FLOOD09787

• County: Delta
• Community: Cedaredge
• Stream Name: Surface Creek
• Basin: Gunnison
• Title: Floodplain Delineation for Cedaredge, Colorado
• Date: 12/8/2003
• Prepared For: CWCB
• Prepared By: Gared Grube
• Floodplain - Doc Type: Floodplain Report/Masterplan

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web site for this gaugmg station IS given In Appendix B. The four different hydrological
methods described below were used. They were then compared to one another and the most
rea~onable result was used.

The ewes Regional Regression equation for the Lower Gunnison River Subregion gave a
resulting IOO-year peak discharge of 1525 cfs. The USGS Regional Regression equation for the
Nonhwesl region of Colorado gave 1446 cfs for the IOO.year peak discharge. HEC-FFA
indicated that the IOO-year peak flow would be 1240 cfs and the Weibull Method resuhed in a
peak discharge of 1135 cfs . The details for all of these methods can be found in Appendix C.
When the results were compared and a discussion was held with Tom Browning at the ewes.
the 1240 cfs peak discharge was determined to be the best candidate for the floodplain study.
The HEC.FFA output of 1240 cfs was used due to the accuracy. the use of other variables (skew
coefficient), and the values were comparable to the values of the other methods.

5,4 Hydraulic Analysis

The hydraulic analysis began by importing the GPS data into ArcYicw. By using HEC-GeoRAS
and XTooL<;, two ArcYiew extensions, the cross-sections of the river reaches were converted into
a Geometric Data import file that could be exported and imported into HEC-RAS, as described
above. The geometric data was imported into HEC-RAS through the geometric data view using
the "Import Geometric Data" bUllon from the File pull-down menu. The XS interpolation tool in
between reaches was used from the Geometric Data view to create cross-sections with-in the
river reach that are one thousand feet of one another. The XS interpolation tool was only used
when the cross-sections were farther then one thousand feet. The geometric data was then
modified by entering Manning's n values, and specifying the hydrologic conditions. The
Manning's values were determined from photographs that were taken during the surveying
procedure. The photographs can be found on the attached CD. The height, with, and/or
diameters of the bridges and/or culverts were then entered into HEC-RAS. Entrance and exit
coefficients of 0.7 and 0.1 were entered in for the culverts and a weir coefficient of 2.6

Hi were entered in for all deck roadways. Expansion and contraction coefficients of 0.3 and

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