EM 1110-2-1100 (Part III)
30 Apr 02
and breaking waves (i.e., breakpoint bars and the shoreline). Overall, field measurements demonstrate great
variability in the shape of the longshore transport distribution profile across shore.
(6) Stresses exerted by waves vary in the cross-shore direction, generally decreasing from the breaker
zone to the shoreline, but not necessarily in a uniform manner due to the presence of bars and troughs on the
beach profile. The longshore current also has a characteristic profile, and because sand transport is a result
of the combined waves and currents, its distribution will be related to their distributions. Theoretical
relationships for the cross-shore distribution of longshore sediment transport have been postulated (e.g.,
Bagnold 1963, Komar 1977, Walton 1979, McDougal and Hudspeth 1981, Bailard and Inman 1981);
however, they have not been shown to reproduce field measurements well. Using data from their field and
laboratory experiments, Bodge and Dean tested five existing cross-shore distribution relationships, which
were concluded to give from fair to poor correlation with measurements. Bodge and Dean based on the work
of Bagnold (1963) also proposed a relationship for the cross-shore distribution of longshore sediment
transport which assumes that sediment is mobilized in proportion to the local rate of wave energy dissipation
per unit volume, and transported alongshore by an ongoing current
1 M
( E Cg ) VR
qx(y) ' kq
(III-2-22)
d Mx
where qx(y) is the local longshore transport per unit width offshore, y represents the cross-shore coordinate,
kq is a dimensional normalizing constant, d is the local water depth in the surf zone (including wave-induced
setup), E represents the local wave energy density, Cg is the local wave group celerity, and VR is the local
longshore current speed. Equation 2-22 can be expanded by assuming shallow water conditions, small angles
of wave incidence, and assuming a nonlinear value for Cg = (g(H+d))1/2
3
1
H
dH
H
d
2
ρg
(H%d)
VR
qx(y) ' kq
2
(III-2-23)
%
8
dy
2 (H%d) dy
H%d
in which H is the local wave height in the surf zone. Equation 2-23 represents conditions landward of the
breakpoint; seaward of the breakpoint, qx(y) = 0 under the assumption that no energy dissipation occurs. In
application of Bodge and Dean's relationship, the dimensional constant kq may be determined by integrating
the distribution qx(y) across the surf zone, and equating this quantity to the total longshore sand transport rate
QR.
(7) The model (solid lines in Figure III-2-21) was compared with field data, and predicted the general
trend of the measured transport distribution fairly well for one case (Figure III-2-21a), but shifted the cross-
shore distribution slightly shoreward relative to the measured data for the second case (Figure III-2-21b).
Comparison of the model with laboratory data indicated that the model generally overpredicted transport in
the mid-surf zone (especially for the plunging/collapsing and collapsing cases) and modeled the near-
shoreline transport distribution to a more reasonable degree than previous approaches.
g. Application of longshore sediment transport calculations.
(1) Littoral budgets.
(a) A littoral sediment budget reflects an application of the principle of continuity or conservation of
mass to coastal sediment. The time rate of change of sediment within a system is dependent upon the rate
at which material is brought into a control volume versus the rate at which sediment leaves the same volume.
The budget involves assessing the sedimentary contributions and losses and equating these to the net balance
of sediment in a coastal compartment. Any process that results in a net increase in sediment in a control
Longshore Sediment Transport
III-2-43
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