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I I I I I I I I I I I I I I I I I I I 4.0 4.1 4.2 FLOODPLAIN EVALUATION Introduction A floodplain evaluation was done for each of the majDr drainage basins for the 10-, 50-, and 100-year events for developed basin runoff conditions. Also, floodways were determined for the 100-year frequency. The HEC-2 Water Surface Profile Computer Program, developed by the Army Corps of Engineers, was used in the evaluation. 4.3 Desion Criteria Channel cross-section locations were chosen along each drainageway so that prominent drainage features were represented. The spacing between cross- sections was limited to no greater than 1000 feet (the average spacing was no greater than 500 feet). The coordinate data for each cross-section was determined by Landmark, Ltd. from 1 "=200', 2-foot contour interval tDpographic mapping. The left and right bank locations were chosen at points where either the grade changed dramatically orthe ground cover changed. Channel distances between cross-sections were measured from 1 "=200' mapping. Roughness coefficients for channel and overbank areas were estimated by field inspection. The roughness, or Manning's "n", values varied considerably and ranged from 0.020 for paved surfaces to 0.070 for heavily vegetated areas. Expansion and contraction coefficients were chosen respectively as 0.3 and 0.1 for open channel reaches and 0.5 and 0.3 fDr bridge transition areas. Culvert crossing structures were analyzed using the special bridge routine. Significant storm sewer systems were accounted for by subtracting the flow capacity of the system from the design discharge. The storm sewer systems in the upper Jackass Gulch basin, the culverts under Jackass Hill Road, and the 60" RCP outfall for Jackass Gulch were the only storm sewers considered to be significant. The "30" RCP in the Rangeview Gulch basin and other small storm sewers or culverts, which have minimal capacity and have high potential for blockage, were ignDred in the analysis. The bridge structures on Lower Dad Clark Gulch were modeled by cross-sectional data obtained from J.F. Sato & Associates. The City Ditch flume structure just downstream of these bridges was assumed to collapse during a major storm event. Therefore, it was not considered as an obstructiDn in the analysis. 4.4 Starting water surface elevations for profiles beginning at the South Platte River were taken from the Flood Insurance Rate Maps for Arapahoe County (Map Number 08005C0065F, April 17, 1989). The starting elevations fDr Rangeview Gulch discharging into Lee Gulch were taken from the Lee Gulch Little Creek FHAD, October 1977. The 10- 50- and 100-year events were analyzed for the existing channel conditions with developed basin runoff rates. Also, a floodway analysis based on a 0.5 foot rise in the 1 OO-year water surface was performed. The drainage patterns dDwnstream of Santa Fe Drive are poorly defined for all three drainage basins and separate CDmputer runs were done for shallow flooding areas. Analvsis FDur profile runs were analyzed for each basin: the 1 O-year flood profile, the 50- year flood profile, the 100-year flood profile, and the 1 OO-year floodway profile. The floodway was determined by encroaching on both sides of the floodplain until either the water surface or energy grade line rose 0.5 foot or the channel banks were reached, whichever came first. Since the floodway was limited by the channel banks, the water surface did not rise 0.5 foot in all areas of the drainageway. In fact, for some areas, the floodplain was completely within the bank limits. Rangeview Gulch had tD be analyzed in two steps. First, the reach upstream of the railroad tracks was analyzed to determine how much flow continues north along the east side of the tracks and how much flow spills over the tracks (the split-flow option was used for this). A second profile was then run from the South Platte River to the railroad tracks using a lower discharge based on the first analysis. On Jackass Gulch, a flow separation occurs downstream of Santa Fe Drive. SDme flow goes along the north side of Mineral Avenue and SDme flDW goes along the south side. The amount of flow which split to the nDrth was estimated based on the prDportion of flow on that side of the street. The south side was analyzed using the entire 1 OO-year discharge. On Lower Dad Clark Gulch, two separate runs were analyzed: one run considering no flood attenuation at McLellan Reservoir (0100 = 1780 cfs) and one with flood attenuation (0100 = 870 cfs). Also, since the lower basin floodplain is bounded on the north side by a berm which acts as a levee, a separate 100-year floodplain was analyzed for the condition without the berm. Flood Problems The potential flood problem areas that presently exist are listed below: Rangeview Gulch. . Several houses along South Curtice Street between Rangeview Park and Turtle Lake are inundated by flood waters. . The mobile home park just west of Santa Fe Drive will be affected by shallow flooding due to stDrm water overtopping the AT&SF railroad embankment. 5