EM 1110-2-1100 (Part III)
30 Apr 02
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 H1 (WISTRT requires the significant wave height Hsig), 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 KSPM 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.
d. Deviation from potential longshore sediment transport rates.
(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
To
1
To m R
(III-2-18)
QRNET / Q '
Q (t) dt
R
o
III-2-26
Longshore Sediment Transport
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