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Inflow Design Flood Determination <br />• ~-~. <br />Although inflow floods will usually not be equivalent to the maximum <br />probable flood, it is recommended that the maximum probable flood be used i <br />i <br />in design of the subject structure, in order that potential changing <br />conditions downstream in the distant future will not require additional <br />changes in spillway sizing or flood routing from the subject structure. <br />In determining the maximum probable flood, probable maximum thunderstorm <br />data was considered, as well as probable maximum general storm data, and <br />snowmelt runoff. It is felt that the most critical flood would result <br />from a probable maximum thunderstorm occurring during a period of snowmelt, <br />at which time the surface soils would either be frozen or saturated. <br />• Detailed calculations for both the probable maximum thunderstorm and the <br />probable maximum general storm are outlined in Tables 3, 4 and 5. <br />Hydrographs for the subject storms are included in Tables 6 and 7. The <br />general process for determining these inflow quantities are outlined in <br />succeeding paragraphs of this Section. <br />The Minnesota Dam drainage area is located in Probable Maximum Thunderstorm <br />Zone Two. The maximum one hour point rainfall amount, as detailed in the <br />"Design of Small Dams", is 7.0 inches. The area reduction factor used to <br />convert the one hour point rainfall to a basin one hour rain is 0.9. The <br />resulting basin probable maximum thunderstorm is therefore equal to 6.93 <br />inches. <br />CJ <br />VI-11 <br />