CETN IV-15
Rev. September 1999
commonly required to represent periods of engineering and geomorphic significance, from 3-
5 years (dredging cycle at inlets) to 30-50 years (project life span; time scale for cyclic ebb-shoal
welding). Data sets reflecting the longer durations are required to develop the sediment budget
for spatial scales reflecting a regional approach. However, seasonal and year-to-year variability
should be considered and can contribute to the uncertainty in a sediment budget or form the basis
for a sensitivity analysis.
for each sediment budget calculation and denotes the existence of a complete self-contained
sediment budget within its boundaries (Dolan et al. 1987). Bowen and Inman (1966) introduced
the concept of littoral cells (Inman and Frautschy 1966) within a sediment budget. The southern
California coast lends itself to this concept, with evident sources (river influx, sea-cliff erosion),
sinks (submarine canyons), and coastal geology (rocky headlands) defining semicontained
littoral cells and subcells (Komar 1996, 1998). A littoral cell can also be defined to represent a
region bounded by assumed or better known transport conditions or natural and engineered
features such as the average location of a nodal region (zone in which Qnet ~ 0) in net longshore
transport direction or a long jetty.
Step 5: Consider Net and Gross Transport Rates. For cells of the regional budget that may
capture a portion of the left- or right-directed transport, both of these components must enter in
the formulation. Examples include submarine canyons and inlet channels that capture both left-
and right-directed transport; inlet weirs that may trap a portion of the left- or right-transport rate;
and initial beach response at a long groin or headland feature, which may indicate accretion
associated with left- and right-directed transport. Caldwell (1966) considered the gross transport
rate as a potential indicator of shoaling for inlets in the vicinity of Cape May, New Jersey.
Jarrett (1977, 1991) balanced potential longshore energy flux calculations with measured beach
and tidal inlet change to solve for net and gross rates of longshore sand transport. Bodge (1993)
focused on the inlet and its adjacent beaches and emphasized the importance of considering the
gross components of longshore sediment transport, especially for inlets that act as sediment
sinks. The gross transport rate can also provide an upper limit for the net, left-, and right-
directed rates (Shore Protection Manual 1984).
Step 6: Assign Values and Uncertainties. Known, estimated, or easily obtained values and
their associated uncertainties are assigned to source, sinks, and engineering activities within the
sediment budget. This step should represent a low level of effort to assess quickly the integrity
of the macrobudget (discussed below) and to uncover problems before detailed analysis begins.
Detailed data analysis is discussed in Step 8 (below).
Every measurement has limitations in accuracy and contains a certain error. For coastal and inlet
processes, typically direct measurement of many quantities cannot be made, such as the long-
term longshore sand transport rate or the amount of material bypassing a jetty. Values of such
quantities are obtained with predictive formulas or through estimates based on experience and
judgment, which integrate over the system. Therefore, measured or estimated values entering a
sediment budget can be considered as consisting of a best estimate and uncertainty. Uncertainty,
in turn, consists of error and true uncertainty. A general source of error is limitation in the
measurement process or instrument. True uncertainty is the error contributed by unknowns that
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