FLOOD02591 - Laserfiche WebLink

Home Browse Search

SECTION IV HYDROLOGIC ANALYSIS Previous Studies Two previous hydrologic studies have been prepared for Monument Creek. These studies are Flood Plain Information Mommu:nt Creek, Colorado Springs, Colorad;) prepared by the U.S. Arrny Corps of Engineers (COE) in May 1971 and the F.E.M.A. Flood Insurance Study revised in 1989. The F.E.MA. Flood IIl!;urance Study appears to have used the hydrology from theCOEstudyintheirrepon. The COE study used a regional gage analysis [0 detennine the peak discharge for Monument Creek because the limited stream flow records available for Monument Creek were judged inadequate to develop reliable peak frequency curves. The regional gage analysis used a procedure developed during the study of the Arkansas River and its tributarie~ above John Martin Dam. The procedure utilitts the relationship between drainage area to the ratio of peak discharge for a given flood and the peak of the unit hydrograph for the basin. Using the information developed in the John Martin Dam sludy, frequency curves were developed for Monument Creek al various locations. Intermediate Regional Flood and Standard Project Flood discharges were developed for the basin. The Standard ProjecI Flood discharge at the confluence with Fountain Creek was determined 10 be 63,400 cubic feet per second (cfs). The Intermediate Regional Flood discharge was determined to be 32,000 cfs al the confluence. The IntelTl1ediate Regional flood is defined as having an ~verar.e fl'f<quency of occurrence nf nnce in 100 years, which is also known as the 1000yeardischarge. I ! I I i , 1935 Monument Creek flood was documented in the Twenty-Eighth Biennial Report of the State Engineer to the Governor of Colorado (State Engineer, 1939). The peak discharge of the flood just above the mouth of the Fountain Creek confluence (approximately 75 miles down~tream of Pikeview) wasestimaled al 50,000 cfs in the report The flood flow frequency analysis was conducted based on the recorded peak discharge data at Pikeview. A program developed by the COE called HEC WRC (COE 1985) was used to conduct the analysis. To incorporate the 1935 flood into the flood flow frequency analysis al Pikeview, the peak discharge was adjusted according to the ratio of the respective drainage areas (204 sq.mi. / 238 sq.mi X 50,000 cfs). This resulted in an approximate peak flow of 42,860 cfs at Pikeview. To determine the effects of including lhe 1935 flood discharge in the flood flow frequency analysis. three options were examined: 1. Use of recorded peak discharge data from 1939-1949 and 1976-1989 withoulthe 1935 flood. 2. Vse of recorded peak discharge data from 1939-1949. 1976-]')89,and inputting the 1 935 flood as an annual peak discharge. 3. Vse of recorded peak discharge data from 1939-1949, 1976-1989, and inputting the 1935 flood as an historic event. Gal!e Station Analvsis The flow in Monument Creek has been monitored by the U.S. Geological Survey (USGS) at Pikeview. Colorado, for the periods betwe<:n Cktober 1938 and September 1949 and between January 1976 and the currenl year. The discharge gage, identified as hydrologic unit 11020003. is located approximately 0.7 miles downstream from Dry Cll:ek and \,200 feet upstream from the Interstate 25 Bridge between the Rockrimmon and Nevada Avenue exits. The watershed drainage area at the gage is 204 square miles. A flood flow frequency analysis for Ihe Monument Creek basin was conducted so runoff model results could be compared to the analysis_ The annual peak discharges;!! the Pikeview gage were documented in Maglliwde and Frequenc)' of Floods in the U.S. reports (DOl. 1939-1949). and in U.S. Geological Survey W/JIa Resollrces Data for Colorado reports (DOl. I \176.1989). In addition. the peak disdlarge of lhe The results of the analysis are presented in Table 5. Computed llnd expected probability flows forvanou.s exceedance probabilities are shown for each of the three options. The Prol;ranl determined computed discharges and confidence intervals from thc basic flood frequency curve. The expected probability flows were determined by acljusung the b4Sic flood frequency curve 10 incorporate the effects ofuncenainty in app]ication of the curve. The authors of the Guidelines for Determining Flood Flow Frequency (DOl, 1982) indicate that the expected probability flows may be more valid than the compuled flows tor analysis based on small dala sets. Since lhe IIlli1lysisofthe Pikeview Illllleconsists or only 2501' 26 years of reeord, it is recumml'ltded that Ihe upccled probability flows be tLsed for CQmparing the Pikeviewfl!2edatawilhresultsoflherainfalllrunoffmodcl. From the review of lhe data presented on Table 5, il appears thaI the magnitude of predicted discharges is extremely sensitive 10 the manner in which the 1935f1 oodis incorporJled inlothe gage record. Option 1 (not including the 1935 flood) resulls in an under-estimation of discharges. Option 2 (inputting the 1935 flood as an annual peak discharge) result:; in an over estimation of extreme event peak flows Oplion 3 (inputting lhe ]935 flood as an histork event) 1'0'-1 1'0'-2