THE OREGON COAST

Tide 14'5Extreme High Tide - The highest projected tide that can occur. it is the su m 0 1 the
Sff highest predicted tide and the highest recorded storm surge. Such an "e would
STATE OF OREGON to be expected to have a very long recurrence interval. I n some locations, thenetiffect of
DIVISION OF STATE LANDS in ft. a rain induced freshet must also be taken under consideration. The extreme high
M.L L.W tide level is used by engineers for the design of harbor structures.

12.63 Highest Measured Tide - The higest tide actually observed on the tide staff.

14 10.3 Highest Predicted Tide - Highest tide predicted by the Tide Tables.
Typical Days Tide 8.38 Mean Higher High Water - The average height of the higher high tides observed
over a specific time interval. The intervals are related to the moon's many cycles
12-- which range from 28 days to 18.6 years. The time length chosen depends upon the
re finem ent required. The datum plane of MHFIW is used on National Ocean Sui-vey
charts to reference rocks awash and navigational clearances.

7.62 Mean High Water - The average of all observed high tides. The average is of both
higher high tide: ......... . ......... ................... ................... .............. ... . highs, high tide 10.
- the higher high and of the lower high tide recorded each day over a specific time
.......... .......... The datum of MHW is the boundary between upland and tideland. It is used
9 period
....................
on navigational charts to reference topographical features.
.......... i.......... .......... .......... .......... .......... .......... .......... .......... . ...... ......... ......
Also called half-tide level. A level midway between mean high
lower high Ide 4.58 Mean Tide Level
water and mean low water. The difference between mean tide level and local mean
.......... .......... .......... ........ ... i......... ... .... 7
........ .......... 1 - !
sea level reflects the asymmetry between local high and low tides.
.......... ..................... .......... .......... .... . . ......... ... . ........ . 6
4.51 Local Mean Sea Level - The average height of the water surface for all stages of the
tide at a particular observation point. The level is usually determined from hourly
.......... .................. ..... . . ......... .......... 5
height readings.
... ......... .......... .................... .......... . . . ....... ......... . ......... .......... 4
4.11 Mean Sea Level - A datum based upon observations taken over a number of years
at various tide stations along the west coast of the United States and Canada. It is
. ......... ......... .......... ..... . ...... ..........
higher low tide: officially known as the Sea Level Datum of 1929, 1947 adj. and is the most
7 i..........i.......... ......... .......... i 2 common datum used by engineers. IySL is the reference for elevations on U.S.
a : : 7 .
Geological Survey Quandrangles, The difference between MSL and Local VISL
.......... ......... . ......... ........ .......... ......... ......... flects numerous factors ranging from the location of the tide staff within an
.......... .......... ......... .......... i..........
lobal weather patterns.
estuary to 9
.... ...... .......... .......... ....... . ......... .......... .0
e

1.54 Mean Low Water - The average of all observed low tides. The average is of both the
......... ......... .. .... . ...... .......... .......... .......... ....... .......... ... .. -I- lower low and of the higher low tides recorded each day over a specific time period.
The datum of MLW is the boundary between tideland and submerged land.
.......... ..........
! .......... .......... ......... . ......... .......... ..........
lower lo@ tide 0.00 Mean Lower Low Water - The average height of the lower low tides observed over
_3, a specific time interval. The datum plane is used on Pacific coast nautical charts to
refe7rence soundings.
24 hours -47 2.9 Lowest Predicted Tide - The lowest tide predicted by the Tide Tables-
Note: Specific elevations are based on six Vearsof tide observations at the Oregon State University U -3.14 Lowest Measured Tide - The lowest tide actually observed on the tide staff.
Marine Science Center Dock on Yaquina Bay. Values have been reduced by the National Ocean
Survey (for merly the Coast and Geodetic Survey). The elevations differ from estuary to estuary _-3.5 Extreme Low Tide - The lowest estimated tide that can occur. Used by
and from different points within an estuary. The exception is MLLW which is zero by definition. navigational and harbor interests,

Figure 13. Tidal elevations as measured in Yaquina Bay (from Hamilton, 1973).

Tides are an important factor in coastal erosion in that they
govern the hour by hour level of the sea and hence the position of the
shoreline and the zone where ocean waves expend their energy. In
particular, spring tides may bring water levels high up on sea cliffs
and foredunes so that the waves attack the coastal properties directly.
Such high spring tides have been shown to aid in the erosion of Siletz
Spit and to have had an important part in causing the 1978 breaching
of Nestucca Spit. Tides are also significant to the currents which
occur in estuaries, especially in the inlets of bays and estuaries.

I. Tsunami

Tsunami are large waves generated on the ocean surface, usually
by an earthquake producing a displacement of the sea floor, The most
common source of significant tsunami reaching the Oregon coast come
from earthquakes in and around Alaska. Two have struck the Oregon
coast in recent years--28 March 1964 and 16 May 1968. Their impact on
the coast is described in Schatz, et al. (1964), Schatz (1965) and Wilson
and Torum (1968).

Figure 14 shows the heights of tsunami waves arriving at various
Oregon coast sites during the 1964 episode. It is seen that at each
location there is a series of waves, the first wave not always being
the largest in the series. In that episode, the maximum wave heights
were approximately 10 feet. About the same heights were recorded in
the 1968 tsunami.

Maximum destruction from the tsunami occurred along the shorelines
of bays and estuaries rather than on the open ocean coastline. This
is because at least initially the heights of the tsunami waves increase
as they are wedged into the constricting confines of the estuaries.
For example, the 1964 tsunami damaged bridges and dwellings along the
shores of the Necanicum and Neawanna Rivers (Figure 15), the damage
being estimated at $276,000. Other hard hit areas were Cannon Beach
($230,000), the Waldport-Alsea area ($160,000), Florence ($50,000) and
Coos Bay ($20,000).

There are few reports of tsunami wave destruction along the ocean
shorelines. During the 1964 tsunami four children were drowned as their
family slept on the beach at Beverly Beach State Park. This low-lying
campground was evacuated twice in 1965 as a result of tsunami warnings
(Stembridge, 1975, P. 103). There may be more destruction from future
tsunami as many dwellings have been recently constructed in vulnerable
areas close to the beach. In addition to low-lying areas that have little
or no elevation above the beach, foredune areas can in general also be
expected to be vulnerable to tsunami runup. This is because the foredunes
commonly have a general oceanward slope that permits the tsunami runup to
continue with no direct obstacles. In contrast, areas with sea cliffs
should not be endangered as the cliff will reflect most of the energy
of the tsunami waves.

1240W 1220 W 1200 W
CAPE FLATTERY/ I
FRIDAY HARBOR 2.3( R
NEiH BAY - 4.7(R I VICTORIA ILEGEND
APPROX. LOCATION 5 TIME
LAPUSH 5 3(R I OF TSUNAMI WAVE FRONT
% % EVERETT APPROX. LOCATION a TIME 481 N
HOM R. 14 1 OF CREST OF SPRING TIDE
SEATTLE 0.8 F 2.3 1 R) FIGURES ARE HEIGHTS (in ft)
BREMERTON OF MAXIMUM TSUNAMI WAVE
HOLAH- .I 4(R) BASED ON RISE I. R ) OR
TA
WRECK CREEK 4 9(R FALL( F) ABOVE TIDE LEVEL
DATA FROM SPAETH AND
C) BERKMAN, 1967 ; SCHATZ,
OCEA SHORES-9.7(R I st al , 1964 j HOGAN, ot a ,
RAYe HARBO R WHIPPLE a LUNDY, 1964-,
U.S. COAST GUARD

WILLAPA SAY

\ TIDE CREST
SEAVIEW - 12.5(R i WASHINGTO14
ILWACO - 4.5 R 1 01 0 0 0
0 S? n 0
2 COLUMBIA
CAPE DISAP OINTMENT RIVER
5.7 ( R WAVE HEIGHT ABOVE MEAN HIGH WATER
FEET
0 TSUNAMI
ASTORiA-2.4\(R) 0 FRONT 10 460N
NEHALEM R. Ll 0
TILLAMOOK %DAY 10
Ik': I TILLAMOOK
TILLAMOOK R, L 0
10
0
@71 0 o
2 0 0) DEPOE BAY L 0
10
NEWPORT
co "I OUINA BAY L
0 YA 0
I TOLEDO CO'RVALLiS
ALSEA BAY
SIUSLAW R. L I'D
0
EUGENE
UMPQUA R. L 10 440N
WINCHESTER SAY 0
I
10

COOS BAY 0

to
BANDON COQUILLE R. 0
It 11
CAPE
BLANCO CHETCO R. 10
07 08 09 10 11 12 13
TIME IN HOURS - G.M.T. MARCH 28, 1964

OREGON

420N
RESCENT CITY
19.7 (R CALIFORNIA

Figure 14. Maximum heights of tsunami waves recorded at tide
stations or by observations along the Washington-Oregon coast (from
Wilson and Torum, 1968).

