The longshore current is determined by solving the longshore momentum

equation under the assumption of alongshore uniformity. Kraus and Larson

(1991) discuss the governing equation employed in NMLong together with

underlying assumptions and limitations. In NMLong-CW, the same

differential equation as in the original model is solved to calculate the cross-

shore distribution of the longshore current, with the difference that wave

properties are described in a relative frame of reference. Another difference

compared to NMLong is the possibility of specifying an arbitrary current in

NMLong-CW that might be generated, for example, by tidal motion. In the

following, the governing equation is reviewed with emphasis on the changes

made. Reference is made to Kraus and Larson (1991) for more complete

discussion.

In NMLong-CW, after the wave transformation calculations described in

Chapter 3 have been performed, the longshore current is computed from the

alongshore momentum equation including lateral mixing, bottom friction,

and external forcing. The equation is,

1 *dS*xy

- *f*by =

- *R*w - *R*lc

ε*h*

(34)

ρ *dx*

where

originating from waves, tide, and external current)

ε = lateral mixing coefficient

directed alongshore

(e.g., tide), respectively.

The lateral mixing coefficient is parameterized as (Kraus and Larson 1991),

ε = Λ*Hu*m

(35)

41

Chapter 5 Longshore Current Model

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