C150094 Red Mesa Ward Hydrology Study

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Red Mesa Ward Site Hydrology TABLE 1 Summary Of PMP Estimates Storm Type Duration (Hours) Total Precipitation (Inches) General 6 5.6 General 12 7.7 General 18 9.2 General 24 10.3 General 48 13.5 General 72 14.9 Local 6 8.2 Appendix A contains the detailed calculation sheets with relevant figures and tables taken from HMR No. 49. The local storm PMP computation yields hourly incremental precipitation values throughout the duration storm. The most intense precipitation occurs during the third hour of the storm. For this hour, the procedure breaks down the rainfall into 15-minute increments. For input to the hydrologic model (HEC-1), it was necessary to break up the entire event into 15-minute increments. Appendix A contains the hyetograph showing how the data was entered into the HEC-1 model.' It is assumed that all precipitation occurs as rain (as opposed to snow or hail) and is immediately available as runoff. Unit Hydrographs Spreadsheets were used to develop dimensionless unit hydrographs for each subbasin. The unit hydrographs were developed in accordance with the guidance provided in Chapter 4 of the Flood Hydrology Manual, a USBR publication. Following is a description of the variables and calculations employed in determining the unit hydrograph for individual basins. Appendix B contains pertinent excerpts from Chapter 4 of the Flood Hydrology Manual. 1. Drainage Area. Subbasin boundaries, shown on Figure 1, were imported to AutoCAD where areas were queried electronically. 1 Wright Water Engineers used a program called Graphical HEC-1,which provides a Windows interface for HEC-I. 991-077.070 Wright Water Engineers, inc. Page 2 September 2002