Figure 15. Destruction at Seaside from the March
1964 tsunami (from Wilson and Torum, 1968).

The occurrence of tsunami along the Oregon coast is very sporadic
and unpredictable. However, there is a strong probability that another
will occur within the next 10 to 20 years. In addition to the estuary
areas, almost any foredune or low-lying area can be a potential site for
destruction. Unfortunately, with our present understanding of tsunami
it is difficult to predict their expected runup for a specific area.
Studies following the occurrence of a tsunami show a great deal of
variability in the amount of runup along the coastline and the amount
of resulting destruction, a variability which at present is poorly
understood and cannot be predicted. For this reason, at the present
time it is best to be cautious when building on foredunes and in
low-lying areas susceptible to tsunami runup.

J. Sea Level Changes

Changes in sea level with respect to the land have important
consequences to coastal erosion. With the melting of the Pleistocene
ice sheets, which most recently began about 30,000 years ago, water
was returned to the oceans causing a rise in sea level. At first
this sea level rise was rapid, but about 7,000 years ago it slowed
down appreciably (Komar, 1976, p. 154-7). For the past 34 years there
appears to be at most a 1.5 mm (0.005 ft.) per year rise in sea level
(Hicks, 1972). Hicks determined this rate from long-term tide gauge
records obtained on all coasts of the United States. If one averages
the recordings of a tide gauge for the entire year, an average water
level for that year is obtained. Over the years this level can change,
indicating an apparent long-term change in the sea level. The result
obtained at a particular coastal site will depend on whether the land
there is stable, rising or lowering, as well as on any actual sea level
change. For example, on the east coast of the United States the land is
sinking so that the apparent sea level rise is still larger than the
1.5 mm/year value (Figure 16). As a consequence, in the long term
(tens of years to centuries) the shoreline there tends to migrate
landward, resulting in shoreline erosion and endangering dwellings
constructed too close to the beach. In contrast, much of Alaska is
rising at a geologically rapid rate, much greater than 1.5 mm/year.
This results in the land emerging from the sea with the shorelines
receeding (Figure 16). If sea level is presently rising at the rate of

time (years )

10 20 30 40 50
+20

East Coast

0
7; Oregon Coast

- eu

Alaska

-401

Figure 16. Schematic of water level changes
.@@East -Coast
,7-
0 ego'
@@A Ira s k c

on the Oregon coast as compared to the East coast
and the coast of Alaska, based on the data of
Hicks (1972).

1.5 mm/year, then the Oregon coast must be rising at about the same
rate as the tide-gauge records at Astoria, Crescent City (California),
and Friday Harbor (Washington), analyzed by Hicks (1972), all show no
apparent sea-level changes over the years (Figure 16). Excepting the
yearly fluctuations which have many causes, the long-term trends show
a nearly unchanging sea level. In contrast to the east coast, the
apparent lack of a rising water level with respect to the land along
the Oregon coast should act as a deterrent to coastal erosion. However,
as recently as 7,000 years ago the sea was probably transgressing
rapidly over the Oregon coastal zone, producing erosion. Insufficient
time has passed since then for the coast to come to equilibrium with
the present sea level, so that the general erosion of the rocky headlands
and terraces is more a response to that past rise in sea level than due
to any present-day rise.

III. EROSION DUE TO JETTY CONSTRUCTION

The earliest erosion problems on the Oregon coast were associated
with the construction of jetties at the entrances to bays and estuaries.
Most of these were installed early in the century, but subsequently have
been repaired and in some cases lengthened. Of interest are the causes
of the erosion resulting from jetty construction or extension. An
examination of these problems also provides information about the
littoral drift of sand along Oregon's coastal beaches.

The most dramatic and famous case of erosion due to jetty con-
struction on the Oregon coast is that which occurred on Bayocean Spit
opposite Tillamook Bay. Following the installation of a single north
jetty, sand accumulated to the north side of that jetty, resulting in
sand deposition to the north (Figure 17). At the same time, erosion
occurred along most of the length of Bayocean Spit and south past the
community of Cape Meares. Somewhat earlier the resort community of
Bayocean Park had been developed on the spit; its homes and buildings
were progressively undermined by the erosion and lost (Figure 18), so
that eventually the entire town disappeared. Erosion to the spit
culminated in November 1952 when storm waves combined with high tides
to break through the spit at its narrow mid-section, the northern half
of the spit becoming an island. The newly breached area became the main
inlet to the bay; the former inlet with the jetty began to close. For
this reason, in 1956 the U.S. Army Corps of Engineers built a dike
across the new inlet, closing it, and the inlet with the jetty opened
once again. The story of the development of Bayocean Spit and its
subsecuent erosion is documented at length in Terich (1973) and in
Terich and Komar (1974).

Manhatten Beach

Rockaway Beach

jetty

0
CL

0 3
krn

E3 erosion
V-1 deposition

Cape
Meares

Figure 17. Patterns of beach deposition and
erosion resulting from construction of the north
jetty at the entrance to Tillamook Bay (from
Terich and Komar, 1974).

The sand accumulation to the north of the Tillamook jetty together
with erosion along the spit to the south (Figure 17) led early studies
to conclude that the jetty construction had blocked a large net littoral
drift of sand toward the south. Such patterns of erosion and deposition
(beach sand accumulation) are typical of the blockage of a net littoral
drift, diagramed schematically in Figure 19A, such as has commonly
occurred on the east coast of the United States and in southern
California. As previously mentioned it is now believed that this and
other beach areas of the Oregon coast have essentially zero net littoral
drifts (Figure 19). If a very large net littoral drift did occur in the
Bayocean Spit area, then Cape Meares to the south (Figure 17) should
have acted similar to a jetty, blocking the drift with a large
accumulation of sand on its north side; there is none.

The erosion of Bayocean Spit has to be understood in terms of
having occurred under conditions of a zero net littoral drift. Even
n @B@
y Be.

with a zero net drift there can be local rearrangements of beach sand
produced by the jetty construction. Such changes are best seen where

two jetties are constructed rather than a single jetty as at the
Tillamook Bay entrance.

i910

19-32

,01 1,

Figure 18. Erosion on Bayocean Spit leading
to the loss of the natatorium with an indoor
swimming pool (from Terich and Komar, 1974).
z @--'

A. NET LITTORAL DRIFT
@_@ <0CEAN
net I i floro I

drift

deposition wove C
rest

erosion

B. ZERO NET DRIFT

OCEAN wave crests

------------

erosion
erosion
deposition deposition

Figure 19. Schematic of shoreline changes
(deposition and erosion) produced by jetty con-
struction in areas experiencing a net littoral
drift versus an area such as the Oregon coast
where there is a zero net littoral drift.

Large shoreline changes also occurred following construction of
a pair of jetties at the entrance to the Siuslaw River near Florence
(Figure 20). It is seen that where two jetties are constructed, there
is beach sand accumulation both to the north and south, immediately
adjacent to the jetties. This deposition and shoreline advance occurs
because an embayment is formed between the newly constructed jetty and
the pre-jetty shoreline. Before jetty construction, the shoreline
curved inward toward the inlet and was in equilibrium with both the
ocean waves and with the currents coming in and out of the inlet.
Jetty construction eliminated the inlet currents acting on that curved
portion of shoreline, leaving only the waves. The waves broke at
angles to the curved shoreline and so moved sand into the embayment
,until it completely filled with sand (Figure 21). Once the embayment
filled and there was a smooth and nearly straight shoreline parallel to
the dominant waves, then a zero net littoral drift once again prevailed.
After that stage is reached there are no additional large-scale adjust-
ments of the shoreline due to the presence of the jetties. Therefore

a new equilibrium is achieved, and shoreline changes do not continue
indefinitely. In the case of the blockage of a net littoral drift
(Figure 19A), the only equilibrium that could occur following jetty
construction is if the sand accumulated on the updrift side of the
jetties until it is able to pass around the jetties to the downdrift
side.

PACIFIC OCEAN N

1974

'09 '914 1915
-1889 1915

SJUSLAW

SIUSLAW JETTIES @@300
High tide shoreline advance due to meters
jetty construction. Based on Corps

of Engineers.

