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I I I I I I I I I I I I I I I I I I I 17 that the natural channel crossed Colfax Avenue in line with Lewis Street. A 4' x 4' box culvert crosses Colfax at this point, but smaller culverts have been added to each end. A network of culverts now carries flow from 14th Place and Miller Street to l5th Place and Lewis Street, but is is incapable of convey- ing floods of even moderate magnitude. When the flow reaching the storm sewers exceeds their capacities, the water ponds in the low area at 14th Place and Miller until it reaches Colfax. This ponding causes extensive flooding in the "sump" area, and has been experienced on several occasions in the past. Once the water surface becomes high enough to reach Colfax, flood water will flow easterly on the street. The street grade is extremely flat be- tween Miller and Kipling, so Colfax has only a small water carrying capacity, and the water tends to flood the street more than flow- ing along it. This continues until the water overtops the center- line of Colfax and then flows north on Miller Street. Consider- able flooding occurs in the sump area when the flow reaches this level, resulting in a degree of detention storage and an associated reduction of the flood peak. The ponding thus repre- sents a benefit to downstream properties, while creating a major hazard in the ponding area. The flooded structures include resi- dences and commercial buildings. significant channel section. Several small swales drain easterly through this area, and will carry the lOO-year flood at a depth of less than two feet. This shallow flooding will inundate several large commercial structures. However, the flood plains in the area are difficult to define, due to the relatively flat nature of the land and shallow flows. A second reason for the lack of a distinct channel west of Nelson Street is that the Agricultural Ditch crosses the basin between Quail and Simms Streets. Since the drainage basin upstream of the ditch is small, it is reasonable to presume that "normal runoff" from it has been collected by the ditch since its construction. As a consequence, the basin west of Quail has probably experienced only infrequent flooding. upstream (west) of Nelson Street, the drainage channel is not defined, due primarily to the fact that the peak flood flows near the upstream end of the basin are too small to create a The flows in this area will also be affected in the future by a storm sewer on Simms Street which will convey the five- year flow reaching Colfax and Simms to Lakewood Gulch. This diversion of flow out of the Dry Gulch basin greatly reduces the lOO-year flow in Dry Gulch. For the purpose of this analysis, the storm sewer on Simms Street is presumed to be in existence, and the effect of the ditch is ignored. The latter assumption is required by the UDFCD, since it is impossible to evaluate the flow which would be in the ditch at the time of the flood.