system of classifying and identifying Oregon's coastal beaches and dunes

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

A System Of Classifying & Identifying
Oi'egonrs Coastal Beaches & Dunes

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1979 Oregon Coastal Z one Management Association, I nc.

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
Summary of the State-of-the-Arts
II. BEACH AND DUNE IDENTIFICATION:
A System of Classifying and Identifying Oregon's Coastal Beaches
and Dunes

III. PHYSICAL AND BIOLOGICAL CONSIDERATIONS:
PhysicaZ 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: I
Beach and Dune Irtiplementation 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 PZanning 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, Eugene,
Illustrations prepared by Lorraine Morgan, Newport, Oregon
Cover photo by Christianna Crook, Newport, Oregon

ftoperty of CSC Library

A SYSTEM OF CLASSIFYING AND IDENTIFYING,
L
OREGON'S COASTAL BEACHES AND DUNES

U - 3 . DEPARTMENT OF COMMERCE NOAA
COASTAL SERVICES CENTER
2234 SOUTH HOBSON AVENUE
CHARLESTON I SC 29405-2413

by
Christianna Stachelrodt Crook

Research Associate
OCZMA, Beaches and Dunes Study Team

Kathy Bridges Fitzpatrick
Editor and Project Administrator

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

June, 1979

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

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PREFACE

The following report presents the results of an in-depth analysis
of beach and dune identification and classification systems conducted
by the Oregon Coastal Zone Management Association, Inc. This report
constitutes one element of an overall analysis of planning for and
managing beaches and dunes as required by Oregon's Beaches and Dunes
Goal.

This report was prepared by Christianna Crook, OCZMA Beaches and
Dunes Study Team Research Associate, with assigtance from other Study
Team members composed of Carl Lindberg, Project Director, Wilbur
Ternyik, Project Coordinator, Arlys Bernard, Project Secretary, and
Kathy Fitzpatrick, Project Administrator.

in addition, valuable review and comments were 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 Cortright, Oregon Department of Land Conservation and Development
Jim Lauman, Oregon Department of Fish and Wildlife
Jim Sternbridge, 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 Mecone, 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

Additionally, OCZMA extends special appreciation to the following
individuals for their valuable input and direction and for their signifi-
cant contributions to this report:

Bill Burley, Program Biologist, The Oregon Natural Heritage Program
of the Nature Conservancy
Don Leach, District Conservationist, U.S.D.A., Soil Conservation
Service

Dr. Paul Komar, Department of Oceanography, Oregon State University
Cathy Mecone, Planning Research Associate, Coos-Curry Council of
Governments
Dr. Leonard Palmer, Department of Earth Science, Portland State
@ University
'Dr-. James Stembridge, Coastal Resource Specialist, Oregon State
Soil and Water Conservation Commission, and
Wilbur Ternyik', Owner/Operator, Wave Beachgrass Nurser'y

Finally, OCZMA expresses its sincere appreciation to the University
of Washington Press for permission to reprint illustrations from
Vascular Plants of the Pacific Northwest by C. Leo Hitchcock, et. al.,
(1955-1969).

TABLE OF CONTENTS

Chapter Page

Preface ................................................ i
List of Figurgs ........................................ iv
1. Introduction ........................................... 1

Ii. Beach .................................................. 1

III. Foredune ............................................... 10,

IV. Interdune Forms ........................................ 26

A. Deflation Plain
B. Seasonally Wet Interdune Area
V. Interior Dune Forms - Vegetated ........................ 46
A. Hummock Dunes
B. Surface Stabilized Dunes
C. Older Stable Dunes
D. Parallel-Ridge Dunes
Vi. Interior Dunes - Nonvegetated .......................... 73
A. Transverse-ridge Dunes
B. Oblique-ridge Dunes
C. Recently Reactivated Forms
Vii. Glossary of Terms ...................................... 91

LIST OF FIGURES

Figure La2e

1. Steepened profile characteristic of an eroding
beach ................................................. 2

2. Beach cusps ........................................... 3

3. Sand deposition around a beach grass windbreak ........ 10

4. As European beachgrass is buried, new shoot and
root growth develops at the dune surface .............. 11

5. Beach, foredune and deflation plain supply sand
to interior open sand areas .................... I ...... 73

6. Transverse-ridge dunes form approximately
perpendicular to northwest summer winds ...... 74

7. Oblique-ridge dunes form obliquely to both
northwest and southwest dominant seasonal winds ....... 78

8. Accumulation and advance of the oblique-ridge
dune .................................................. 79

9. The ridge position of the oblique dune is modified
by smonal winds ..................................... 79
10. Transverse-ridge dunes riding up over the flanks
of an oblique-ridge dune which terminates in a
precipitation-ridge ................................... 80

11. Eastward expansion of deflation plain ................. 82

12. Small blowout within beach grass environment .......... 86

13. Parabola blowout moving through forested dunes ........ 88

I. INTRODUCTION

Beaches and dunes are found on those accumulations of sand which
occur intermittently along the Oregon coast. They range in size from
small pocket beaches between headlands to expansive dune sheets more
than twenty miles long and three to four miles wide. Varying combina-
tions of physical factors (e.g. wind, vegetation, and moisture, etc.)
are capable of producing widely diverse beach and dune landforms. Each
landform exhibits discrete physical capabilities as well as characteris-
tic sensitivies to man's activities.

Dune forms exhibit varying states of stability. Areas of open sand
occur where dune topography is controlled only by sand and wind. Lightly
vegetated areas are considered to be in an active state and are continu-
ously subject to erosion and accretion. Dunes are conditionally stabilized
when they have sufficient vegetative cover to withstand wind erosion.
Other dune forms can be surface stabilized or older (semi-cemented)
stable. That is, they may exhibit vegetation with a thin layer of soil,
or may have vegetation and extensive soil layer with semi-cemented
underlying sands.
This report classifies, describes and discusses the physical and
biological nature and general capabilities of coastal Oregon sand
landform types. In addition, a checklist of physical and biological
features characteristic of each type is included to assist with field
identification. A glossary of terms used in this report is presented
in the concluding section.

II. BEACH

A relatively narrow, sloping zone of unconsolidated materials
extending from the low tide line Landward to the uppermost
line of effective wave or tidal action.

A. Geomorphology

Beach materials range in size from fine sand, to pebbles and even
small boulders and are supolied from the erosion of coastal cliffs, the
reworking of ancient and recent coastal sand deposits, and from riverine
sediment loads. Sand supply and beach formation processes occur in
seasonal cycles in which beaches commonly experience sand removal
during the winter and are rebuilt by the more gentle wave action
associated with summer weather activity. Beaches are the coastline's

1Illustrations indicate the degree of magnification or reduction.

primary defense against the erosive action of storm waves.

Sand dunes, cliffs and drift log accumulations may occur at the
landward side of beaches. Some parts of the coastline are repeatedly
interrupted with headlands, creating small pockets of cobble beaches.
Elsewhere the seaward margins of vast dune sheets create extensive
beaches.

1. Stable beaches

In terms of sand availability, beaches may be stable, eroding or
accreting. A stable form is one which experiences neither a net loss
nor gain in beach materials on an annual basis. Gentle summer waves
replace the same amount of sand on the beach as was lost offshore during
winter storms. Beaches which are presently stable include Sand Lake in
Tillamook County and the region between the mouth of the Umpqua in
Douglas County and mouth of the Coquille River in Coos County.