Figure 20. Compilation of shoreline changes
resulting from jetty construction at the mouth of the
Siuslaw River, based on old ground surveys and aerial
photographs. The 1889 shoreline predates jetty con-
structi on (from Komar, et al . ,1976a).

The sand that fills the shoreline embayments produced by jetty
construction must come from somewhere, and most of it comes from shore-
line erosion at greater distances from the jetties (Figure 19B). Thus
a symmetrical pattern of erosion and deposition results with beach
sand accumulation immediately adjacent to the jetties,* both to the
north and south, and with erosion at greater distances from the jetties
(Figure 19B). This contrasts with the asymmetrical pattern where the
jetties block a net littoral drift, the shoreline advancing seaward on
the updrift side and erosion occurring on the downdrift side (Figure 19A).
As in the case for jetty construction on the Siuslaw River (Figure 20),
the patterns of erosion and deposition may not be perfectly symmetrical
due to the different sizes of embayments created on the two sides of the

jetties. In.that example the embayment to the north was much larger
than that to the south. This required a larger amount of sand to fill
the embayment and resulted in greater erosion to the north to supply
that sand. Because it is only about 6 miles from the Siuslaw jetties
north to Heceta Head, there was a relatively short stretch of beach
from which to erode sands to fill the embayment; thus the amount of
erosion per unit length of shoreline was large. In contrast, to the
south there is a very long length of beach and a smaller embayment to
fill; the amount of erosion per unit shoreline was smaller there, nearly
negligible. The amount of shoreline retreat produced by jetty construc-
tion in areas, such as the Oregon coast where there is a zero net
littoral drift, is a function of the size of the embayment to be filled
adjacent to the jetty and the length of beach over which erosion occurs
to supply the sand.

OCEAN

post -jetty equilibrium
s
r
hoeline A-00 shoreline erosion

pre - je

Figure 21. Schematic of the filling of the shoreline
embayment created between the newly constructed jetty and the
pre-jetty shoreline, the waves causing sand transport into the
embayment until it is completely filled and a new straight
equilibrium shoreline is achieved.

The coastal erosion associated with the construction of the jetties
on the Siuslaw River inlet produced no problems in that the erosion
was confined to the early part of the century soon after jetty construc-
tion. At that time the coastline to the north was undeveloped so no
homes were in the path of the erosion.

The erosion problems at Bayocean Spit were similar, but complicated
by the fact that only a single north jetty was constructed rather than
a pair of jetties. As at the Siuslaw jetties, there was sand accumulation
to the north of the jetty (Figure 17) until the embayment produced by

jetty construction was filled and the shoreline straightened. There was
probably some erosion further to the north, but the length of beach there
is long so that there was only a small amount of erosion per unit shore-
line length. Sand also accumulated at the northern tip of Bayocean Spit
even though a true embayment was not formed by jetty construction. This
accumulation was in the form of a large shoal which developed seaward of
the south side of the inlet (Figure 17). That sand apparently came from
the erosion of Bayocean Spit, so there was something of a symmetrical
pattern of deposition and erosion (Figure 17), similar to that of the
Siuslaw jetties. The erosion of Bayocean Spit was large because the
eroded sand came from a short length of beach, Cape Meares being to the
immediate south. In 1976 a south jetty was constructed at the inlet,
forming a true embayment with the pre-jetty shoreline. As at the Siuslaw
inlet and other Oregon-coast inlets, the embayment filled until the shore-
line was straight with the shoreline along the remaining length of Bayocean
Spit. That filling required some additional sand, again derived from
further erosion of the spit. However, since that time, erosion of the
spit and the Cape Meares area has been small since a new equilibrium
exists in which no further sand is required for deposition next to the
south jetty.
On the Oregon coast and other areas of zero net littoral drift,
once jetties are constructed and sand has filled the embayments to
either side, the jetties can subsequently be extended without producing
additional major shoreline readjustments and erosion. This is especially
true if the jetties are perpendicular to the coastline trend, extending
straight out to sea. For example, the jetties on the Siuslaw River inlet
(Figure 20) could be extended without causing renewed erosion problems,
and the proposed extension of the south jetty at Tillamook Bay will not
cause significant additional erosion on Bayocean Spit. However, where
jetties are oblique to the shoreline trend, as at the Yaquina Bay entrance
(Figure 22), jetty extension produces some additional sheltering from the
waves to the enclosed side and results in further sand accumulation next
to the jetty and some further erosion at greater distances from the jetty;
this was the case with the extension of the Yaquina Bay jetties in 1971.

The filled embayment areas to either side of inlet jetties are
dependent upon the presence of the jetties. If the jetties are allowed
to degrade then there may be some erosion to these filled areas. A
possible example of this may be the recent erosion at Nedonna to the
immediate south of the jetties on the Nehalem River. As at the other
inlets, following the construction on the Nehalem in 1917, the embayments
to either side filled and the shoreline there advanced seaward. No
further work has been done on these jetties, however, and they have
deteriorated to the point that they are covered with water at high tide.
The shoreline again curves back inward into the inlet, but not as much
as prior to jetty construction so that further erosion might be expected.
The community of Nedonna Beach was developed on the south embayment fill,
in an area that was underwater before jetty construction.

ENTRANCE TO
YAQUINA BAY, OREGON

High tide shoreline advance due to
jetty construction. Based on Corps NEWPORT
of Engineers surv ys and recent
aerial photograph:.

YAQUINA
BAY

-N-

me,er,
Za

Figure 22. Compilation of shoreline changes
resulting from jetty construction and then later
extension at Yaquina Bay. The 1830 shoreline
predates the jetty construction. The south jetty
extension occurred between 1940 and 1974 and is
seen to have produced some shoreline advance due
to the additional protection caused by the jetty
extension (from Komar, et al . , 1976a) .

All of the jetty systems on the Oregon coast, with the exception
of the Columbia River jetties, were studied by Komar, et al. (1976a)
to determine patterns of erosion and deposition. All were shown to
conform to the pattern of deposition adjacent to the jetties with
erosion at greater distances. This provides strong evidence that there
is indeed a zero net littoral drift prevailing along the coast of
Oregon as discussed earlier.
0

IV. SAND SPIT AND FOREDUNE EROSION (SILETZ SPIT)

Bayocean Spit eroded due to jetty construction. But other spits,
such as Siletz and Nestucca Spits, have also suffered episodes of
erosion without the presence of jetties. Their storm wave erosion
problems are therefore attributable mainly to natural causes, man
playing a minor role in the processes. This section will deal with
such natural erosion to the fragile sand spits, especially that which
has occurred on Siletz Spit as the problems there have been extensively
studied (Rea, 1975; Rea and Komar, 1975; Komar and Rea, 1976b; McKinney,
1977; Komar and McKinney, 1977).

Prior to 1960, Siletz Spit appeared much as it had for hundreds of
years; over most of its length there were low hummocky dunes, active or
sparsely covered with dune grasses. Development of the spit began in
the early 1960's. A road was cut along its length and artificial lagoons
were carved into the bay-side of the spit. A scatter of houses appeared.

Following spit development, beach erosion first appeared during
January 1971 when a series of storms cut into the foredunes upon which
homes had been built. Several homes were in the path of the erosion,
but riprap placement halted the erosion advance before they were
seriously threatened.

Erosion returned in the winter of 1972-73. A major storm occurred
over the North Pacific in late December 1972 (Figure 8), generating
wave breakers up to 23 feet in height (Figure 9). The waves cut into
the foredunes, quickly threatening several homes. A house still under
construction was left unprotected and so was undermined by the.retreating
dune bluff and collapsed onto the beach (Figure 23), Riprapping began
on Christmas Eve to protect the other homes, However, these houses
were initially defended only on their seaward sides, empty lots to either
side being left unprotected. Foredune erosion and retreat continued in
these empty lots, flanking the riprap fronting the homes, necessitating
the placement of rocks along their sides as well as fronts (Figure 24).
The result was groups of homes situated on promentories extending out
onto the beach, supported by riprap on three sides.

Erosion has returned in varying degrees in subsequent winters. It
was particularly severe during the winter of 1975-76 and again in 1977-78
when a storm generated breaking waves about 23 feet high, and at a time
of high Spring tides. The combination of large waves plus high tides
nearly breached the spit (Figure 25). This same storm did breach
Nestucca Spit, (see Section V).

