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 C*g ) *V*R

(III-2-22)

where *q*x(y) is the local longshore transport per unit width offshore, *y *represents the cross-shore coordinate,

setup), *E *represents the local wave energy density, *C*g is the local wave group celerity, and *V*R 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 *C*g = (*g*(*H*+*d*))1/2

3

1

2

ρ*g*

(*H*%*d*)

2

(III-2-23)

%

8

2 (*H*%*d*) *dy*

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, *q*x(y) = 0 under the assumption that no energy dissipation occurs. In

application of Bodge and Dean's relationship, the dimensional constant *k*q may be determined by integrating

the distribution *q*x(y) across the surf zone, and equating this quantity to the total longshore sand transport rate

(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.

(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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