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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\GSOC Ets:,~'iES changes in the basin have altered the recorded flows during the period of the gage from what they would be naturally. These will be adjusted using standard procedures to achieve a proper flow frequency relationship. The emphasis will be on the validity of peak flows that occurred before the construction of Gross Reservoir. This effort will include visiting and inspecting the gage itself, checking its calibration, including the recording of flows higher than 600 cfs, and discussing the gage accuracy with the USGS. A hydrodynamic model of the Eldorado Springs gage could be developed if the gage data are accurate enough to support this activity and if a hydrodynamic analysis of this reach adds significant information to the hydrology task. The gage data would have to be sufficient for detailed reconstruction of event stage hydrographs for several events that exceed 600 cfs. If a preliminary analysis of the gage data indicates that this item is warranted, then a dynamic HEC-RAS model will be developed for this river reach for approximately 1 mile upstream and downstream of the gage. An inflow hydrograph will be estimated based on the existing stage-discharge rating curve and would be used as input to the model. The inflow hydrographs will be adjusted until the model stage hydrograph at the gage matched the recorded stage hydrograph. This procedure would be conducted for 50 percent of the suitable gaged hydrographs and a revised rating curve would be developed for the gage. The results would be validated using the remaining gage records. This analysis would require additional survey data for this reach. After adjusting recorded flow values, a statistical analysis on both annual and partial duration flow values will be completed for the two major basin gages (Rollinsville and Eldorado Springs). We do not believe that the data exist to separate mixed events into rainfall and snowmelt only for this evaluation or that this provides particularly meaningful results. Hydrology - Rainfall-Runoff Modeling Ayres Associates has reviewed the South Boulder Creek Interim Hydrology Study (Taggart 7/2000) with respect to the completion of a final hydrologic analysis that will be completed integrally with the spatially varied flow hydraulics. The hydrology needs to be fine-tuned and adjusted to provide proper spatial and flow information to the 2- dimensional hydraulic analysis. The final model will reflect future basin conditions. The basic model seems to be well thought out and configured to produce reasonable results. We would suggest using the same UDSWMM model, but are willing to use HEC-HMS if you prefer the use of a different event-based model. Regardless of the model used, we offer the following thoughts to be considered as part of making refinements to the hydrology: . We understand that there is some concern regarding the peak flows resulting from the rainfall-runoff model being significantly higher than the flows projected from the gaging station data. We believe that the major reason for the difference between the rainfall-runoff modeling results and the statistical evaluation of the gage data is the routing of the floods through Gross Reservoir. This reservoir has a significant and real impact on flood peaks even though it does not have officially allocated flood storage volume. This is, however, an institutional issue that is not only relevant to establishing base line conditions, but also to the future alternatives analysis. This really needs to be resolved as 16