TM 5-850-1
Reduction of wave height through the entrance may be
site, the depth of water, the condition of the sea bottom, its
predicted more accurately by using diffraction and
function in the harbor, and the equipment suitable and
refraction diagrams. For the construction of diffraction
available for its construction.
and refraction diagrams, refer to Shore Protection
3-3. Jetties.
Manual, Vol. I (app A).
3-2. Breakwaters.
a. Locations and alinement.
a. Function. Harbor jetties function to prevent the
A breakwater is
normally the most costly single item required for the
movement of littoral drift into the entrance channel and are
harbor development.
Special care is required to
required in the case of natural harbors located in estuaries,
minimize the length and height. The following factors in
in rivers, in lagoons, or other areas where sandbars or other
the location and alinement of breakwaters shall be
offshore accumulations or silt and debris must be cut
considered:
through for navigation channels; to ensure the required
(1) Minimum height. Locate the breakwater in
water depth.
the shallowest water consistent with harbor area
requirements.
location and alinement of jetties are as follows: (1) Number
(2) Flank protection. Join headlands and rock
required. Use two jetties where feasible. Where funds are
limited or other restrictions apply, one jetty in the updrift
outcrops to natural abutments on shore to prevent
side may be used.
flanking.
(2) Length. Jetties shall line the entrance channel
(3) Foundation conditions. Where there is a
through the offshore bar and extend a sufficient distance
choice, locate the breakwater along hard or sandy
past to reach the required water depth with allowance for
bottom, avoiding locations with poor foundation
assumed silt accumulation.
conditions.
(3) Cleansing velocity. Current flow through the
(4) Channeling incident waves. Aline so that
entrance channel shall be adequate to scour and remove
the refraction and reflection of incident waves are away
silt accumulations.
Where the natural current is
from the entrance and toward the shore.
(5) Sheltering entrance.
inadequate, use offshore jetties to restrict the channel to
Stagger the
reach the cleansing velocity.
breakwater head on the weather side of the entrance
(4) Silting. Where possible, orient the jetties
with respect to the breakwater head on the lee side (fig
perpendicular to the clittoral drift.
(5) Anchorage. Anchor the jetty on shore to
being blown against the lee breakwater head. This
configuration improves the entrance conditions and
prevent flanking.
c. Form. The forms of jetties are as follows: (1)
increases sheltering in the harbor.
(6) Alinement. Avoid a concave shape or one
Parallel jetties. Use this form where the harbor entrance is
with reentrant angles, as entrapped waves will cause
not the mouth of a river having a pronounced flow or where
major disturbance in such areas.
the configuration of the existing, natural estuary indicates a
(7). Seiche. If the occurrence of a seiche
prolongation in the form of parallel walls.
(2) Divergent jetties.
Use this form in tidal
appears likely, realine the breakwater to reshape the
harbor basin or provide structures for dissipating wave
estuaries or in lagoon inlets where ebb and flood flows are
energy.
about equal. Under this circumstance, there is a tendency
for a parallel channel to silt up due to the reduction in
b. Types.
There are two main types of
volume of the influence tide. The slope of the divergence
shall be limited to about 2,000 feet per mile so that a bore
breakwaters, the mound type and the wall type.
(1) Mound-type breakwaters. These types of
will not be created.
(3) Convergent jetties. Use this form in lieu of
breakwaters are generally constructed from natural rock,
parallel jetties where the attenuation of waves incident on
concrete block, a combination of rock and concrete
the harbor entrance must be promoted by lateral
block, or concrete shapes such as tetrapods,
expansion; that is, where the run-in is not adequate or
quadripods, hexapods, tribars, modified cubes, and
where the wave traps are insufficient or undesirable.
dolosse. The mound type may also be supplemented in
each case by concrete monoliths or seawalls to break
3-4. Anchorage basins.
the force of the waves and to prevent splash and spray
from passing over the top.
a. Location and size.
(2) Wall-type breakwaters. These breakwaters
The factors affecting the
can be classified as concrete-block gravity walls,
location and size of anchorage basins are as follows: (1)
concrete caissons, rock-filled sheet-pile cells, rock-filled
Isolation. Locate the anchorage basins near the entrance,
timber cribs, or concrete or steel sheet-pile walls. The
away from channels, out of traffic, and in
type of breakwater to be used is usually determined by
the availability of materials at or near the
3-2
Previous Page
