Shoreline erosion in Virginia

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

SHORELINE EROSION
IN VIRGINIA

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Scott H ardaway
Gary Anderson

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57.2
.V51 5 SEA GRANT PROGRAM
no. 31 Marine Advisory Service
Virginia Institute of Marine Science
College of William and Mary
Gloucester Point, Virginia 23062

Copies of this publication may be ordered
for $ 1. 00 each from the Sea Grant Communications Office,
Virginia Institute of Marine Science, Gloucester Point, VA 23062

SHORELINE
EROSION IN VIRGINIA

SCOTT HARDAWAY
GARY ANDERSON

Virginia Institute of Marine Science
Gloucester Point, VA 23062

Property of CSC Library

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

Designed and Edited by Kevin Gray

cm
N-) Cn October 1980
C"
A Sea Grant Advisory Service of the
42 College of William and Mary

4@ Educational Series No. 31

ACKNOWLEDGEMENTS

This publication is the result of research sponsored by the Virginia
Institute of Marine Science Institutional Sea Grant Prbgram, supported
by the Office of Sea Grant.

Thanks go to jean Belvin and Cynthia Gaskins for typing the numerous
drafts. Special appreciation goes to Cheryl Teagle for composing the final
manuscript.

TABLE OF CONTENTS

SHORELINE EROSION IN VIRGINIA ............................................. 1

CAUSE ................................................................. I

EFFECT ................................................................ 3

SHORELINE VARIABLE ................................................... 4

COASTAL PROCESSES .................................................... 5

WIND AND WAVES ........................................................ 5

CURRENTS ............................................................. 5

BEACH ................................................................. 5

SHORE TYPES ........................................................... 5

SEDIMENT BANKS ....................................................... 6

MARSH SHORELINES ..................................................... 7

BARRIER BEACHES AND SPITS ............................................ 8

MAN MODIFIED ......................................................... 9

SHORE PROTECTION METHODS ........ ..................................... 11

VERTICAL RETAINING STRUCTURES (Bulkheads, Seawalls and Revetments) ....... 12

GROINS AND JETTIES ................................................... 14

BREAKWATERS (Solid and Floating) ........................................ 15

SUBMERGED SILL ...................................................... 15

DESIGN CONSIDERATIONS ............................................... 16

RIPRAP REVETMENTS ................................................... 16

BULKHEADS .......................... ................................ 17

GABION REVETMENT ................................................... 18

GROINS AND JETTIES ................................................... 18

SUBMERGED SILL ...................................................... 19

BREAKWATERS ........................................................ 19

VEGETATIVE CONTROL ..................................................... 21

MARSH GRASS ......................................................... 22

RIVERBANK SLOPE CONTROL ............................................ 22

iii

INTRODUCTION

Virginia has over 5,000 miles of tidal shoreline. Several different shore
types occur in the Tidewater region including the low-lying barrier islands
of the Eastern Shore, the ocean front headland -barrier spit of southeastern
Virginia, and the shores of the Chesapeake Bay and other estuaries which
range from high bluffs to tidal marshes. In order to put shore erosion in
proper perspective as a natural phenomenon, one must examine the recent
geologic history of the region.
Much of shoreline erosion is a direct product of high energy storms
like hurricanes and northeasters. The rate and amount of erosion along a
specific shoreline may vary from year to year. The rate of erosion will
depend upon the following factors: (1) storm frequency; (2) storm type
and direction; (3) storm intensity and duration; and (4) resulting wind
tides, currents, and waves. Also, the presence of man-made structures
(bulkheads, groins, etc.) will modify the erosion process, increasing or
decreasing it to a degree depending on the type, location, design, etc.,
of the structure (O'Connor, et al., 1978).
The problem of shoreline erosion is most acute when coastal property
with improvements is threatened by a rapidly receding shore bank. Many
waterfront properties are bought and developed each year with little or no
consideration of the shoreline situation. Consequently, additional money
must be spent for erosion protection structures.
Shoreline protection structures must be adequately designed and cor-
rectly placed to be effective under the severest of storm conditions. In-
adequate installation or design may result in failure or deleterious effects
to adjacent waterfront properties. In many cases a structure is not needed
and protection of a shore bank may be accomplished by vegetative means,
such as the planting of appropriate grasses, shrubs or vines to stabilize the
bank, beach or nearshore area.
Virginia's coast is a dynamic and active environment as well as a beautiful
place to live. Sound judgement in coastal development is essential to effec-
tive control of shoreline erosion. -C. Scott Hardaway

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SHORELJ`N.--@.
EROSION JN, VIRG IN`j@X

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THE CA USE tributary estuaries are a geologi- melt waters began to raise the
cally young portion of the Vir- level of the oceans. The rising
The Chesapeake Bay and its ginia coastal system. About sea level caused the shoreline and
tributaries are drowned river val- 15,000 years ago, the ocean shore- coastal system to slowly migrate
leys of the ancestral Susquehanna line was about 60 miles east of upward and westward across the
River system (Figure 1 ). Drowned the Virginia Capes and sea level continental shelf. Today's estu-
river valleys where seawater from was some 300 feet lower than aries are formed as the rising sea
the ocean and freshwater from it is today. Much of the ocean's level floods the topographically
upland rivers mix freely are called water was locked up in the great low river and stream valleys.
estuaries. The circulation in the ice sheets which covered the As sea level continues to rise, the
estuaries is influenced by the northern half of North America coastal lands of Virginia con-
astronomical tidal conditions, the during the Late Pleistocene glacial tinue to flood.
amount of fresh water run-off epoch. As the glaciers began to The process of shoreline mi-
and the shape of the basin. melt and recede in response to a gration is better known as shore-
The Chesapeake Bay and its gradually warming climate, the line erosion. In the estuaries of

Figure 1 - Chesapeake Bay and its tributaries.