In each instance foredune erosion did not occur over the entire
length of the spit. Instead, it was limited to two or three zones, each
some 200 feet of spit length. This localization of dune erosion was
governed by the positions of rip currents as previously discussed
(Section II). The seaward flowing rip currents transport sand offshore,
hollowing out embayments into the beach berm. At times these embayments

reach across the entire beach and begin to cut into the foredunes on
Siletz Spit, setting the stage for severe erosion. The major erosion
itself occurs during a severe winter storm which produces high wave
conditions along the coast. The large waves are able to move ashore over
the deep water of the embayments with little loss of energy, swashing
directly against the base of the foredunes, cutting them back. Such
embayments are seen in Figure 26 at the time of the December 1972
erosion; major foredune erosion occurred shoreward of the most pro-
nounced embayment.

28 December 1972

4@7,,W

19 January 1973

Figure 23. Destruction of house under construction on
Siletz Spit due to the rapid wave erosion of the foredunes
upon which the house was being built (from Komar and Rea, 1976).

Figure 24. House left on a promentory of riprap on Siletz Spit as
adjacent unprotected empty lots continued to erode (Photo by P. D.
Komar, 23 January 1973).

T,
Wi

-7

IMM @@

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

Figure 25. Erosion during the winter of 1977-78'alona the narrowest
portion of Siletz Spit, nearly leading to its breaching (Photo by P. D.
Komar).

"n@

'N' 41

Figure 26. Embayments cut out of the beach and into the
foredunes on Siletz Spit leading to property losses during
December 1972 and January 1973, produced by seaward flowing
rip currents (from Komar and Rea, 1976).

Thus the positioning of the rip currents during the winter governs
the locations of maximum beach and foredune erosion. This usually
changes from one winter to the next so the areas of erosion are not always
the same. At present we are unable to predict where the rip currents
will form. But once it is seen where they are positioned we can anticipate
that these could be potential erosion sites. During some winters they
remain relatively fixed in position and so are able to hollow out large
embayments; such conditions are most conducive to major erosion.
During other winters the rip currents migrate somewhat north-south,
probably when waves arrive obliquely to the coastline, and do not form
as large embayments; consequently the potential for erosion is smaller.

During severe episodes of erosion on Siletz Spit, the foredunes
were cut back some 100 feet around the lengths of ocean-facing lots.
Studies of aerial photographs dating as early as 1939 show that such
erosion has occurred repeatedly in the past, but that following the
erosion, beach sand is washed and blown into the eroded zone eventually
rebuilding the foredunes (Rea, 1975; Rea and Komar, 1975; Komar and Rea,
1976b). The following sequence of events is revealed by aerial
photographs and is typical of many cycles of erosion and accretion of
the foredunes: (1) high storm waves erode an embayment or vertical*
scarp into the foredunes; (2) subsequent high tides deposit drift
logs in the eroded embayment; (3) lower energy waves during the summer
build a wide beach; (4) the logs behind the beach trap sand that is
either blown off the beach or washed there by the waves at high tides;
(5) wind-blown sand continues to accumulate around the logs, sometimes
aided by dune grasses, until the foredunes are re-established; (6)
erosion again occurs to repeat the cycle. If uninterrupted, one complete
cycle can take from ten to fifteen years. Figure 27 illustrates the
process of dune reformation (steps 4 and 5) in a small embayment cut
into the foredunes. The criss-crossing matrix of logs is seen to be
effective in trapping sand to re-establish the foredunes. Drift logs
can therefore be an important agent in the reformation of foredunes.

Such cycles of erosion and foredune accretion have occurred repeatedly
in the past, shown by the sequence of aerial photos of the Siletz Spit.
It is also indicated by the presence of sawed drift logs buried within
the foredunes, revealed by the erosion. Many homes built on Siletz
Spit were constructed on foredune areas that had previously been eroded
away and then reformed as described above--erosion which occurred as
recently as the 1950's and early 1960's, just before spit development.

In summary, erosion of foredune areas can be very rapid, removing
some 100 feet of property in two or three weeks. The erosion is mainly
centered in the lee of rip currents which hollow out embayments into
the beach. Maximum erosion occurs under large storm waves, and is also
aided by the high water levels of Spring tides, Following erosion the
foredunes may be re-established by beach sand washing and blowing into
the eroded zone; drift logs aid in dune reformation by trapping the
wind-blown sands.

%
,4

, V.1

113

!,oe

Figure 27. Drift logs washed into an embayment cut by a rip
current on Siletz Spit, now actively trapping wind-blown sands and
beginning to reform the foredunes (from Rea and Komar, 1975).

It would have been preferable if development on Siletz Spit had
been prohibited in the approximately 100 feet zone where foredunes are
susceptible to rapid wave undercutting and erosion. Then the natural
cycle of erosion followed by dune rebuilding could have continued.
Instead, the presence of the homes necessitated the placement of huge
quantities of riprap to the detriment of the spit's appearance (Figure 28).
Much of this riprap was placed on an emergency basis, without the benefit
of correct engineering procedures. As a result this riprap is being
progressively washed away (Figure 29) and will have to be replaced at
additional cost to the homeowners.

jQ;,
t""Iff-al"", 14, t'.
---- I

Figure 28. Large piles of riprap employed on Siletz Spit
to protect the homes built on the foredunes,

Ivor-

Figure 29. Erosion of riprap on Siletz Spit by a
series of storms, exposing the dune sands to wave attack
(from Rea and Komar, 1975).

V. OTHER AREAS OF FOREDUNE EROSION OR POTENTIAL EROSION

Other sand spits and foredune areas have suffered erosion on the
Oregon coast in addition to that which has occurred on Siletz Spit.
Even though erosion may not have been noted in the recent past, all
foredune areas have the potential for rapid wave erosion due to their
negligible resistance to wave attack. This section will discuss other
sand spit and foredune areas that have eroded or have the potential for
future erosion.

A. Nestucca Spit Erosion

The erosion of Nestucca Spit, Figure 30, is comparable in extent
and in processes to the erosion on Siletz Spit, already discussed
(Komar, 1978). The erosion has threatened a number of homes at the
Kiwanda Shores development to the south of Cape Kiwanda, necessitating
the placement of large quantities of riprap even before house construc-
tion was complete (Figure 31). Maximum erosion occurred during the
winter of 1977-78; under the onslaught of 23-foot high breakers at a

N
. ..........
. .........
. .........
. .........
........... ..
......................
. ....................
...................
CAPE
KIWANDA
...................
....................
0 i

kilometers
KIWANDA,@- RIV51?
BEACH PACIFIC Wro
CITY

LAJ

Q) w
e (j (@
BREACH

71 E
....... ...
...........

Figure 30. Nestucca Spit, showinq the
areas of foredune erosion and breaching- during
February 1978 (from Komar, 1978).

. .. .......

Figure 31. Homes to the south of Cape Kiwanda protected
by riprap placed due to the erosion of the foredunes upon
which they were being constructed (from Komar, 1978).

time of high Spring tides on 5 February 1978, the.resulting erosion
broke through the spit near its southern end (Figure 32). This is the
only known occurrence of the natural breaching of a sand spit on the
west coast of the United States (the breaching of Bayocean Spit,
discussed above, was not natural in that the ultimate cause was the
construction of jetties). Fortunately, the breached site at Nestucca
Spit was well away from any developments and so threatened no dwellings.
However, it did demonstrate the fragile nature of sand spits and their
general unsuitability for development.

-..Mat

Figure 32. The breach in Nestucca Spit produced by
a combination of unusually high storm waves and hiqh
Spring tides in early February 1978 (State of Oregon,
Highway Department photo).

The erosion processes on Nestucca Spit are very similar to those
on Siletz Spit. Rip current embayments again played a role in deter-
mining the centers of maximum erosion. However, the beach sand on
Nestucca Spit is finer than on Siletz and therefore the beach slope is
somewhat less (see Section II). This causes the rip current embayments
on Nestucca Spit to be wide and shallower than on Siletz, and they do
not cut as far back into the foredunes. The result is that foredune
erosion on Nestucca Spit tends to cover a longer length of coastline,
but does not reach as far inland. During the time of maximum erosion
and breaching, sawed drift logs were observed within the eroding
foredune scarp. As at Siletz, this indicates that these foredune areas
have eroded before, since logging began in the coastal watersheds about
the turn of the century.

B . Netarts Spit

Netarts Spit has a total length of about 5 miles, and is wide to
the north but very narrow in its middle section. The spit is covered
with dunes, the highest reaching nearly 50 feet high. To the south
the dunes are apparently old as they are covered with Sitka spruce.
Elsewhere the dunes are vegetated only with low pines or sparse dune grasses.

