FLOOD09787 - Laserfiche WebLink

Home Browse Search

I I I I I I I I I I I I I I I I I I I (Browning. 2002). Cedaredge ha\ a drainage area much larger then this limiting area, so the Rational Method could not be used. In addition. 200 acres is a small enough area that the use of an extensive computer based analysis would be unnecessary. and therefore. the Rational Method is not considered as a hydrologic analysis method for thili project. If previously repol1ed stream flow records are only 3,,'aiJable for around fifty years, a regression analysis of the data can be used to determine the peak flood discharges for the IOO-year recurrence interval. A flood-frequency curve was produced from USGS gage stations through the water year of 1993 for each hydrologic region classified by the USGS. A flood-frequency curve graphically shows the relationship of the annual peak discharges to the annual exceedance probability obtained from the Log-Pearson probability distribution. Annual exceedance probability is the percent probability that a flood magnitude would be exceeded in anyone year. A recurrence interval is the reciprocal of the exceedance probability, which is then multiplied by 100 to obtain a percentage. It is the average time interval between exceedances of any given flood. The flood frequency curves were created from 328 gauging stations on unregulated streams having at least 10 years of data (USGS, 2()(x)). The generalized least-squares regression method is an appropriate method for developing regional regression equations of stream flow characteristics. Base 10 logarithmic transformations are used to normalize the data and to obtain linear relationships. For more information refer to USGS-Analysis of the Magnitude and Frequency of Floods in Colorado (USGS. 2(x)()). The CWCB has also developed regional regression equations that can be used to determine the I(X)-year peak discharge at given locations based upon the drainage area leading to that point and the regions where they exist. The CWCB warns that these equations should only be used for approximate floodplain delineation where no hydrologic data are available. Major drainage basins were separated into geographic and hydrologic subregions by the CWCR IOO.year flow values for studied streams were collected with their related drainage areas for Colorado's major drainage basins. Regression equations were created using a best.fjt regression technique with GB-Stat. which is a statistical software program. The program fitted values of drainage area and loa-year flow values to different equations and recogmzes regression correlations. For more 13