A computer program for the two-dimensional diffusion hydrodynamic model which is based on the diffusion form of the St. Venant equations where gravity, friction, and pressure forces are assumed to dominate the flow equation will be discussed in this section.

The DHM model consists of a 1-D channel and 2-D flood plain models, and an interface sub-model. The one-dimensional channel element utilizes the following assumptions:

.......... (1) infinite vertical extensions on channel walls (figure 13),
(2) wetted perimeter is calculated as shown on figure 13a,
(3) volumes due to channel skew is ignored (figure 13b), and
(4) all overflow water is assigned to one grid element (figure 14).

The interface model calculates the excess amount of water either from the channel element or from the flood plain element. This excess water is redistributed to the flood plain element or the channel element according to the water surface elevation.

This FORTRAN program has the capabilities to simulate both one-and two-dimensional surface flow problems, such as the one-dimensional open channel flow and two-dimensional dam-break problems illustrated in the preceding pages. Engineering applications of the program will be presented in the next section.

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