|
<br />The 100-yr peak flow estimated by the regression equation for plains region of eastern Colorado
<br />is in close agreement with the value estimated by the USACE using Snyder's method and CN
<br />values of 72, 81, and 85 (Table 3), However, this estimate is more than two times the value
<br />estimated by the regression equation for watersheds in Kansas; about 2,5 times that given by the
<br />more recent regression equation for watersheds in the Republican River basin; more than three
<br />times the value obtained from the previous study by USACE (1980); and more than four times
<br />that given by the regression equation for Region 2, Nebraska, This estimate is judged to be
<br />overly conservative,
<br />
<br />The other estimates included in Table 8 indicate 100-yr peak flows ranging from 11,417 to
<br />13,314 cfs (i.e" within a range of about 14%), The 100-yr peak flows estimated by the
<br />regression equations for the Republican River basin; watersheds in Kansas; and Region 2,
<br />Nebraska are 11,417; 12,471; and 6,491 cfs, respectively. The upper l'mit values iven b
<br />re ression equations are 15,527' 16,961; and 11,943 cfs, res ectivel. In view of this, a 100-yr
<br />peak flow 0 ,12,145 cfs (Table 8) appears to be a reasonably conservative estimate. Since
<br />potential future urbanization and other developments in the sub-watersheds are to be accounted
<br />for, then the slightly higher value of 13,314 cfs (Table 8) may be adopted,
<br />
<br />
<br />The estimates shown in Table 8 indicate 25-yr peak flows ranging from 540 to 6,808 cfs.
<br />Estimates based on previous studies vary from 3,600 to 4,913 cfs, For reasons explained above,
<br />a 25-yr peak of 6,288 cfs (Table 8) appears to be a reasonably conservative estimate. Since
<br />potential future urbanization and other developments in the sub-watersheds are to be accounted
<br />for, then the slightly higher value of 6,808 cfs (Table 8) may be adopted,
<br />
<br />Often, there is significant difference between peak flows of specific return periods estimated by
<br />statistical analysis of observed annual peak flows and those estimated by rainfall-runoff analysis,
<br />The precipitation depth, duration, and temporal distribution of precipitation for the storm of a
<br />given return period used in the rainfall-runoff analysis is generally assumed to occur uniformly
<br />over the entire contributing drainage area (CDA), The observed annual peak flows may have
<br />resulted from storm events of different patterns covering only a portion of the CDA, This
<br />appears to be the case for the June 14-15, 1997 storm event used for the indirect peak flow
<br />estimate of Frenchman Creek (CWCB, 1997), A 100-yr 24-hr storm event in a watershed may
<br />not necessarily produce a peak flow equal to the statistically estimated 100-yr peak flow at its
<br />outlet. Usually, the rainfall-runoff analysis provides a conservative estimate,
<br />
<br />Draft Letter Report
<br />Independent Review of Hydrologic Analysis for Frenchman Creek
<br />Contract No, DACW41-00-D-0026-000 I
<br />
<br />Flood Plain Management Services Special Study
<br />Holyoke, Colorado
<br />.June 18,2001
<br />
<br />Page 14
<br />
|