FLOOD10350

• Designation Number: 578
• County: Delta
• Community: Orchard City
• Stream Name: Surface Creek, Cedar Run and Tongue Creek
• Title: Floodplain Information Report: City of Orchard City, CO
• Date: 9/1/2004
• Prepared For: Orchard City and CWCB
• Prepared By: Love and Associates
• Floodplain - Doc Type: Floodplain Report/Masterplan

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interval of interest is selected, a weighted estimate of
the peak discharge can be computed for a site using
the regression equation for the appropriate region and
the peak-discharge value from the flood-frequency
curve.

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Weighted estimates are used for unregulated
streams to reduce the time-sampling error that may
occur in a station flood-frequency estimate. This time-
sampling error is associated with the length of record
for a gaging station. A station with a short period of
record may have a large time-sampling error because
its record may not be representative of the actual flood
history of the site based on a large number of years.
The observed period of record has the possibility
of falling within a wet or dry climatic cycle. The
weighted estimate of flood frequency should be a
better indicator of the true value because the regres-
sion estimate is an average of the flood histories of
many gaging stations over a long period of time
(Thomas and Lindskov, 1983).

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Table 2. Basin characteristics and the range of values used
in the analysis

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Basin characteristics
Drainage-basin area, in square miles
Mean annual precipitation, in inches
Mean drainage-basin elevation, in feet
Mean drainage-basin slope, in foot per foot

Range of values
5.5 to 988.0
7.0 to 49,0
2.805 to 12,200
0.081 to 0.562

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Sites near Gaging Stations on the Same
Stream

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Peak discharges for sites near gaging stations on
the same stream can be estimated by using a ratio of
drainage area for the sites near the ungaged sites and
the gaged sites. This method is considered to be reli-
able when the drainage-area ratio is between about
0.5 and 1.5 and when the two sites have similar
drainage-basin and climatic characteristics. If the sites
of interest have similar basin and climatic characteris-
tics and meet the drainage-area-ratio requirement,
peak discharges can be computed by the following
equation:

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QT(u)

x
QT(g)(AJ Ag) ,

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10 Analysis of the Magnitude and Frequency of Floods in Colorado

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where

QT(u)

is the peak discharge, in cubic feet per
second, at the ungaged site for T-year
recurrence interval;
is the weighted peak discharge, in cubic feet
per second, at the gaged site for T-year
recurrence interval;
is the drainage area, in square miles, at the
ungaged site;
is the drainage area, in square miles, at the
gaged site; and
is the average exponent for drainage area for
each flood region as follows:

QT(g)

Au

Ag

x

Flood region

Mountains

Rio Grande

Southwest

Northwest

Plains

Exponent
0.69
0.88
0.71
0.64 c:
0040

The following is an example calculation to
determine the lOO-year peak discharge for an ungaged
site near a gaged site on the same stream in the moun-
tain region. The drainage area at the ungaged site is
given as 350 mi2 and at the gaged site is 450 mi2. The
weighted discharge for the lOO-year peak at the gaged
site is given as 11,500 ft3 Is.

1. Check that the drainage area ratio AjAg is between
0.5 and 1.5. That ratio is as follows:

AJ Ag = 350/450 = 0.78

which meets the ratio requirement.

2. Compute the discharge at the ungaged site using the
specified values in equation 3:

QIOO(u) = 11,500(350/450)0.69

9,670 tt3/s.

Ungaged Sites

(3)

Peak discharges at ungaged sites can be
computed using the appropriate regional equation
shown in table 1. For sites on streams that cross
regional boundaries, results from more than one of the
regional equations need to be weighted as described
below.