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I I I -19- 2. 6 miles downstream from the dam is the first building that would potentially be subject to flood damages if Sheriff Dam were to fail. However, as subsequently discussed, routing computations demonstrated that this structure would be well above the maximum water surface elevation resulting from dam failure plus base flood discharge during the fu 11 PMF. I I I I I I I I I I I I I I I I 3. 7.2 miles downstream from the dam is Knotts Ranch, the first resi- dence (and series of buildings) that would be adversely impacted by large flood flows and dam failure. To accurately characterize Trout Creek in the study reach, field surveys were conducted to establish channel cross section characteristics and first- floor building elevations at five locations, as well as characteristics of all road crossings of the creek. Figure 4 indicates the cross section locations. The study reach of the creek was photographed on a reach-by-reach basis for use in determining channel On" factors. Longitudinal channel slopes were derived from USGS topography. Flood Routing Results - The National Weather Service model DAMBRK was utilized to route dam breach flood flows downstream through the study reach. DAMBRK is considered the state-of-the-art dam failure routing model presently available. D. L. Fread, in his report entitled "DAMBRK: The NWS Dam Break Flood Forecasting Model," (NWS, July 1984), provides the following descrip- tion: . "The model consists of three functional parts, namely: (1) descrip- tion of the dam failure mode, i.e., the temporal and geometrical description of the breach; (2) computation of the time history (hydrograph) of the outflow through the breach as affected by the breach description, reservoir inflow, reservoir storage charac- teristics, spillway outflows, and downstream tailwater elevations; and (3) routing of the outflow hydrograph through the downstream valley in order to determine the changes in the hydrograph due to valley storage, frictional resistance, downstream bridges or dams, and to determine the resulting water surface elevations (stages) and flood-wave travel times." Dam breach parameters assumed in our evaluations were as follows: Crest elevation Final breach bottom elevation Base width of breach Side slopes of breach Time for total breach development Water elevation at initiation of breach 9730 9682 60 feet IH:IV 45 minutes 9731 Flood routing results downstream of the dam through the study reach indi- cate that the summer cottage, located at section 3 shown on Figure 4, is well above PMF flood levels including dam break discharge. Therefore, no signifi- cant impact would occur at this location in the event of a dam break during a flood. Conversely, the results indicate that during the full PMF, the failure