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MOUNTAIN HOUSE NEIGHBORHOOD' 'PROJECT INITIAL STUDY Hydrology and Hydraulics Studies The project hydrologic engineers, Condor Earth Technologies(CET), completed extensive hydrologic and hydraulic modeling for the project. The hydrologic modeling was completed using the U.S. Army Corps of Engineers HEC-1 hydrologic model, a powerful and commonly-used hydrologic model. The hydrologic model parameters generally follow appropriate County and general modeling guidelines. The rainfall runoff analysis was completed using the National Resources Conservation Service(formerly the Soil Conservation Service [SCS])curve number method. Infiltration and runoff parameters are generally determined from the terrain and soil hydrologic groups. The SCS unit hydrograph was used to develop the design flow hydrographs. Output from the hydrologic model was used for the design of the conceptual master drainage features and the storm drainage system of the Neighborhood"F"facilities. The Curve Number(CN)and subbasin lag times have been estimated using standard methodologies for purposes of the study. The hydraulics of the existing creek channels were investigated. The U.S. Army Corps of Engineers HEC-RAS program was used. This program utilized discharge information from the hydrologic model to predict the behavior of the flow in the on-site creeks. The model predicts many variables but of particular interest in this project are average channel velocities and water surface elevations. Mountain House Creek was studied in detail, and modeled throughout its reach above and below Byron Road. The model inputs were viewed as appropriate by the Initial Study consultants and comply with standard practice. Master Storm Water Drainage Plan The hydrology and hydraulic models were used as the main tools to develop a Master Storm Water Drainage Plan (hereinafter referred to as the Master Drainage Plan)for the complete Mountain House New Community. The discussion below briefly summarizes the types of recommendations and improvements that would be needed for the complete development of the new community. Following this discussion is a more specific discussion concerning the portions of the Master Drainage Plan that would be constructed as part of the Neighborhood"F"project. The overall Master Drainage Plan can be divided into several portions: improvements to south and north sections of Mountain House Creek; improvements to off-site dams; improvements to the road crossings; internal storm drainage facilities; and water quality basins. The location and general descriptions of these features are shown in Figure 3. Mountain House Creek Improvements. The Creek improvements generally consist of removing or amending farm crossings and enlarging the channel cross section to carry the design 100-year flow. Most of the existing farm crossings in the southern portion of the Creek would be removed or amended to reduce flow constrictions. Portions of the channel would be enlarged to carry flood flows with freeboard. Downstream of Byron Road, the channel would be amended to include a diversion/by-pass channel and significant channel enlargement downstream to the Dredger Cut and Old River. Irrigation Dams 1 and 2. There are two irrigation dams west of the project site. Analysis of these dams demonstrated that they do not have the capacity to handle the ultimate design 100-year flow. Irrigation Dam 1 (upper dam)would be refitted to a new spillway to safely pass the 100-year flow. Irrigation Dam 2 (lower dam)functions adequately during minor to moderate flood flows. However, the 100-year flow would overtop the dam. No improvements to this dam are proposed. However, the hydrologic model assumes that this dam would fail and 100-year design flows would account for this failure. CVOBSWCVNnw.9PMr RNAL1Nn7A1S=Y-ruen.DDC(7/20/00) 92 V.J. HYDROLOGY AND WATER QUALITY