2. Eroding beaches

An eroding beach is one which annually experiences net sand loss.
This can result from continuous excessive erosion, diminishing beach
sand supplies, or both. Erosion occurs primarily during vigorous winter
storms and may be heightened by such factors as a high spring tide
which effectively increases wave height. A reduction in beach sand supply
may result from dams, riprap, jetties, commercial removal, or other
structures or activities which modify beach material transport or near
shore currents.

Eroding beaches can often be recognized in their earlier stages
by the development of a steeper than usual profile (Figure 1). Some

Figure 1. Steepened profile characteristic of
an eroding beach.

eroding beaches may contain noticeable eroded embayments, or cusps,
which are the result of local rip currents carrying sand away from the
beach (Figure 2). Beaches presently in an eroding state include the
beach from Peter Iredale Park, north to the Columbia River south jetty
in Clatsop County and the area between Blacklock Point and Floras Lake
in Curry County.

Figure 2. Beach cusps.

3. Accreting.beaches

Any beach which has a low-tide margin experiencing net seaward
growth due to an annual increase in sand supply is considered to be
accreting. The development of small persistent dunes seaward of the
foredune is indicative of this sand accumulation process. With continued
accretion, the developing dunes will eventually grow and join to form a
new foredune seaward of the previous one. Accretion at a site may be
the result of escalated erosion elsewhere, beach or spit migration,
natural and/or induced changes in off-shore currents, or structures
and activities which modify beach material transport. To a limited
extent, a beach may experience accretion due to the sand-trapping
ability of dune grass. This is only likely to occur, however, when the
near shore environment already favors accretion. Examples of accreting
beaches include the area adjacent to the community of Surf Pines in
Clatsop County, the north end of South Beach in Lincoln County, and
just north of Gold Beach in Curry County.

B. Vegetation

High winds, waves. and cyclic tidal inundation'severely restrict
vegetative growth in the beach zone. However, many types of seeds
germinate easily in wet sand and a few hardy species may be found on
the higher beach slopes in the summer and fall.(Wiedeman, et. al.,
1974, P. 25Y. Such plants are not found growing in great profusion.
In fact, it may be that only one or two species, if any, will occur in
a given beach area. These may occur as isolated individuals, and will
commonly experience burial or destruction during the period of winter
storms.

Three species which occur most commonly on the beach include
American sea rocket (Cakile edentula), European sea rocket (Cakile
maritima) and honkenya Although not p'eculiar to
h
arenaria), sea lyme-grass
beaches, European beachgrass Ammophila
(Elymus, mollis) and seashore bluegrass (Poa macrantha) are occasionally
found in this zone.

C. Attractions and Limitations

The beach is a highly attractive site for numerous recreational
activities ranging from beachcombing to operation of off-road vehicles.
It lends itself well to both solitary and group activities and, as a
geologic feature, seems to be relatively tolerant of most transient
activities. Management of this landform should consider such issues
as (1) the desirability of allowing vehicle traffic and significant
pedestrian use in the same areas, and (2) harrassment of shore bird
species by various recreational activities..

.D. Identification Checklist

The beach can be recognized by the following characteristics:'

1. The landward boundary of the beach may be characterized by one of
the following:
a. drift log accumulations,

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b. foredune ridge, or

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2. Beaches may consist of fine to medium grained sands and exhi,bit a
relatively gentle slope.

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3. A steeper slope profile is exhibited by those beaches which consist
of pebbles and/or boulders.

4. Erosion is sometimes caused by stationary rip-currents which eat
embayments or cusps into the beach and foredune.

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5. Accreting beaches may often be recognized by the development of
embryo dunes seaward of the foredune.

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6. The most commonly occurring beach vegetation includes the following:

a. American sea rocket (Cakile edentula),

flowers purple
to white

b. European sea rocket (Cakile maritima), similar to Cakile edentula
except for the fruit, and'

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c. Honkenya (Honkenya peploides).

flowers
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white

III. FOREDUNE

First ridge of sand situated immediateZy above the high tide
tine and paratleZ to the beach.

A. Geomorphology

The present day foredune of the Oregon coast has developed primarily
in the last forty years as a result of the introduction of European beach-
grass (Ammophila arenaria). First introduced for sand stabilization in
the Coos Bay area in 1910 and the Clatsop Plains in 1935, this species
became naturalized to the coastal sand areas. It spread along the
coast forming a nearly continuous barrier ridge along the shore. Euro-
pean beachgrass prefers sites of continuous sand deposition. It grows
seaward until it is halted by wave action at the high tide line. Embryo
dunes form here at the landward edge of the beach in conjunction with
vegetation and drift log accumulations where the velocity of the wind
decreases suddenly, depositing the sand load (Figure 3). Continued

010

Figure 3. Sand deposition around a beach grass
windbreak.

deposition may lead to the burial of the original obstacle. The driftlogs
remain buried forming the base of the foredune but European beachgrass
(Ammophila arenaria) can survive seasonal sand burial of up to three feet
and for that reason is the primary foredune building agent. As sand
builds up around the base of the plant, new roots and shoots grow

from the stem joints (Figure 4). This traps more wind-blown sand above,
while holding underlying sand within the complicated root network below.

Figure 4. As European beachgrass is buried, new
.shoot and root growth develops at the dune 'surface.

The dune thus increases in height and width until it merges with
adjoining dunes to form a barrier ridge along the upper beach. This
dune-ridge area stops and holds most of the sand blowing in from the
beach and continues to grow until it reaches the maximum height
dictated by local conditions, usually up to twenty-five or thirty feet.
It may be bounded on the east by deflation plains, interior dunes,
cliffs, marshes, lakes or estuaries.

The foredune is a naturally occurring geomorphic feature which, to
some degree, acts as a dike during ocean storms. It's function transcends
that of a simple barrier-wall because it has a sponge-like ability to
absorb and mute the force of storm waves. Hitting the foredune, wave
energy is dissipated over, around and, most critically into the dune.
However, while it can act as an effective shock absorber, the foredune
can occasionally sustain considerable damage during storms and may be
unable to provide sufficient storm protection to inland sites, thus
allowing adjacent deflation plains or hummock dunes to be exposed to
the full force of maritime storms. In such instances, the area of
potential hazard is extended beyond the foredune to include additional
inland sites. Any excavation into, or construction modification of
the foredune may increase this hazard potential.

Foredunes are among the most dynamic of landforms and will naturally
vacillate between periods of being active (subject to wind and wave
erosion and breaching) and being conditionally stable (wind stable but
subject to wave erosion and breaching).

The term "foredune" is applied to this dune ridge wherever it
occurs along the Oregon coast. This feature varies considerably in
appearance, however, and to some degree in function, between the north
coast (Clatsop County) and the central-south region.

The foredune ridge which occurs. in the central and southern counties
commonly varies between ten to thirty feet in height, is twenty to
fifty feet wide at the base and often appears as a distinctive sea-wall
ridge particularly when viewed from the beach. Storm hazard in this
area is primarily associated with erosive storm waves and wind, impact
from solid debris carried by the waves, and flooding of inland sites.