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Virginia, shoreline erosion is a
TABLE 1.
continuing prOCC55 which has been
operating for several thousand
years. Rates of erosion are de- AVERAGE SHORELINE EROSION RATES - TIDEWATER VIRGINIA
pendent upon specific shoreline
variables and varying storm condi- YORK RIVER
tions, according to location. Lo- NORTH SIDE EROSION RATES AVERAGE
cally, a shoreline may appear
stable or actually accrete sedi- Gloucester Co. - 0.5 ft/yr 0.4 ft/yr
ments. However, such a situation King and Queen Co. - 0.3 ft/yr 0.4 ftlyr
is anomalous and is usually SOUTH SIDE EROSION RATES AVERAGE
short-lived. York Co. 0.9 ft/yr
James City Co. 1.8 ftlyr - 1.2 ft/yr
Shoreline erosion on a daily New Kent Co. 0.9 ft/yr
basis is minimal. Severe erosion
occurs during periods of high JAMES RIVER
energy storms such as northeasters NORTH SIDE EROSION RATES AVERAGE
and hurricanes. Therefore, the Newport News 0.8 ft/yr
rate of erosion at any specific James City 0.1 ft/yr - 0.45 ft/yr
location depends upon the fol- SOUTH SIDE EROSION RATES AVERAGE
lowing conditions (Riggs, et al., Isle of Wight Co. 1.8 ft/yr
1978): Surry Co. 1 .2 f t/yr - 1.5 ft/yr
1. Storm frequency RAPPAHANNOCK RIVER
2. Storm type and direct- NORTH SIDE EROSION RATES -AVERAGE
ion Lancaster Co. 0.6 ft/yr
3. Storm intensity and du- Richmond Co, 0.6 ft/yr - 0.6 ft/yr
ration SOUTH SIDE EROSION RATES AVERAGE
4. Resulting wind tides, Middlesex Co. 1 .0 ft/yr
current and wave storm Essex Co. 1.2 ft/yr - 1.1 ft/yr
surge CHESAPEAKE BAY
Seasonal wind patterns vary in WESTERN SHORE EROSION RATES AVERAGE
the Chesapeake Bay region. From Gloucester Co. 0.6 ft/yr
late fall to spring the dominant Hampton 1.0 ft/yr
Lancaster Co. 1.4 ft/yr
wind direction is from the north Mathews Co. 0.8 ft/yr
and northwest. During the late Northumberland Co. 1 .0 ft/yr
spring, the dominant wind shifts York Co. 1.5 ftlyr - 0.9 ft/yr
to the southwest and continues so EASTERN SHORE EROSION RATES AVERAGE
until the following fall. Northeast Accomack Co. 1.5 ft/yr
storms which occur from late fall Northampton Co, 0.7 ft/yr
to early spring are associated. with Fisherman's Is. +11 ft/yr - 1 .0 ft/yr
eastward moving storm fronts. SOUTHERNSHORE EROSIONRATES AVERAGE
.... .. . .....
Frequently there is a period of Virginia Beach 1 .7 f t/yr
intense north or northeast winds Norfolk 1.2 ftlyr
Nansemond 1 .2 ft/y r - 1.4 ft/yr
following the passage of the front.
Hurricanes can occur from mid- -----------
summer to late fall, Hurricanes shorelines of these tributary es- line exposure to the northwest,
are less frequent than other storms tuaries are highly dissected by north and northeast directions
but sustained winds of 74 knots numerous lateral tidal creeks. from where the severest seasonal
and wave heights of over five feet From about 18SO to 19SO winds originate. Individual seg-
make hurricanes an unwelcome the Virginia Chesapeake Bay and ments of shoreline have experi-
visitor to waterfront property its tributaries lost over 21,000 enced erosion rates of more than
owners. acres of land to shoreline erosion. seven feet per year. However,
one or two feet per year is more
EFFECT Average shoreline erosion rates common. For the 2,365 miles of
for this period are shown in Table estuarine shoreline measured, the
There are over 5,000 miles of 1. The bay side of the Eastern average rate of erosion is about
shoreline along the Virginia por- Shore, the Peninsula, west side 0.7 feet per year (Byrne, et al.,
tion of Chesapeake Bay and its of the Bay and the south side of 1979). The Virginia Institute of
tributaries. The major tributaries the tributary estuaries have the Marine Science defines 5evere
are the James, York, Rappahan- highest relative erosion rates. erosion as any shoreline segment
nock and Potomac Rivers. The This can be attributed to Shore- with a rate of two or more feet

per year (Figure 2). Shoreline (the length of open mediately behind the
erosion becomes a problem when water facing the shore- sediment beach (if pre-
coastal property with improve- line), the wind speed, sent) or shoreline. For
ments (house, cottage, etc.) are direction and duration, a given recession rate,
threatened by a rapidly receding and the nearshore water the bank height deter-
shore bank. depth, mines how much ma-
terial enters the estu-
SHORELINE VARIABLES 2. Depth offshore - shal- arine system.
low water, such as tidal
Many variables affect the es- flats, helps reduce wave 4. Bank composition -
tuarine shorelands of Virginia. energy better while tight clay or well ce-
The importance of any given deeper water in the mented sand resist eros-
variable depends on the site. nearshore area allows a ion better than soft
Some of the important variables greater proportion of clay or uncemented
include: the wave energy to sand.
reach the shore.
7. Wave height - this vari- 5. Width and elevation of
able is in turn depend- 3. Bank height - the height sand beach - a sand
dent upon the fetch of the shore bank im- beach is a natural buf-
fer to wave activity.

6. Abundance of vegeta-
tion - vegetation (grass-
es and vines) on the
shore bank nearshore
CID
and beach help hold
the sediment and baffle
wave action.

7. Shoreline geometry -
the general shape of the
shoreline. Irregular
shorelines like marshes
tend to break up wave
V.,
energy better than
straight shorelines.

lo 8. Shoreline orientation -
the general geographic
direction the shoreline
faces along with the
fetch, influences the de-
gree of exposure to
wind wave attack.

9. Boat wakes - waves
from boat wakes may
severely affect a shore-
line which is close to or
on a boat channel,

NE
.=Rlj In addition, the presence of
man-made structures (bulkheads,
groins, etc.) modify the process
of erosion by increasing it or
NORFOLK
decreasing it to a degree depend-
ing on the type, location and de-
\/,\-b

sign of the structure.

