FLOOD00875

• County: Grand
• Community: Granby
• Title: Floodplain Information Report - Town of Granby
• Date: 3/1/2002
• Prepared For: Granby
• Prepared By: CWCB
• Floodplain - Doc Type: Floodplain Report/Masterplan

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ENGINEERING METHODS

Hvdrolol!ic Analvsis
This FIR calculates flow depths in flood prone areas from two flooding sources. The CWCB has
developed regional regression equations for computing lOO-year flood-flow values in
undeveloped basins. The CWCB equations are documented in a publication entitled Guidelines
for Determining lOO-Year Flood Flows for Approximate Floodplains in Colorado (CWCB,
January I 999). However, these equations were developed based on actual gaging data and are
primarily used for basins larger than at least one square mile. Since the drainage basin areas in
this FIR are significantly less than one square mile, empirical methods are required to estimate
runoff.

The "SCS Method," as documented in Technical Release No. 55, Urban Hydrology for Small
Watersheds, is commonly used in ruraI areas that do not have detailed intensity-duration-
frequency curves, such as Granby. The Soil Conservation Service (SCS) document entitled
"Procedures for Detennining Peak Flows in Colorado" (supplement) incorporates and
supplements TR-55 for Colorado and was incorporated in this analysis. The TR-55 method uses
the lOO-year, 24-hour storm event, time of basin concentration, and "Curve Numbers," which
represent basin imperviousness to estimate instantaneous peak discharges within the basin.

Rainfall depths were determined from isopluvial maps contained in the TR-55 supplement and
verified with previous studies. The estimated lOO-year, 24-hour rainfall depth for Granby is
approximately 2.60 inches. Drainage basins were delineated on 2-foot contour interval maps
provided by the Town, and are shown on Figure 3 in the appendix. SCS Soil Survey maps for
Grand County were used to detennine soils groups, while curve numbers were taken from TR-55
and the supplement. Table 1 presents summary of the hydrology calculations for each design
point shown on Figure 3. Detailed calculations are shown in the appendices.

a e . lV( ro Ol!Y: u
loo-year
Design . Sub- . Tot Area Peak Runoff
Point Basins (acres) (ers)
Aa Al 123 67
Ab Al,A2 198 68
Ac AI, A2, A3 210 70
Ba Bl 48 69
Bb Bl,B2 58 76
Bc B3 42 50
Bd Bl - B4 106 128
Be B5 144 213
Bf BI - B5 250 310
Bg BI - B8 339 279

TbllHdl S

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