Erosion of Netarts Spit by wave attack was apparently a threat to
the southern portion prior to 1940. Dicken (1961, p. 57) suggests that
there is evidence that the spit may have broken through early in the
century in its narrow portion. In 1940 the State of Oregon developed
Cape Lookout State Park at the spit's southern end. The State construct-
ed a wood piling bulkhead backed by riprap along 600 feet of the park
where erosion had apparently been occurring (Figure 33). On the basis of aerial
photographs, Stembridge (1975, p, 80) estimates that erosion has
amounted to only 10 to 15 feet since 1939.

4,q

Figure 33. The wood piling bulkhead built on Netarts
Spit to stop wave attack of the dunes.

The dune bluff facing the ocean is vegetated and also testifies to
the fact that little or no erosion has occurred in recent years. At
times the waves do reach the base of the slope, making a slight notch
into the dunes, but this has been minor. It would probably take a very
unusual combination of storm waves, high tides and a storm surge to
cause appreciable erosion on the spit. At present, the chief problem
is due to visitors cutting paths through the dune vegetation (Figure 34)
which may lead to wind erosion of the dunes.

01 AV;

Figure 34. Degradation of the dunes on Netarts Spit due to
visitors cutting a path from the beach to the state park.

The sand on Netarts Spit is fine and the beach slope very low. As
a result, the rip current embayments are extremely broad and shallow,
so much so that they do not appear to play any significant role in beach
erosion as was the case on Siletz and Nestucca Spits. This may be a
major factor in the general lack of erosion problems on Netarts Spit.

C . Nehalem Spit

Nehalem Spit is an area of low dunes that have been conditionally
stabilized by European beach grass, there having been a dramatic growth
of the grass in the area since 1939 (Stembridge, 1975, Figure 31). On
the basis of aerial photographs, Dicken (1961, p. 66) estimated that the
spit has eroded 5 to 10 feet over a 21 year period (0.25 to 0.5 feet
per year). As discussed earlier (Section III), there was shoreline
progradation adjacent to the Nehalem inlet jetties following their
construction in 1910-19. But subsequently that area has also been
eroding due to the progressive deterioration of the jetties.

The bluff in the Manzanita area cut by this long-term erosion is
now nearing many homes (Figure 35) built a number of years ago. The
erosion is progressive, rather than periodic and raDid as at Siletz
Spit, so these homes are probably not in any immediate danger. Further
south, on Nehalem Spit itself, a number of new homes have been recently
constructed on the foredunes close to the beach (Figure 36). There are
signs of wave erosion of the foredunes in this area so there may be some
potential danger to these houses. Sands are also being actively moved
by the winds, which may also lead to problems. Unfortunately, no one
has made a detailed study of this area.

I NIS

Figure 35. Long-term progressive erosion in Manzanita,
now nearing some of the homes.

A
A 13 ,
Abu

-low

I'll EM

4
*7@

Figure 36. Homes built on Nehalem Spit in an area of
active foredunes susceptible both to ocean wave attack and
wind erosion.

D. Alsea Spit

Alsea Spit may be the one spit.on the Oregon coast that is accreting
rather than undergoing long-term erosion. This is to be hoped for as
the spit is presently undergoing intensive development over its entire
area.

Stembridge (1975, p. 115-120) compared aerial photographs of 1939
and 1974 of the area and found that the south tip of the spit has shown
maximum accretion, an average of 10 feet per year. The accretion progres-
sively decreases in amount northward along the length of the spit, until
at about 1.6 miles north of the inlet it becomes zero with erosion
occurring still further to the north. Erosion rates of up to 2 feet per
year have been occurring along the bay side of the spit.

This accretion of Alsea Spit may be related to high sediment yields
from the Alsea River as suggested by Stembridge (1975, p. 120). Because
the accretion is maximum at the south tip of the spit, it may instead
have resulted from a southward migration of the inlet itself, such
migrations being common for inlets without jetties. If this is the case,
then at some future date the inlet migration could reverse and move back
to the north. This would cause erosion on the spit, especially at its
south tip, so it would be best not to develop that area.

E. Seaside - The Necanicum River Inlet

An example where foredunes have been eroded by inlet migrations is
provided by the Necanicum River inlet at the north edge of Seaside. In
past decades the position of this inlet has alternately migrated north
and south over a distance of about 4,000 feet. In 1948, for example,
it moved well to the south to the very edge of the Seaside community,
endangering the sewage treatment plant there. It then moved back to the-
north causing erosion of the f6redunes south of Gearhart.

Seaside and the Necanicum inlet area are at the southern portion of
the Clatsop Plains and thus have an abundance of sand. Because of this,
whenever the Necanicum inlet migrates away from an area, sand rapidly
accumulates to form a foredune. At present, such an active foredune
is found north of the inlet at the south edge of Gearhart. However,
such foredunes adjacent to the inlet are very susceptible to erosion
bY renewed inlet migration.

In 1967 the inlet migrated to the north and a spit of foredunes
began to develop to the south as a continuation of Seaside. A developer
quickly placed riprap over that foredune area so the inlet would not
migrate back and reclaim the area. The intention was to construct

dwellings on the newly accreted area, but the riprap was placed without
a permit and so has been under litigation ever since. The inlet has
periodically been attempting to migrate back to the south and has been
progressively eroding and undermining the riprap at its northern tip.
In the meantime, foredunes have accumulated in the area on top of the
riprap. In 1978 these active dunes were bulldozed flat, covered with
sludge from the sewage treatment plant and seeded with grass.

Due to their natural migrations, such inlet areas without jetties
are particularly dangerous to develop. The strong currents in the inlets
can undermine the riprap unless done with jetty-scale material. The
deep-water of the inlet also allows ocean waves to reach the shoreline
with little loss of energy so that inlet areas can also suffer from
wave attack. They are also particularly susceptible to overwash by
tsunami waves (Section II). In the particular case of the Necanicum
inlet, there will be continuing problems with wind-blown sands due to
the particular abundance of sand there.

The Necanicum inlet also provides an example of foredunes and
bay-shore properties being eroded by currents within the estuary itself.
The Neawanna Creek enters the Necanicum estuary on its north side to
the east of Gearhart. In the past few years, the flow of the Neawanna
has been eroding the bay-side of Gearhart, necessitating the placement
of riprap to protect homes there. It strikes the bay side of the
foredunes south of Gearhart and is actively eroding them (Figure 37).

Figure 37. Bay-shore erosion at Gearhart, caused by the
flow of the Neawanna Creek against the property.

Such bay-side erosion could also pose a threat to any dwellings placed
in the area. Bay-side erosion has similarly occurred on Siletz Spit
where the flow of the Siletz River impinges on the spit after flowing
across the bay (Figure 38). The erosion there has been aggravated by
the placement of the Siletz Keys landfill. Prior to the landfill,
river flood waters were able to spill into the south part of the bay
(open arrows of Figure 38), but after the landfill blocked these channels

DIKE
0
C
SILETZ KEYS

SILETZ

E3 A Y

-------------------

Spit Width. W

Figure 38. Bay-shore erosion on Siletz Spit
where the Siletz River strikes the backside of the
spit. The erosion has been aggravated by the
placement of landfills such as Siletz Keys which
prevents flood-waters from spilling into the south
portion of the bay (from Rea and Komar, 1975).

all of the flood waters were jetted against the spit (Rea, 1975; Komar
and Rea, 1976b). This bay-side erosion has progressively narrowed
the spit, and together with the ocean-side erosion (Section IV), may
cause a breaching of the spit (see Figure 25). Old, well-vegetated
dunes may also be eroded by bay or estuary currents; an example is the
north shore of the Siuslaw River.

F. Cannon Beach (Breakers Point)

Not all foredune areas are located on sand spits are associated with
inlets. There are examples where foredunes have formed fronting sea
cliffs or older well-vegetated dunes. One example is found in the Cannon
Beach area to the north of Elk Creek (Figure 39), a portion of which is
presently undergoing development. There are clear signs that the forma-
tion of this foredune is quite recent, probably less than 100 years old.
At times storm waves cut into the foredunes, much as on Siletz Spit,
removing as much as 30 feet during a single storm (Rosenfeld,
1979). This erosion has exposed sawed drift loas, again much as
observed on Siletz Spit (Section IV), indicating dune accumulation
since logging began in the area. Backing the northern portions of this
foredune are higher, older dunes covered with trees. A clear erosion
scarp, now covered with grass, has been cut into the seaward-facing
side of these older dunes (Figure 39). This indicates that not too
long ago erosion proceeded all the way up to the older dunes, entirely
removing the foredunp,,. That.erosion must have been an unusual
combination of extreme storm waves, high Spring tildes and a storm
surge, producing an event analogous to the 100 to 200 year flood in
a river. Since that event the foredune sands have been accumulating
with the exception of the 30 feet or so that is periodically eroded
by more common winter storms. Like the river floodplain which is
covered by the 100-year flood, this foredune area and others like it
are not desirable locations for permanent dwellings.

