Laserfiche WebLink
<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I Figure 5 Lyons, Colorado, May 7, 1969 <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />8 <br /> <br />HYDROLOGIC AND HYDRAULIC DETERMINATIONS <br /> <br /> <br />Flood Characteristics <br /> <br />Floods in the study area are usually caused by general rainstorms and <br /> <br /> <br />cloudburst storms during the period May through September. Floods re- <br /> <br /> <br />sulting from prolonged heavy rainfall over the watershed are character- <br /> <br /> <br />ized by high peak flow of moderate volume and duration. The cloudburst <br /> <br /> <br />type is a high-intensity, short duration rainstorm which produces a <br /> <br /> <br />flood of high peak flow, short duration and small volume of runoff. <br /> <br /> <br />Peak flooding at Lyons will usually occur within a few hours after a <br /> <br /> <br />single rainfall event. <br /> <br />Flood potential also exists from the rapid melting of heavy snow cover <br /> <br /> <br />in the late spring. Flood flows resulting from snowmelt are character- <br /> <br /> <br />ized by moderate peaks, large volume and long duration. This type of <br /> <br /> <br />flood is usually not as damaging as those resulting from rainstorms. <br /> <br /> <br />Nevertheless, severe flooding may occur when rainfall accompanies the <br /> <br />snowmelt. <br /> <br />Hydrologic Analysis <br /> <br />A hydrologic analysis was carried out to establish the peak discharge <br /> <br /> <br />- frequency relationships for floods of 10-, 50-, 100- and SOO-year <br /> <br /> <br />recurrence intervals for each stream in the study area. <br /> <br />For South St. Vrain Creek and Middle St. Vrain Creek, floodflow fre- <br /> <br /> <br />quency data were based on regional relationships for statistical para- <br /> <br /> <br />meters of log-Pearson Type III distribution (reference 11). The <br /> <br /> <br />regional relationships were developed through statistical analyses of <br /> <br /> <br />streamflow records at ten USGS gaging stations located in the upper <br /> <br /> <br />St. Vrain Creek and Big Thompson River basins (references 2, 5, 6, 7, <br /> <br /> <br />8 and 10). Figure 6 presents discharge-frequency curves developed from <br /> <br /> <br />the hydrologic analysis of the Middle and South St. Vrain Creeks. <br /> <br /> <br />Table 4 summarizes the flows of Figure 6. Table 3 lists these gaging <br />