The foredune in the Clatsop area is commonly an extremely broad,
low appearing feature. While it may reach heights of more than twenty-
five feet, the gradient is so gentle, often five degrees or less, that it
has a less distinctive sea-wall appearance. Storm hazard on the north
coast (north of Seaside) is more commonly associated with inundation from
sand than from erosion or debris impact, Wave run-up may be considerably
reduced by the extensive traverse associated with the long, shallow,
off-shore area and the extreme width of the foredune. However, storm-
velocity winds are capable of transporting generous quantities of sand
available in this region considerably inland. The gentle gradient
common to Clatsop foredunes may also offer less obstruction to the wind.
Thus, the "functional" width of the foredune (or the area impacted by
maritime storms) in the Clatsop area can be as great as 800 feet
depending on local conditions (Leach, 1978). The "foredune" under this
designation, may contain more than one "ridge" so the term "foredune
area" may be more applicable (Ternyik, 1978). Furthermore, while storm
associated sand deposition can cause sand-blasting type damage and may
result in subsequent excavation costs, this activity may be more aptly
designated as "nuisance" rather than true "hazard" to life or property.

1. Active foredunes

Sand dunes are in an active state when they possess insufficient
vegetative cover to retard wind erosion. In this condition the sand
dune is experiencing active accretion and/or erosion.

On a static or accreting beach, an active foredune will commonly
evolve towards the conditionally stable state. As the active foredune
grows in height (up to thirty feet), it becomes an increasingly effective
barrier and progressively less sand is deposited on the lee side of the
dune and other sites inland. This offers somewhat greater protection
from storm winds, but also seriously limits all fresh beach sand supplies
to interior open sand areas.

Active foredunes may be most numerous in central and north coast
areas. They occur, for example, in Clatsop County west of Slusher and
Sunset Lakes and in Tillamook County at Neskowin North.

2. Conditionally stable foredunes'

When foredunes exhibit sufficient vegetative cover to retard the
erosive effects of the wind, they are termed conditionally stable,
Obviously, the stability of a given foredune is conditional u*Pon the
maintenance of the vegetative cover.

While the conditionally stable foredune may not have any greater
resistance to wave erosion than does an active foredune, it appears to
recover more quickly from wave overtopping (Ternyik, 1978). However,
any conditionally stable sand dune is prone to reactivation upon
disturbance of the vegetative cover.

Examples of conditionally stable foredunes presently occur between
Sunset beach and Gearhart in Clatsop County and at the community of
Bayshore in Lincoln County.

B .Vegetation

1. Active foredunes

The active foredune receives such a substantial sand supply that
it is occupied almost exclusively by European beachgrass (Ammophila
arenaria). Some native dune grasses such as sea lyme-grass (@@us
mollis) may be found here but occur less commonly as they are less
tolerant of continual sand burial.

2. Conditionally stable foredunes

The increasing height of the conditionally stable foredune restricts
the inland passage of salt spray and sand. A new environment is thus
created on the crest and the lee side of the foredune which is reflected
in the vegetation at this site. European beachgrass (Ammophila arenaria),
the most significant species which occurs on the foredune, becomes less
important because it prefers the more fertile sites of sand deposition.
Other species less tolerant of salt spray and sand deposition become
established. The first to become established include, among others, such
herbs as beachpea (Lathyrus japonicus), coast strawberry (Fragaria
chiloensis) and seashore lupine (Lupinus littoralis), Later successional
species may include such woody shrubs as salal (Gaultheria shallon), or
kinnikinnick (Arctostaphylos uva-ursi) and an occasional shore pine
(Pinus contort-aT

The lee side of a foredune may exhibit vegetation characteristic
of conditional stabi'lity and yet be experiencing erosion and undercutting
on its windward side. Such circumstances indicate that the foredune was
formerly in a conditionally stable state long enough for some vegetative
succession to take place, and has only recently begun to experience
erosion. The remaining foredune ridge has apparently been able to
provide sufficient protection to the lee side to maintain existing
vegetation. Should the oceanward side experience temporary in-filling
with logs, sand and beach grass during the summer it could deceptively
give the appearance of a completely conditionally stable foredune.

C. Attractions and Limitations

While the foredune may not be a primary recreational attraction in
itself, it nonetheless experiences moderate recreational traffic. This
is partially because it is a barrier which must be traversed in order to
reach the beach. It is also used as a sheltered 'base camp' from which
forays to the beach are made. However, because of its hummocky, semi-
stable surface and the sharp tips of the European beachgrass, pedestrian
traffic usually follows open pathways. This activity, referred to as
"trailing", results in the development of open mobile sand trails.
However, the continual replenishment of fresh sand in active dune areas
commonly maintains sufficient fertility for beach grass regeneration.
Thus, trailing is not necessarily a serious problem unless it is
desirable for the active foredune to become conditionally stable
(OCCOC, 1975). Conversely, the trailing which results from the
passage of motorbikes and other off-road vehicles inhibits beach grass
regeneration and creates troughs from which blowouts can develop.

The proximity of this landform to the ocean renders it at once
highly attractive and yet extremely hazardous as a site for permanent
structures. Construction of permanent structures which either project
onto the beach or require any excavation of the foredune endangers the
site and adjacent areas. Erosion intensification, interruption of
natural sand movement, and wind-blown mobile sands are potential hazards
associated with such disturbance. Additionally, the installation of
riprap to protect structures impedes the natural flow of beach and
foredune materials, possibly resulting in beach starvation at the site
or elsewhere. Goal 18 specifically addresses the problem of development
on active and other foredunes. No active dune, by definition, should
harbor permanent structures as they may be subject to inundation or
undermining due to moving sand. Permanent structures should be reserved
for permanent landforms. Management of this landform should consider
the highly dynamic mobile nature of this land/sea interface area.

D. Identification Checklist

The foredune can be identified by the following characteristics:

1. A sparsely or thickly vegetated sand dune ridge five to twenty-five
feet high, running parallel to the beach.

2. The foredune is bordered by the beach and possibly drift logs on the
west.

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3. It commonly occurs adjacent to:

a. hummoc k dunes, or

b. the deflation plain on the east.

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4. The active foredune may exhibit storm surge cuts. Logs and debris
may be found here.

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5. An eroding, formerly conditionally stable foredune may continue to
exhibit vegetation characteristics of its conditionally stable
state. (Coast strawberry is shown on the eroding face of a
formerly conditionally stable foredune.)

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6. Drift logs may be exposed at the base of the windward face of the
active foredune, particularly in the winter.

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7. The vegetative cover of the active foredune is rather sparse and
consists almost exclusively of European beachgrass (Ammophila
arenaria).

8. The conditionally stable foredune exhibits a very dense vegetative
cover.

9. Sea lyme-grass (Elymus mollis) may also occur occasionally on the
foredune in lesser amounts.

1/3

10. Species of the pioneer community found on the lee side of the
conditionally stable foredune include:

a. coast strawberry (Fragaria chiloensis),

flowers white to
pinkish

b. beach morning-glory (Convolvulus soldanella),

flowers light pfnk
v to rose

c. beachpea (Lathyrus japonicus), and

flowers
violet

d. gray beachpea (Lathyrus littoralis).

flowers white
to pink or
purple

11. Four species appearing later in succession include:

a. seashore lupine (Lupinus littoralis),

flowers purplish
blue

b. seaside tansy (Tanacetum douglasii),

flowers yellow

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3.2 3.2.

c. western bracken fern (Pteridium aquilinum)

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112

d. pearly everlasting (Anaphalis margaritacea)

heads white

12. By the time the later successional species appear, many of the
pioneer species are no longer apparent. The following shrub species
belong to this latter successional group:

a. salal (Gaultheria shallon

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fruit
dark purple
to black' flowers
white to pink

b. evergreen huckleberry (Vaccinium ovatum),

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flowers
white to pink

c. kinnikinnick (Arctostaphylos uva-ursi)

flowers
white to
pink

d. hairy manzanita (Arctostaphylos columbiana)

112 G; flowers white to
pale pink

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IV. INTERDUNE FORMS

Interdune forms include those low areas between dunes which are
often subject to the controlling factors of wind action, and/or high
water table.

