Response of Adjacent Beaches

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of morphologic change. Although specific morphologic response and navigability

conditions depend on the waves, tide, wind, bay area, channel depth, distance between

jetties, length of jetties, amount of dredging, location of dredged material placement,

and other factors, certain general morphologic responses will occur. Principal

morphology and navigation issues relating to such engineering actions are:

1. Response of adjacent beaches.

2. Formation of ebb shoal and flood shoal.

3. Interruption of natural bypassing.

4. Inlet stability.

5. Navigation reliability.

6. Response of bay.

7. Ebb shoal collapse.

8. Cost of construction and maintenance dredging.

For each of these, one must consider the time scale, magnitude, and spatial extent.

Development of inlet and regional sediment budgets is a first step in inlet functional

design (e.g., Kana and Mason 1988; Rosati and Kraus 1999, 2001; Byrnes et al. 2003;

Rosati 2005). A carefully formulated sediment budget provides information on the

balance of net and gross longshore transport rates for different physical extents of the

coast, long-term trends in shoreline change, and sediment sources and sinks. It also

identifies gaps in needed information for assessing the present and future morphology.

1. Response of Adjacent Beaches

If an inlet experiences a dominant direction of longshore sediment transport, the typical

response of the adjacent beaches is up-drift accretion and down-drift erosion. If the inlet

is in a nodal region of longshore transport such that the long-term net rate is zero,

shoreline response as accretion on both sides can result from jetty construction.

However, as opposed to the situation where jetties interrupt appreciable longshore

sediment transport (large net transport rate), an equilibrium shoreline configuration at

nodal points may be reached within relatively few years, as found by Komar et al.

(1976) for Pacific northwest coast inlets. Porous jetties can cause erosion of the up-drift

beach by allowing sediment to leak through to the inlet channel, increasing dredging

maintenance, and this loss is especially deleterious to the down-drift beach. Sand

tightening of porous jetties near to shore can provide an immediate beach-growth

enhancement (Creed et al. 1994).

Jetty construction at an existing inlet will confine the ebb-tidal current and push the ebb

shoal offshore from its original location. Flanks of the ebb shoal not located in the ebb-

tidal jet may migrate onshore and give the appearance of accretion by longshore

transport on the down-drift side of the inlet, until the abandoned portions of the ebb

shoal disappears, removing this sand source. The down-drift and, possibly, up-drift