These three classes of engineering actions induce different magnitudes and time scales
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.
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