Each year, flood control projects and storm channel systems are constructed by Federal, State, county and city governmental agencies and also by private land developers, which accumulatively cost in the tens of billions of dollars. Additionally, floodplain insurance mapping, zoning, and insurance rates are continually being prepared or modified by the Federal Emergency Management Agency. Finally, the current state-of-the-art in flood system deficiency analysis often results in the costly reconstruction of existing flood control systems. All of these flood control or protection measures are based upon widely used analysis techniques, which commonly are not adequate to represent the true hydraulic/hydrologic response of the flood control system to the standardized design storm protection level. The main drawbacks in the currently available analysis techniques lie in the ability of the current models to represent unsteady backwater effects in channels and overland flow, unsteady overflow of channel systems due to constrictions, such as culverts, bridges, and so forth, unsteady flow of floodwater across watershed boundaries due to two-dimensional (horizontal plane) backwater and ponding flow effects.
In this report is developed a diffusion hydrodynamic model, which approximates all of the above hydraulic effects for channels, overland surfaces, and the interfacing of these two hydraulic systems to represent channel overflow and return flow. The overland flow effects are modeled by a two-dimensional unsteady flow hydraulic model

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