CETN IV-20
September 1999
GIVEN: A 1,000-m-wide inlet will be cut through the center of a barrier island that has length
∆x = 10 km. From analysis of shoreline-position data, the barrier island erosion rate has been
estimated as ∆y/∆t = 0.3 0.1 m/year/m for the past ∆t = 10 years (see Figure 2). The active
depth of transport for the barrier (the range over which the shoreline is assumed to translate
parallel to itself, defined as the absolute elevation from the berm crest to depth of closure) is
estimated as DA = 10 1 m. In the absence of other information, assume that uncertainty in
transport rates is 40 percent of the given value. Net longshore sediment transport is to the north
and is estimated as Qnet-N = 150,000 60,000 m3/year at the northern boundary of the barrier.
Based on deposition rates at a weir jetty south of the project area, the right-directed component
of the net transport (for a land-based observer) is estimated as QR-S = 200,000 80,000 m3/year
at the southern boundary. Because of the similarity in wave fetch and bathymetry, the right-
directed component of the net transport is assumed to be the same for the northern portion of the
barrier, QR-N = 200,000 80,000 m3/year. Beach fill F = 200,000 10,000 m3 was placed along
the entire barrier at the start of the 10-year period. Assume that the north and south boundaries
are sufficiently removed from the inlet's influence, and transport rates at these boundaries of the
barrier do not change after the inlet is cut.
Qnet-N = 150
QL-N = ?
N
QR-N = 200
∆y/∆t = - 0.3 m/yr/m
F = 200,000 m3
∆x = 10 km
1000 m
QL-S = ?
Qnet-S = ?
QR-S = 200
Note: Q in 1000s m3/yr
Figure 2. Example: pre-inlet data
FIND: (a) Formulate a pre-inlet sediment budget. (b) Calculate the adjacent beach shoreline
change immediately after sandtight jetties are constructed (initial impoundment). (c) Once
impoundment is complete, assume the inlet is a gross sink. Assume that 50 percent of the
material transported to the ebb shoal accretes on the shoal, and the remainder is passed to the
channel. Assume that 50 percent of this material shoals in the channel, and the remainder is
accreted within the flood shoal and bay. Calculate the volume change and net transport rates for
the adjacent barriers, ebb shoal, channel, and flood shoal/bay. (d) Determine the sediment
budget representative of a partial bypassing to the downdrift beach. Assume that 25 percent of
the material entering the ebb-tidal shoal is bypassed to the downdrift beach, 25 percent is
transported to the channel, and the remaining 50 percent deposits on the ebb-tidal shoal.
SOLUTION: The solution follows the recommended procedure for developing a sediment
budget. Refer to CETN-IV-15 (Revised) for additional discussion of the procedure. Also,
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