Figure 2 - Green lines indicate erosion
rates of two or more feet per year.

COASTAL PROCESSES

WIND AND WA VES

Waves are created by wind.
The size of the waves is a func-
tion of fetch (distance over Ilk
water which the wind blows),
wind velocity and depth of
the water. Storm winds generate
large waves which do the most
damage to shorelands on the
open bay and ocean. During
northeast storms and hurricanes
the water level itself may be
KEY
dramatically elevated (storm Sand
Surge) so the wave attacks the
Clayey-Sand
shore at elevations higher than Organic Muck
normal.

CURRENTS Figure 4 The cypress fringe damping erosion wave forces and trapping sedi-
ments is illustrated in this figure. The cypress trees can form a natural bulkhead
There are basically two types which dissipates wave energy and traps sand.
of currents which figure @ro-
minently in the coastal processes
acting in the Chesapeake Bay shore. It is generated by waves SHORE TYPES
system. These are tidal currents which strike a shoreline at an
and longshore currents. Tidal ngle. Waves and currents work- Five basic types of shoreline
currents are generated by the
periodic rise and fall of he ing along a shoreline mold a shore- exist in the Virginia coastal
t line's configuration and cause the system. These are:
astronomical tide. These tidal movement of sediment to and
currents are most noticeable at the from the shore. The movement of 7. Swamp forest
entrances to harbors and tidal sand either parallel to shore, 2. Sediment banks
creeks. onshore or offshore is known as 3. Marsh
The longshore current moves littoral transport. Although sand 4. Barrier beaches and
parallel and adjacent to the may move along a shoreline in spits
either direction, there is usually a 5. Man-modified
net movement in one direction.
Their distribution is a function
of topography and the regional
BEACH slope of the Chesapeake Bay
drainage system.
P-LE AN.-I
Beaches, no matter how ex- Swamp forests occur as rivers
tensive, are natural landforms flood plain vegetation (Figure 4).
resulting from wave action and As sea level rises and floods the
represent a buffer zone between upland river basins, the flood
the land and water. During storm plains become the receding shore-
periods, waves may carry much line. In Virginia, extensive reaches
of the beach sands offshore to of swamp forest shorelines occur
S, form a bar (Figure 3). This in the upper portions of the
helps d issi pate the energy of tributary estuaries and their lateral
L w
the waves before they reach creeks.
ACCI TI01
LE A the shore. With the return of Swamp forests contain a
calmer weather, the bar sands variety of tree species including
slowly migrate back to the beach. Bald Cypress (Taxodium disti-
Longshore sand movement usually chum), Black Gum (Nyssa syl-
-ET- occurs also. The direction of vatics), and Tupelo Gum (Nyssa
'Rol- beach sands movement must be aquatic). The massive above
Figure 3 Creation of nearshore bar taken into account when designing ground root system and flared
during a storm. coastal structures for erosion trunk of the Bald Cypress make
protection (groins, breakwaters it relatively resistant to wave
and bulkheads). action and erosion. However,

its vulnerability to flooding makes tern Shore in South Accomack off large chunks of fastiand.
swamp forest shorelines undesir- County and Northampton Abating active erosion of a
able for development. County. Low sediment banks sediment bank requires vegetat-
( < 14 feet) occur mostly along ing the face of the bank. To
SEDIMENTBANKS the small creeks and embayments best accomplish this, the slope
on the west side of the Chesa- of the bank should be reduced.
Sediment banks are composed peake Bay. A recommended slope gradient
of varying mixtures of gravel, Active erosion - of sediment is 4:1 (4 horizontal to I vertical).
sand, silt and clay (Figure 5). banks produces almost vertical However, it may be possible to
They range in height from a exposed scarp with fallen trees affect vegetative bank stabilization
few feet to over 100 ft. above and logs littering the beach and on as much as a I: I slope if the
mean high water. Usually, a nearshore area. Erosion of high bank is "dry" and has no ground-
sand beach will exist along the banks is caused by rainwash and water springs "weeping" out.
base of the bank with much of groundwater which saturates the Once the eroding bank is
the beach sand coming from face of the bank causing sliding stablized by grading, the toe or
erosion of the bank. and slumping. Wave action base must be protected against
Sediment banks occur as in- during storms causes additional waves and high water which will
terstrearn divides (between creeks) slumping by undercutting the undercut the sloped bank and
in the Chesapeake Bay system. base of the high bank. Low banks eventually renew the erosion pro-
The higher banks ( > 15 feet) are are most vulnerable to wave blem. There are basically two
found along the lateral tributaries activity which will overtop the methods for accomplishing this.
and on the bay side of the Eas- bank during storms and carry One is to build up a sand beach,

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ov
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Figure 5 - Erosion of sediment banks is the primary source of material for the beaches along the estuarine shoreline.
(Upper right) storm waves undercut the high bank causing slump blocks, along with the vegetative cover, to slide
onto the beach. In this illustration, the slump blocks are reduced by further wave action, leaving behind debris of
fallen timber.

the other is to harden the toe
of the bank (refer to section of
"Shore Protection Methods").
Sand can be trapped by groins, W
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submerged sills and breakwaters.
Care must be exercised in em-
N,
placin- those structures to pre-
vent sand starvation of adjacent
shorelines. The enhanced beach
"A
helps dissipate wave energy against
the bank. However, during large
7
storms, water levels and waves
may overtop an established beach
and severely attack the sediment
bank. In many cases, a beach
is not enough to prevent bank
4
erosion, consequently, the toe
of the bank must be hardened.
Hardening the shoreline can
be done by building a bulkhead,
-Z,
These
seawall o r revetment.
methods are often costly and must
be constructed properly to pre-
vent premature failure.
If a sediment bank shoreline
is exposed to small distances of
open water, it may be possible
to plant a marsh grass frin@e
(Figure 6). Marsh grass wi
baffle wave action and also help
trap sand. This has been done
effectively in many areas and it - ------
can be a viable and relatively
inexpensive measure to slow
TZ, A
shoreline erosion.

