ASSUMPTIONS
Channel infilling can occur through an arbitrary combination of bedload transport,
which decreases channel width; and through suspended load transport, which decreases
channel depth. Bypassing can be represented by suspended load passing over the channel
and by re-suspension and transport of material that has been deposited in the channel. Such
processes are depicted schematically in Fig. 2. Assumptions underlying the model are:
1. Infilling by bedload can create a shoal at the edge of the channel and thereby
constrict the channel (bank encroachment). Encroachment decreases channel width.
2. Sediment can be deposited directly into the channel.
3. The slope of the channel remains constant. (After dredging, slumping may occur to
achieve the angle of repose, and this process is neglected.)
4. The channel does not erode on the down-drift side. (This assumption will be
removed in a future version of the model.)
5. Channel slopes are sufficiently mild that flow separation and secondary circulation
(which can cause sediment near the bed to move against the upstream flow direction) do
not occur or can be neglected.
6. Sediment transport along the channel, as by tidal action or a river current, is
negligible or has constant along-channel gradient at the cross-section of interest.
version of the model.)
7. Cross-channel (longshore) transport is predominantly unidirectional.
(This
assumption is easily eliminated in numerical solution of the model.)
8. 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 at that time 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 numerical
version of the model can readily 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, the project depth.
If the channel becomes narrower because of growth of the updrift side by bedload
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)
Kraus and Larson
3
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