A. Deflation Plain

Eroad pLain which deveZops immediateZy inZand from the foredune
and is wind scoured to the ZeveZ of the summer water tabte.

1. Geomorphology

A deflation plain is created by wind removal of dry sand particles
inland from the foredune. Sand is thus removed only down to the lowered
summer water table, because groundwater moisture binds sand particles
together at this level. This may result in standing water during the
winter when the water table is naturally higher. Sand transported inland
from the deflation plain is deposited in interior dune regions.
The best conditions for development of this landform probably occur
behind a conditionally stable foredune, where the inland flow of sand,
which could otherwise contribute to dunal development at this site, is
cut off and the wind has more available energy for transport. If an
active foredune exists, sand supplies are more readily passed over the
ridge to form hummock dunes or open-sand dunes.

Examples of old, inactive deflation plains occur in some areas
considerably inland from the foredune area. These are commonly
occupied by forest communities and the water table is still quite high.
These features probably developed adjacent to a foredune, but merged
inland by subsequent beach accretion.

2. Vegetation

Components of the vegetation communities in the deflation plain
vary according to specific site factors and the stage of successional
development. In the early successional stages, grass, rush, and
sedge communities occupy progressively wetter sites. Later successional
stages are characterized by a low scattered shrub community, followed
by the development of tall shrub thickets and eventually a shore pine
forest (U.S.D.A., 1972, pp. 84-98). Water stands at, or near the surface
most of the year in these sites.

The grass community occurs where water stands on the surface for
two to three months of the year (vegetation covers about 80% of the
ground). A few scattered shrubs and dwarfed shore pine are occasionally
found in this community.

The rush community is found in sites where the water table stands
on the surface for three or four months of the year (vegetation covers
about 90% of the ground). Isolated shrub and tree seedlings also occur
here. Coast willow (Salix hookeriana) seedlings are the most numerous.

Water stands on the surfa ce of the sedge community for at least
six months of the year. Vegetation covers about 95% of the ground.

3. Attractions and limitations

The primary attractions of the deflation plain appear to be the
viewing of waterfowl and, when drained, the development of housing.
Many deflation plain marsh communities provide habitat for a number of
waterfowl, which is lost when the site is developed. Even when drained,
the water table remains quite high and septic tanks, drainfields, and
other buried structures may not be appropriate. Flotation and failure
of such structures could occur with resulting pollution,

4. Identification check-list

The deflation plain may be identified by the following features:

1. It is a low plain bordered on its ocean side by a foredune.

44,

2. The deflation plain may be bordered on the eastern boundary by:

a. hummock dunes or,

b. transverse-ridge dunes.

0
@'S

..........

"
J"'I"M11"IrIND TRIZZ"!
N'WN" ' , I
MI

3. It may contain standing water several months of the year.

4. Important species of the deflation plain grass community include:

a. European beachgrass (Ammophila arenaria),

12
lodicu)es

"'10

b. seashore lupine (Lupinus littoralis)

flowers purplis-h blue

c. false dandelion (Hypochoeris radicata)

1.6

flowers yellow

d. coast strawberry (Fragaria chiloensis)

flowers white
to pinkish
rXI

e. yellow-eyed grass (Sisyrinchium californicum)

flowers yellow

1/3

5. The following plants are most common in the rush community:

a. spring-bank clover (Trifolium wormskjoldii).

flowers - purplish-red
often white-tipped

2.5

b. Rushes are important here. (They may be identified by a round
stem.)

(1) sickle-leaved rush (Juncus falcatus)

- rI
4
1
20
1@", YS
wd

1/3

(2) brown-headed rush (Juncus nevadensis)

6. The most important species of the sedge community are:
a. Sedges (which have stems with edges) are the most critical
species. They include:

(1) slough sedge (Carex obnupta)

1/3

indsi i
(2) hindi s sedge (CaLe-x--hl 1

paci f i ca)
b Pacific silver weed (Pote it i I I -a-P--(

flowers bright yellow

C. creeping buttercup (Ranunculus flammula)

flowers
yellow

d. king's gentium (Gentiana sceptrum)

flowers blue to
pur.plish-blue

1.2

7. The low, scattered shrub community is characterized by the following
species up to six feet in height:

a. coast willow (Salix hookeriana) dominates,

b. salal (Gaultheria shallon),

flowers
white to pink

c. evergreen huckleberry (Vaccinium ovatum),

flowers - white
to pink

d. wax myrtle (Myrica californica), and

fruit reddish-
brown

e. shorepine saplings (Pinus contorta).

8. The tall shrub thickets follow the low, scattered shrub association
and is composed of the foregoing shrubs and trees ranging in height
from s'ix to twenty feet.

9. The next successional stage on the deflation plain is the shorepine
forest. The plant community includes the following:

a. shorepine dominates (Pinus contorta). See drawing above.

b. sitka spruce (Picea sitchensis)

411

J, 14- '12

c. occasionally trees of:

(1) wax myrtle (Myrica californica), and

00"

(2) coast willow (Salix hookeriana)

C
J,

d. a sparce occurrence of shrubs:
(1) salal (Gaultheria shallon), and
P
VVJ
-Y11
flowers
white to pink
, wll@
A

IIt

evergreen huckleberry (Vaccinium ovatum)

white to
flowers
pink

e. slough sedge (Carex obnupta) is common.

1/3
b"

B. Seasonally Wet Interdune Area

Interdune sites which are commonZy occupied by standing water
onZy part of the year.

1. Geomorphology

Low lying sites which contain surface water during some part of the
winter are found in association with many sand landform types. Swales
between oblique-ridge dunes, the basal area between hummock or trans-
verse-ridge dunes, and low areas within older stable dune units, all
provide examples of sites which may be wet part of the year. Water table
is probably high even during the summer when all trace of surface water
has disappeared.

2. Vegetation

Although the surface can range from bare sand through marsh associa-
tions to mature forest, some vegetative cover is most common. Any of
those communities occurring in the deflation plain may be found here.
Some sites may exhibit mottled clayey soils indicative of prolonged
saturation.

3. Attractions and limitations

Characteristic attractions and limitations of these sites would
be similar to those of the deflation plain. Any development proposals
for occasionally wet areas in older stable dune forms should address
the potential water table limitations here.

4. Identification check-list

Occasionally wet interdune sites may be identified by the following
characteristics:

1. The site exhibits standing surface water during only part of the

-7-

Mp,,@
54
"Vv 'W
j,"alp
"N
a,

IV
"I q

2. Occasionally wet interdune areas may occur as follows:

a. vegetated areas between open sand dune ridges,

@7'7111_1 "I

.. ... ..... M, l,'W
. . . . . ......

;,77@

J116

b. nonvegetated sites between open sand dune ridges, or

"IN

c. swales between parallel-ridge dunes,

d. low sites in older forested dunes (may be affected by underlying
impermeable iron lenses), or

e. swales between hummock dunes which may be a functional extension of
the adjacent deflation plain.

V. INTERIOR DUNE FORMS - VEGETATED

A. Hummock Dunes

FieZds of vegetated sand dune mounds most commonZy occuring
inZand from the foredune or defZation pZain.