MARSHSHORELINES

Marsh shorelines are extensive
marsh (wetlands) plains (Figure 7)
or narrow fringes in front or river-
ward of sediment banks. Marshes
also occur along the flood plains 7" P,
of embayed lateral creeks. These ;r,
creeks are subject to daily tidal
fluctuations and usually have a
defined channel.

Figure 6 - Fringe marshes offer ex-
cellent natural buffers to erosion. The
marsh grass greatly reduces wave energy
acting on the shoreline.

Figure 7 - Extensive marsh plains occur
along many low lying areas bordering
the Chesapeake Bay.

Figure 8 depicts tidal wetlands 10). This situation exists along maintain a state of equilibrium.
of Virginia and the vegetative the barrier islands of Virginia's There is usually little input of
zonation of plant species. There Eastern Shore and to a lesser sand from actively eroding fast-
are 212,000-225,000 acres of extent along the Bay facing shore- land. When a severe storm im-
tidal marshes in Virginia. In line of Accomack, Mathews, and pinges on a barrier system, beach
addition to their value as an York Counties. and dune sand can be carried
essential link in the estuarine Barrier beaches generally ope- through a breach in the dune
food chain, marshes also act as rate with a limited amount of line and deposited on the marsh.
effective buffers to erosion of sand. The beach and dune system These features are called wash-
the fastland. Their low elevation react to existing weather condi- overs. Washovers reduce the
and matted root system make tions by attempting to attain and amount of sand available to the
marshes more resistant to wave
erosion than sediment banks ex-
posed to the same fetch con-
clitions.
Extensive marshes act as large
sponges to help reduce the flood
hazard in some coastal areas.
Fringing marshes occurring along
sediment banks greatly reduce
wave action. Actively eroding t3
marsh shorelines are characterized
by exposed and undercut peat
banks (Figure 9). Marshes are
also valuable to waterfowl as a
source of food and habitat.

BARRIER BEACHANDSPITS

Figure 9 The factors involved in erosion of the brackish marsh shorelineare,
A beach system is comprised illustrated. During low tide, wave action erodes the softer underlying peat (13),
of a beach, a dune, and a marsh undercutting the firm surface layer (A). The peat shelf breaks into blocks that fall
complex behind the dune (Figure into blocks that fall into the water, leaving U-shaped notches along the bank.

BRACKISH WATER MIXED COMMUNITY TYPE XII
(excluding upland species - pines, cedars, etc.)

SALTMARSH CORDGRASS SALTBUSH TYPE IV
TYPE I
BIG CORDGRASS
BLACK NEEDLERUSH TYPE V
TYPE III
SALTGRASS MEADOW
SALTMARSH BULRUSH TYPE 11

OLNEY THREESQUARE SEA LAVENDER

Figure 8 - Vegetative zonation of plant species.

MAN MODIFIED

Man modified shorelines are
any of the previous shore types
which have been altered in some
fashion by man (Figure 73).
These alterations or modifications
r-, 7 include bank grading, beach
44, nourishment, and protective struc-
z
tures. Much of Virginia's coast-
J line has been developed in one
way or another. Over 800 miles
of Bay shoreline has a housing
density of six or more structures
per mile.
4"'
Shoreline protection to a large
degree has been haphazard and
piecemeal in nature. Along any
given shoreline, the number of
different protective -methods often
Figure 10- Extensive low-lying dune shorelines occur along bay shores in Hamp- equals the number of people
ton, Mathews County and Northumberland County. living there. This often is in-
active beach, forcing the beach described previously. effective in controlling the pro-
blem of shoreline erosion. For
to react to a reduced sand supply Development on the barrier example, an improperly designed
by retreating. In many instances beaches and spits is precarious or poorly constructed bulkhead
this retreat exposes the surface due to the shifting nature of may fail, creating problems for the
of marsh peat covered by pre- sand. Ocean shorelines are the owner and adjacent neighbors.
vious washovers (Figure 11). most dangerous because of high Groin installations often do well
Spits are active sand features wave energy potential generated to protect the updrift shoreline by
found at the clowndrift end of over long fetch conditions, high trapping sand, but may cause
barrier beaches. They are also storm surge and deeper nearshore serious sand starvation and erosion
defined as tongues of sand moving water depths. Rigid structures clowndrift.
across the mouths of lateral (cottages, bulkheads and seawalls)
creeks. One example of a spit is placed on the dunes and beaches Rather than using this ap-
Willoughby Spit where the Hamp- usually create adverse effects proach, shoreline erosion should
ton Roads Bridge Tunnel enters by causing loss of beach and be addressed on a reach basis
Norfolk. This mile long sand spit subsequent undermining of the with full consideration for the
is said to have formed in a single structure during severe storms. net effectiveness of the structural
hurricane storm in 1806 (Figure
72).
Sand spits may advance across
small creek entrances if enough
sand is moving alongshore which
results in temporarily closing the
creek. Larger lateral creeks
maintain their channel opening by
tidal flushing. However, even these
naturally maintained channels are _4
not deep enough for continued
use by some vessels. This navi- ft
gation problem is solved by
maintenance dredging of the chan-
nel and/or stabilizing the inlet -r
with jetties.
Some barrier beaches lack a hw
Z10-
marsh complex behind the dune
system. Such shorelines exist
along the south shore of
Chesa
rtt;"L

peake Bay and between Cape Figure 11 The dune shorelines recede by washovers during storms. This often
Henry and Sandbridge. They also exposes a peat horizon which was once a living marsh behind the dune system
retreat by the same process as bayward of its current position.

or other ' methods employed low
(Byrne, et al., 1978). A reach is
a shoreline unit where there is a '17
mutual interaction between the
forces of erosion and/or the sedi-
4N7
ment supply. It may be advan-
tageous for a waterfront com- 401@@
T-r
munity to seek out advisory or
engineering services, whether pub-
lic or private, to insure a sensible
approach to their shoreline situ-
17
ation.

&
Figure 13 - Wooden bulkheads are a r.. I F
QJP!!R-A@ ajj!F',z',@ip
popular method of slowing shoreline
erosion. If properly installed, they 0 N
will generally last 10-15 years.

Figure 12 Willoughby Spit as seen
from the air on March 3, 1973.

