FLOOD03181

• County: Weld
• Community: Greeley
• Stream Name: Cache La Poudre River
• Basin: South Platte
• Title: Master Floodway Plan
• Date: 8/1/1983
• Prepared For: Greeley, CWCB
• Prepared By: Simons Li & Associates Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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The only major tributary of the Cache 1a Poudre River within the study

reach is Eaton Draw. Eaton Draw enters the north bank of the Cache 1a Poudre

River between 6th Avenue and the Union Pacific Railroad. Eaton Draw has a

peak discharge of 2,000 cubic feet per second (cfs) for the 10-year~ecurrence

interval and 4,470 cfs for the 100-year recurrence interval. The drainage

area of Eaton Draw is 42.1 square miles at the confluence with the Cache 1a

poudre River.

The soils in the study area are of two predominant types, namely Aquo11s

and Aquents and Bankard Sandy Loams (reference 1). The Aquo11s and Aquents

are found on bottom lands and floodplains. They are deep, poorly drained

soils subject to flooding. The water table is typically close to the surface

in spring and recedes to a four-foot depth or more by late summer. The soils

are suited for grassland or wetland vegetation and wildlife. Bankard Sandy

Loams are deep, somewhat excessively drained soils found on floodplains. This

soil unit is also best suited for grassland or wildlife habitat.

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Hydrology

Flood discharges for the 10-, 50-, and 100-year floods of the Cache la

Poudre River were provided by the COE (reference 2). The 500-year discharge

was obtained from a straight-line extrapolation of these values on 10g-

probability paper (reference 2). Peak discharge-drainage area relationships

for the Cache la Poudre River are given in Table 1. The drainage basin of the

Cache la Poudre is shown in Figure 5.

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Table 1. Summary of Discharges.

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Peak Discharges in cfs

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Flooding History

Major floods on the Cache la Poudre River are generally caused by intense

rainfall from localized thunderstorms occurring in May through September.

Flooding potential is increased due to snowmelt in May through July.

Damaging floods along the Cache la Poudre River occurred in 1876, 1884,

1904, 1917, 1923, 1947, 1949, 1951, 1964, 1965 and 1983. The largest recorded

flood occurred in 1917 and had a computed discharge of 13,000 cfs. The flood

of 1965 had a discharge of 3,480 cfs (reference 2).

Flooding in June of 1983 was the result of rapid snowmelt compounded by

general area-wide rainfall, Frequent late spring snowfall in the mountainous

portion of the drainage basin combined with cool spring weather resulted in

little snowmelt until late May. The peak recorded discharge was 6,270 cfs

(State Engineer's estimate) and occurred June 22, 1983. This peak flow

corresponds to approximately a 50-year flood. Photographs taken during early

June during a flow of approximately 4,500 cfs are shown in Figures 2, 3,

and 4.

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Flooding Source
and Location

Drainage Area
(sq.mi. )

10-Year 50-Year 100-Year 500-Year

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CACHE LA POUDRE RIVER

At mouth 1,890

3,100 7,100 9,400 17,000

Downstream of Eaton Draw 1,875

3,500 8,000 10,700 19,000

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Upstream of Eaton Draw

1,825

2,600

6,000

8,100

14,500

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At 25th Avenue

1,810

3,400

7,200

9,600

17,000

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Source:

"Flood Insurance Study, City of Greeley, COlorado, Weld
1979, U.s. Department of Housing and Urban Development,
Insurance Administration.

County, "
Federal

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