Coastal shoreline defense structures Self-taught education unit

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

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Ccoastal Shoreline Defense Structures

By Thomas Barnard

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

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

hardware
H

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

filter cloth

sheet pile anchor pile

0
10

NOTES fender pile

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

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

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

ile@ 19973
OWO_
P%�

Printed on recycled paper.

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

splash apron

armor rock

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

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

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

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

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

2
marsh peat surface

MLW ------

:D%
armor rock NOTE

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

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

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

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

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

P- A' A`

filter cloth NOTES

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

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

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

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

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

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

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

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

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

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

waves.

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

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

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

waves.

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

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

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

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

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

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

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

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

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

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

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

Sill Sill is a continuous low-profile breakwater structure.

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

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

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

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

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

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

Coastal Shoreline Defense Structures
Exam Questions

1. Define the following:
1. Shoreline defense structure

2. Sea level rise

3. Riprap

4. Deadman

5. Armor stone

6. Breakwater

7. Geotechnic material

8. Longshore drift

9. Jetty

10. Downdrift

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

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

Explain.

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

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

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

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

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

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

8. Discuss three different types of breakwaters.

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

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

11. What are the differences between groins and jetties?

12. What is vegetative control of erosion?

When is it likely to work and not work?

Explain your answer.

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

Explain, (You can ignore cost.)

Literature Cited

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

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

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

NOAA COASTAL SERVICES CTR LIBRARY

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