ERDC/CHL CHETN-IV-34
June 2001
ends of the jetties is about 4 m, and the channel depth is 6 m. The grain size at this beach is
relatively fine, 0.17 mm, and has a fall speed of 0.02 m/s. The typical longshore current speed
under fair-weather wave conditions is estimated to be 0.2 m/s, a fairly strong current because of
the strong longshore component of the wind at this site.
Find: If it is assumed that the sediment plume can be observed half way through the water
column, is it possible that the downdrift portion of the plume consists primarily of 0.17-mm
sand?
Solution: Under typical wave conditions, waves cannot break in the channel, and resuspension
will be minor. Therefore, we assume the suspended sand originates from the surf zone and is
carried by the rip current along the updrift jetty to be captured again by the wind-generated
longshore current and transported past the jetties. We take ∆z =2 m to be the maximum elevation
in the water column that can be observed downdrift and assume some 0.17-mm particles are
present at the water surface at the updrift (starting) end of the plume. Then by Equation 6,
interpreting the distance W as the maximum length of travel,
ha U a
4 0.2
W=
∆z =
2 = 20 m
hc V f
4 0.02
In this calculation, we set the ratio ha/hc = 1 to maximize the distance. The fine sand suspended
to the water surface and carried by a 0.2-m/s current can only travel 20 m before dropping below
2 m in the water column. Therefore, it is concluded that the surface portion of the plume
observed in the photograph must consist of fine material such as clay that has small fall speed
and of organic material that floats. Because such plumes can appear after a storm, it may be that
the material consists of clay and organic particles washed into the ocean during the runoff caused
by the storm.
ADDITIONAL INFORMATION: This CHETN was written by Dr. Magnus Larson, University
of Lund, Sweden, and by Dr. Nicholas C. Kraus of the U.S. Army Research and Development
Center (ERDC), Coastal and Hydraulics Laboratory. The research was jointly supported by the
Coastal Inlets Research Program, Inlet Channels and Adjacent Shorelines Work Unit, and by the
Coastal Navigation and Sedimentation Program, Diagnostic Modeling System Work Unit.
Questions about this CHETN can be addressed to Dr. Nicholas C. Kraus (601-634-2016,
Fax 601-634-3080, e-mail: ).
This CHETN should be cited as follows:
Kraus, N. C., and Larson, M. (2001). "Estimation of suspended sediment trapping
ratio for channel infilling and bypassing," ERDC/CHL CHETN-IV-34, U.S. Army
Engineer Research and Development Center, Vicksburg, MS.
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