<br />
<br />Overbank flow velocities should be expected to be hazardous in Lyons and upstream.
<br />Downstream from Lyons, special care must be taken in road and building
<br />construction since modifications of the flood plain can alter flood patterns and
<br />inundate lands not now affected by flooding.
<br />
<br />45. Army [U.s.] Engineer District, 1972, Flood plain information: LowerSt. Vrain Creek, Volume III,
<br />Boulder County, Colorado: Omaha, Nebr., Prepared for Urban Drainage and Flood Control
<br />District, City of Longmont, Boulder County, Colorado, 26 p.
<br />
<br />The study reach of 51. Vrain Creek for this report extends 8.2 miles from a county
<br />road bridge crossing 3.1 miles west of Longmont, Co., to a road bridge 1.7 miles east
<br />of Longmont, at the Boulder County-Weld County line. Left Hand Creek, a major
<br />tributary, joins 51. Vrain Creek from the south downstream from Longmont. St. Vrain
<br />Creek has a slope of 22 ftlmi and drains an area of 373 sq mi in the study area. The
<br />channel bed is from 200 to 320 feet wide and contains shifting sand bars. Above
<br />Longmont, flood plain development is limited to pasture and croplands. In and
<br />around Longmont (1970 population 23,009), flood plain development includes
<br />commercial, industrial, residential, and municipal sewage treatment uses.
<br />Obstructions to flood flows include 9 bridges, 3 culverts, vegetation, raised road
<br />surfaces and floatable materials from industrial development in the flood plain.
<br />Floods occur from May to September. Serious floods are most often caused by
<br />rainfall runoff augmented by snowmelt runoff. Peak flooding from a single rainfall
<br />event will usually occur within a half day after the rain, with the duration of
<br />flooding lasting from a few hours to a half day. The intermediate regional flood
<br />and the standard project flood will have peak discharges below Left Hand Creek of
<br />13,200 and 39,000 cubic feet per second, respectively. Upstr.eam from Longmont an
<br />area of rural land would be inundated 2,000 ft wide by an IRF; the SPF would average
<br />4,300 ft. wide.
<br />
<br />46. Army [U.S.] Engineer District, 1973, Flood plain information: North Platte, Nebraska; North
<br />Platte River and South Platte River: Omaha, Nebr., Prepared for City of North Platte and
<br />Nebraska Natural Resources Commission, 23 p.
<br />
<br />Land in the floodplains of this study area include residential and commercial
<br />developments, but is primarily agricultural at present. There is pressure to continue
<br />developing in flood prone areas and large floods are possible. At the confluence of
<br />the North and South Platte Rivers, about 4 mi southeast of North Platte, NB, the
<br />combined drainage areas of the rivers is 59,200 sq mi. Most of the city lies between
<br />the rivers and almost all of it is subject to flooding which can last for several days.
<br />The floodplains of the rivers vary from 5,500 to 13,100 ft in width. There is higher
<br />ground suitable for development; the city originated on the floodplain and has
<br />continued to grow there. It is expected that floodplains will continue to be
<br />encroached upon unless control measures are imposed. Heavy rains combined with
<br />snowmelt, or snowmelt alone, can cause flooding. Peak flows generally occur in March
<br />through October. No record of floods exist for North Platte, but gaging records
<br />indicate that the estimated 7,000 cubic ftlsec capacity of the channel of the North
<br />Platte River has been exceeded numerous times. The 20,000 to 30,000 cfs capacity of
<br />the South Platte River has been exceeded only once. In an Intermediate Regional
<br />Flood, peak discharges of 13,500 and 60,000 cfs are predicted on the North Platte and
<br />South Platte Rivers, respectively. In a Standard Project Flood, peak discharges of
<br />
<br />16 Bibliography of Water-Related Sludles, South Platte River Basln--Colorado, Nebraska, and Wyoming
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