FLOOD05038

• County: San Miguel
• Community: Norwood
• Basin: Gunnison
• Title: FloodPlain Information Report Town of Norwood
• Date: 9/1/2000
• Prepared For: Norwood
• Prepared By: CWCB
• Floodplain - Doc Type: Floodplain Report/Masterplan

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ENGINEERING METHODS

Hvdroloeie Analvsis
The Norwood 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, approximation methods developed by the USGS were used in this
analysis.

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

Equation 1
Regional Regression Equations for Norwood

QllMl = 213.8(A)o.601

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

For Equation I, the QlOO 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 is only valid for
the range of drainage areas as shown next to the equation.

The loo-year flow for the three drainages were estimated by utilizing the regional regression
equations presented in Equation 1. Table 1 presents loo-year flow estimate for each drainage at
the downstream Town limits. As shown, the western flood hazard area has a tributary area less
than 1 square mile, which is outside the bounds of the equation. Because of this small area, the
West SFHA will not be included in the CWCB designation.

sIma lse 81"2es or orw
Drainage lOO-year
Floodinl! Source Location Area (miz) Disehal'2e (ers)
West SFHA at dls Town Limits 0.79 Outside Bounds
Cedar Street SFHA at dls Town Limits 2.72 390
Ed Joe Draw at dls Town Limits 22.90 1,404

Table 1
E l' ted D' h Ii N ood

Norwood should regulate the approximate floodplains on a case-by-case basis. FEMA requires
that a detailed floodplain analysis must be performed by a professional engineer and submitted
by the developer or development proponent for development projects involving more 5 acres or
more than 50 units (whichever is more restrictive).

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