1. Geomorphology

Hummock dunes, like the foredune, are primarily created by the
mound building activities of European beachgrass (Ammophila arenaria).
These dunes form as fields of hillocks rather than linear ridges because
there is no natural linear vegetation and sand accumulation boundary
(such as the beach), and sand supply and wind patterns are rendered
inconsistent and discontinuous by the foredune barrier ridge.

Hummock dunes most commonly occur either immediately inland from
the active foredune or inland from the deflation plain. Sand is supplied
from either or both of these source regions. Patches of hummock dunes
are also found within open sand areas on occasion. In general, hummock
dunes range from ten to thirty feet in height and twenty to thirty feet
at the base. In their active form they may be only sparcely vegetated
and thus actively migrating. Active hummock dunes occur in the south-
western region of Bayocean Spit in Tillamook County and west of Lily
Lake in Lane County. A vegetative cover sufficient to make hummock
dunes wind stable creates a conditionally stable form, examples of which
are found at the north end of Bayocean Spit in Tillamook County and at
South Beach in Lincoln County.

2. Vegetati'on

The most prevalent vegetation found on hummock dunes is European
beachgrass (Ammophila arenaria). Other secondary components include
seashore lupine (Lupinus littoralis), seashore bluegrass (Poa macrantha),
and coast strawberry (Fragaria chiloensis). Later successional growth
is similar to that on the lee side of the conditionally stable foredune
and may frequently exhibit shrubs or dwarfed trees in the more protected
sites. The depressions in those hummock dune areas which have a high
winter water table may be occupied by marsh vegetation including sedges
and rushes.

3. Attractions and limitations

This area has attractions to both pedestrian and off-road vehicle
recreation. Some areas have been used for home sites.

The proximity of active hummock dunes to the active foredune and
their natural interaction is of critical importance. Often one blends
into the other imperceptably and the boundary is somewhat hypothetical.
One cannot be managed in isolation from another.

Hummock areas are often used by off-road vehicle recreationalists
but limited visibility can create hazards, particularl for pedestrians.
y

Those areas which have a high winter water table may develop
"quicksand-like" conditions in the low areas (U.S.D.A., 1972, p. 76).
High water table areas are also highly sensitive to development and
would be particularly unsuitable for either spotic tanks or buried
pipelines due to the possibility of structure flotation and failure
(Ibid, P. 83).

High wind scour and blowout potential also limit the development
possibilities of this dune landform. Stabilization planting should be
undertaken and consistently maintained during and after any construction.

4. Identification check-list

Hummock dunes can be identified by the following characteristics:

1. Hummock dunes occur where clumps of vegetation cause deposition of
windblown sand

2. They occur as fields of individual, vegetated sand hummocks

ALAU.

3. Hummock dunes occur either:

a. leeward of an active foredune, or

........ ..

b. inland from the deflation plain, or

At.

@4'

c. in isolated hummock fields within open sand areas.

0, "POR
6@'
"N"
1411 "111,
@W&5111'
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4p
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4. European beachgrass (Ammophila arenaria) forms the primary
component of the vegetation community of active hummocks.

12
lodicules

'110

5. Other species which may occur here include: oft

a. sea lyme-grass (Elymus mollis)

lo@
113

A
'A

lodicules

b. seashore lupine (Lupinus littoralis)

flowers
purD.l i sh-bl ue

'12

c. seashore bluegrass (Poa macrantha), and

1/3

d. coast strawberry (Fragaria chiloensis).

flowers white to
pinkish

Hummock dune areas which have experienced prolonged successional
growth indicating conditional stability may display the following
species:

a. kinnikinnic (Arctostaphylos uva-ursi),

flowers
white to pink

7.5 @2

b. evergreen huckleberry (Vaccinium ovatum),

flowers
white to pink
50

c. salal (Gaultheria shallon),

flowers
white to pink

Ix
'k,@j z '12
9 6

d. shore pine (Pinus contorta), and

m -

e. sitka spruce (Picea sitchensis).

B. Surface Stabilized Dunes

Dunes of any form which possess a weakZy developed thin soil
and underlying unconsolidated sands.

1. Geomorphology

These dunes have been stabilized with vegetation long enough for
soil to begin forming. This process may have taken hundreds or possibly
a few thousand years.

Surface stable dunes are wind stable so long as the weakly developed
soil is not seriously disturbed. The underlying sand materials are prone
to reactivation, particularly if excavations are oriented toward pre-
vailing winds. Parabola dunes commonly occur in this landform. Occasionally,
buried soils and iron bands may impede permeability. Surface stable
dunes occur in the central portion of the Tillamook Spit in Tillamook
County and are intersperced throughout the dune sheets in Lane, Douglas
and Coos Counties.

2. Vegetation
2*

Forest associations occur most commonly in this unit although
meadow communities are not unknown. Native herbs, shrubs and trees, not

unlike the later successional species found on conditionally stable
foredunes, are found here although species proportions, size of
individuals and biomass density are considerably different, Because
of tho favorable environment provided by an abundance of moisture,
mild climates and surrounding forest productivity, coastal forests,
particularly the shrub layer, may be nearly impenetrable.

3. Attractions and limitations

The forested surface-stabilized dunes offer attractive sites for a
number of man's activities because they are sheltered and somewhat more
stable than most other dune forms. Such reactional activities as camping
and picnicking, as well as the placement of dwellings and other structures,
are popular activities.

Caution must be used in developing these sites, however, because of
some inherent limitations. The dunes are only surface stabilized and are
prone to reactiviation if the surface vegetation and soil are disturbed.
Furthermore, windfallen trees are common along the edge of new clearings
posing potential hazard for develoPment.

Many surface stable dunes are underlain by older, buried soils and
iron lenses which restrict vertical permeability. This is typical of the
subsurface stratigraphy in open dune sand and older stable dunes as well.
Septic tank viability could be threatened by local high water due to these
impermeable lenses.

Water drawdown could also be a problem in this unit if stabilizing
vegetation is affected. Pollution of the groundwater is also a
potential problem for the development of this landform.

4. Identification check-list

Surface-stable dunes can be identified by the following characteris-
tics:

1. Dunes having a thin, weakly developed soil,

-'F -77

2. The sands underlying the soil are unconsolidated and will easily
reactivate with suffitient disturbance.

A-1

3. Forests most commonly cover the surface-stable dunes.

'0 -20 _MN!

4. The forested, surface-stable dune is characterized by the following
vegetation:
a. The understory may consist of, W!
(1) salal (Gaultheria 4
shallon-)
/1
flowers - white
to pink

N, :?
@N

13 6

59
western bracken fern (Pterldfum a Jflinum)

N -@-Vlv

'-V KV@"""@

@2 0

'2
(3) tree 7UPine (Luoinus abore

flowers
p yellow
rarely white,
Z5 b
luish or
Purp7ish

I-N

2.5

heel

(4) evergreen huckleberry (Vaccinium ovatum) and

flowers white to
pink

(5) western rhododendron (Rhododendron macrophyllum).

flowers pink
1.2
to deep rose
rarely white

(6) The more open sites may contain kinnikinnick (Arctostaphylos
uva-ursi) and

flowers
white to pink

T5 @2

(7) hairy manzanita (Arctostaphylos columbiana).

flowers white to
pale pink

b. The forest canopy consists primarily of shore pine (Pinus contorta)
but also may include:

112

(1) Western hemlock (Tsuga heterophylla) in moist sites,

IIM@ 'r,
017
2
tl\

1/3

(2) sitka spruce (Picea sitchensis)

40,

(3) douglas-fir (Pseudotsuga menziesii)

Xl@

(4) western red cedar (Thuja plicata) and

(5) red alder (Alnus rubra)

A0.

