CETN IV-15
Rev. September 1999
describing the sediment budget of regions directly adjacent to the inlet are not unique (that is,
different formulations are possible).
Coastal engineering analyses involve two types of longshore transport rates. The net longshore
transport rate is defined as the difference between the right- and left-directed littoral transport
over a specified time interval for a seaward-facing observer,
Qnet = QR QL
(2)
in which the leftward-directed transport QL and rightward-directed transport QR are taken as
positive. The gross longshore transport rate is defined as the sum of the right- and left-directed
littoral transport over a specified time interval for a seaward-facing observer,
Qgross = QR + QL
(3)
An inlet channel has the potential to capture the left- and right-directed components of the gross
longshore transport of sediment, and the inlet system may bypass left- and right-directed
longshore transport. Thus, knowledge of the net and gross transport rates, as well as the
potential behavior of the inlet with respect to the transport pathways, may be required to
correctly represent transport conditions within the vicinity of inlets, as emphasized by Bodge
(1993).
STEPS IN FORMULATING A SEDIMENT BUDGET: In the following section, a
methodology for creating a sediment budget is outlined.
Step 1: Consider a Regional Approach. For accurate representation of a local project area,
especially in the vicinity of inlets, a sediment budget is formulated with its lateral boundaries
located well beyond the apparent (expected) local project boundaries. A regional sediment
budget might include one or more barrier islands, several inlets, headlands, and pocket beaches
to fully capture the past and potential future changes in the sediment transport.
In the United States, inlets have been stabilized by jetty construction for periods typically
ranging from decades to centuries. Such navigation projects have the potential to influence the
transport of sediment on the adjacent beaches for many kilometers, a distance that may extend
well beyond what is considered the direct area of the inlet. Thus, a regional sediment budget that
incorporates adjacent barrier islands, bay regions, underlying geology, estuarine and riverine
impacts, and perhaps several inlets, may be required to assess the impacts of past and future
projects. Engineering activities at navigable inlets and other data required for an inlet sediment
budget have a high degree of uncertainty. Examples of these data include dredging quantity,
location, and littoral quality; adjacent beach-fill volumes, initial cross-shore and longshore
adjustment, and littoral quality; and limited ebb- and (more commonly) flood-tidal shoal
bathymetric coverage. In addition, uncertainty is introduced by the need to assess the degree to
which structures block, reduce, and modify the sediment-transport pathways and magnitudes
(Kraus and Rosati 1998a, 1999).
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