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

3.0

HYDROLOGIC ANALYSIS

Yampa River
Upstream of Maybell:

for 50-year frequency: k= 3027, c= 0.221
for 100-year frequency: k= 7798, c= 0.114

3.1

Existing Data

For locations where existing d~tailed hydrologic analyses were available, those analyses were used
to obtain peak 50-year and 100-year flows. For some other locations, statistical analyses of
stream gages provided 1;ly the USGS were used to compute the 50-year and 100-year flows.

Yampa River
Downstream of Maybell:

for 50-year frequency: k= 17.21, c= 0.855
for 100-year frequency: k= 11.38, c= 0.915

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 Maybell, 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.

3.3 Eauation Selection Process

The CurveFit computer program analyzes the input data using a library of 25 equations. The
output displays the constants and the correlation values for each equation. The best fit equations
are chosen based on the correlation value closest to 1.0. For this report an additional step was
included in the selection of an equation. Of all the possible equations which fell within a
correlation value greater than 0.95, the least complex equation was chosen. The same equation
format was selected for both the 50-year and 100-year flow calculations.

3.2 New Hvdrologic 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.

3.4 Hvdrologic Findings

Table 3 shows the results of this analysis in tabular form. Figure 5 shows the relationship
between 50-year or 100-year flow and drainage area, as described by these equations, in graphical
form.

The graphical interpretation of the computed flow values on theYampa River indicated a distinct
change in hydrology near the Maybell gage, at the mid-point of the study reach (see Figure 5).
For this reason, two separate sets of equation coefficients were obtained to define thc hydrology
upstream and downstream of the Maybell 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.

10

11