FLOOD10503 ‎(2)‎

• County: Clear Creek
• Basin: South Platte
• Title: Clear Creek County Hydrologic Analysis Document Correspondence
• Date: 2/1/1998
• Floodplain - Doc Type: Educational/Technical/Reference Information

)

hydrologic regions because of widely ranging, complex basin physiography and climate. The
five hydrologic regions include the mountain region, Rio Grande region, southwest region,
northwest region, and the plains region.

The study area for this report (and most of Clear Creek County) is within the mountain region as
defined by the USGS. The explanatory variables for the mountain region flood-frequency
equations include drainage area and mean basin slope. For the purpose of this study, the
drainage areas and basin slopes for the watersheds of interest were measured from the USGS
1 :50,000 scale County Series mapping for Clear Creek County. The regression equations and
variables used for this study are presented in Appendix A.

At the time of this hydrology study, the USGS report is considered to be in draft format.
Potential revisions to the regression equations for the plains region were preventing the final
publication from being released. Revisions to the mountain region equations were not
anticipated by USGS staff at the time of this report, and therefore the equations were deemed to
be valid by CWCB staff.

There are five existing small reservoirs located in the upper Clear Creek watershed. Four of the
reservoirs are located on South Clear Creek, and one is located on the Clear Creek mainstem just
below Georgetown. None of the five reservoirs are operated or maintained for the purpose of
flood control in the watershed. Therefore, it is assumed that peak flood flows are essentially
unaffected by the reservoirs. Storage and routing for flood control purposes was completely
) ignored for all five of the reservoirs. The peak flows presented in this study represent relatively
natural, unregulated basin conditions in the upper Clear Creek watershed.

Methodolol?V for Previous Corps of Engineers Analysis
The Corps of Engineers completed a hydrologic study for Clear Creek County in 1990 under
contract with the Federal Emergency Management Agency. After careful review by CWCB
staff, and in light of new information and regression equations that were not available at the time
of the Corps study, some of the peak flow results developed by the Corps have been superseded
by this study. However, the original Corps analysis and results for the Clear Creek mainstem
downstream from its confluence with West Fork Clear Creek are considered to be valid and were
accepted for this study.

For the 1990 Corps study, discharge-frequency relationships were based on a combined-
population snowmelt/rainfall discharge frequency analysis. Rainfall discharge frequency
relationships were estimated using the Environmental Protection Agency's (EPA) Storm Water
Management Model (SWMM) as modified by the Missouri River Division of the U.S. Army
Corps of Engineers. Snowmelt discharge frequency relationships were determined from an intra-
basin regional analysis. The regional analysis was developed for Clear Creek by relating the
drainage area to the mean flood logarithm and the standard deviation of the logarithms of the
annual snowmelt peak discharges at two gaging station locations located on Clear Creek
(Lawson and Golden). The rainfall and snowmelt discharge-frequency curves were then plotted
for each location, and the combined-population curve at each location was calculated. More
detailed information on the Corps analysis can be found in the [mal report (reference 2).

5