FLOOD10390 ‎(2)‎

• County: Moffat
• Community: Craig to Dinosaur National Monument
• Stream Name: Yampa River
• Title: Floodplain Information Report - Yampa River from Craig to Dinosaur National Monument
• Date: 4/1/1996
• Prepared By: CWCB
• Floodplain - Doc Type: Floodplain Report/Masterplan

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3.0 HYDROLOGIC ANALYSIS

3.1 Existing Data

For locations where existing detailed hydrologic analyses were available, those analyses were used
to obtain peak 50-year and lOO-year flows. For some other locations, statistical analyses of
stream gages provided by the USGS were used to compute the 50-year and 100-year flows.

Published floodplain studies prepared by FEMA for Craig, Colorado provided 50-year and 100-
year flows at the upstream limit of this study, above the confluence with Fortification Creek.
They also provided flows below that confluence, a short distance downstream. The USGS
performed frequency analyses on gage locations at Mayben, near the middle of the study reach,
and at Deerlodge Park, at the downstream study limit. In addition, USGS analyses were available
for two gages on tributary streams.

3.2 New Hydrologic Information

Hydrologic analyses of the 50-year and 100-year flood flows for the remainder of the hydrologic
points along the study reaches of the Yampa River were accomplished by interpolation of the
known flow values. The known values and corresponding drainage areas were entered into a
curve fitting computer program that selects equations which provide a close description of the
plotted curve. The curve fitting program computed the flows at the other hydrologic points on
the Yampa River locations by using the drainage areas at each location for which flows needed
to be calculated.

The graphical interpretation of the computed flow values on the Yampa River indicated a distinct
change in hydrology near the Mayben gage, at the mid-point of the study reach (see Figure 5).
For this reason, two separate sets of equation coefficients were obtained to defme the hydrology
upstream and downstream of the Mayben gage. The form of the equations for the Yampa River
below Craig is:

Q=k* AC

where:

Q = 50-year or 100-year flow;
A = drainage area;
k = a constant; and
c = an exponent

The form of the equation is the same for the entire study reach, but the coefficients are different
for the two portions.

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