FLOOD07152

• County: Delta
• Community: Orchard City
• Stream Name: Sruface Creek, Cedar Run, Tongue Creek
• Basin: Gunnison
• Title: Orchard City, Colorado 100-Year Floodplain Delineation
• Date: 2/20/2003
• Prepared For: CWCB
• Prepared By: Gared Grube, Thomas Gilman
• Floodplain - Doc Type: Floodplain Report/Masterplan

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4,3.2 Guidelines For Determining lOO-Year Flood Hows For Approximate Floodplains in
Colorado, CWCB, 1999

The Colorado Water Conservation Board (CWCB) developed regional regression equations for
different subregions in Colorado, Major drainage basins were separated into geographic and
hydrologic subregions and 100-year flow values for studied streams were collected with their
related drainage areas for Colorado's major drainage basins, Regression equations were created
using a best-fit regression technique with GB-Stat, a statistical software program, The program
fitted values of drainage area and 100-year flow values to different equations and recognized
regression correlations, Certain geographic areas within the state have been excluded from the
CWCB Guidelines where insufficient data exists,

The procedure for using these equations consists of establishing the major drainage basin and the
hydrologic subregion where the stream reach is located, The corresponding regression region is
then selected, the drainage area is found by referencing topographic maps or published data, and
the discharge is calculated from that area using the appropriate equation that can be found in the
CWCB document. Orchard City is located in the COL-3: Lower Gunnison River Subregion, The
corresponding equation for the 100-year peak flow is as follows:

Q = 180.8x A 578

Where:

Equation J

Q = The estimated flood magnitude (cfs)
A = Drainage Area (m?)

(USGS, 2000

For more details of these equations refer to the Guidelines for Determining 100-year Flood
Flows for Approximate Floodplains in Colorado (CWCB, 1999),

4.3.3 Probability Curve Fitting Data From Gage Stations Using the Weibull Method

A statistical approach to establishing the peak flow value can be taken using the Weibull
Method, Hydrological flow data is obtained from an existing source and the values are entered
into an Excel spreadsheet and ranked from high flow to low flow, The ranks of the flows are

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