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SHORE PROTECTION METHODS

The problem of shore erosion is often overlooked :0j?joio!0-:0:0lolo* T
by the prospective or present owner of shoreline I I
property. Average rates of erosion calculated for
discrete segments of a shoreline rarely present the
true picture. Some areas remain stable for many
years and then suffer extreme erosion. These erosion 0 0 0:1 0! 0 o 0 0 0
rates reflect the relative severity of erosion in one WALE
area versus another. In response to sudden increases
in the erosion rate, many landowners install forms
of shore protective structures whether it is the PILE SHEETING
proper structure or not. In many instances im-
0 0. 0 1' 0a 0 0 0. o:
proper structures only serve to accelerate the erosion.
Failures stem from the use of improper materials,
improper installation, or poor advice. No patent
answers exist because each situation is unique; nor
is there a guarantee that any system will solve the
problem. Construction of proper shore protective ELEVATION
structures is also expensive. In many cases, the
cheapest solution is not always the best. There
are several protection shore rnethod5 which, if pro- WALE,,,, dbNPILE
perly designed for a particular estuarine shoreline,
can be effective in abating erosion. Some of the
methods are: SHEETING

VERTICAL RETAINING STRUCTURES
(BULKHEADS, SEAWALLS, REVETMENTS) `*-T I E ROD
The respective definitions of these structures WALE\
vary little although different construction materials I
are sometimes associated with each structure (Figures
74A & B, 15, 76A & B). Vertical structures act to
retain the fastland material and to prevent wave
induced erosion. In one way, they can be thought of ANCHOR PILES
as defensive structures. However, in many situations, PLAN
vertical structures lead to the loss of the beach which
may front them.

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AAw"'.,
It 'no
6@ -Asa
tie
ING4

kad"APC

Figure 14A - Wooden bulkhead. Figure 14B (top right) timber sheet - pile bulkhead.

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TOPSOIL AND SEED
4'-66'. ROUNDING 0.1
1'-6" min ELEV. 9.0

ELEV. 8.75'--,,, r
I'-d' min.
STONE RIP-RAP 2 FT THICK
(25 % 300 1 bs., 25 % 30 bs. POURED CONCRETE
50 % wt. 150 lbs.) (Contraction it. every 10'
2 FILTER CLOTH OR
-.q--GRAVEL BLANKET I FT. THICK
EXISTING BEACH (200 SIEVE to 3", 50% F 1/2
A--ELEV. 0.00' M.S.L. OVER REGRADED BANK

ELEV. 1.00'

mppm"wm'm'-
r

4M4jL

Figure 15 - Concrete seawall (top). Figure 16B (bottom) small riprap revetment. Figure 16A (middle) riprap revetment.

During times of abnormally high wdter, waves PLANKS
STAGGERED
may overtop the beach and strike the bulkhead.
Most of this energy is reflected off the wall. The I- - G. 1. BOLTS
reflected wave then meets the next incoming wave WATER LEVEL DATUM
and creates a zone of extreme turbulence where TIMBER WALE
they meet. This turbulence causes scouring of
beach material near the base of the bulkhead. Over
a period of time, this can lead to the deterioration
of the beach if there is an insufficient input of TIMBER SHEET
ROUND PILING
sand. In addition, age or improper construction can PILING
lead to small cracks in a bulkhead. This can result
in leaching of fine material from behind the bulk-
head. In many instances, erosion will continue on
each side resulting in the flanking of the structure. SECTION

ROUND
GROINS A ND JETTIES PILES G. I. BOLT
WALE
Groins and jetties are vertical structures oriented -PLANKS
perpendicular to the shoreline. Groins are used on GERED
open shorelines to trap the littoral transport of sand \T I BER WALE
parallel to the beach. When functioning properly, \WASHERS
groins will widen and heighten a beach. PLAN
jetties are structures used to define and protect
an inlet or harbor entrance from shoaling. Shoaling

4, Z' %
"T

_J
is usually the product of the littoral transport of
sand towards and into the inlet. jetties can also
serve to make inlet access easier by reducing wave
height (Figures I 7A & B).
Imperative in the success of groins is an adequate
supply of sand to quickly fill the groins and then
begin bypassing sand. An inadequate supply of
sand can lead to accelerated erosion clowndrift of the
groin. In some cases, this accelerated erosion can
flank the groin rendering it totally useless (see Figure
78).

Figure 17A Groin field. Figure 17B (upper right) Timber
sheet-pile groin.

Figure 18 - Groin which has been flanked by severe erosion.

BREAK" TERS (SOLID AND FLOATING) reduction in wave height can be achieved. The small
waves are the first to be dampened out.
A solid breakwater is a structure usually con-
structed of stone, which serves to reduce wave
height (Figures 19A & B). This reduction of wave
height reduces the erosive power of the waves striking SUBMERGED SILL
the beach or entering a harbor. As solid breakwaters
work to resist the full power of the incoming waves, Although the submerged sill is new to the Chesa-
the structure must be massive. If not designed peake Bay, it was first employed over 40 years
and placed properly, they can lead to accelerated ago. It consists of a detached structure constructed
nearshore currents which can cause erosion. Used parallel to the shore along the extent of the eroding
incorrectly, they can halt the parallel littoral sand shone. To date, sandbags, gabions and wood have
transport leading to accelerated erosion downdrift. been used as construction materials.
A different type of breakwater becoming popular The principle involved is known as a perched
is the floating breakwater (Figure 20). Numerous beach. As illustrated (Figures 27A & B) the desired
designs have been proposed and some have been effect is to elevate the profile of the beach. If suc-
tested. Two general categories, rigid and flexible, cessful, a protective layer of sand exists at the base
have arisen with various benefits added to each. of a cliff or bulkhead. Because this layer is signi-
Most designs have a floating structure tethered to ficantly higher than the unprotected backshore
the bottom. Instead of working to totally eliminate height, additional protection is afforded during
waves from passing the structure as does a solid storm elevated water levels. This system offers
breakwater, floating breakwaters act as filters to protection from all wave angles and at higher water
the incoming waves by reducing the wave energy levels. In essence, it resembles a "permanent" bar
behind the structure. In some cases substantial situated at the step of the beach.

