FLOOD09736

• County: Statewide
• Basin: Statewide
• Title: Estimating Flood Flows from Mixed Populations in Colorado
• Date: 1/1/1978
• Prepared For: State of Colorado
• Prepared By: CWCB
• Floodplain - Doc Type: Educational/Technical/Reference Information

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describe thunderstorm type floods, The thunderstorm flood season in Colorado
is from the middle of July through October,
The widely publicized Big Thompson Canyon flood disaster upstream of
Loveland, Colorado, on July 31, 1976, was a result of an intense thunderstorm
cell which dropped up to ten inches of rain in a few hours over the basin. _
The principal sources of sno~~elt floods are the winter snowpack areas in
the high mountains along the continental divide as shown in the ERTS-LANSAT
aerial photo mosaic of Figure 2,
The spring runoff typically begins with the gradual melting of the winter
snowpack after the first spring warming trend. If this warming trend con-
tinues for eight to ten consecutive days in a basin with a snow-water content
of more than 150 percent of average, flooding can develop, The melting cycle
is characterized by a smooth diurnal fluctuation in discharge rate with each
successive daily peak discharge slightly greater than in the previous day.
The cumulative hydrograph exhibits a stair-step shape and occurs over a period
of several days. Experience has shown that if a single cold day or cold front
interrupts the melting cycle, the rising waters will stabilize until the
stair-step process is started allover again. Snowmelt floods generally yield
a large volume of runoff water in comparison to thunderstorm floods.
Floods in 1941 and 1949 in La Plata County, and those in June 1983, along
the Cache 1a Poudre River in Fort Collins and Greeley, along Clear Creek and
its tributaries in Silver Plume and Georgetown, and along the Arkansas River
in Fremont and Chaffee counties, were principally due to melting snow.
During the months of May and June and the early part of July there is a
potential for flooding due to rain falling after spring snowmelt has already
increased runoff, Sometimes such rain is over a small part of a basin and the
resulting flood is of short duration and may often go unnoticed in the lower
reaches of a large drainage basin, In some cases, however, rainfall may be
heavy and widespread enough to noticeably affect peak flows throughout a
basin.
Flooding along the Colorado River in Grand Junction in July of 1884,
flooding in June of 1965 at higher elevation communities including Georgetown
and Frisco, and flooding in June of 1983 along the Gunnison River at Delta and
Grand Junction and along the Colorado River at Grand Junction, are examples of
flooding from rain on melting snow,
Ice jams are formed when floating ice moves downstream and become lodged
at some constriction, such as a bridge, The jam forces water to be diverted
from the stream channel causing a flood. The ice jams can also break up sud-
denly causing a surge of water to flow downstream as the "reservoir" that was
formed behind them is suddenly released, Ice jams occur in slow moving
streams where prolonged periods of cold weather are experienced, Sometimes
ice jams are broken up with dynamite or aerial bombs. Flooding from ice jams
in Colorado have occurred on the White River of Rangely (1955, 1962, and 1983)
and at Meeker (1973) and on the Gunnison River at Gunnison.
Most dam failure floods have characteristics similar to thunderstorm
floods. There is an abrupt increase in flow, a sharp peak, and a steady
decrease in discharge after the peak, Dam failure floods may occur at any
time.
Although few lives have been lost in floods caused by dam failure, pro-
perty damage can be high, There have been at least 130 known dam failures in
Colorado since 1890, Two dam failures in Colorado have resulted in
Presidential Major Disaster Declarations, These were the failure of the Lower
Latham Reservoir Dam on April 12, 1973, and and subsequent flooding of the
Town of Kersey, and the failure of Lawn Lake Dam on July 15, 1982, which

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