FLOOD10502 - Laserfiche WebLink

Home Browse Search

2.2 PRECIPITATION DATA Using TP-40 (USDOC, 1961), the USACE estimated 1-, 2-, 10-, 25-, 50-, 100-, and 500-year (500-yr) point precipitation depths for durations of 15 minutes to 24 hours. Values for the 25- and 100-yr return periods with some corrections are shown in Table 2. The 100-yr values estimated from NOAA Atlas-2 (NOAA, 1973) are within 1 to 5 % of these values. Therefore, the values shown in Table 2 are judged'to be reasonable and are used in this study, The 5-minute precipitation depth is taken to be 0.37 times the 30-minute precipitation depth (USDOC, 1961). The 48-hour precipitation depths shown in Table 2 are estimated from TP-49 charts (USDOC, 1964), Table 2 Point Precipitation Depths in Frenchman Creek Watershed Duration Precipitation Depth (inches) 25-yr 100-yr 5 min. O,67a 0.87a 15 min. 1.31 b 1.69b 30 min. 1.82b 2.35b 1 hr, 2.30 3.02 2 hrs. 2.61 3,27 3 hrs 2,70 3,58 6 hrs, 3.10 3,99 12 hrs 3,50 4.40 24 hrs, 3.84 4.93 48 hrs 4,10 5.50 a Taken to be 0.37 times the 30-minute precipitation depth (USDOC, 1961) b Re-computed based on TP-40 charts and tables (USDOC, 1961) 2.3 HYDROLOGIC CHARACTERISTICS OF SUB-WATERSHEDS To estimate Soil Conservation Service (SCS) curve numbers (CNs), the USACE used soils information available in soils survey reports of the area (USDA, 1976) and divided the 19 sub- watersheds into three categories with CN values of 72, 81, and 85, respectively. A value of 72 was used for two sub-watersheds, 81 for seven sub-watersheds, and 85 for ten sub-watersheds (See footnote b, Table 4), However, site inspections by USACE persollilel indicated that the basin should have significantly higher infiltration than indicated by the Soil Survey Reports (USDA, 1976) and that parts of the upper basin and large areas in surrounding basins dead end behind stabilized dunes. This suggests that the CN values should be significantly lower. The average annual surface runoff for the region is reported to be about 1,0 inch (Geraghty, et aI, 1973), The same average annual surface runoff is reported for northwestern Kansas and southwestern Nebraska. The average annual precipitation in these three regions are 17.63 inches at Holoyoke, Colorado; 16,88 inches at St. Francis (northwestern Kansas); and 18.29 inches at Draft Letter Report Independent Review of Hydrologic Analysis for Frenchman Creek Contract No, DACW41-00-D-0026-0001 Flood Plain Management Services Special Study Holyoke, Colorado June 18,2001 Page 3