FLOOD09787 - Laserfiche WebLink

Home Browse Search

I I I I I I I I 4.3.2 Guidelines For Determining JOO-Year Flood Flows For Approximate Floodplains in Colorado, CWCR, /999 The Colorado Water Conservation Board (eWeS) has also developed regional regression equations for different subregions in Colorado. The procedure for using these equations consists of establishing the major drainage basin and the hydrologic subregion where the stream reach is located. The corresponding regression region is then selected. the drainage area is found by referencing topographic maps or published data. and the discharge is calculated from that area using the appropriate equation that can be found in the ewes document. 4.3.3 Probability Curve Fitting Dala From Gage Stations Using the Weibull j\1ethod I I I I I I I A statistical approach to establishing the peak flow value can be taken USing the Weibull Method. Hydrological flow data is obtained from an existing source and the values are entered into an Excel spreadsheet and ranked from high flow to low flow. The ranks of the flows are then divided by the total years plus one (the Wiebull fonnula given below) and multiplied by tOO to give a percentage. Where: r P = Recurrance Probability p=- n+t R = rank. and rquat;on 2 N = total number of data records. The values are plotted on a semi-log graph with flow versus the Wiebull fonnula and the X-axis is reversed to created probability. A line of best fit is fixed to the values and an equation is produced. The probability of a lOG-year flow is produced and entered into the equation coming up with the loo-year flow. 4.3.4 HEC-FFA Flood Frequency Anal)'sis I I HEC-FFA is the Army Corps of Engineers Hydrologic Engineering Center's Flood Frequency Analysis computer program. This program allows existing stream flow data to be entered from gauging stations or other sources. It will then calculate the peak discharges for the loo-yr flow I I 19