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

Figure 39. Foredunes at Breakers Point, Cannon Beach@,
4-1 A@106__

backed by older, well-vegetated dunes into which waves at
some time cut a near-vertical scarp.

VI. SEA CLIFF EROSION

Although not as dramatic as the rapid erosion of foredunes, the
long-term, progressive erosion of sedimentary sea cliffs along the Oregon
coast remains an important problem for the coastal planner and resident.
Certainly on a coast-wide basis, more homes are threatened by sea cliff
erosion than by eroding foredunes. This is because many of our coastal
communities (Cannon Beach, Lincoln City, Newport, Waldport, Bandon,
Brookings, and numerous others) are located in areas of eroding sea
cliffs. Most of these are built on the flat areas of marine terraces,
consisting of Pleistocene marine sandstones overlying mudstones of older
ages. These rocks are susceptible to wave attack to form the familiar
sea cliffs (Figure 40) seen along much of the Oregon coast.

This section will examine the processes of sea cliff erosion
(including landslides), what is known about their recession rates, and
what attempts have been made to protect them from wave attack and the
success or lack of success of such attempts. Examples of problems with
eroding sea cliffs on the Oregon coast are cited.

A. Processes of Erosion

Erosion of sea cliffs is often viewed as a process of wave attack
undermining the cliff followed by landsliding. This view is somewhat
oversimplified as other processes are also involved including groundwater
sapping and direct erosion by rainwash (especially important in Oregon).
The Pleistocene terrace sandstones that form a primary component of the
Oregon sea cliffs are only weakly cemented and so are easily eroded away
by rainwash and groundwater. The sand so washed away, or that which
has dropped from the cliff as a minor landslide, tends to accumulate at
the base of the cliff as a tallus pile, sloping toward the sea (Figure 40).
Most often the waves are more important in periodically removing this
tallus accumulation than in directly attacking the sea cliff itself. The
amount of tallus found at the cliff base can give some idea as to the
frequency of wave attack in a particular area (Figure 41). A large
accumulation, especially one with vegetation growing upon it, indicates
that a long period has elapsed since storm waves were able to reach the
sea cliffs. This was the case at Taft until the winter of 1977-78 at
which time unusually severe winter storms removed the extensive tallus
accumulations (Figure 42). A large quantity of drift logs had been
removed from the beach fronting Taft, and this too may have played a
role in the renewed erosion of the sea cliff.

The absence of any tallus accumulation at the base of the sea cliff
indicates a very recent episode of water erosion. Where the fronting
beach is narrow, such erosion may occur nearly every winter so only
minor tallus accumulations may be found during the summer months. Such
areas are generally those that show the maximum rates of overall sea
cliff recession. At other areas, Taft being an example, wave attack
occurs infrequently and the tallus may accumulate over several years

before again being eroded away; such areas generally show smaller rates
of cliff recession.

_IW ""jL

ep,

'rn'T, -7.

Figure 40. Typical sea cliffs of the Oregon coast formed
by erosion of marine terraces. The upper photo shows a thin
layer of Pleistocene terrace sands overlying older Tertiary
mudstones with an apparent dip to the left. The lower photo,
from the Lincoln City area, is a sea cliff composed entirely
of terrace sandstones, and is seen to be more susceptible to
erosion processes.

Figure 41. The extent of tallus accumulation at the base of the sea
cliff can give some indication of the frequency or recency of wave attack.
The upper photo from Taft shows a considerable accumulation with the
development of vegetation, indicating an extended period of time since
wave erosion, in this case the logs possibly offering some protection
(compare with Figure 42 of the same area after erosion during the winter
of 1977-78). The middle photo from Gleneden Beach shows a sea cliff with
a large tallus accumulation but no vegetation, indicating no wave attack
for perhaps 5 to 10 years. The lower photo is from the same area, the
severe storms of the winter of 1977-78 having washed away all of the
tallus accumulation.

4""

Figure 42. Sea cliff erosion at Taft during the winter
of 1977-78. Compare with the first photo of Figure 41 of
the same area, noting the loss of logs on the beach and the
loss of the vegetated tallus slope.

The presence of the tallus slope offers some support to the sea
cliff. Once it is removed landsliding usually quickly follows, respond-
ing more to this loss of support than from actual wave undercutting of

the cliff. In most areas the landsliding consists of only small sections
of the cliff dropping down onto the beach (Figure 43). This minor
slumping, together with rainwash and groundwater sapping, produces a
slow to moderate progressive retreat of the sea cliff and loss of
property.

Ak
@x All
@o"

tAl 4 @7
7 Zi4

Figure 43. Small landslides are an important process to
sea cliff erosion, especially where the cliff is composed of
terrace sandstones.

At times, however, large landslides can occur that suddenly remove
several acres of land. Important to their generation is the geometry of
the sea cliff, including its height and the orientation of the geologic
strata forming the cliff. Large landslides are likely to occur in areas
where the older rocks underlying the Pleistocene terrace sands slope in
the seaward direction as the sliding of the rock mass can occur along
this bedding. Byrne (1964) has estimated that such stratigraphically
seaward-dipping terrace deposits are present along more than half of the
coastline north of Waldport. One such area is Newport where in 1943
an area of about six acres progressively slid seaward, dropping down
20 feet in the process (Figure 44). More than a dozen homes and other
structures were lost, some of which originally had been well back from
the cliff edge (North and Byrne, 1965; Stembridge, 1975). Figure 45
summarizes the sea cliff retreat over the years in the Newport area, a
retreat brought about mainly by landsliding.

'AIL,

.40;

Figure 44. Large landslides in the Jumpoff Joe area of
Newport.

Another factor important to the generation of large landslides is
the presence of groundwater which lubricates the slide, increases the
weight of the material, and may also produce a pore-water pressure.
Compiling the occurrences of major landslides as reported in coastal

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1967 Share
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Built 1939-1907 4-

o t.,Sou rce:
mbridge, 19761

Figure 45. The sea cliff retreat in the Jumpoff Joe
area of Newport as documented by Stembridge (1975) from
aerial photographs. The property losses here are due
almost entirely to large landslides.

newspapers, Byrne "1963) showed that they occur almost exclusively
k
during the months of October through April (Figure 46). Although wave
attack may play some role in the winter increase in landsliding, the
increased precipitation appears to be more important in that most of
the newspaper accounts indicated that sl iding occurred during or im-
mediately after extended periods of torrential rains. In more recent

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Landslides on the Oregon coast have received considerable attention.
Byrne (1963), North (1964) and North and Byrne (1965) document land-
sliding on the northern coast from Florence to the Columbia River. The
various reports of the Oregon Department of Geology and Mineral Industries,
discuss the hazards from coastal landsliding (for example, Schlicker,
et al., 1973). Schlicker (1956) reviews landsliding in general, and
Prestedge (1977) discusses the mechanics of landsliding with specific
reference to the Oregon coast and the engineering techniques of
stabilization.

One additional factor is important to sea cliff erosion--the
human factor. Figure 47 illustrates how people can have an impact on
the erosion rate by carving graffiti and in some cases even cutting
tunnels into the sea cliffs. Considering that natural sea cliff reces-
sion rates often amount to only a few inches per year, this human factor
cannot be viewed as negligible.

rIT,

0.0-6;

Figure 47. Graffitti carved into a sea cliff at Lincoln
City, having a significant effect on the long-term cliff re-
treat rate.

B. Rates of Sea Cliff Erosion

Of relevance to planning is the Iona-term recession rate of sea
cliffs and the potential for landslides removing large blocks of property

in a short time. Landslides have already been discussed, and in most
cases their Dresence or potential is reasonably clear. The progressive
recession of sea cliffs is important for determining What.distances
homes or other structures should be set back from the eroding sea
cliff so that they are not destroyed before their anticipated life
time of use.

A standard procedure for determining long-term cliff recession rates
is through the use of sequences of aerial photographs. There are many
difficulties and inherent uncertainties in this procedure so that the
amount of erosion measured has to be large if the measured rates are to
exceed the uncertainties. This means that the procedure gives best
results in areas that have hiqh rates of cliff recession or if there
is a very long period of time represented by the available aerial
photographs so that even though the rate may be small the total amount
of erosion over that period of time is large enough to measure.

