ERDC/CHL CHETN-IV-35
June 2001
Mathematical Model for Rapid Estimation
of Infilling and Sand Bypassing at Inlet
Entrance Channels
by Nicholas C. Kraus and Magnus Larson
describes a mathematical for rapid estimation of rates of infilling and bypassing at inlet entrance
channels located on sandy or gravel shores. Infilling is assumed to occur by cross-channel
transport. The calculation procedure requires information typically available or estimated in
coastal navigation projects and is intended to provide guidance for projects where detailed
studies cannot be performed. The procedure can be applied to any channel that meets the basic
assumptions.
BACKGROUND: Navigation channels issuing through an inlet entrance intercept sediment
moving alongshore. 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 (assumed to be predominantly sand or gravel) moving across a channel can reduce
channel width by accumulating on the updrift side or it can be deposited along the bottom of the
channel, reducing channel depth (Figure 1). After equilibration, the side slope of an entrance
channel dredged in open water on a sandy shore will typically range between 8 and 10 deg
(Buonaiuto, Kraus, and Bokuniewicz 2000), so that there is considerable vertical exaggeration in
Figure 1.
Sediment can pass over the channel by moving in suspension, and material deposited in the
channel can be resuspended and transported out. A channel traps sand arriving to it from either
side and, if the material remains within the channel, gives a measure of the gross longshore
transport rate along a coast. Sand entering a channel may be transported seaward during ebb-
tidal flows, and into the bay during flood-tidal cycles. For channels with riverine sources,
shoaling may also result by deposition of upland sediments. Bypassing (sand moving over,
through, or around the channel) can occur to either side as well, and the bypassing rate in the
predominant (net) direction to the downdrift beach is typically required in coastal inlet projects.
In Figure 1, hp is the authorized project depth, the minimum allowable channel depth. The
authorized project depth, authorized width at the bottom, and authorized side slopes define the
channel cross section. Allowable dredging tolerances for the bottom (called overdepth, typically
0.3 to 0.6 m (1 to 2 ft) and for the side slopes account for dredging inaccuracies and define the
allowable pay cross section of the channel.
Estimates are made to determine the increase in dredging and cost that will accompany channel
deepening and/or widening. The deepening and widening may be done under existing authority
or in response to a change in authorization. In addition, deepening and, sometimes, widening
may be done under a plan of advance maintenance dredging. In advance maintenance dredging,
a certain length of channel is deepened (and, possibly, widened) to reduce dredging frequency.
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