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

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

4.3 Hydrologic Analysis

There are several hydrologic methods that may be used to determine the peak discharge for the
stream reach including the Rational Method, regional regression equations, statistical analysis of
gauge data, and Rainfal1lRunoffModels, The Rational Method can only be used for small
catchments with areas ofless than 200 acres (Browning, 2002), Unfortunately, all of the streams
in Orchard City have a drainage area much larger then this limiting area, so the Rational Method
could not be used and was not considered as a hydrologic analysis method for this project, The
other methods that were used depended on the amount of information that was available,
especially the data from recorded gauging stations.

If previously reported stream flow records are unavailable or limited, regional regression
equations based upon the drainage area leading to that point and the regions where they exist can
be used to detennine the peak flood discharges at a given location, The USGS and the CWCB
have developed these equations, however, the CWCB warns that they should only be used for
approximate floodplain delineation where no hydrologic data are available, Regional regression
analysis was completed for both Cedar Run and Tongue Creek If a stream-gauging station in
the area exists, it is used to obtain information on flood heights and discharges. Statistical
analysis is conducted using the existing gauge data and includes the Weibull Method and the
HEC-FFA computer program, These statistical methods were conducted for Tongue Creek where
a gauging station existed at the lower study limit, If there are no stream flow gauging records
available for the stream being studied but a detailed analysis needs to be performed then a
rainfall! runoff model will need to be conducted to determine the hydrology ofthe channel. The
HEC-HMS Rainfall/RunoffModel, in conjunction with HEC-GeoHMS, an ArcView extension,
was used for the detailed hydrological analysis of Cedar Run, All of these methods are discussed
in more detail below.

4.3,1 Analysis of the Magnitude and Frequency of Floods in Colorado, USGS, 2000

A flood-frequency curve was produced from USGS gauge stations through the water year of
1993 for each hydrologic region classified by the USGS, A flood-frequency curve graphically

14