The earliest aerial photogaph coverage of substantial stretches of
the Oregon coast dates back to 1939. The areas covered by those 1939
photos are diagramed by Stembridge (1975, p. 193). Coverage in the
1940's is scarce, but in the 1950's to the present many more flights
were carried out. This forty years of coverage is adequate so long as
the cliff recession rates are moderate to high, but if low (less than
2 to 4 inches per year) then the total amount of erosion that has occurred
can barely be measured with any certainty by aerial photo techniques.
Stembridge (1975) gives a coast-long summary of the cliff recession
rates, based upon the 1939 and 1967 aerial photos and upon field inspec-
tions. Table 2 summarizes his estimated erosion rates for backshores of
various compositions. The terrace deposits are seen to have a wide
range of recession rates, from less than 1 foot per year to greater than
20 feet per year. The large recession rates are found in areas suscep-
tible to landsliding, such as the Jumpoff Joe area of Newport, already
discussed. The largest recession rates are for areas of recent sand
deposits, the rapid erosion of Bayocean Spit being the primary example
(see Section III). Stembridge (1975) does discuss the erosion rates he
found for a number of areas along the Oregon coast, as well as presenting
the overall summary of Table 2.

Smith (1978) has determined coastal changes for Lincoln County,
again using aerial photographs (1939, 1959 and 1973). Erosion rates for
that 34-year period range from amounts too low to measure to a maximum
of about 240 feet in the Jumpoff Joe area of Newport. The mean amount
of erosion was about 20 feet, giving a mean rate of 7.1 inches per year
for that 34-year period. Included in that average are some basalt
headlands with very low rates of erosion. Excluding those areas from
the' county-wide average leaves an average of 9.2 inches per year, an
average for the coast consisting of sedimentary terraces or unconsolidated
materials (the sand spits). Smith found a great deal of variability
in recession rates along the Lincoln County coast, so that these averages
should not be applied to estimate the recession rate of sea cliffs in

some particula.r area. It would be wise for each county to conduct a
study similar to that of Smith (1975) in Lincoln County.

Table 2. Ranges of maximum backshore erosion rates (after Stembridge,
1975)

Backshore Range of Maximum Erosion
Composition (in feet per year) Examples

Igneous (basalt) <0.1 to >0.3 Cape Foulweather,
Heceta Head

Metamorphic <0.1 to >1.0

Sedimentary <0.5 to >2.0 Cape Kiwanda,
Cape Arago

Terrace Deposits <1.0 to >20.0 Lincoln City,
Jumpoff Joe

Recent Sand Deposits <10.0 to >100.0 Bayocean Spit

C. Methods of Sea Cliff Protection

Several methods have been employed on the Oregon coast in attempt
to prevent or slow the erosion of sea cliffs, Those most commonly used
are riprap and a variety of sea walls. The sea walls may be constructed
of concrete or logs; drift logs taken from the adjacent beach are
sometimes employed. Groins that project out across the beach to trap
part of the littoral sand drift have not been used on Oregon beaches,
and probably would not be effective due to the lack of a littoral drift.

All of the protective devices must act to defend the sea cliff from
wave attack. In many cases this defense is against only the wave swash
rather than the full force of breaking waves. In such cases, a low wall
of logs fixed in place at the base of the sea cliff or just in front of
the tallus slope is adequate. Great masses of riprap are really needed
only where there is severe and direct wave attack. The weight of the
riprap does have the added advantage of helping to prevent landsliding
as it weights the toe of the cliff. Solid concrete walls have the same
effect, but have the disadvantage that they can reflect the wave energy
which induces erosion of the beach adjacent to them. This can lead to
the undermining of the sea wall and its failure and collapse onto the
beach. Log walls and riprap may be partially destroyed by wave attack, but
seldom completely fail like a concrete wall.

'MMEZ t7ta

''k

. . . . .. ...........
'MA,

Figure 48. A variety of sea cliff protection approaches have been
employed on the Oregon coast, mainly involving log sea walls, concrete
sea walls an d riprap.

None of these protective schemes completely halt the sea cliff
recession unless they extend to the full height of the cliff. If they
cover only the base of the cliff, the bare upper portion will continue
to suffer some erosion by rain wash and groundwater sapping. This
retreat of the top of the cliff will continue until the overall slope of
the cliff is decreased, at which time it may become vegetated. But
before that stage is reached, the top of the cliff could retreat by
several feet, but at a lower rate than before protection was provided to
the lower portion of the cliff.

There are arguments against any form of sea cliff protection. First,
they can be expensive and would be unnecessary if dwellings were set
back an adequate distance from the cliff edge. As discussed in Section II,
the erosion of sea cliffs in most cases provides the principal source of
sand to the Oregon beaches; cutting off this source by extensive protec-
tion will lead to the long-term diminishment of our beaches. And finally,
the huge piles of riprap or concrete sea walls can be unsightly, destroy-
ing the aesthetic value of the coast that originally attracted people
there.

VII, THE COASTAL DUNE SHEETS

Sections IV and V of this report dealt largely with foredune erosion,
whether the foredunes are located on sand spits such as Siletz and Nestucca,
or fronting sea cliffs and older dunes as at Cannon Beach (Breakers Point).
This section will concentrate instead on the older dunes generally found
more inland. It was pointed out in Section II that such dunes, active
or vegetated, cover about 45 percent of Oregon's 310 miles of coastline.
The best known and most intensely studied is the sheet of active dunes
extending for a distance of 55 miles between Coos Bay on the south to
Heceta Head -near Florence. These dunes and others on the Oregon and
Washington coasts were investigated by Cooper (1958), and most of our
information on Oregon dunes comes from that source. Later contributions
have been made especially by Lund (1973) and various chapters in Dicken
(1961). This section will summarize what is known about the physical
processes important to dune sand movements on the Oregon coast, the
effects of vegetation, and the problems relevant to the management of
these areas.

A. Active Dune Types

In his study of the active dunes of the Oregon coast, Cooper (1958)
identified two principal types, the transverse-ridge pattern and oblique-
ridge pattern (Figure 49). These dune types are somewhat different from
those commonly found in deserts and other coastal dune areas.

The transverse-ridge pattern of dunes originally occurred in nearly
all the major dune localities on the Oregon coast, but since the intro-
duction of European beachgrass its form has been restricted to the Coos

Bay dune sheet. They are asymmetric in cross-section with windward
slopes of 3 to 12 degrees and lee slip faces averaging about 33 degrees,
the steepest possible slope for sliding sand, They vary greatly in
length; a single ridge may be more than half a mile long. They are
not uniform in hei,ght, the ri'dge crest forming a succession of highs and
lows.

" "0'

Figure 49, The two active dune types found on the Oregon coast,
the transverse-ridge pattern and the oblique-ridge pattern, both now
largely confined to the Coos Bay dune sheet. (Lower photo courtesy of
Oregon Department of Transportation.)

Cooper has shown (1958, p. 31-33) that the Oregon transverse-ridge dunes
are not precisely perpendicular to the controlling northwest summer
winds, although they are nearly so. Instead, he found that the trans-
verse-ridges form angles of 11 to 23 degrees to what should be the
perpendicular to the wind, facing more to the landward (Figure 50).
Presumably the dunes also migrate by this same 11-23 degrees to the left
of the wind direction, although Cooper did not demonstrate this to be
,the case.

dune

perpendicular to wind

11 230

11 23"

Figure 50. Cooper (1958) has shown that the trans-
verse-ridge dunes do not align exactly perpendicular to
the wind direction, instead forming an angle of about 11
to 23 degrees, the dune facing (and migrating) more landward.

Transverse-ridge dunes occupy a strip adjoining the beach or
separated from it by foredunes parallel to the shore. Prior to the
introduction of European beachgrass and the formation of grass-
covered foredunes backing the beaches, transverse-ridge dunes covered
the entire area from the beach to the seaward edge of the field of
oblique-ridge dunes. Since the introduction of European beachgrass
their area has been greatly reduced and continues to shrink. On their
inland edge they merge with the field of larger oblique-ridge dunes,
the transverse-ridges sometimes climbing up and over the seaward ends
of the oblique-ridges. They tend to be smallest near the shore and

largest at their inland edge. Average crest to crest distances in fields
of transverse-ridge dunes range from 60,to 160 feet.

Cooper (1958) made a few measurements of migration rates of
transverse-ridge dunes on the Coos Bay sheet. Measurements were obtained
for four dunes with slipfaces over a period of six years. The average
rate of advance was 5.2 feet per year, varying from 2.3 to 9.2 feet
per year. As expected, most of this advance takes place during the dry
summer months of April through August. Also for this reason, the
migrations tend to be toward the south to southeast under the north to
northwest winds prevailing during those months. Cooper also found that
the closer the dunes are to the shore the higher their rates of advance,
resulting from the greater wind speeds closer to shore than further
inland. He found no correlation between dune height and its rate of
advance.

