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-5- HYDROLOGIC AND HYDRAULIC DETER~INATIONS during the 1942 flood caused destruction of five bridges and damages to three additional bridges in the channelized reach, and caused raveling of bank riprap and erosion of levee embankments in this reach even though the flood discharge caused only minor overtopping of the channel banks in Den- ver. The 1942 flood al so breached the agricultural levees in the reach north of Denver and flooded about 350 acres of truck crops, causing heavy damages. FLOOD CHARACTERISTICS Flood flows on the South Platte River may result from rapid runoff of snow melt from the higher foothills and rrountain peaks in the late spring; how- ever, these flood peaks are often reduced by Ch Ittield Dam. Snow melt in the lower foothills and high plains is not normally associated with flood peak s. June 1949 Flood The June 1949 flood was caused by general rain and snow melt. Although the discharge at Littleton was only 5,980 cfs, the flood resulted in extensive rural damages upstream of Denver. Most major flood events on the South Platte River have been associated with thunderstorm activity. Recurrent major storm tracks have been identified which cross the South Platte River watershed and generate most flood flows. May 1973 Flood A snowpack of up to 170 percent of normal combined with early warm tempera- tures at high elevations and prolonged rainfall caused the May 1973 flood. The flood had a peak at Henderson of 33,000 cfs, about double the peak at Denver of 18,500 cfs. This flood caused extensive agricultural damage due to erosion and sediment. Sources indicate that the base flow in the South Platte from snow melt was approximately 1,000 cfs, indicating that the ma- jority of the flooding was attributed to runoff which originated in the met- ropolitan area below the flood control dams. HYDROLOGIC ANALYSIS A hydrologic analysis was carried out to establish the peak discharge/fre- quency relationships for floods of 10-, 50- and lOO-year recurrence inter- vals for the South Platte River in the study area. The 100-year discharges used in this report were suppl ied by a study by ;.I"rrick and Company in May 1983. The 100-year flood flows developed by Merri~~ were based on the Storm Water Management Model (SWMM) computer program developed by the Env i ronmen- tal Protection Agency and modified by the U.S. Ar~v Corps of Engineers and Urban Drainage and Flood Control District. The discharges provided by the Merrick study are based on fully developed conditions. June 1965 Flood Thi s flood was caused by severe thunderstorms over Pl um Creek and Cherry Creek which later joined other thunderstorms over the Kiowa and Bijou Creek basins. This combination of events resulted in flows of 110,000 cfs at Lit- tleton and 40,300 cfs at Denver. These flows caused $300 million in damages in the Denver metropolitan region alone. The 50-year discharge profile for the South Platt~ River from Chatfield to Sand Creek has been estimated by plotting the 10-y!ar and 100-year peak dis- charges on probability paper and interpolating between these events at vari- ous locations. The two-year discharge profile was fxtrapolated in the same manner. The discharge profile is shown on Figure 3. Typical South Platte River 100-year storm hydrographs are shown on Figuroc 4 and 5. HYDRAULIC ANALYSIS A hydraulic analysis was performed to determine the water surface elevations for the 10- and 100-year storm events. The elevations were computed using