FLOOD09983 - Laserfiche WebLink

Home Browse Search

4.1 III. MODEL APPLICATION 4.1 General The MUDFlOW model has been designed to permit a threE!-phase approach t.o simulating hyperconcentrated sediment flO\~s on alluvial fans. The first phase is to model the rainfall/runoff from the upstream watershed. The excess rainfall is routed overland from watershed and to the main watershed channel. In the second phase, sediment is added to the water as it enters the channel and the hypercon- cE!ntrated sediment flow is routed down the ~latE!rshed channel to the fan apex. This often may require routing of the mud flow through a steep watershed canyon to where it debouches onto the alluvial fan. An outfllow hydrograph from the watershed is created defining the discharge and sediment concentration at each time step. The-tJ outflow hydrograph from the watershed can be routed over the alluvial fan using the existing channel if one exists, permitt"ing the channe"1 to overflow onto the fan or, assuming the flow is completely unconfined and free to chose its own path over the fan. To perform a simulation with the model, the following procedure is suggested: 1. Field reconnaissance: Confirm the type of flow event expected, such as mud flow, mud flood, granular debris flow, or clear,w'ater flow. Identify and characterize potenti al sediment sources. Choose a representative sedi ment sample for estimation of viscosity and yield stress parameters from Table B.1 in Appendix B. 2. Using topographic maps, layout a grid system to enclose the possible flow area. A compass system should be establ ished. It is suggested that the southerly direction be designated as the flow direction and the east direction be placed on the left. The grid system will then be oriented so the USE,r views the gri d system in the direct i on of fl ow. 3. Identify grid elevations, roughness val ues, and other parameters, create a data file for the grid system, 'I. Identify and defi ne any channels on the all uvi a 1 fan. 5. Conduct a hydrologic analysis of the watershed to determ"ine excess rainfall. 6. Route the excess rai nfa 11 to the hn apex us i ng the MUDFLOW mode I or other runoff simulation method. 7. Use the MUDFlOW model to route the n ows over the all uvi a 1 fan. 8. Ca 1 i brate model results as requi red by vari at i on of roughness val ues and sediment concentrations.