where

Λ = nondimensional coefficient

The velocity *V *constitutes the alongshore component of *U*, that is, *U *=

(*V*2+*U*c2)1/2, where *U*c is the mean cross-shore velocity. Lateral mixing as

described here for a depth-averaged model is an approximation to the more

complex process of vertical and horizontal transfer of momentum as derived

by Putrevu and Svendsen (1992).

The forcing associated with a local wind is given by:

ρa

(36)

ρ

where

by the WAMDI group; see Equation 29)

ρa = density of air

ϕ = wind direction (*W *and ϕ defined in the same way as for

the current; see Figure 1)

It is possible to specify an external current, assumed to be associated with

some large-scale circulation not resolved by NMLong-CW. To represent this

current in the model, a forcing is derived from,

(37)

component of the external current (*U*cs is the cross-shore component of this

current taken to be equal to total cross-shore current *U*c, that is, the cross-

shore current is specified and not calculated in NMLong-CW). If no waves

and wind are present, Equation 34 will produce the specified external current

distribution (compare Equations 34 and 37). To represent the roller, an extra

term should be added on the right side of Equation 34 according to

The quadratic bottom friction is calculated by means of a rapidly

evaluated square-wave approximation (Nishimura 1988; Kraus and Larson

1991),

(38)

where

42

Chapter 5 Longshore Current Model