Laserfiche WebLink
<br />Final parameters are similar to those at Igo Creek tributary indicating a moist soil condition (low <br />initial losses) and a well-drained soil. Hydraulic conductivity estimates are slightly lower that <br />that of Igo Creek tributary with a larger impervious area attributed to the extensive rock <br />outcrops. The SCS CN model uses a curve number of 80 indicating a poor Class B soil. Field <br />observations confirm the poor condition of ground cover. <br /> <br />The HEC-HMS model simulation is seen beginning to gradually deviate to higher floods beyond <br />the 25-year flood. The 1 DO-year flood estimated using HEC-HMS is 31 percent greater than the <br />mean frequency curve. Since the point is within acceptable limits, it is difficult to judge whether <br />this deviation should be of concern. Additional points beyond the 100-year flood would be <br />required to determine if this deviation from the mean frequency curve persists. The SCS CN <br />method appears to perform well at this site. This site contains only three main soil types, which <br />supports the assumption of homogeneity inherent in lumped modeling. . <br /> <br />4.2.3 Owl Creek tributary <br /> <br />Section 4.1 described some of the data challenges facing additional analysis at Owl Creek <br />tributary. All of the flow values in the systematic records may be affected by errors in the rating <br />curve. Adjusting the systematic values using the rating curve described in the aforementioned <br />section may mitigate this. This, however, opens a debate on the reliability of all rating curves <br />used to establish the stage-discharge relationship. It was decided to accept the original data as <br />published (Cochran, et aI, 1983). <br /> <br />Additional issues face this location. Initial analysis of the flood frequency relationship indicated <br />a I DO-year flood near 14,000 cfs. Livingston and Minges (1987) were unable to develop a <br />frequency relation using the station data and estimated a 100-year flood of 13,300 cfs using <br />simulation. Both of these values are in excess of the envelope curve of peak discharges for a <br />basin of this size and location (Jarrett, in review). Figure 14 illustrates the envelope curye <br />developed using sites along the Palmer Divide, Front Range foothills, and eastern Colorado. <br />Basins are separated by high and low topographic relief. High topographic relief is defined as <br />basins having more than about 300 feet of relief in 3 miles or less (Jarrett, in review). All <br />locations used in this analysis are near the Cheyenne Ridge. <br /> <br />The primary reason for this discrepancy is believed to be sampling error. Owl Creek tributary <br />just meets the criteria for a high topographic relief basin. This also provides a conservative <br />estimate of the envelope value. The envelope curve value for Owl Creek tributary is <br />approximately 11,000 cfs. While this is less than previous estimates of the lOO-year flood, it is <br />unlikely the envelope curve would have such a high return period. The actual return period of <br />the envelope curve is not known. Jarrett (verbal communication) suggested a return period of <br />5,000 years as a reasonable value. Using this as a scaling factor, the two large floods recorded at <br />Owl Creek tributary may be very near to the "true" I DO-year flood.l <br />18 <br /> <br />J <br />