more complex conditions, such as time-varying wave conditions, large shoreline angles, variable longshore

wave height (perhaps due to diffraction), multiple structures, etc., numerical models can be used in many

instances.

(1) Overview.

(a) The numerical model GENESIS (Hanson 1987; Hanson and Kraus 1989; Gravens, Kraus, and

Hanson 1991) is an example of a one-line shoreline change model that is supported for use both on personal

computer and mainframe systems (see Cialone et al. (1992)) and has a companion system of support programs

(Gravens 1992). GENESIS has been applied to numerous coastal engineering projects, and it calculates

shoreline change due to spatial and temporal differences in longshore transport as produced by breaking

waves. GENESIS is used in conjunction with grid-based wave transformation models that develop values

of breaking wave height and angle for various representative wave periods and approach azimuths along the

coast. Shoreline change at grid cells along the coastline is computed in the time domain as a function of these

computed values of the breaking wave height and angle.

(b) As discussed by Hanson (1987) and Hanson and Kraus (1989), the empirical predictive formula for

the longshore sand transport rate used in GENESIS is

(III-2-42)

The nondimensional parameters *a*1 and *a*2 are given by

5

(III-2-43)

ρs

2

&1

(1 & *n*) (1.416)

16

ρ

and

7

(III-2-44)

ρs

2

&1

(1 & *n*) *m *(1.416)

8

ρ

where *K*1 and *K*2 are empirical coefficients, treated as calibration parameters, and *m *is the average bottom

slope from the shoreline to the depth of active longshore sand transport. The factors involving 1.416 are used

to convert the *K*1 and *K*2 coefficients from use with rms wave height to use with significant wave height

(which is the statistical wave height required by GENESIS). That is, Equation 2-43 is presented such that

coefficients *K*1 and *K*2 should be viewed as calibration parameters that are to be adjusted to match measured

positions of shoreline change (Hanson and Kraus 1989).

(c) The first term in Equation 2-42 corresponds to Equation 2-7, and accounts for longshore sand

transport produced by obliquely incident breaking waves. The second term in Equation 2-42 is used to

describe the effect of another generating mechanism for longshore sand transport, the longshore gradient in

breaking wave height. The contribution arising from the longshore gradient in wave height is usually much

III-2-80

Longshore Sediment Transport

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