ERDC/CHL CHETN-IV-35
June 2001
the channel depth). Bypassing of the channel is represented by suspended load passing over the
channel and by resuspension and transport of material that has been deposited in the channel.
Assumptions underlying the model are as follows:
a. Infilling by bed load can create a shoal at the edge of the channel and thereby constrict
the channel (bank encroachment). The encroachment decreases the width of the channel.
b. Sediment can be deposited directly into the channel.
c. The slope of the channel remains constant. (After dredging, in particular, slumping may
occur to achieve the angle of repose, and this process is neglected.)
d. The channel does not erode on the downdrift side.
e. Channel slopes are sufficiently mild that flow separation and secondary circulation do not
occur or can be neglected.
Sediment transport along the channel, as by tidal action or a river current, is negligible.
f.
(Transport by ebb and flood currents along the channel will be introduced in a future
version of the model.)
g. The cross-channel (longshore) transport is predominantly unidirectional.
(This
assumption can be eliminated in numerical solution of the model.)
h. Material that is deposited in the channel can be resuspended and leave the channel, and
the rate of resuspension is proportional to the depth in the channel and the rate of
deposition.
Figure 2 illustrates the conceptual framework of the model for the situation of transport directed
to the right, assumed to be the dominant direction of transport. A general version of the model
can treat both left- and right-directed transport. Immediately after dredging, the channel has
width W0 and depth h0. The ambient or natural depth in the vicinity of the channel is ha. As
sediment is transported to the channel, it can become narrower by filling from the side and
shallower by filling from the bottom. The coordinate z measures elevation from the bottom of
the dredged channel. It is convenient to work with elevation from the dredged bottom rather
than depth; conversion to depth below the navigation datum can then be made through
knowledge of z, h0, and hp.
If the channel becomes narrower because of growth of the updrift side by bed-load transport and
deposition into the channel, the width of the channel at a given time is
W ( x, t ) = W0 - x(t ),
for x < W0
(1)
The transport rate qR per unit length of channel near the updrift side of the channel can be divided
into the bed-load transport rate qbR, the rate qdR of suspended material deposited into the channel,
and the rate qsR of suspended material passing over the channel from the right. For that portion
of channel crossing the surf zone, the transport rate per unit length at the channel can be
estimated as the total transport rate Q multiplied by the ratio of length of channel exposed to the
longshore transport to the total width of the surf zone.
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