FLOOD07100

• County: Larimer
• County: Weld
• Community: Cache La Poudre River Basin
• Basin: South Platte
• Title: Vol I Flood Hazard, Dam Safety and Flood Warning-Cache La Poudre River Basin
• Date: 10/1/1981
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Flood Mitigation/Flood Warning/Watershed Restoration

The frontal-type storm occurs when a cold air mass moves into the

region from the nDrth at the same time that moist air is transported

into the region by a low pressure system south of the region. The

moist air is lifted along the cold front and curved westward under the

influence of the low pressure cell to the south. Precipitation becomes

general behind the cold front.

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Cloudbursts, due to daily temperature variatiDns, occur during the

summer in the mountain regions and over the plains area within about 75

miles of the foothills. The morning sun results in greater heating

over the mountains than over the plains, A flow of air up the mountain

slDpes soon originates. Thunderheads form over the mountains. The

thunderstorms, which begin about noon in the mountains, move in a

northeastward direction and are dissipated over the plains. The

intensities of the thunderstorms are influenced by the volume of moist

unstable air in the region.

MDSt floods alDng the Cache la Poudre River have been caused by

rainfall adding tD snowmelt in the month of June. It is possible,

however, for severe flooding to occur from rainfall alone -- as

evidenced by the disastrous Big Thompson River flood,

Discharge-Probability Relationships. The discharges of floods of

selected probabilities are listed in table 6, The 100- and SOO-year

flood discharges at selected locations are displayed on plate 2,

The hydrologic studies used to derive the discharges are presented
in Volume II. Discharges in the mountains were determined by use of
the Massachusetts Institute of Technology Catchment (MITCAT) Model.
Discharges in the plains were determined by synthetic unit hydrographs.
From the flood history and from table 6 and plate 2, the attenuation of
peak discharges as floodwaters leave the mountains and crDSS the plains
is evident. In this analysis, attenuation results from valley storage.

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