directional band. In Example III-2-6, wave data are more accurately represented by calculating a

representative wave period for each of the given wave height bands for the given directional band. Each

example requires the transformation of offshore wave data to breaking conditions, and subsequent

computation of the associated longshore transport rate. The former can be accomplished using

Equations 2-14 and 2-16 or using the program WISTRT (Gravens 1989). Both require knowledge or input

of the offshore wave height *H*1 (WISTRT requires the significant wave height *H*sig), associated period *T*, angle

relative to the shoreline *α*, and water depth associated with wave data. The longshore transport rate can then

be computed directly by Equation 2-7b. The program WISTRT uses a *K *value and a breaker wave height-to-

depth ratio different than those used here. It also requires the percent occurrence associated with the given

wave condition. The ACES program "Longshore Sediment Transport" (Leenknecht, Szuwalski, and Sherlock

1992) also provides a method for calculation of potential longshore sediment transport rates under the action

of waves. Again, different constants K and κ are utilized than those presented here. Both WISTRT and the

ACES programs use individual wave events as input, rather than an extended time series of wave information.

A program for processing the WIS time series to obtain values of sediment transport is presented in Gravens,

Kraus, and Hanson (1991).

(d) Note that both Examples III-2-5 and III-2-6 employed the same wave data, but Example III-2-6

computed the transport for discrete wave height bands whereas Example III-2-5 computed the transport for

a single, band-averaged wave height. The transport computed in Example III-2-6 is more than double that

in Example III-2-5. This difference is due to the nonlinear dependence of the transport equation on breaking

wave height. If, for example, wave heights are Rayleigh distributed and the waves are all of uniform period,

the transport rate computed using the distribution of wave heights will be about 1.53 times larger than that

computed using only the band-averaged wave height.

(e) Bodge and Kraus (1991) and others (e.g., Kraus and Harikai 1983; Gravens, Scheffner, and Hubertz

1989; Gravens 1990a) have observed that use of the CERC formula (with the *K*SPM coefficient) and WIS

hindcast wave data have yielded potential longshore sand transport magnitudes that are two to five times

larger than values for the region as estimated from dredging records, bypassing rates, or volumetric change.

The longshore sand transport rate determined in Example III-2-6 represents the potential longshore transport

rate, which depends on an available supply of littoral material. Consideration of the availability of littoral

material; location, type, and condition of coastal structures; and sheltering specific to the project shoreline

may contribute to a lower actual longshore transport rate. It is recommended when using hindcast wave data

to predict potential longshore sand transport rates that other independent measures or estimates of longshore

transport be used to supplement the potential transport estimate.

(1) Temporal variations and persistence.

(a) Longshore sediment transport is a fluctuating quantity which can be depicted as shown in

Figure III-2-7 where positive sediment transport is defined as positive in value if toward the right for an

observer looking seaward from the beach, and negative in value if sediment transport is toward the left as

noted previously and consistent with notation utilized by Walton (1972), Walton and Dean (1973), Dean

(1987), and others. In terms of "QR," on Figure III-2-7 the net longshore sediment transport rate is the "time

average" transport given by

1

(III-2-18)

R

III-2-26

Longshore Sediment Transport

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