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I I I I I I I I I I I I I I I I I I I AYRES ~SSOCQ~(;,=-.,S . We will review the decision on unit hydrograph use for appropriateness. However, we do not see this characteristic as subject to significant change. . As part of developing the Resource Atlas, we will conduct a thorough watershed site evaluation including conducting selected surveys of connecting channels to provide a more sound basis for definition of flood conveyance elements. We do not expect this to have a significant impact on the analysis, especially above the canyon mouth. . Hydrologic calibration will be done using the split sample technique where available data permits. The specific locations for this calibration will evolve from the inventory of existing information on both South Boulder Creek and its tributaries. This calibration may be done for streamflow hydrographs, high water marks, paleohydrologic information, or whatever reliable information can be made available. The new rainfall-runoff model will be done for the entire basin even though it does not produce all the discharge values we will be using through the spatially varied flow reaches. The final hydrology values actually evolve from a combined hydrologic- hydraulic model. The hydrologic model will provide a peak discharge at the canyon mouth (where the vast majority of the watershed has contributed its flow). Hydrologic calibration will be done primarily at the canyon mouth where a long-term gage exists. Flow at the canyon mouth is then routed downstream using 2-dimensional hydraulic techniques with hydrographs from side tributaries being added to the hydraulic model at the appropriate locations. As such, the side tributaries can be separately computed and hydrographs input by another technique, if you prefer, such as CUHP. Paleohydrology As previously mentioned Dr. Bob Jarrett will complete the paleohydrologic aspects of this project. In fact, his work is already complete, but is, as yet, unpublished. This work has been completed independently as an employee of the USGS as part of their activity as a "cooperating entity" providing technology transfer. Our staff geomorphologist, Bill Spitz, will work directly with Dr. Jarrett to coordinate the remaining paleohydrologic work and its documentation so it is effectively integrated into the hydrology. Ayres Associates has demonstrated the ability to work with the USGS on this type of technology transfer. This work is primarily for the purpose of: . Providing reasonable rare event discharge values on ungaged tributaries . Providing information on high outlier flood values on a discharge frequency relationship and gaining additional reliability for mid-range frequency floods (50- to SOO-year events) for which few data points and insufficient records exist to otherwise define . Providing a proper perspective or frame of reference on the frequency and spatial distribution of experienced floods, especially those familiar to the public 18