Hydrologic Issues for Dams - Association of State Dam Safety Officials

Hydrodynamic modelMIKE 21 BACKGROUNDThe version of Mike 21 used in this study simulates water level variations and flows inresponse to a variety of forcing functions in water bodies. The water levels and flows areresolved on a rectangular grid covering the area of interest when provided with thebathymetry, bed resistance coefficients, wind field, hydrographic boundary conditions,etc. The system solves the full time-dependent non-linear equations of continuity andconservation of momentum. The solution is obtained using an implicit ADI finitedifference scheme of second-order accuracy.Mike 21 parameters:The only changes to the modeling parameters used for all individual inundation modelswere to the Manning’s n values used for boundary resistance. Higher values were used incanyon areas than in the floodplains. A single value was used for the entire model. Noeffort was made to vary Manning’s n by land use. Each 30-meter grid cell wasrepresented by a single ele vation value and a single roughness coefficient. The damfailure hydrograph generated by HEC-1 had an initial steady flow and a constant baseflow added at the end of the hydrograph. Model stability problems occurred withoutadding these to the hydrograph.Table 2. Parameters used in Mike 21.ParameterValue usedEddy Viscosity 5 m 2 /sWind ConditionsNo windDry Depth0.04 mFlood Depth0.10 mManning’s n - C anyons 0.06 to 0.08Manning’s n - Floodplain 0. 04 to 0.05ResultsThe assumptions used produced con servatively large flows from the simulated damfailure; the Manning’s n-values were considered conservativelylow; and many of thedetailed features of the surface geom etry were not represented in the 30-m DEMs.It was concluded that the inundation maps developed using these data typically over-expected inundation areas that could occur as a result of a dam predict failure.A primary purpose of this study was to identify dams for which flooding due to damfailure could cause major risk to people and property. It is the dam owner’s responsibilityto submit inundation maps to CA OES for review and approval in accordance withguidance issued by CA OES. Based on the results of this study various dam owners weredirected to have dam failure inundation studies made.Paper 17 – DeVries 122