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V. TWO-DIMENSIONAL FLOOD SIMULATIO!1 ON ALLUVIAL FANS 5.1 Advantages of Two-Dimensional Modeling Unconfined flooding over an alluvial fan should be analyzed as a two-dimensional flow problem. Some of the advantages of two- dimensional, unsteady flow simulation with a finite difference routing algorithm are: * Flows can be simulated over roughness. Split flow, shallow channels can be modeled. complex topography and flow and flow in multiple * Flood wave attenuation can be analyzed while routing the flood hydrograph. * Channel, street and overland flow and the interflow between them can be simulated. * The effects of flow obstruct:ions such as buildings, walls or levees which limit storage or constrict flevl paths can be modeled. This chapter presents routing model, FLO-2D. presented, followed by a an overview of the two-dimensional flood A brief description of the model is discussion of the theory. 5.2 Description of the FLO-2D Model The multifaceted computer model" FLO-2D, was designed to route water floods or mudflows in a channel or over an unconfined floodplain. The model uses a central difference, finite difference routing scheme (an explicit numericcll technique) and uniform grid elements for the application of the continuity and momentum equations. The important attribut<~s of the F1,O-2D model are: * Overland flow on unconfine!d surfaces is modeled in two- dimensions. * Channel flows are routed l,dth a variable geometry cross section to represent river reaches. * The flow regime can vary between subcritical and supercritical. * Return flow to the channel from t,he floodplain is simulated. * Viscous mudflows can be simulated. * Buildings and obstructions are simulated by assigning area and width reduction factors to the grid elements. 16