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I I . I I I, I I \1 I f I I I I,' I I I I I 4/8/05 CDM 0611\40892\PREP-FHAD\S2.DOC 4/5/05 cje Section 2 Study Area Description and Cherry Creek Reservoirs), trans mountain diversions, water rights diversions, and gravel mining/ water supply storage operations. Balancing the quantity and quality of these flows is an ever-increasing challenge in the South Platte River basin. The influence of upstream reservoirs and trans mountain diversions on stream flows in the Study Area is discussed in Section 3 - Hydrology. 2.5 Flooding History Floods on the South Platte River and its tributaries that have caused significant damage have been observed and recorded since 1844 (FEMA 1995). Significant floods observed during recent times on the South Platte River occurred in 1921, 1933, 1942, 1949,1957,1965,1973, and 1995. Flood events on the South Platte River are generally characterized by high peak flow and/ or long flow duration. High peak flow events are usually associated with thunderstorms and sustained rainfall, while long duration flooding is typically the result of snowmelt runoff and slow-moving frontal systems. Recent floods of significance to the Study Area, as documented in UDFCD (1984) are described below. The flood of June 1921 was the result of heavy rainfall over the entire South Platte River watershed. The peak discharge at Denver was a relatively moderate 8,790 cubic feet per second (cfs), while flood volume was approximately 200,000 ac-ft. The high flood volume resulted in overbank flooding from Waterton to Brighton with flood widths ranging from 1/2- to 1-1/2-miles in areas upstream of the City of Denver. The flood of September 1933 was a flash flood with peak discharge of 22,000 cfs and a volume of 36,000 ac-ft. The flood at Denver was caused by intense rainfall on Plum, Little, and Big Dry Creeks. The maximum discharge passed through Denver with only minor overtopping and relatively small damages due to effective channel improvements. Damages in downstream rural areas were quite high. The May 1942 flood had a peak discharge of 10,200 cfs at Denver, resulting in extensive damages in rural areas below the channelized reach that extends through the city. Sustained high flows during the flood caused the destruction of five bridges and damages to three additional bridges in the urban reach. Although the flood discharge caused only minor overtopping of the channel banks in Denver, bank riprap was raveled and levee embankments were eroded. This included the agriculture levees north of Denver, which resulted in severe crop damage over approximately 350 acres. The flash flood of 1957 originated on Sand Creek from intense local rainfalL Peak discharges reached 24,000 cfs on the South Platte River just below the Sand Creek confluence and 14,800 cfs at the Henderson gaging station. Heavy rural damages were sustained from Sand Creek to Brighton. The June 1965 flood was caused by the combination of severe thunderstorm events over the Plum, Cherry, Kiowa, and Bijou Creek watersheds. The heavy rainfall resulted in flows of 110,000 cfs at Littleton and 40,300 cfs at Denver. The Denver metropolitan area sustained $300 million in damages due to this flood. The May 1973 flood was caused by a snowpack of up to 170 percent of normal conditions combined with early warm temperatures at high elevations and prolonged rainfalL The flood had a peak flow of 33,000 cfs at Henderson and 18,500 cfs at Denver. Agricultural areas incurred extensive damage due to erosion and sediment deposition. Baseflow in the South Platte River from snowmelt was approximately 1,000 cfs (UDFCD 1984), indicating that the majority of the flooding was the result of runoff originating from the metropolitan area. The 1995 estimated high flow resulted from a high snowpack with prolonged rainfall throughout the beginning of summer. The Henderson gage recorded average daily flows of 4,000 to 6,000 cfs for the months of May, June, and July. Damage was not quantified. 2-2