Varies

@*-ARMOR STONE

CORE STONE
(Quarry Run)

CROSS-SECTION

T17

_tat
04

IN-
v, L

4
11 __411 I'll

Figure 19A - Small stone breakwater. Figure 19B (upper right) generalized cross section of a large stone breakwater.

7:7

@ngwln

-5p,
rjW@%JZ

Figure 20 Floating
tire breakwater.
x,
4

A0
74'

N;&

711

FIN

Mff 'P'0%-V
1000

The sources of material to fill the system come DESIGN CONSIDERATIONS
from three areas. The first area is that which drifts
along parallel to the beach. The second area is that B UL KHEA DS, SEA WA L LS, R E VETMEN TS
which is moved onshore from offshore, and the
third area is that which is introduced by rain runoff Effective designs for these structures vary with the
erosion of the cliff face behind the structure. The forces which these structures must deal with. How-
percentage of input from these three sources varies ever, certain considerations may apply according to
with the location. Beach nourishment can also be the location.
used to fill the system.
As with any structure, the submerged sill has its RIPRAP RE VETMENTS
drawbanks. It can be a swimmer and boating hazard.
Also, it can be too effective in halting the parallel 1. The stone or rubble should be placed as
transport of sand along a beach. Construction opposed to dumped at the site.
materials such as the sandbag are susceptible to ice Rationale: Placing assures good inter-
damage and vandalism. It is generally restricted to locking of the individual stones.
areas which have significant volumes of sand within The proper slope for the material
the beach system. can be easily attained.
16

2. Stone or rubble of sufficient size to resist
the maximum expected wave forces should
be used.
Rationale: Many failures of riprap
structures stem from insufficient
stone size. Waves during storm W.W1111
10 1
conditions remove the material too
'NOW"'
small, thus leading to failure of the
whole structure.

3. A protective apron should extend from
the toe of the structure.
Rationale: Although the rough
nature of a riprap structure removes
a portion of a wave energy, some is IS-
still reflected. When this reflected
wave meets the next incoming wave,
an area of intense scouring occurs
immediately in front of the structure.
This scour undercuts the structure, A _Ri @ 44"t'. r,
allows it to slump, and eventually Figure 21 A - Sandbag sill.
leads to its failure.
PROFILE BEFORE INSTALLATION OF SILL
4. The use of filter cloth is strongly recom-
mended.
Rationale: By their nature, riprap
structures are porous. However, this MHW
porosity can lead to the failure of MLW
the structure. The constant flooding SAND VENEER OVERLYING
of the structure due to tidal action NOTE INTERSECTION OF 5TORI 11GII SEMI- CONSOLIDATED SEDIMENTS
and the passage of ground water and WIITER ANO SIORe IN Sol. CSE,
rain runoff through the structure
can leach fastland material fro m PROFILE AFTER INSTALLATION OF SILL
behind it. Filter cloth acts to pre-
vent this leaching of material. It is
emplaced first and then the stone is
placed on top of it. ------ SIL-LTO-PE-RCH TH-E- REACH -------------- -STORM HIGH WATER
-MHW
- - - - - - - MLW

LEVEL OF B@ACH
BULKHEADS BEFORE PERCHING"
Vertical sheet pile bulkheads can be constructed Figure 21 B - Perched beach concept.
from a variety of materials. Among these are wood,
steel, aluminum, asbestos concrete and concrete.
Gabions can also be used to construct vertical struc- 2. As a minimum, depth of penetration
tu res. should be equal to the exposed portion.
Rationale: A common cause of
bulkhead failure is inadequate pene-
WOOD BULKHEADS tration. Many people assume that
the beach will remain in front of
7. Vertical tongue and groove sheet pile is the bulkhead. However, in storms,
preferred, waves can remove the beach leaving
Rationale: All treated wood warps a minimum length of sheet pile
and shrinks after exposure to the penetrating the stable bottom. The
elements. The tongue and groove weight of the cliff material being
configuration reduces the risk of retained causes the bulkhead to
spaces opening between the sheet topple. In other cases, the inade-
pile which would allow fastland quate penetration of the bulkhead
material to leach through the open- allows the retained fastland to leach
i ng, Tongue and groove cannot out the bottom of the bulkhead.
reduce this risk in cases of faulty To insure adequate penetration, it is
construction. necessary to establish where the

stable layer begins. In most areas return wall joins the fastland. To
throughout the lower Chesapeake prevent the flanking of the bulk-
Bay, this layer generally coincides head, the return wall should be
with a reddish brown clay layer which entrenched in the fastland. In addi-
exists under the beaches. tion, a riprap cornice may be ne-
cessary. This riprap serves to reduce
3. Emplacement of filter cloth is recom- the scout of waves focused at the
mended. ends of the wall.
Rationale: The application of filter
cloth behind a tongue and groove
bulkhead acts as it does with riprap. GABION RE VETMENT
Because the wood shrinks and warps,
small openings can appear between 7. GGbions should be entrenched into the
the sheet pile. The filter cloth then clay layer below the beach (Figure 22).
acts to prevent the leaching of the Rationale: As with other vertical
fastland material out through these structures, some scour can occur on
openings. It is also very helpful at the seaward side of the structure.
the junction of two structures. As Gabions placed on top of sand will
an added measure of protection, be undermined, causing the structure
the bulkhead can be backfilled with to fail.
gravel or other coarse material and
then regular fill. 2. Filter cloth should be placed under, or on
the backside of the gabion revetment.
4. Adequate tiebacks and backfill are ne- Rationale: Gabions are similar to
cessary. riprap in their use. The filter cloth
Rationale: A common cause of prevents leaching of fine grained
bulkhead failure is the lack of sub- material from behind the structure.
stantial system of tiebacks. Common The filter cloth beneath the structure
mistakes are: tiebacks too close helps prevent undue settling of the
to the bulkhead, spaced too far structure.
apart, tieback material is incapable
of resisting the stresses placed on it, GROINS AND JETTIES
and insufficient size cleadmen or
screw anchors. Groin systems vary greatly depending on wave
climate. The following comments concern an imper-
5. Weep holes may be necessary. meable, fixed height groin.
Rationale: In certain areas, ground
water collects behind a bulkhead. 7. Tongue and groove sheet pile should be
It then becomes necessary to release used.
this pressure by having small openings Rationale: As with a bulkhead,
through the bulkhead. These open- the tongue and groove helps pre-
ings are stuffed with wads of filter vent the leaching of material through
cloth to prevent leaching of fine the structure. In most areas a mini-
materials. They are usually placed mum of two-inch thick sheet pile is
above the mean high water mark. necessary.
6. Riprap at the toe of the bulkhead may 2. Round pile should be placed on alter-
be necessary. nating sides at an equal spacing.
Rationale: In certain high energy Rationale: This type of configuration
areas, the scour associated with provides good strength and inhibits
bulkhead jeopardizes the integrity undue flexing which can result if the
of the structure. To minimize this structure does not fill equally on
impact, an apron of riprap should each side.
be placed at the base of the bulkhead.
The approach also provides a re- 3. Ratio of sheet pile penetration to exposure
medial repair for a failed vertical should, as a minimum, be equal.
retaining structure. Rationale: Initially, the groin is a
free standing structure. Until it is
7. Return walls should be well tied into full, waves can vibrate a structure
the fastland. out of the bottom if the penetration
Rationale: On an open bulkhead, is inadequate. In addition, if the fill
waves can be focused where the is differential, i.e., one side only,

