FLOOD10348

• County: Clear Creek
• Community: Clear Creek County
• Basin: South Platte
• Title: Hydrological Analysis - Type 15 FIS - Clear Creek County
• Date: 9/1/1990
• Floodplain - Doc Type: Floodplain Report/Masterplan

CLEAR CREEK HYDROLOGY FOR CLEAR CREEK COUNTY

Peak Flows Based on Ratio of Draina2e Areas

One potential approach for determining peak flows for the subbasins upstream of Lawson is to
use the ratio of drainage areas as a guide for the ratio of peak flows. Once the peak flow at
Lawson is established, the drainage area for a particular subbasin is compared to the drainage
area of Clear Creek at Lawson. The peak flow at Lawson is multiplied by the drainage area ratio
to determine the peak flow at the point of interest.

A variation to this approach is to look only at the drainage area within a particular elevation
range, both at Lawson and at the point of interest, and then calculate a ratio. For example, the
ratio of the drainage area below 10,000 feet or the ratio of the drainage area between 9,000 feet
and 12,000 feet. The presumption for making this choice would be that only that portion of a
drainage basin or subbasin that falls within the selected elevation range has a bearing on the
flood hydrology of that basin or subbasin.

The listing below shows the drainage areas for Clear Creek at Lawson depending on which
elevation range is selected:

Drainaee Areas at Lawson (in square miles)
CWCB CWCB CWCB CWCB CWCB CWCB
COE Total < 12.000' < 11.000' < 10.000' < 9.000' > 9.000'
< 12.000'
144.9 146.9 120.9 56.3 22.0 7.4 113.5

With the above explanation in mind, the table that follows shows what the peak flows would be
if the ratio approach were valid. There is no adjustment factor for small areas versus large areas;
every square mile of drainage area generates the same amount of flow. The entire table is based
on the 100-year snowmelt flow calculated by the Corps of Engineers for Lawson. That flow is
2330 cfs. There are seven columns of flows; they are listed below:

Column 1
Column 2
Column 3
Column 4
Column 5
Column 6
Column 7

COE Fwws
Flows Based on Ratio of Total D. A.
Flows Based on Ratio of D. A. Beww 12,000'
Flows Based on Ratio of D. A. Beww 11,000'
Flows Based on Ratio of D. A. Beww 10,000'
Flows Based on Ratio of D. A. Beww 9,000'
Flows Based on Ratio ofD. A. Between 9,000' and 12,000'

In each case the particular drainage area for the subbasin in question is compared to the particular
drainage area for Clear Creek at Lawson. For example, in Column 3 the drainage area at Lawson
that is below 12,000' is compared to drainage area for any subbasin that is below 12,000'.