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<br />Flood Peaking Time. In addition to factors such as flow depth and <br />velocity, the peaking time of flood runoff affects the nature of the <br />flood hazard; the peaking time of floDd runoff is the time from the <br />runoff-producing rainfall to the time the flood reaches its maximum <br />discharge. Also peaking time is generally related to duration. It <br />would be expected that a rapidly rising stream would have a rapid fall <br />and a short duration of flDoding. Table 7 indicates the peaking time <br />of floods from mountain rainstorms at various locations in the basin. <br />It also indicates the time from the start of rainfall runoff to other <br />flood events pertinent to flood warning. <br /> <br />. <br /> <br />Peaking times in the mountains were obtained by the MITCAT model. <br /> <br /> <br />Peaking times in the plains were developed in the following manner. <br /> <br /> <br />The peaking time of the main stem near the c.onfluence with the North <br /> <br /> <br />Fork of the Cache la Poudre River is taken as a starting point, This <br /> <br /> <br />was developed by the MITCAT model. Downstream frDm the cDnfluence, the <br /> <br /> <br />MITCAT model was not used. A synthetic flood hydrograph for the 100- <br /> <br /> <br />year flood was routed downstream, The time at which this hydrograph <br /> <br /> <br />reached various points downstream was noted. This was added to the <br /> <br /> <br />peaking time of the main stem flood near the edge of the mountains (the <br /> <br /> <br />confluence with the North Fork) to obtain the total peaking time. <br /> <br /> <br />Floods were assumed to originate from the main stem as North FDrk <br /> <br /> <br />floods are slower. <br /> <br />The peaking time of flood runoff is more graphically illustrated <br /> <br /> <br />on Plate 3. At the extreme lower end of the basin, peaks can occur in <br /> <br /> <br />1 day or more. In the mountains, minor tributaries may have only an <br /> <br /> <br />hour or less from rainfall runoff to flood peak. <br /> <br />PHYSICAL CHARACTERISTICS <br /> <br /> <br />Stream and Valley Characteristics. The slope of the Cache la <br /> <br />Poudre River averages from 3 to 10 feet per mile across the plains. <br /> <br />The depth of the Cache la Poudre River channel in the plains ranges <br /> <br /> <br />from approximately 5 to 14 feet with the average being about 10 feet. <br /> <br />. <br /> <br />28 <br />