CETN IV-15
Rev. September 1999
shore-based observer, can be calculated by using the left- or right-directed waves,
respectively, with the methodology as outlined in CETN-II-19. Often the magnitudes of the
calculated net, left-, and right-directed potential longshore sand-transport rates do not agree
with accepted values for the site. However, the relative magnitude between the left- and
right-directed transport can be applied in a sediment budget with an accepted value for net
longshore sediment transport to adjust the magnitudes. Wave height, period, and direction
data allow construction of wave rays or orthogonals (Shore Protection Manual (SPM) 1984,
Chapter 2) as indicators of pathways of sediment transport.
Inlet flow speed and direction data as indicated by current meters or drogue movement give
the relative magnitude of sand-transport rates and pathways. For example, measurements of
the current from Ocean City Inlet, Maryland, indicated that the flow to the northern part of
the bay was considerably greater than that to the southern part (Dean and Perlin 1978). This
information can be adapted to proportion the relative magnitude of the bay-directed sand
transport to different parts of the bay.
The rate of relative sea-level rise may represent a contributing factor to the observed beach
change. The long-term beach loss ∆ysl because of an increase S in relative sea level is
(Bruun 1962, 1988; Komar 1998)
Lc
∆ysl =
S
(6)
B + Dc
for which Lc is the cross-shore distance from datum to the long-term depth of closure Dc .
Other types of coastal process data useful for formulation of a sediment budget include the
following:
River-flow speed, fluvial sediment grain size, and sediment availability as a possible
sediment source to the coastal environment.
Wind speed and direction, sediment grain size, and availability as a potential aeolian
sediment source to or a sink from the coastal environment.
Sediment characteristics (e.g., median size, size distribution, mineral content) as natural
tracers for sediment movement.
Step 9: Use Residuals to Balance Individual Cells. As presented in Equation 1, balanced
individual sediment budget cells and the macrobudget sum to zero all sources, sinks, and volume
changes associated with engineering activities. Inman (1991) considered recording an
unbalanced sediment budget cell (a cell with a nonzero residual in Equation 1) as a region
requiring more definition and investigation of the unknown processes. Knowledge of the
residual may also be useful to bracket the uncertainty range for the data sets (Kraus and Rosati
1999).
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