FLOOD07461

• County: Yuma
• Community: Wray
• Stream Name: North Fork Republican River
• Basin: South Platte
• Title: Hydrology for Floodplain Information Report
• Date: 3/21/1977
• Prepared For: CWCB
• Prepared By: CWCB
• Floodplain - Doc Type: Floodplain Report/Masterplan

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4. Streamflow Records. Records of streamflow are available at two loca-
tions within the North Fork Republican River basin. At the Colorado-
Nebraska State line gage (USGS 8230) there are 45 years of record available
(1932-1976). The gage at Wray (USGS 8220) has an intermittent record of
18 years (1937-1946, 1952-1957, and 1963-1964).

5. Flood History. The North Fork Republican River basin is known to have
had very little recorded history of substantial flooding. The Colorado-
Nebraska State line gage has recorded only one flood event in its 45 year
history. Most flood damage over the years has been confined to the city
of Wray. Information on discharges for historical floods tha.t have
occurred in Wray is very limited because operation of the gaging station
at Wray was discontinued during periods of heavy rainfall. Before 1961
most of the flooding in Wray was due to runoff from the hills directly
to the south and north of town. Since the construction of six Soil Con-
servation Service dams in 1961 south of the main sections of the city,
there has been little problem with excessive runoff from these hills.
Flood damage has not been a very large problem outside of Wray due to the
lack of development in th" flood plain. Much of the flooding that takes
place is caused by cloudburst type thunderstorms and inadequate bridges
and culverts.

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Table 1 contains several fairly large rainfalls which have occurred
at Wray. There are, with few exceptions, no r"cords of corresponding dis-
charges. An envelope curve of historic peaks was plotted using the 13
stations from the upper Republican River Regional frequency study. The
13 stations are described on table 2. The study itself is shown on chart 2~

6. Flow Frequency Anal YSJ~s. Peak flow frequency curves were prepared for
a point just downstream of Wray and at the Colorado-Nebraska State line.
Several methods, including two analyses of streamflow data, two sets of
regression equations, and a synthetic computer modeling technique, were
used in obtaining the best possible frequency curves.

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The Storm Water Management Model (SWMM) was used as the synthetic
modeling technique. This technique utilizes the principle of dividing
(discretizing) the basin into subcatchments of 135 acres on the average.
Basin parameters are then measured and input into the model. The design
hyetographs for the various recurrence interval rainfalls were obtained
from "United States Weath"r Service Technical Paper Number 40 for the
Western United States." An adjustment was made for the size of the drain-
age area and then the values were plotted on log-probability paper. A
straight line was then drawn through the 10 and 100 year, 1, 3, and 6 hour
rainfall amounts, extendec~ in both directions and adjusted to reflect
estimated areal distribution effects. The 500 year, 100 year, and 10 year
rainfall amounts were then read from these rainfall frequency curves shown

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