A knowledge of dune migration rates is important in planning
measures of dune control or in keeping structures out of the path of an
advancing dune. As pointed out by Cooper, his few measurements over
a period of six years have to be viewed as the maximum for long-term
over-all advance as there may be periods of temporary stabilization
with no advancement. Additional study needs to be made of migration
rates of these transverse-ridge dunes on the Oregon coast.

The oblique-ridge pattern of dune formation identified by Cooper
(1958) occurs only on the Coos Bay dune sheet. They are much larger
than the transverse-ridge pattern, forming a series of ridges 4,000 to
5,500 feet in length, aligned with their lengths roughly in an east-west
(onshore-offshore) direction (Figure 49). They are highest at a point
somewhat shoreward of their landward ends, both in absolute altitude
and in height above the immediate base. Cooper measured an average
height of 185 feet for ten major dunes. The ridges are spaced rather
evenly, particularly at their seaward parts, where the average inter-
crest distance ranges between 500 to 650 feet.

On their landward ends the oblique-ridges blend with a ridge of
sand that connects them together, the resulting pattern being described
by Cooper as a rake, the oblique-ridges forming the teeth of the rake.
The connecting ridge is part of the precipitation ridge that has a
landward-facing slipface, slowly moving inland and progressively
burying the forests that usually lie in the path (Figure 51). This
inland advance always appears to be slow (Cooper gives no measurements,
however), tending to be somewhat more rapid where the ridge is low.

The oblique-ridge dunes are oriented such that their crests are
oblique to both the summer north-northwest winds and to the southwest
winds of winter. Most important, they do not migrate, but instead
remain fixed in position except for minor shifts with no consistent
trend. In cross section the steepest side is usually on the north.
During the summer the eroding northern slope is smooth-faced and the

south side has a prominant slipface below which is a gentler slope.
leading to the floor of the adjacent corridor. In most places the
windward slope is almost as steep as the slipface. During the summer
the oblique-ridge behaves much as a giant transverse-ridge and sand
is moved to the southeast. During the winter a slipface of sorts forms
on the north side, which is subject to frequent mass slumping, so that
even during the winter there is a northward sand transport in the
midst of the rain.

r 7al,

Figure 51. A precipitation ridge of the Coos Bay
dune sheet migrating slowly landward, burying trees in
its path. (Photo courtesy of Oregon Department of Geology
and Mineral Industries.)

B. Vegetation Effects

The native flora of the Oregon coast did not provide species capable
of building substantial foredunes. Thus prior to the 1940's extensive
active dune fields existed, sands blowing inland from the beaches to
provide a plentiful supply of sand. In about 1910 European beachgrass
(Ammophilia arenaria (L.)) was first brought into the Coos Bay region.
European beacFg-rass took a firm hold and has subsequently spread along
the coast producing in many places a prominant foredune where none
existed before. These developing foredunes have largely cut off the
sand supply from the inland dunes.

The most noticeable effect has been the shrinking of areas covered
by transverse-ridge dunes. With the sand supply cut off, the winds
erode the dune sands down to the summer groundwater level. so that
vegetation can quickly take hold. Areas formerly covered by active
transverse-ridge dunes have been converted into deflation plains since the
1940's. Comparisons of aerial photos of that period with more recent
photos reveal dramatic changes (Figure 52). In places, the areas of
open active sand have narrowed by nearly half in 30 ye ars (Lund, 1973).

277-021 W,

Figure 52. An example of the effects of the introduction of
European beachgrass to the Oregon coast at Coos Bay, diminishing the
extent of active dune sands and encouraging the formation of foredunes
and deflation plains (left - 1939, right - 1975). (Photos courtesy of
U.S. Army Corps of Engineers, Portland District.)

C. Older Vegetated Dunes

The active dune fields found on the Oregon coast achieved their
present development during the last few thousands of years as the sea
rose to its present level. Cooper (1958) discusses the abundant evidence
that a similar history of dune development occurred earlier during the
Pleistocene, also at times of submergence (times of glacial melting).
There appears to have been at least two such episodes of dune formation
on the Oregon coast. The dune fields formed during these earlier
episodes are now generally well vegetated with forests of pine and spruce
and with at least some soil development. They vary considerably in the
amount of cementation of the old dune sands beneath the soil cover.
Although vegetated forms of transverse-ridge and oblique-ridge dunes
cannot'be recognized, vegetated precipitation ridges provide good
evidence for the landward extent of the old dune fields.

Most of these older dune fields are adjacent to important bodies of
modern dunes, indicating that in the earlier cycles, dune development
followed processes similar to those of the present fields. Particularly
large areas exist to the east of the active Coos Bay dune sheet,
especially to the north of Florence and to the immediate north of Coos
Bay. Portions of these old forested dunes are also found on sand spits
such as Bayocean and Netarts, and in terrace areas such as around
Newport. Cooper (1958) provides a series of maps showing the aerial
extent of the old vegetated dune fields.

Where these old dune sands are uncemented, removal of the vegetation
cover can result in their rejuvenation. Natural examples of this are
commonly found adjacent to the beaches where wave erosion cleared some
of the dunes of vegetation. This leads to a blowout, removing dune
sands from the exposed portion. If the effective wind is unidirectional,
then the blowout can develop into a parabola dune, a trough blowout of
major size with large terminal and lateral walls. Parabola dunes grow
progressively in length in the direction of effective wind, and more
slowly in width. According to Cooper (1958, p. 75), most have developed
in areas protected from the summer winds and are hence mainly under the
influence of the winter's southwest winds. For this reason, most
develop northward to northeast. In addition to originating near the
beach, a number of parabola dunes have also formed along the margin of
the Coos Bay dune sheet. The extensive field of active dune sands to
the north of Sand Lake is basically a large parabola dune, the largest on
the Oregon coast (Cooper, 1958, p. 75). Although these examples of
parabola dunes were formed naturally, man's removal of the vegetation
covering the older dunes can similarly bring about rejuvenation and the
development of a blowout or parabola dune.

The Clatsop Plain extends from the Columbia River south to Tillamook
Head, and is largely covered by a series of vegetated dune ridges. These
dune ridges are long linear features extending approximately north-south,
roughly parallel to the modern-day beach. Long linear lakes, marshes

and creeks occupy the lows between the dune ridges.. The vegetation cover
of the Clatsop Plain has undergone extensive changes in the last 100-150
years; these changes are documented by Hanneson (1961, p. 85).

Although vegetated, the dunes of the Clatsop Plain are not as old
as the other vegetated dunes discussed in this section. They were
formed during the last several thousand years as the sea neared its
present level. Following sea level rise, the beach built out and the
dune ridges developed on the accreting land, formed by the abundant
sand supplied by the Columbia River. Cooper (1958, p. 123-6) recognizes
three stages of progradation on the basis of three groups of ridges.
Shoreline advance appears to be continuing, although the picture has
been somewhat complicated by the construction of the jetties at the
mouth of the Columbia. Dicken (1961, p. 73) calculated, for example,
that the maximum growth of the beach between 1944 and 1960 was about
500 feet, some 30 feet per year. As discussed in Section V, the excess
of sand at Seaside has resulted in problems with blowing sand, and
several hundred thousand cubic yards of sand have been removed from
the Seaside beach since 1960 (Stembridge, 1975, p. 45).

Beneath their vegetative cover, the dune sands of the Clatsop
Plain are loose. As already discussed for the older dunes of the Oregon
coast, removal of the vegetation cover can lead to blowouts and dune
rejuvenation.

REFERENCES CITED

Aguilar-Tunon, N. A., and P. D. Komar. 1978. "The Annual Cycle of
Profile Changes of Two Oregon Beaches." The Ore Bin. Department
of Geology and Mineral Industries, Portla@-d_, Oregon. 40(2):25-39.
14 pp.

Bowen, A. J., and D. L. Inman. 1966. Budget of Littoral Sands in the
Vicinity of Point Arguello, California. U.S. Army Coastal Engineering
Research Center Technical Memo No. 19. Fort Belvoir, Virginia. 56 pp.

Byrne, J. V. 1963. "Coastal Erosion, Northern Oregon." Essays in Marine
Geology in Honor of K. 0. Emery. University of Southern California
Press, Los Angeles, California. (11-33). 24 pp.

Byrne, J. V. 1964. "An Erosional Classification for the Northern
Oregon Coast." Association of American Geographers Annals.
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