TM 5-850-1
CHAPTER 5
COASTAL PROTECTION
5-1. Shoreline stabilization.
action. Revetments are the lightest because they are
a. Methods. The shoreline stabilization methods
designed to protect shorelines against erosion by currents
or light wave action.
can be generally classified as artificial nourishment and
b. Selection of basic stabilization method.
protective construction.
(1) Artificial nourishment.
(1) Artificial nourishment.
The artificial
(a) Advantage. The artificial supply benefits
nourishment methods include the following:
(a) Offshore dredging.
not only the shoreline where it is placed but other shores as
The material
well.
dredged elsewhere is deposited in a ridge offshore and
(b) Disadvantages. Temporary changes in
updrift of the beach to be stabilized.
(b) Stockpiling. A beach is placed updrift,
the shoreline due to individual storms are not prevented by
artificial nourishment. The method is not suitable for
but not offshore, of the denuded area from which
stabilization of areas abutting buildings, pavements, or
material is derived for replenishment of the downdrift
other structures. Also, the amount of supply must be
area.
(c) Continuous supply. A pumping plant
balanced against the amount of decretion. An oversupply
causes accretion, which may be detrimental; an inadequate
located on the updrift jetty at a harbor entrance
supply will be ineffective in producing the desired stability.
bypasses the sand across the inlet to the eroding shore.
(2) Protective construction.
(d) Direct placement. This method is a
(a) Advantages.
Protective structures
variation on the use of a stockpile in that the fill is
require little maintenance as compared to the continuing
completed at one time over the entire area to be
supply requirement involved in the use of artificial
protected.
(2) Protective construction.
nourishment. Furthermore, the results are more positive
The following
than with the use of artificial nourishment.
protective structures may be used for shoreline
(b) Disadvantage. In the highly developed
stabilization:
(a) Breakwaters. Breakwaters reduce the
areas, correction of localized deficiencies is not feasible. In
such areas, protective structures must be installed over an
wave force reaching the shore. Offshore breakwaters
extensive length of shoreline because their use in one
are more costly than onshore structures and are seldom
location tends to produce decretion in adjacent downdrift
built solely for shore protection, rather they are
areas.
constructed mainly for navigational purposes.
A
c. Layout and design for stabilization by artificial
breakwater protecting a harbor area provides shelter for
nourishment. After the selection of artificial nourishment as
all types of marine vessels.
(b) Jetties.
the method of shoreline stabilization, layout and design can
Jetties are generally
take into account the following factors: (1) Rate of loss of
employed at inlets in connection with navigation
beach material. The loss rate may be measured by one of
improvements.
They control sand movement and
the following methods: (a) Measure the quantity of littoral
shoaling in channels. Jetties are similar in structure
current and the solid content of the suspended sediments.
though larger than groins and sometimes extend from
Use the difference in quantity between the updrift and
the shoreline seaward to a depth equivalent to the
downdrift ends of the site.
channel depth desired for navigation purposes.
(c) Groins.
(b) Take beach-profiles over a period of time
Groin is a barrier-type
and determine the loss rate by section.
structure that extends from the backshore into the littoral
(c) Approximate the loss rate from aerial
zone. The basic purposes of a groin are to interrupt
photography or maps of changes in the shoreline. Use the
longshore sand movement, to accumulate sand on the
rule that a loss of 1 square foot of surface area represents a
shore, or to retard sand losses.
(d) Shoreline armoring.
loss of 1 cubic yard of beach material. This rule has been
Bulkheads,
found applicable for exposed seacoasts; for less exposed
seawalls, and revetments are wave-resistant walls used
shores, it results in a conservative approximation.
to armor the shore and provide a definite landwater
boundary at a given location. The distinction between
seawalls, bulkheads, and revetments is mainly a matter
of purpose. In general, seawalls are the most massive
of the three because they resist the full force of the
waves. Bulkheads are next in size; their function is to
retain fill, and they are generally not exposed to severe
wave
5-1
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