|
<br />n 1\.....,.,...
<br />.U:.)i;jl~'J~
<br />
<br />discharging 123,000 cubic feet per second (cfs). At the same location, the 100-year
<br />flood would discharge 94,000 cfs and would crest at 4,104 feet mean sea level (msl),
<br />while the SOO-year flood would discharge 240,000 cfs and would crest at 4,108 feet
<br />msl. Historic flood data and future flood projections are presented for the entire
<br />study area. This study is intended for use in making land use planning and
<br />management decisions concerning flood plain utilization.
<br />
<br />43. Army [U.S.] Engineer District, 1970, Flood plain information: Crow Creek, Cheyenne,
<br />Wyoming: Omaha, Nebr., Prepared for City of Cheyenne, 24 p.
<br />
<br />The drainage area of Crow Creek, a tributary of the South Platte River, is 253 sq mi
<br />including the southern portion of Cheyenne. The creek provides the city with its
<br />water supply and sewage disposal. Study area extends from Interstate Highway 25
<br />downstream 6.3 miles to sewage disposal plant. Most of the channel has been
<br />straightened and confined by roads, levees, and fills and drops about 18 ft/mi in
<br />slope. Industrial, commercial and residential development in the flood plain is
<br />gradual but steady with pressures for development likely to continue. Eleven
<br />bridges cross the creek, possible obstructions to major flood flows. Elevated
<br />roadways and inadequate culverts also create problems. Intense thunderstorm
<br />rainfall during spring and summer months possibly preceded by heavy snowmelt
<br />causes most floods. Although it has been almost 40 years since the last major flood,
<br />June 1929, future floods of the same size are still possible. Intermediate regional
<br />flood with a peak discharge of 5,500 cfs would overtop all bridges except on
<br />Highway 25 and the railroad bridges, covering an area about 600 ft wide upstream of
<br />Morrie Avenue. A standard project flood with an estimated peak discharge of
<br />17,000 cfs would inundate about 1,000 ft in the same reach. The depth of flood water,
<br />together with the rapidly rising waters, and high velocities can cause substantial
<br />damage to industrial, commercial, and residential areas. Recommendations for flood
<br />protection are not included.
<br />
<br />44. Army [U.s.] Engineer District, 1972, Flood plain information: Upper St. Vrain Creek, Volume IV,
<br />Boulder County, Colorado: Omaha, Nebr., Prepared for City of Longmont and Boulder County,
<br />24 p.
<br />
<br />The study reach extends upstream from the confluence of two streams at Lyons
<br />(drainage area at Lyons 219 sq. mi.), 3.7 miles along the north St. Vrain Creek, and
<br />1.5 miles along the south St. Vrain Creek, and downstream from the confluence point
<br />along St. Vrain Creek for 5.2 miles. St. Vrain Creek slopes between 42-48 ft/mi. The
<br />channel bed has cobbles and. small boulders. Development in the flood plain
<br />includes residential and commercial uses. Pressure for more flood plain
<br />development is expected as Lyons (population 958) continues to grow due to the lack
<br />of suitably sloped land in the area plus the attraction of living near the stream.
<br />Eleven bridges, raised road surfaces and residential development can obstruct flood
<br />flows. Flooding (9 floods in the last 108 years) has occurred from May through
<br />September as the result of snowmelt runoff combined with rainfall. Peak flood is
<br />usually reached within several hours of the rainfall event. Flood duration is normally
<br />short but can be prolonged by continued rapid snowmelt. The intermediate regional
<br />flood and the standard project flood will have peak discharges of 10,700 and
<br />28,300 cubic feet per second, respectively. In addition to scattered residences along
<br />the flood plains, the IRF can be expected to flood large portions of the town of Lyons.
<br />
<br />BIBLIOGRAPHY 15
<br />
<br />
|