EM 1110-2-1100 (Part V)
31 Jul 2003
EXAMPLE PROBLEM V-3-1 (Concluded)
4. Protection of the bank toe depends on the performance of the beach berm during storm events.
A worst case scenario can be evaluated first by considering the adjusted beach slope and shoreline
area with storm wave conditions prior to reduction by the nearshore breakwaters. Using the
analytical model of Kriebel and Dean (1993) or a numerical, storm erosion model (e.g., SBEACH,
Larson and Kraus 1989) gives the results (for the worst-case, 50-year storm event scenario) that
the storm berm would remain with a width of 1.5-3 m. The actual erosion would be expected to
be significantly less due to reduction in the storm wave energy as a result of wave diffraction
through the gaps. The gap width, Lg=30 m was selected for design.
5. Wave energy is also transmitted over the top of nearshore breakwaters during elevated water level,
storm surge events. A wave transmission model (see Part VI-5-2) capable of predicting wave
energy over and through submerged, reef-type breakwaters was used for analysis with crest
heights of + 1.2 m, +1.5 m and 1.8 m above mlw datum. During the 50-year design storm, wave
heights immediately behind the breakwaters are reduced about 60 percent, 54 percent, and
46 percent, respectively for these three breakwater crest elevations. These transmitted waves then
propagate shoreward and are further dissipated by the beach salients. With the proposed beach
fill in place, a breakwater crest elevation of +1.2 m (mlw) is selected to limit the transmitted,
design wave height to about 1.2 m. This is the same, diffracted, design wave height opposite the
gaps and is dissipated by the storm berm.
Note: Further details of this example problem are found in Appendix A of Chasten et al. 1993 and was
developed by Mr. Ed Fulford of Andrews Miller and Assoc., Inc., Cambridge, Maryland. It is taken from
a real project on the Chesapeake Bay for the community of Bay Ridge near Annapolis, Maryland.
Construction was completed in July 1991 and postconstruction monitoring commenced soon after and at
a November 1991 survey. As of 2000, the project has performed as expected with subdued salients
forming behind each breakwater resulting in the overall stability of the shoreline. Numerous, significant
storms occurred and the project prevented erosion of the bank area to protect the roadway and sewer
pipeline. The project has been well-received by the residents of the community as a result of the stability
of the shoreline and the enhancement of the recreational beach area.
(1) Background and definitions. Groins are the oldest and most common shore-connected, beach
stabilization structure. They are probably the most misused and improperly designed of all coastal structures.
They are usually perpendicular or nearly at right angles to the shoreline and relatively short when compared
to navigation jetties at tidal inlets. As illustrated schematically in Figure V-3-23, for single and multiple
groins (groin field) the shoreline adjusts to the presence of the obstruction in longshore sediment transport.
Over the course of some time interval, accretion causes a positive increase in beach width updrift of the groin.
Conservation of sand mass therefore produces erosion and a decrease in beach width on the downdrift side
of the groin. The planform pattern of shoreline adjustment over 1 year is a good indicator of the direction
of the annual net longshore transport of sediment at that location.
(a) Groins are constructed to maintain a minimum, dry beach width for storm damage reduction
(Figure V-3-11) or to control the amount of sand moving alongshore. Previously stated purposes such as
trapping littoral drift are discouraged for this implies removal of sand from the system. Modern coastal
Shore Protection Projects