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I I I I I I I I I I I I I I I I I I I AYRES l\SS~:::;~l\~:r:'7:S an institutional issue first, so it can be appropriately handled in the hydrologic analysis. This means that a storage level frequency analysis for Gross Reservoir may be needed. . Another potential major impact on rainfall-runoff modeling is the spatial distribution of rainfall. It has been widely known that large rainfall amounts only very rarely occur over large (greater than 50 square miles) contiguous areas over the mountains and along the foothills due to the orographic impact of the mountains. It is much more common for there to be distinct isohyetal pattern to flood producing storms. We found this to be the case for the master plan hydrologic study Mr. Laiho completed for Fountain Creek, which is positioned very similarly to South Boulder Creek with respect to the mountains. Unfortunately, the rainfall patterns along the Front Range do not follow a standard spatial distribution, again due to the differences in the mountain topography. We suggest that hydrometeorologic simulation of storm patterns and transposed scenario storms be used to help determine rainfall patterns. This will then be used to complete a sensitivity analysis on the runoff produced from a varying rainfall pattern having different storm orientations. . We do not believe that evaluating storm movement by itself is particularly meaningful due to the lack of adequate data. . We agree with the manner the railroad grades were modeled by Taggart with respect to inadvertent detention and that this may be part of the reason the rainfall-runoff model produces higher values than the statistical analysis of peak flood flow rates. Rainfall-runoff modeling of the sub-basins impacted by the railroad grades may be the best way to adjust (back out) gage records for this impact. . High altitude infiltration should be re-evaluated as our experience (in the North Boulder Creek watershed) indicates that a surprising amount of runoff becomes interflow due to rock fractures and rubble, reducing the direct runoff peak. This is due to a higher infiltration rate than one would expect from what would appear to be bedrock. . The Task 2 Climatology evaluation will include a re-evaluation of NOAA Atlas rainfall information using updated sources, including the occurrence of large rainfall amounts at high elevations. That information will be used to adjust model rainfall input if necessary. . We will conduct a sensitivity analysis on rainfall loss algorithms in the model and make appropriate adjustments, including the method for accounting for losses, if necessary. . Flooding in this basin is known to be a mixed event phenomenon of rainfall- runoff and snowmelt runoff (with rare exception, such as the 1938 flood). We do not believe that completing a snowfall accumulation and ablation model is necessary. However, some attention to snowmelt contribution to runoff should be included. This could be done based on adjusting hydrograph base flow to reflect the time of the year the associated rainfall event occurs and the appropriate probability of snowmelt runoff magnitude. It is important to note from a hydrometeorologic perspective that very extreme floods in this area (more infrequent that once every 50 years) are late summer and early fall intense rainstorms, rather than mixed events. 17