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<br />Commerce City is characterized by, broad, gently rolling terrain.
<br />The South Platte River flows from south to north, draining the
<br />western part of the city. The main drainageway through the center
<br />of Commerce City is Sand Creek, which flows from the southeast,
<br />northwesterly to its confluence with the South Platte River just
<br />south of Interstate 270 at the iwestern corporate limit. The
<br />streambed of the South Platte River ranges in elevation from 5,127
<br />feet at the southern limit to 5,09i feet at the northern boundary.
<br />The streambed of Sand Creek ranges from 5,215 feet at the
<br />southeastern corporate limit to 5,097 feet at the confluence with
<br />the South Platte River. The vegetation in the undeveloped
<br />floodplain areas is predominantly. grama-buffalo grasses and some
<br />scattered native ,box elder and cottonwood trees.
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<br />The city's climate generally is simpar to that of the high plains.
<br />It has a wide daily range in tempe~ature with a yearly normal daily
<br />maximum of 640F and a minimum of 330F, with approximately 15 inches
<br />of annual precipitation.
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<br />The soils. in the South Platte and 'Sand Creek floodplain areas are
<br />predominantly sandy loamswith moderate infiltration (Reference 1).
<br />Development within thefloodplain~, mostly industrial, has been
<br />limited due to the poor structural i properties of the soils in the
<br />floodplains. W.ith the increasing value of land, development
<br />pressures are inc~easing within flo~dplains and, if permitted, could
<br />result in increased flood heights and, additional damage and
<br />destruction.
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<br />2.3 Principal Flood Pr.oblenis
<br />
<br />Major floods have. occurred on'the S<;luth Platte River and Sand Creek
<br />since 1844. During that period,'H floods occurred on the South
<br />Platte and 10 notable floods occurred on Sand Creek.
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<br />The most significant fLpods of reqent times on the South Platte
<br />River occurred in 1912, 1921, 1935,: 1942, 1965, a.nd 1973 for which
<br />discharges of 13,000 cubic feet per ,second (cfs), 8,790 cfs,22,000
<br />cfs, 12,320 cfs, 10,200 cfs, 40,300 ch, and 33,000 ch,
<br />respectively, were recorded at the penver stream gage. The floods
<br />of 1896, 1912, 1917, 1921, 1933, abd 1938 on Sand Creek were not
<br />recorded wi th regard to specific di s~harges. Measurements were made
<br />to estimate Sand Creek :discharges fior the fOllowi,ng years: 1948,
<br />10,500 cfs at the mouth; 1957, 25,5do cfs at Yosemite Street; 1965,
<br />18,900 cfs just downstream of Toll ~ate Creek; and 1973, 5,630 cfs
<br />at East 49th Drive (References 2, 3, ;4, 5, 6 and 7).
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<br />The major cause of flo,odson the South Platte:1Hver and Sand Creek
<br />are cloudbursts of "intensive rainstorms' which normally occur during
<br />the periOd of May through Augusf.'~he South Platte RIver' qooding'
<br />also is aggravated by s!lowmeit runoff, on the tribl1tary streams
<br />during the rainstorm period.
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