FLOOD00550

• County: Clear Creek
• Title: Engineering Division Technical Report Type 15 Flood Insurance Study Clear Creek County
• Date: 9/1/1990
• Prepared For: Clear Creek County
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Flood Documentation Report

'.

.
'10 j ~,

.'

Rainfall runoff hydrographs produced by SWMM for the 2-,
5-, 10-, 50-, 100- and 500-year storms are shown in Figure 2 for
Lawson. A comparison of peak discharges for the 2- through
100-year rainfall events are shown in Table 5 for the
statistical analysis of stream gage records and the SWMM modeled
discharges. Table 6 lists the recurrence interval and
corresponding rainfall discharges for different locations in the
basin.

TABLE 5
Results of SWMH Calibration
Rainfall Discharge Frequency at Lawson Gage

Recurrence Peak Discharge in c.f.s.
Interval statistical SWMH Difference
(years) Analysis Model c.f.s. Percent
2 .352 390 38 10.8
5 /;605 630 25 4.1
10 ;;817 840 23 2.8
50 1430 1340 -90 -6.3
100 1760 1850 90 5.1

Snowmelt Discharqe Frequencv Curve
------

The snowmel t discharge frequency curves were determined
from an intra-basin regional analysis utilizing streamflow
records at the Lawson and Golden streamgages. The regional
analysis was developed using the mean flood logarithm and
standard deviation of the logarithms for annual snowmelt peak
discharges and were plotted versus the drainage area of 147
square miles at Lawson and 399 square miles at Golden. The mean
logarithm and standard deviation were obtained from frequency
analyses derived at the two locations using annual snowmelt peak
flows. Snowmelt peak flows were obtained from the same USGS re-
port that rainfall peak flows were found. For Lawson, the mean
flood logarithm was 3.00 with a standard deviation of logarithms
of 0.16. For Golden, the mean logarithm was 3.16 with a stan-
dard deviation of 0.20. These values were plotted versus drain-
age area and are shown in Figures 3 and 4. Snowmelt frequency
curves were computed at various locations in the study area us-
ing the log Pearson Type III distribution equation, with mean
flood logarithms and standard deviations of logarithms being ob-
tained from Figure 3 and 4, respectively. A regionalized skew
coefficient of -0.10 for snowmelt-runoff for the South Platte
River basin was used (Jarrett, 1987). Table 7 lists the
recurrence interval and corresponding snowmelt discharges for
various locations in the study reach.

6