geometrical idealizations of the modeled area are not difficult to justify 
since the majority of undisturbed beaches (i.e., beaches without jetties, 
detached breakwaters, or inlets in the immediate vicinity) have well-defined 
features which can be easily idealized. Beaches which are heavily influenced 
by natural or artificial boundary conditions cannot be modeled by the present 
or any other one-dimensional erosion model. Complex cases such as these must 
be modeled by computationally expensive two-and three-dimensional numerical 
models which more completely represent the physics of the flow regime. These 
models are not, with available techniques, economically feasible for studies 
such as this in which multiple simulations of random storm events are re- 
quired; additionally, the dune and fluid interaction has not yet been well 
represented in such models. 
Governing equations 
135. The governing equations of the model can now be presented. The 
most important concept of the model, and one which separates it from most 
other beach erosion models, is the specification of a sediment transport rela- 
tionship which states that the cross-shore sediment transport rate in the surf 
zone, Q, , is a function of the dissipation of wave energy. This is written 
as 
Q, = k (D - Day) (7) 
q 
where k is an empirical coefficient which was determined by Moore (1982) to 
have a relatively constant value of 2.2 x 107° mtn (0.001144 ft/lb). The 
dissipation term, D , is given by 
dF 
dz (8) 
where F represents the energy flux evaluated with linear wave theory. The 
dissipation D can be reduced to the following simple form: 
1/2 dh 
D = const h an (9) 
in which "const" is the product of known constant factors. The term Deg 
is defined as the equilibrium dissipation resulting when the offshore pro- 
file is in equilibrium according to Equation 6. It has been found that the 
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