C. Older Stable Dunes

OZder sand dunes of any fo2wi which possess both a deep, weZZ-
deveZoped soit and moderateZy cemented underZying sand.

1. Geomorphology

This dune type has bee'n stabilized with a vegetative cover long
enough for a relatively deep soil to develop and for the underlying
sands to acquire some stability. The sub-surface sands exhibit varying
degrees of cementation. The iron bands and buried soils which are found
in the surface stable dune occur more frequently here and are more
pervasive.

Although these sands will form a cliff where excavated, sloughing
and landsliding are common. This is often intensified when saturation
occurs due to subsurface impermeable iron lenses.

This landform may contain layers of loose sand overlyinq or underlyinq
the semi-cemented strata but it is wind stable throughout the cemented
layers.

2. Vegetation

Forests, often the coastal climax forest, most commonly occur here
although natural grass areas may be found as well. The same species as
occur in the surface-stable dune classification also occur here. A
more even mixture of forest species is often found with less predominance
of shore pine, and the forest canopy may be more dense with a resulting
less dense shrub layer.

3. Attractions and limitations

The older stable dune presents an attractive site for recreational
activities and residential development. The limitations associated with
mobile sands do not exist with this landform. When excavated, the
semi-cemented sands will maintain a cliff and are wind stable, However,
sloughing is common, particularly in the winter months. This tendency
is accentuated during groundwater recharge months by the impermeable iron
bands which commonly run horizontally through this unit. Infiltering
groundwater is concentrated above these bands, saturating the sands
causing collapse where cliffs exist. This same phenomenon, which forms
a perched water table, results in conditions highly unfavorable to
septic tank siting, consequently, septic tank failure is not uncommon in
this landforTn.

4. Identification check-list

Older stable dunes can be identified by the following characteristics:

1. The presence of a moderately well developed soil.

n1i,

"uM
..... ......

5", Ri

2. The underlying sands are somewhat consolidated and often exhibit
horizontal iron bands which offer varying resistance to erosion and
impede vertical percolation of groundwater.

3. Underlying sands will form a cliff where cut, but sloughing is
common.

4. Vegetation species which occur on the older-stable dune are essentially
the same as the surface-stable dune but species proportions vary.
Shore pine and salal are less dominant. The following species are
common to this landform:

a. western rhododendron (Rhododendron macrophyllum

flowers - pink to
deep rose, rarely white

b. western hemlock (Tsuqa heterophylla)

c. douglas-fir (Pseudotsuga menziesii)

AWK@-@
"Ali
IN

1/3

d. sitka spruce (Picea sitchensis)

vm
W

e. -western red cedar (Thuga plicata)

'12

D. Parallel-Ridge Dunes

AZtipZe sand dune ridges which occur more or Zess paraZleZ to,
and inZand from, the foredune.

1. Geomorphology

Each ridge in a group of parallel-ridge dunes originally formed as a
foredune on an accreting beach. As the beach grew seaward, vegetation
advanced to keep pace with the upper beachline. New dune mounds and
eventually a new foredune developed oceanward of the previous one.
Continued accretion resulted in the eventual development of a series of
parallel ridges bordering the beach. The sand dunes of the Clatsop
Plains are classic examples of this type. Here the dunes are very broad,
gently sloping features aligned in a general north-south direction, paral-
lel to the beach. Other, often discontinuous examples occur in
association with accreting beach areas, such as in those areas of jetty
construction. Parallel-ridge dunes appear to be developing at the north
end of South Beach in Lincoln County. A very pronounced example of this
feature occurs adjacent to the north end of Heceta Beach in Lane County.
These are extremely steep ridges, unlike the Clatsop Plains variety.

2. Vegetation

Vegetation associations in parallel-ridge dunes become increasingly
diverse and mature progressing inland, and range from European beachgrass
(Ammophila arenaria) on the existing foredune, landward through native
herbs, shrubs and forest s.pecies, many of which have been planted through
sand stabilization projects (e.g. Clatsop Plains).

3. Attractions and limitations

This landform probably has the same attractions and limitations as
conditionally sta-ble hummock dunes and surface stabilized dunes. Due
to reactivation of a major portion of the sand in the Clatsop Plains, most
examples of this landform are in a conditionally-stable state although
areas of surface-stabilized conditions do occur.

4. Identification check-list

Parallel-ridge dunes can be identified by the following characteristics:

1. They occur in groups, running more or less parallel to the beach.

2. The Clatsop Plains variety commonly possess a very gentle angle of
slope, whereas limited occurrences on the central and south coast are
steeper.

"N."WENMR

"@Q
.......... S z"I",

Ip J,
'A 'Y ... .....
ag

3. Because they occur in a region of accreting beaches, portions of a
newly forming foredune may occur seaward of the present foredune.

@Vwl
IM

@'Tv

VI. INTERIOR DUNES - NONVEGETATED

This category includes those large areas of active sand which are
located primarily on the sand sheets (sand deposits of considerable depth
and breadth overlying subsurface coastal terraces) along the central
Oregon coast. These dunes are mostly vegetation free and therefore,
are formed primarily in response to wind and sand supply. Moisture and
topographic factors provide morphological controls of secondary importance.

The western boundary of open dune sand areas is generally located
east of the deflation plain, but is occasionally found immediately adjacent
to the foredune. The western section is essentially a nonvegetated
equivalent of the hummock dunes. Open sand areas derive their sand supply
from the deflation plain and foredune (Figure 5). However, open sand

Figure 5. Beach, foredune and deflation plain supply
sand to interior open sand areas.

landforms located downwind from conditionally stable foredunes or
vegetated deflation plains could dwindle due to sand starvation because
little sand escapes beyond these well-vegetated landforms. Open sand
dunes move inland under the influence of onshore winds.while the sand
supply is.captured by the foredune. Consequently, open sand areas are
growing ever smaller, as increasingly large deflation plains are formed
in their wake.

The pattern of dune development and reactivation is nowhere more
complicated-than on the open sand sheets. Sand deposited in these areas
during Pleistocene and post-Pleistocene sea level fluctuations has been
"reworked" several times, varying from one area to another. Because
of this, buried soils, iron bands, islands of mature forest, and actively
eroding older dune strata are common features coexisting within active
sand areas.

A. Transverse-ridge Dunes

Low northeast1southwest oriented, nonvegetated sand dune ridges
which most commonty migrate in a southeasterZy direction.

1. Geomorphology

Transverse-ridge dunes are primarily features of the summer environ-
ment. They are undula 'ting, sinuous ridges which are formed essentially
perpendicular to the northwest winds of summer and which are greatly
modified in shape during winter storms. Their orientation is northeast/
southwest; migration takes place in semi-parallel ridges moving in a
southeasterly direction (Figure 6).

Figure 6. Transverse-ridge dunes form approximately
perpendicular to northwest summer winds.

Transverse-ridge dunes are a low relief feature, five to twenty
feet high composed of a gentle sloping windward face (five to twenty-five
degrees) and a relatively steeper slip face (sixty to seventy degrees).
The distance between crests is highly variable, but generally ranges
between seventy-five and 150 feet. Where breaches occur, these dunes
reveal a highly complex interior cross-bedding. This feature is produced
when layers of sand from an advancing dune are deposited obliquely on
the dune in its path.

