EM 1110-2-1100 (Part V)
31 Jul 2003
The primary function of nearshore breakwaters is to reduce the offshore sand transport during storms.
Hence, these structures help retain sand on nourished beaches for longer intervals. However,
overtopping can result in a net seaward flow of water and sand through the gaps between breakwater
segments during storm events. The breakwater can also reduce the onshore sediment movement
during normal, swell wave conditions that naturally rebuild the beach. The structure blocks the
return of sediment to the beach. Following breakwater construction, a new equilibrium between
onshore and offshore transport will be established.
(d) Minimum dry beach width, Ymin. Figure V-3-21 schematically displays the mhw shoreline for normal
wave conditions. During storms, some gap erosion, e will occur to impact the minimum, dry beach width,
Ymin required for shore protection.
A conservative estimate of the gap erosion, e, can be made using analytical models for the dynamic
response of natural beach profiles to storm effects (e.g., Kobayashi 1987; Kriebel and Dean 1993).
Part III-3-2. gives a general description of these methods and also example applications of the
Kriebel and Dean (1993) analytical model. The theory is for open coastal beaches so that wave
diffraction in the gap area and wave overtopping to increase the return flows through the gap region
are not considered, but tend to offset each other. An Example Problem is given as Part V-3-1.
Dynamic, numerical, cross-shore sediment transport models (e.g., SBEACH, Larson and Kraus 1989)
could also be applied in the gap area to estimate the erosion potential, e (see Part III-3-2). These two-
dimensional (vertical) models also do not consider wave diffraction and return flows in the gap area.
These results would be a worst case scenario and the actual erosion can be expected to be
significantly less. A general, three-dimensional, wave, current, and sediment transport model is
clearly needed in this area.
Hughes (1994) presents a complete discussion of scaling laws as applied to predicting cross-shore
sediment transport in physical, hydraulic models.
(4) Nontraditional designs. Most nearshore breakwaters built in the United States and foreign countries
for shore protection have been rubble-mound type structures. Availability of materials and construction
equipment have made construction costs relatively inexpensive.
Several patented, nontraditional devices have been tested in the United States. These have been
precast concrete units or sand-filled geotextile tubes and bags. If constructed to the same dimensions
as rubble-mound structures, they may produce similar functional performance. Their success (or
failure) has been a function of structural stability of the units during storm conditions and their
durability over an economic life. Their functional success (or failure) has also been dependent on
maintaining the design crest elevation for wave energy reduction. Proper attention to foundation
design to minimize settlement must be given for precast, concrete units.
Some nontraditional designs reduce the bottom footprint to minimize impacts on benthic organisms.
And, the costs for removal and/or adjustments to reduce downdrift impacts on adjacent shorelines
can be significantly less than for traditional, rubble-mound designs. The need to reduce impact on
the environment is increasing the necessity for further research and comprehensive field testing
programs for nontraditional designs. (See Part V-3-5 for further details.)
Shore Protection Projects
V-3-57
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