ERDC/CHL CHETN-IV-34
June 2001
Estimation of Suspended Sediment
Trapping Ratio for Channel Infilling
and Bypassing
by Magnus Larson and Nicholas C. Kraus
describes a method for estimating the percentage of sand-sized material placed in suspension by
breaking waves and carried by a cross-channel current to either fall into a navigation channel or
travel across it. Required inputs are channel width and depth, depth in the vicinity of the
channel, depth-averaged current velocity perpendicular to the channel, and sediment fall speed.
The procedure is applicable to inlet entrances that experience breaking waves. The percentage of
material deposited into or passing a channel is an input to the channel infilling model described
in Kraus and Larson (2001).
CALCULATION METHOD: Navigation channels issuing from an inlet intercept sediment that
is moving alongshore (Figure 1). The longshore transport may be generated by wave- and wind-
generated currents, and by the longshore component of the flood-tidal current entering the
channel. Sediment can pass over the channel by moving in suspension, and it can be deposited on
the channel bottom and, possibly, resuspended and transported out of the channel (Kraus and
Larson 2001). This Technical Note presents a method for estimating the percentage of
suspended material transported alongshore (perpendicular to a channel) that will be deposited in
a channel (the trapping ratio), from which the amount passing over the channel is determined.
The following assumptions are made:
a. Transport occurs mainly in the surf zone, for example, in Sections 2, 3, and 4 in Figure 1.
This assumption will be removed in later development of the model to describe deep-
draft channels and transport seaward of the surf zone.
b. Deposition in the channel is controlled by gravity through the sediment fall speed Vf.
Upward diffusion by turbulence and vertical currents in the channel is neglected; giving a
conservative estimate of infilling.
c. The flow across the channel can be described by a simple parameterization based upon
continuity (recirculation in the channel is neglected).
d. The velocity is uniform through the water column.
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