FLOOD07945

• Designation Number: 563
• County: Grand
• Community: Hot Sulphur Springs
• Basin: Colorado Mainstem
• Title: Floodplain Information Report
• Date: 5/1/2002
• Prepared For: Hot Sulphur Springs and CWCB
• Prepared By: Montgomery Watson Harza
• Floodplain - Doc Type: Floodplain Report/Masterplan

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ENGINEERING METHODS

Hvdroloe:ic Analvsis
The Town of Hot Sulphur Springs FHBM presents approximate floodplain delineations.
Hydrologic infonnation used for the FEMA approximate floodplain delineations are not available
or were not documented by FEMA. However, the Colorado Water Conservation Board
(CWCB), using regional regression methodologies for computing 100-year flood-flow values,
has developed acceptable hydrologic infonnation for undeveloped basins.

The CWCB regional regression equation methodology is fully described in a publication by the
CWCB entitled "Guidelines for Detennining 100-year Flood Flows for Approximate Floodplains
in Colorado". The CWCB regression equation for this area is shown in Table 2.

Sub-Re ion Name

uations for Town of Hot Sui
Discharge Equation
(cfs)

QIOO = 79.5(A)o724

Table 2.
Sub-
Re ion ID

COL-I

Upper Colorado River

I to 950

For the equation in Table 2, the QIOO is the 100-year peak flow in cubic feet per second, and A is
the upstream (contributing) drainage area in square miles. The regression equations are only valid
for the range of drainage areas as shown. Calculation worksheets are shown in the appendix.
The 100-year peak discharge for each of the undeveloped approximate floodplains is shown in
Table 3.

Table 3. Summary of Calculated Discharl!es
Drainage lOO-year
Flooding Source Drainage Area Description Area (mil) Discharge (cfs)
Colorado River At Hot Sulphur Sprinl!s 837.63 10,400
Heimbaugh Creek At mouth 1.76 120

Peak flow records are available at one USGS streamflow gages within the Town. USGS Bulletin
17B procedures were used to calculate peak discharges for specific return intervals at the gage.
The estimated IOO-year discharge using this procedure is shown in Table 4, while more detailed
calculations are shown in the Appendix. Although the drainage basin areas are essentially
identical, indirect flood storage in reservoirs within the basin likely has an impact on peak flows
and makes the regional regression calculations less accurate. Because the gage flows measure
actual peak flows, where available, 100-year peak discharge as estimated from gage flows using
Bulletin l7B procedures should be considered more accurate than the regional equations and
used for analysis and design.

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