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I I I I I I I I I I I I I I I I I I I all sections at bridges were obtained from field measurement. Typical cross sections along the study reach are shown on the plan and profile sheets. Cross sections of all bridge structures are shown on Sheet 16. flow paths toward the river. In this reach, the breakdown of these flows was approximated by evaluating street capacities and street gradients. These are generally shallow flooding areas of 1 to 3 feet in depth. No water surface elevations were developed in this reach. Estimates of channel and overbank roughness coefficients were made after field investigation of the study area. The range of "Manning's n values" used in this analysis was the fOllowing: - FUTURE FLOODS - Channel Overbank .015 to .040 .035 to .120 Flood Frequency and Discharge Floods of the same or greater magnitude as those that have occurred in the past will occur in the future in the Harvard Gulch and West Harvard Gulch basins. The fully developed condition of the basin will cause frequent high runoff during annual thunderstorm events. However, the street systems of curbs and gutters and storm sewer systems, and the channel improvements within the basin will ade- quately handle all but the more extreme events. Starting water surface elevations were obtained at control sections for Harvard Gulch and West Harvard Gulch. The control on Harvard Gulch was a broadcrested weir flow section over Logan Street, and on West Harvard Gulch, a broadcrested weir section over Platte River Drive. For Dry Gulch, the starting water surface elevation was taken from the Harvard Gulch profile since these streams are likely to peak at close to the same time. Two special hydraulic analyses were necessary on Harvard Gulch. In Logan Park, profiles calculated upstream along the main channel to the spill section downstream of Ogden Street, and through the Logan Park Detention pond were "trial and error" balanced to obtain the correct discharge across this weir section. During the extreme events, in this case floods with a recurrence interval nearing 10 years, overbank flooding will occur along Harvard Gulch and Dry Gulch. With the many residential units in the Harvard Gulch floodplain, the damage could be very substantial. In the West Harvard Gulch basin, flood damage would be only minor even during events with a recurrence interval exceeding 100 years. Along the concrete channel from Downing Street to Race Street, the 100-year flow will spill overbank and inundate an extensive resi- dential area. This is a dense urbanized area with very little unobstructed area for conveying these flood flows. In this reach, the main flow conveyance is in the concrete channel and the small areas on either side of the channel. In addition, flow conveyance down Wesley Avenue and a small amount through the residential area was considered. The discharges reported in Figures 1, 2 and J for the 10-, 50- and 100-year flood frequencies represent the relative impact and extent of each flood event. The discharge information is usable not only for floodplain regulation but also for planning, engineer- ing and floodplain improvement. In the urbanized ar,ea from Logan Street to the South Platte River, flood overflows'divide down the many streets and follow numerous The 100-year flood event, based upon recorded historical precipi- tation and other valid data,has a one percent chance of being equalled or exceeded during anyone year. The 100-year flood event is considered by the Urban Drainage and Flood Control Dis- trict, the Colorado Water Conservation Board and the Federal Emergency Management Agency as the flood magnitude for which floodplains should be designated for regulatory and improvement -5-