FLOOD08228

• County: Boulder
• Community: Boulder
• Stream Name: Boulder Creek
• Basin: South Platte
• Title: Urban Drainage and Flood Control District
• Date: 3/1/1990
• Prepared For: Boulder Creek
• Prepared By: Goerge V. Sabol
• Floodplain - Doc Type: Flood Mitigation/Flood Warning/Watershed Restoration

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A - Soil moisture at wilting point
Late summer or fall
DTHETA = 0.35
B - Soil moisture at field capacity
Early summer
DTHETA = 0.25
C - Soil moisture at wet condition
Spring after snowmelt or after
previous storm
DTHETA = 0.10
D - Soil moisture at saturation
Theoretical limit since saturation is not
a practical condition
DTHETA = 0.0

Peak Discharue
8,280 cfs

9,735 cfs

12,690 cfs

17,730 cfs

Comparison of the values listed above with the accepted values indicates
that a 100-year flood peak of 1l,600 cfs could occur with an antecedent soil
moisture that is slightly dryer (more drained) than the soil moisture immedi-
ately after snowmelt. That would be a reasonable assumption for conditions
that often exist in the watershed. A graph of the accepted flood frequency
values and the flood peak discharges from the model under the four assumptions
of antecedent soil moisture is shown in Figure 2.

FINAL MODEL CONFIGURATION

The model was run for the condition of 2 percent imperviousness (RTIMP =
2). This increased the peak discharge for the 100-year event from 9,735 cfs
to 10,000 cfs, a 3 percent increase. This was not jUdged to be significant
and the imperviousness of each subbasin was left as shown in Table 1.

The 100-year flood hydrographs from the model (both for DTHETA = 0.10 and
0.25) were plotted on a graph of the 100-year hydrograph that was developed by
the Corps of Engineers. This is shown in Figure 3. The hydrographs from the
HEC-1 model is similar to the Corps' hydrograph. That is, the rising and
falling limbs of the hydrographs are parallel and the peaks are comparable.

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