the weight of the sand and water 2. Sills should be used in areas with good
can topple the groin. sand supplied on the beaches.
Rationale: Insufficient supply of sand
4. The groins should be constructed in a to the beach can cause deleterious
sequential manner, effects to clowndrift shores.
Rationale: In most areas, there is
a net drift in a certain direction. 3. Although not a general rule, the sill is
Thus, the first groin should be con- usually most effective when placed at, or
structed on the downdrift end of the near the mean low water line.
project. Construction of the next Rationale: This position is usually
groin should begin when the first sufficient to insure adequate back-
groin fills completely. In this way the shore height. When the system fills
beach itself can determine the opti- however, placement too far offshore
mum spacing and length of the groins. generally results in failure.
While this sequential construction is
desirable, repeated contractors' mobi- BREA KWA TERS
lization costs may be prohibitive. Rubble mound (riprap) or gabion breakwaters
act to build up beach material and reduce wave
action against the shoreline during storms. In some
cases their benefits tend to outweigh their expense.

7. Like the riprap revetment, the stone
IT,
-rr,n should be of sufficient size to resist the
maximum expected wave forces.
Rationale: Wave forces during storms
will roll undersize stone off the slope
of the structure.

2. For gapped breakwaters, spacing and dis-
tance offshore must be engineered properly
considering all the design variables for a
given shoreline.
Rationale: Breakwaters placed too
far offshore may be ineffective in
. .... reducing wave forces. Too wide a
gap between breakwater units may
result in increased erosion on the
shoreline midway between the units.

ADVISORY SERVICES

In order to help the shoreline property owners in
Virginia, several agencies offer free advice on shore-
line problems. These agencies are:

-r Virginia Shoreline Erosion Advisory Service
"A" P
0. Box 1024
Gloucester Point, VA 23062
Figure 22 - Gabion revetment. (804) 693-3388

Virginia Institute of Marine Science
Gloucester Point, VA 23062
Attn: Mr. Scott Hardaway
SUBMERGED SILL (804) 642-2111, Ext. 280
These considerations refer primarily to the use of Soil Conservation Service
Warsaw, VA 22572
sandbags to form the sill. Gabions and mortar filled Attn: Mr. Blaine DuLaney
bags are also effective in perching a beach. (804) 333-6931
7. Grain size should be X4 millimeter or larger. Army Corps of Engineers
804 Front Street
Rationale: This size sand is necessary Norfolk, VA 23510
to prevent the bags from loosing Attn: Jim Melchor
material through their pores. (804) 625-8201, Ext. 271

-14

7:1

1! A

*46

AS IN,

xk'44
tths Lit

@A'

A6e- 4

VIP

T,-Aw9t 42M

Building a structure close to the Atlantic Ocean may result in severe damage during storm events. The
photograph on page 11 showf a Virginia coastal residence with pool in the Summer of 1978. The photograph
on this page was taken in October 1979 after a storm of moderate intensity.

. . . ...... ......
21

MA RSH GRASS

Propagation of various species of marsh grass may
be done to curb erosion along an eroding sediment
bank where no marsh exists or to enhance an existing
marsh shoreline. Table 2 lists some plant species
and their adaptability to tidal elevations, water and
soil salinities. Fertilization of new seeds, sprigs or
plugs is essential for good marsh grass growth and
development.
The question arises as to the viability of marsh
grass as an erosion buffer along a given shoreline.
Too much open water where wave forces can build
is not conducive to marsh grass growth. Figure 23
depicts a general rule which may be applied to site
suitability. This does not guarantee success, but
the low cost of the method to abate shoreline
erosion may warrant some experimentation.

RI VERBA NK SLOPE CONTROL

Exposed sediment banks with no vegetative
cover are unstable and susceptible to high rates of
erosion by rainwash, groundwater and wave action.
Getting some type of flora to grow on the surface
of the slope will do much to reduce the erosion.
However, it is often necessary to grade the bank
back to reduce the slope for the planting of grasses.
Table 3 lists some plants which have been used to
control riverbank erosion. Fertilization is almost
essential for a good and quick growth of ground
cover.
Getting vegetation established on a slope is some-
times not enough. The tow or base of the bank
should be stabilized to insure the rest of the graded
slope remains intact. In areas susceptible to high
storm wave conditions, this toe stabilization will be in
the form of a bulkhead or a riprap revetment. In
more protected areas, planning a marsh grass fringe
may be all that is needed.

SHORE FACES MORE THAN 5 MILES OF OPEN WATER
AND A IS 15 FEET OR MORE.