@ Transverse-ridge dunes occur in groups. They are commonly found on
the eastern fringe of the deflation plain. However, Lund (1973) reports
that thirty years ago transverse dunes often extended from the beach east

into the lower part of the oblique-ridge dunes, This occurred prior
to the introduction of European beachgrass (Ammoghila arenaria).

A zone of seasonally wet transverse-ridge dunes is often found on
the eastern fringe of a deflation plain with a high winter water table.
Transverse-ridge dunes commonly extend from the eastern edge of the
deflation plain into the zone of the massive oblique-ridge dunes, often
"riding" up over the surface of the latter.

Transverse-ridge dunes occur in Lane County on the major open sand
strip between the Siuslaw and Siltcoos rivers and on the open sand areas
west,of north Ten Mile Lake in Coos County.

2. Vegetation

Although transverse-ridge dunes comprise a basically open dune sand
unit, isolated areas of vegetation may occur in the depressions between
crests. These are primarily associated with the deflation plain and will
consist of the various plant types associated with that landform.

3. Attractions and limitations

This unit appears to be highly attractive to off-road vehicle users
and, to a lesser extent, pedestrian traffic. It has a high tolerance
level to most recreational activities, however, facilities such as
parking lots and road construction are not suited to this formation.
(In some cases stabilization plantings could render such developments
feasible; however, these are commonly relatively infertile sand areas
(U.S.D.A., 1972, p. 105).

Factors which could create hazards are occasional areas of quicksand
in wet depressions between transverse dune ridges, poor visibility in an
area used by both off-road vehicle enthusiasts and pedestrians, and
inundation or-undermining of structures by moving sand.

4. Identification check-list

Transverse dunes may be identified by the following features:

1. This dune form occurs as a low (five to twenty-five foot) sinuous
ridge with a gently sloping (five to twentyfive degree) windward
face and a steeper (sixty to seventy degree) lee face.

PIP.

'77- 7 7 ", @7

2. Transverse-ridge dunes exhibit a northeast/southwest trend and occur
in groups on the large, open sand areas along the south central coast.

3. Transverse-ridge dunes often ride up over the western flanks of the
massive oblique-ridge dunes.

4. Marsh-type vegetation may occur between ridges where these dunes
overlap onto the deflation plain.

.. ........

@7 o"I

Mr,

B. Oblique-Ridge Dunes

Massive, generaZly easterZy trending and migrating, nonvegetated
ridge dunes found on centraZ Oregon coastaZ sand sheets.

1. Morphology

The most dominant and obvious dune form in the open sand is the
oblique-ridge type. Like the transverse-ridge, it is dynamic in nature.
However, unlike the transverse-ridge, which is produced by unidirectional
wind flow, the oblique dune is formed both by the northwest summer and
southwest winter winds, experiencing its most energetic movement during
winter storms (Ternyik, 1978). Its somewhat sinuous axis is oriented
at an angle (obliquely) to both dominant seasonal wind sources (Figure 7).
Primary controlling factors in the development of the oblique-ridge dune

Figure 7. Oblique-ridge dunes form obliquely
to both northwest and southwest dominant seasonal winds.

are an abundant sand supply, nearly constant onshore winds and coastal
forests which act as wind breaks (Cooper, 1958).

Sands are moved inland by the onshore winds both in the summer
(northwest wind) and the winter (southwest wind). Coastal forests
which exist on the sand sheets break the flow of the low-level winds,

causing them to deposit their sand load at the forest margin. Sand is
thereby deposited at a site until the height of the dune thus produced
equals or exceeds that of the windbreak, Sand is then precipitated over
the eastern face of the dune (of the precipitation ridge) by onshore winds
(Figure 8). In this way, the dune moves slowly inland'inundatina the

A
N

-N

Figure 8. Accumulation and advance of the oblique-
ridge dune.

forest as it goes (Cooper, 11958). The ridge does not actually migrate,
but the ridge crest operates rather like a stationary transverse dune,
the upper lee face of which develops to the south in the summer and to
the north in the winter in response to dominant seasonal wind direction
(Figure 9). The windward face has a gentle slope (five to thirty degrees)

Figure 9. The ridge position of the oblique dune
is modified by seasonal winds,

while the lee face is considerably steeper with a gradient of sixty to
seventy degrees.

The oblique dunes may reach heights of 180 feet along the eastern
edge and may occur in groups with 500 feet or more between ridges. They
attain great lengths, averaging over 3,000 feet while some are nearly
a mile long (Cooper, 1958). They are commonly bounded on their ocean-
ward side by transverse-ridge dune systems and often terminate at their
eastern extremity in a precipitation ridge, actively invading older
forested dunes (Figure 10), A system of oblique-ridge dunes may form a

goo

Figure 10. Transverse-ridge dunes riding up over
the flanks of an oblique-ridge dune which terminates in
a precipitation ridge.

nearly continuous eastward advancing precipitation ridge, often called
the active ridge.

The world's distribution of the oblique-ridge dune is limited to
the central Oregon coast and occurs on the sand sheets of Lane,
Douglas and Coos Counties. Good examples are found between the
Siuslaw and Siltcoos Rivers in Lane County and just north and south
of the Douglas-Coos Counties boundary.

2. Vegetation

Sparse, marsh-type vegetation may occasionally occur in the
depressions between ridges within this landform. These are areas of high
water table and are classified as occasionally wet interdune areas.
Isolated areas of hummock dunes may also occur on the surface of this
landform.

3, Attractfons and limitations

Recreationalists use the oblique-ridge dunes on foot, on horseback
and in off-road vehicles., These dunes dominate the landscape and capture
the imagination of visitors. I'mpact of recreationalists on this landform
is reportedly negligible (U.S.D.A., 1972) although research on Atlantic
Coast dunes indicates that considerable sand transport can occur from
ORV traffic. This phenonemon would not commonly be considered a serious
problem in areas of open windblown sand, however, it could prove to
be a contributing factor to such problems as rapid dune advance on
interdune lakes, such as at Cleawox Lake in Lane County where ORV traffic
is quite heavy.

Oblique-ridge dunes do, however, pose potential hazards for
recreationalists from naturally camouflaged tree cast openings in the
ground.

Although the oblique-ridge dune is poorly suited to the development
of permanent structures (U.S.D.A., 1972), some development has occurred
here. Stabilization plantings, if carried out and maintained properly,
can alleviate potential development problems, A primary obstacle,
however, is that due to the oblique-ridge dunes' mobility and incompata-
bility with legal boundaries, access to key downwind sites for the
purpose of stabilization plantings may not be readily available to the
developer. Water table limitations mentioned in relation to the
surface stabilized and older stable dunes also apply to this dune form.

Recently, concern has been expressed for the survival of these
unique dune forms. It has been predicted that these features could
disappear within seventy-five years due to sand starvation (U,S.D.A.,
1972, p. 110). Those processes, both natural and man-induced which
threaten this feature include:

1. The eastward expansion of the deflation plain due to foredune
stabilization which cuts off sand supply to the area (Figure 11).

2. Stabilizatfon plantings for developmental and protec tion purposes.

3. Natural revegetation of the open dune sand areas. The sand sheets
of the central Oregon coast, on which these dunes occur have
experienced several periods of dune reactivation and subsequent
revegetation in the last few thousand years (Cooper, 1958). Sands
which were reactivated, probably by fires, experienced restabiliza-
tion through the natural readvance of indigenous species, This
pattern of revegetation could well repeat itself today.

