EM 1110-2-1100 (Part V)
31 Jul 2003
Rule 5
Begin with Xg/Yg=2-3 where Xg is the longshore spacing and Yg is the effective length of
the groin from its seaward tip to the design shoreline for beach fill at time of construction.
Rule 6
Use a modern, numerical simulation model (e.g., GENESIS) to estimate shoreline change
around single groins and groin fields.
Rule 7
Use a cross-shore, sediment transport model (e.g., SBEACH) to estimate the minimum,
dry beach width, Ymin during storm events.
Rule 8
Bypassing, structure permeability and the balance between net and gross longshore
transport rates are the three key factors in the functional design. Use the model simulation
to iterate a final design to meet the Ymin criterion.
Rule 9
Consider tapered ends, alternate planforms and cross sections to minimize impacts on
adjacent beaches.
Rule 10 Establish a field monitoring effort to determine if the project is successful and adjacent
beach impacts.
Rule 11 Establish a "trigger" mechanism for decisions to provide modification (or removal) if
adjacent beach impacts are found nonacceptable.
f.
Reefs, sills and wetlands.
(1) Background and definitions. Additional types of shore protection alternatives for both high and low
wave energy coasts function by reducing the wave energy striking the shoreline. Reefs are platforms of biotic
organisms built up to a strict elevation in relation to low tide. Natural reefs require high wave energy to
survive. Wetlands are coastal salt or freshwater marshes that are low-lying meadows of herbaceous plants
subject to periodic, water level inundations. Wetlands are fragile and only survive in low wave energy
environments. See IV-2-11 and IV-2-12 for further details on wetland and reef-type coastlines, respectively.
The word "sill" has evolved to take on two separate identities in coastal engineering. Both meanings imply
wave attenuation in the lee of the structure. A submerged, continuous, nearshore dike to hold sand moving
offshore from a nourished beach is one definition and also labeled a perched beach. Free-standing, low-
profile, continuous shoreline structures to permit establishment of a marsh fringe in the lee of the structure
are also called sills.
(2) Reefs.
(a) Natural types. Coral reefs are massive calcareous rock structures that slowly grow upward by
secretions from simple animals living on the rock surface. They exist throughout the Florida Keys, on both
Florida coasts, the Hawaiian Islands, and the U.S. Island territories and serve significantly lower the mean
wave energy striking the adjacent shore. Fringing reefs border a coast, barrier reefs lie offshore enclosing
a lagoon, and atolls encircle a lagoon. Under favorable growth conditions, coral reefs build upward to form
wide, broad platforms that are exposed at low tide. Thus they cause waves to break and to continue breaking
across the reef.
Oyster reefs can exist in much colder, brackish water conditions of lagoons, bays and estuaries. They
are found along both coasts and in the Gulf of Mexico. They are wave-resistant structures that can
biologically adapt to rising sea levels. Their maximum elevation is related to the minimum time of
inundation in the middle range of the intertidal zone.
V-3-78
Shore Protection Projects
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