1 Introduction
Problem Statement and Objectives
The Numerical Model of the Longshore current (NMLong) (Kraus and
Larson 1991; Larson and Kraus 1991) was developed under the U.S. Army
Corps of Engineers' Dredging Research Program. The original model
calculates nearshore wave transformation, water level change, and wave-
induced longshore current across a single beach profile, under the assumption
of longshore uniformity in both the profile and hydrodynamic processes. The
original NMLong solves the wave energy flux conservation equation,
Carlo simulation technique to describe random waves. Wave energy
dissipation accompanying depth-limited breaking is described in accordance
with the model of Dally, Dean, and Dalrymple (1985). The cross-shore
momentum equation is solved numerically to obtain the water level change,
and the alongshore momentum equation yields the distribution of the
longshore current velocity across the shore. Wind-induced setup and
setdown are included, as well as longshore currents generated by local winds.
Nonlinear bottom friction is computed from a rapidly solved approximation,
and lateral mixing is modeled with an eddy viscosity that depends on the
local orbital velocity and wave height.
In considering the nearshore more generally, both tidal and wind-
generated currents can be comparable to or exceed the strength of the wave-
generated longshore current. Also, currents produced independently by
waves and by wind and tide may be in opposite directions, producing
complex distributions of the current across the shore. The capability of
representing the action of currents in NMLong and the interaction between
the current and waves resulted in a new model with the modified name
NMLong-CW, where CW stands for interaction between currents and waves.
Furthermore, this new model included an algorithm to simulate the
contribution of wave rollers generated by breaking waves.
This report documents the theoretical developments associated with the
enhancement of NMLong that have extended it to NMLong-CW. The new
model is validated by sensitivity tests and through comparison of calculations
to measurements.
Overview of Procedure
The theoretical as well as the numerical formulations employed in
NMLong (Kraus and Larson 1991) served as the starting point for the
development of NMLong-CW. However, wave transformation in NMLong-
CW is computed through the equation for conservation of wave action flux,
1
Chapter 1 Introduction