FLOOD04379

• Designation Number: 525
• County: Dolores
• Community: Unincorporated Dolores County
• Basin: San Juan/Dolores
• Title: Flood Insurance Rate Map - Dolores County, CO, Unincorporated Areas
• Date: 1/1/1978
• Designation Date: 9/1/2000
• Floodplain - Doc Type: Floodplain Report/Masterplan

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ENGINEERING METHODS

Hvdrolol!ic Analvsis
The Dolores County FHBM presents approximate floodplain delineations. Hydrologic
information used for the FEMA approximate floodplain delineations are not available or were
not documented by FEMA. The CWCB has developed acceptable hydrologic information for
undeveloped basins using regional regression methodologies for computing lOO-year flood-flow
values. However, due to a lack of information, these equations are not available for the Western
San Juan River Subregion. Therefore, regional approximation methods developed by the USGS
were used in this analysis for those streams within the San Juan River Basin.

The CWCB methodology is fully described in a publication by the CWCB entitled "Guidelines
for Determining IOO-year Flood Flows for Approximate Floodplains in Colorado". The Dolores
River Subregion (DLR-l) is used for streams in the Dolores River basin. The CWCB equation
IS:

Equation 1
Regional Regression Equation for Dolores River Basin

QIOO = 213.8(Al601

(Only for drainage areas between 2 and 1080 square miles)

The USGS regional regression equation methodology is fully described in USGS Water
Resources Investigations Report 99-4190, entitled "Analysis of the Magnitude and Frequency of
Floods in Colorado". The Southwest Region equation is used for streams in the San Juan River
Basin within Colorado. The USGS regression equation is:

Equation 2
Regional Regression Equation for San Juan River Basin

QIOO = 118.4(A)o.71S

(Only for drainage areas between 5.5 and 988 square miles)

For both of these equations in, the QIOO is the lOO-year peak flow in cubic feet per second, and A
is the upstream (contributing) drainage area in square miles. The regression equation equations
are only valid for the range of drainage areas as shown next to each equation.

The IOO-year flow for approximate floodplains in Dolores County estimated by utilizing the
regional regression equations shown above are presented in Table 1.

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