FLOOD06551

• County: Moffat
• Community: Craig
• Stream Name: Yampa River
• Basin: Yampa/White
• Title: Floodplain Information Report
• Date: 4/1/1996
• Prepared For: Routt County
• Prepared By: CWCB
• Floodplain - Doc Type: Floodplain Report/Masterplan

5.0 INTERPRETATION OF TIllS REPORT

5.1 Hvdrologic Findings

The results of the hydrologic analysis for the Yampa River are considered accurate. - They. are
based in part on existing approved data. The presence of USGS gages at equally spaced intervals
along the study reach also provided accurate data. The one data limitation was the short period
of record (10 years) for the downstream gage (Deerlodge Park).

Based on the results of the regression analysis on the Yampa River, it is evident that there is a
change in the hydrologic characteristics near the Maybell gage. This is the reason for two
separate sets of regression equations. It is not clear why there is such a change. It is possible
that Maybell represents the point where the Yampa River changes from a mountain stream to a
canyonlands and plateau stream, with smaller snowfall contributing to less runoff per square mile.

5.2 Hvdraulic Findings

The hydraulic methods used for this study are similar to those used in more detailed studies.
They are ml}ch more detailed than the typical methods used in approximate studies. The 100-
year flood depths range from 8.1 feet in some broad reaches of the river to 17.3 feet at the
downstream limit of the study Gust upstream of the downstream limit the drainage area essentially
doubles at the Little Snake River confluence). The flood depths are generally consistent
throughout the study reach with differences of only a few feet (11-14 feet for 100-year flow
depths).

5.3 Floodplain Delineations

The primary difficulty in presenting precise delineations of the 50-year and IOO..year floodplains
was the limited quality of the available base mapping for the lower Yampa River. Using USGS
topographic quadrangle maps at contour intervals of 20. or 40 feet to portray flood lines in
unslll'Veyed areas provides limited information. The margin for interpretation is very great,
especially in medium to wide reaches of the river where contours are widely spaced. More
detailed topographic information will be needed in areas of specific interest.

-
This study can be utilized to prepare more detailed floodplain delineations in such areas of
specific interest. Those who are interested in greater detail have three choices for using this
study:

1) They can refer to the detailed study information for the upstream reach near Craig;
2) They can rely on the hydrologic determinations and use other topographic information and
hydraulic models; or
3) They can use the depth calculations with other topographic information from field surveys
or other maps.

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6.0 REFERENCES

1.

Federal Emerflencv Manaflement Aflencv: Flood Insurance Study and Flood Insurance
Rate Maps for the City of Craig; 3/84.

2.

Federal Emerflencv Manafletnent Aflencv: Flood Insurance Study and Flood Insurance
Rate for Maps for Moffat County; 8/82.

3.

U.S Armv Coros of Enflineers: Flood Hazard Information Report for Craig, Colorado;
11/77.

4.

United States Geoloflical Survev: flood frequency analyses for gage sites at Maybell, CO,
Little Snake River, and Deerlodge Park through the 1993 water year. This information
was provided for this project only and is unofficial and unpublished

5.

United States Geoloflical Survev: Water-Data Report, Water Resources Data, Colorado,
Water Year 1993.

6.

Curvefit Prof!Yam: A public domain computer program, version 1.0 by T. S. Cox

7.

Chow. Ven Te: Open Channel Hydraulics, 1959

8.

OANY; An in-house hydraulics computer program (not for public domain)

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