Due to the combination of foredune development and resulting
deflation plain advance, stabilization plantings and natural revegeta-
tion, the oblique-ridge dunes will almost certainly disappear in the
foreseeable future, unless man intervenes.

Figure 11. Eastward expansion of deflation plain.
Sand supply is interrupted by the growing foredune.

4. Identification check-list

The oblique-ridge dune can be identified by the following charac-
teristics:

1. The mas sive size of the oblique ridge dune is probably its most
distinctive characteristic.

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2. It consists of:

a. a long, sinuous inland moving ridge gently sloping on its wind-
ward flanks, and

b. very steeply sloping at its high eastern terminus where it may
be encroaching on older forested dunes.

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3. The slopes of this dynamic landform are most commonly vegetation-
free.

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4. The oblique dune landscape appears as a series of undulating waves
of sand.

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5. Passage of the precipitation ridge may leave exhumed forests in its
wake.

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6. Islands of surface stable dunes are occasionally found within this
open sand landscape.

C. Recently Reactivated Forms

Reactivation of active, conditionally stable and surface stable
dunes can occur when binding vegetation and/or the protective soil
layer is removed and the site is exposed to erosive winds. The amount
of disturbance required for reactivation varies from site to site.
Sensitivity to reactivation will depend upon those factors which influence
sand cohesiveness, including vegetative cover and cementation such
as that which often OCCUrs within the older stabilized dunes. The
orientation of the disturbed site to prevailing winds is also of critical
importance.

1. Blowout

LocaZized zone of moving sand within an otherwise vegetated area,
which forms a depression from wind erosion on its windward side
and an area of accumuZation at its terminus

a. Geomorphology

A blowout is the result of wind scouring within an otherwise
conditionally stable or surface stable dune (Figure 12). A blowout

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Figure 12. Small blowout within beach grass environ-
ment.

may breach the host dune and threaten others in its path, This feature
may be only a few feet across and several feet long in the early develop-
ment stages. However, it can develop into a landform hundreds of feet

across and more than a mile long (parabola dune). Smaller blowouts
are relatively common features of recently stabilized areas. Once a
blowout is begun, especially in the foredune, the trough created tends
to funnel the wind, increasing its velocity, and thereby enlarging the
blowout.

b. Vegetation

This is a vegetation free landform which is surrounded by vegetated
dunes on at least three sides.

c. Attractions and limitations

This feature is probably not suitable for any particular activity
or structure.

The Dotential impacts of a blowout on downwind sites include sand
blasting, sand burial and/or heightened storm impact. Areas which may
be prone to blowout activity (i,e. recently planted sites or beach
grass areas which experience considerable use) should be carefully
watched particularly if significant impact inland is probable.

d. Identification check-list

A blowout may be identified by the following features:

1. A blowout is a somewhat elongated, mobile sand landform which occurs
within otherwise surface stable or conditionally stable dunes.

2. Parabola Dune

Massive unidirectionaZ trough of def'Zation terminating in a
zone of accumuZation within an otherwise vegetated area.

a. Geomorphology

A parabola is essentially a trough blowout of major size which is
enclosed on three sides by older vegetated dunes and on the fourth,
its source area, by open sand usually of the oblique-ridge type (Figure
13). The initial development of a parabola requires a stable vegetated

Figure 13. Parabola blowout moving through forested
dunes.

site downwind with a point of weakness for the moving sand to break
through, considerable volume of source sand, and a unidirectional wind
source. Parabolas move inland from their open sand source, through the
vegetated area, in a direction parallel to the unidirectional wind
source. They can be seen oriented either to the northwest winds of
summer or the southwest winds of winter. They may be a hundred feet or
more across and extend to nearly a mile in length. Parabolas are
named for the similarity that their perimenter bears to the parabolic
curve.

b. Vegetation

This is essentially a vegetation free landform although less active
forms may exhibit occasional vegetated hummocks.

c. Attracti,ons and limitations

Recreationalists are attracted to this landform for walking,
viewing, riding horses, and operating off-road vehicles. The only
limitations on these activities are the sensitivity of the fringe-areas'
vegetation to trampling and the possible nuisance of sand blasting.

Any development involving permanent structures would be subject to
the same limitations as those associated with the oblique-ridge dunes
although sand blasting may be a greater problem in this landform.

d. Identification check-list

A parabola dune may be identified by the following characteristics:

1. This feature creates an elongate finger of sand cutting through old
forested dunes.

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2. The terminus (advancing end) of well-developed parabola dunes
commonly creates a precipitation ridge advancing on older forested
dunes.

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VII. GLOSSARY OF TERMS

Accretion: Oceanward advancement of the beach through the ongoing.
accumulation of sand at its edge.

Active Dunes: Partially vegetated dunes that migrate, grow and diminish
according to wind, sand supply and vegetation cover. (May apply to
the foredune or hummock dunes.)

Beach Cusps: Embayments of various widths and slopes cut into the beach
by the cellular circulation of onshore and offshore currents.

Blowout: Localized zone of deflation within an otherwise vea.etated area.

"Conditionally Stable Dune: A dune which presently has sufficient
vegetation cover to retard wind erosion but whichIs vulnerable to
reactivation upon disturbance of this cover.

Deflation: The erosion of sand or soil by the wind.

Embryo Dune: Low, newly forming dune mounds.

Erosion: To wear away @Uy the action of water, waves or wind.

Lee: The side that is sheltered from the wind.

Older Stable Dune: Dunes of any form which possess both-a deep, well-
developed soil and semi-cemented underlying sand.

Onshore '14inds:'Winds which are moving toward or onto the shore from
open water.

Precipitation Ridge: High, steeply sloping slip face of large oblique-
ridge and paraGola dunes.

Rip Current: A strong relatively narrow current flowing outward from a
shore which results from the inland flow of waves and wind-driven
water.

Sandsheet: Sand deposits of considerable depth and breadth overlying
sursurface coastal terraces.

Surface Stabilized Dune: Commonly forested dunes which possess a thinly
developed soil and are underlain by loose unconsolidated sands.

Wind Stable Dune: Those dunes which possess sufficient vegetation and/or
soil cover to retard wind erosion.

Windward: The side or direction from which the wind is blowing.

REFERENCES CITED

Cooper, William S. 1958. Coastal Sand Dunes of Oregon and Washington.
Geological Society of America, Memoir 72. New York, New York. 169 ppt

Leach, Don. Interview. 1978. U.S. Department -of Agriculture, Soil
Conservation Service, Astoria, Oregon.

Lund, Ernest H. 1973. "Oregon Coastal Dunes," The Ore Bin. State of
Oregon Department of Geology and Mineral Industries, Portland..
Oregon. 92 pp.

Ternyik, Wilbur. Personal Communication. October, 1978. Owner, Wave
Beachgrass Nursery, Florence, Oregon.

U.S. Department of Agriculture, Forest Service. 1972. Resource
Inventory Report for the Oregon Dunes National Recreation Area.
Siuslaw National Forest, Portland, Oregon. 294 pp.
U.S. Department of Agriculture, Soil Conservation Service and Oregon
Coastal Conservation and Development Commission. 1975. Beaches
and Dunes of the Oregon Coast. U.S. Department of Agriculture,
Soil Conservation Service, Portland, Oregon. 161 pp.
Weideman, Alfred M., LaRea J. Dennis, Frank H. Smith. 1974. Plants of
The Oregon Coastal Dunes. Oregon State University Press,-Corvallis,
Oregon. 117--pp.-

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