NORMAL HIGH TIDE

Figure 23 - Marsh site suitability
scheme.
SHORE FACES LESS THAN 5 MILES OF OPEN WATER
AND 8 IS 15 FEET MORE.
S 15 F@EET O@RMOR@E
A @NA @I

HIGH TIDE
tNORMAL
LOW TIDE 1i

TABLE 2: MARSH PLANTS AND THEIR ADAPTABILITY TO TIDES AND SALINITY

TZ"
Adaptability to Adaptability to Water
2.51, Tide Elevation Species and Soil Salinities

MT - MHW juncus roemerianus (Needlerush) brackish and freshwater

MT - MHW; sh Peltandra virginica (Arrow-arum) freshwater

MT - MHW; sh Pontederia cordata (Pickerelweed) freshwater
z"
AL
MT - MHW; sh Sagittaria latifolia (Duck Potato) freshwater . ....
MT - MHW Scirpus americanus (Common Threesquare) brackish and freshwater
MT - MHW Scirpus robustus (Saltmarsh Bulrush) brackish and freshwater
MT - MHW Spartina alterniflora (Cordgrass) salt and brackish water

i@3
MT - MHW; sh Typha angustifolia (Narrowleaf Cattail) brackish and freshwater

M
MHW; sh
T Typha latifolia (Broadleaf Cattail) freshwater
Al
above MHW Distichlis spicata (Saltgrass) salt and brackish water
above MHW Festuca elatior (Fescue, Kentucky 31 salt-tolerant
above MHW Panicum amarulum (Coastal Panicgrass) salt-tolerant
above MHW Panicum virgaturn (Switchgrass) salt-tolerant
above MHW Spartina cynosuroides (Big Cordgrass) brackish and freshwater
above MHW Spartina patens (Saltmarsh Hay) salt and brackish water

dune Ammophila breviligulata salt-tolerant
(American Beachgrass)

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

TABLE 3:
RIVERBANK EROSION CONTROL WITH PLANTS *River birch (Betula nigra)

by Norman T. Beal, Extension Agent Maple species
VPI & SU Extension Division
*Silver maple (Acer saccharinum)
Plants are the ultimate controllers of erosion on
soils of any slope. They vary widely in their ability *Boxedler (A. negundo)
to be successfully established on a bank, and to
thrive. They hold the soil by one principal means- *Fishbait tree (catalpa speciosa)
their roots. Following are categories of plants that
have been successfully utilized for erosion control. *Sweetgum (Liquidambar styraciflua)

GRASSES *Empress tree (Paulownia tomentosa)

*Ryegrass (Lolium species) *Elm species
*Bermudagrass t (Cynodon clactylon) tt American elm (Ulmus americana)
ffil *Lovegrass, (Eragrostis species) Chinese elm (U. pumila)
SHRUBS GROUNDCOVERS

Indian currant t (Symphoricarpus vulgaris) Bamboo t
Florida jasmine tt (Jasminum nucliflorum) dwarf bamboo (Sasa pygmaea) 18"
Trailing roses (Rosa wichuriana, R. max Graf.) golden bamboo (Phyllostachys aurea) 2S'
Sumac t sp. (Rhus glabra, R. typhina) cane reed (Arundinaria tecta 10'
*Lespedeza (Lespecleza sericea) Knotweed t
Forsythia tt (Forsythia suspensa, f.x "Arnold's Mexican bamboo (Polygonum cuspiclatum)
dwarf 8)
pq
Rose acacia t (Robinia hispida) English Ivy tt (Lonicera halliana)

TREES Halls honeysuckle (Lonicera halliana)
Willow species Daylily (Hemerocallis species) 2'
*black tf t (salix nigra)

0, weeping tf t (s. babylonica)
Established from seed
Poplar species
Spreads by rhizomes
Quaking aspen (Populus tremuloides)
tt Roots from tips or joints
Hybrid poplars ttt (p.x)
d
ttt Establish from hardwood cuttings early
Lombardy poplar ttt (p. nigra italica) spring before leaves appear.
1"d
V@ r 40-ilvii-i-
A

R
ISS
ai, ou'll""IrnmEl VIA11-11M

- - -- - - -- ... ....,. ... .... . .

PHOTO AND SKETCH REFERENCES

Figure 3. State of California - The Resources Agency, 1976, Shoreline
Protection in California, p. 22.

Figures 4, 5 and 9. Bellis, Vincent, Michael P. O'Connor and Stanley R.
Riggs, 1975, Estuarine Shoreline Erosion in the Albermarle-Pamlico
Region of North Carolina, pp. 27-43.

Figure 8. Marine Resources Commission, 1974, Wetland Guidelines, p. 32.

Figures 15B, 16B, 17B and 19B. U.S. Army Coastal Engineering Research
Center, 1978, Shore Protection Manual.

Figure 20. DeYoung, Bruce, Enhancing Wave Protection with Floating
Tire Breakwaters.

Figure 21 B. Anderson, Gary L., Robert J. Byrne, David W. Byrd and Gary
M. Chianakas, 1978, Demonstration Project in Low-Cost Shoreline
Erosion Control, p. 13.

Figure 23. Environmental Concern, St. Michaels, Maryland 21663.

Photographs: VIMS Remote Sensing Center, C. Alston photos. p. 11
and p. 20.

All other photographs VIMS/Sea Grant Shoreline Erosion Advisory Service,
C. S. Hardaway photos.

REFERENCES

Byrne, Robert and Gary Anderson. 1977. Shoreline Erosion in Tidewater
Virginia. Special Report in Applied Marine Science and Ocean Engi-
neering No. 111 of the Virginia Institute of Marine Science, Gloucester
Point, VA.

O'Connor, M. P., V. J. Bellis and S. R. Riggs. 1978. Relative Estuarine
Shoreline Erosion Potential in North Carolina, Department of Geology
and Botany. East Carolina University, Greenville, NC.

Riggs, S. R., V. J. Bellis and M. P. O'Connor 1978. Shoreline Erosion and
Accretion: A Process Response of the Shore Zone and Environment
of North Carolina. Department of Geology and Botany, East Carolina
University, Greenville, NC.

DATE DUE

The College of William& Mary

VIMS
SEA
GRANT
PROGRAM

3 6668 14107 1698