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I I I I I I I I I I I I I I I I I I I Several major problems exist in this reach. First, the underground pipe for the first half of the reach leaves no defined surface drainage channel. Consequently, discharges in excess of the pipe capacity (about 100 cfs) would flow down First Avenue, effectively cutting off this area to traffic. This amounts to about 1,000 feet of First Avenue being inundated. The second major problem in this area is the apparent floodplain encroachment in the lower part of the subbasin. Many houses are located along the channel in this area. The problem of overland flow on the auto sales lot is compounded by a further constriction of the underground pipe to a 3.5 foot diameter at its outlet. The inlet upstream of Wadsworth Boulevard is 4-foot in diam- eter. The channel turns abruptly just downstream of the underground pipe, and courses through an apartment complex with a well-defined grass-lined cross section and periodic check structures. Passing through a large box culvert under Sixth Avenue, the channel then crosses several backyard areas and then disappears just before the golf course. In this area, severe inundation of several single-family residences would result from a large storm. The third major problem in this area is the restrictive outlet (48-inch RCP) under Wadsworth Boulevard and the street embankment itself. This would cause additional inundation in the lower reach, due to the backup of water that could not find its way into the culvert. The golf course at the lower end of the South Lakewood Gulch basin provides a natural and effective conveyance for floodwaters, however, a residential subdivision at its lower end is subject to flooding from the golf course discharge. Just upstream of the subdivision, the golf course channel makes an abrupt right-angle turn, which would divert low flows from the subdivision. Higher flood flO\'is \./ould tend to overshoot the turn however, and inundate many of the mutifami Iy units in Meadow Creek subdivision. A rather severe problem exists at the low point of Wadsworth Boulevard which is about 200 feet north of the gulch centerline. The backing up of water at the centerline causes flows in excess of the culvert capacity to overtop Wadsworth at this point, flowing onto the large commercial area on the other side of the street. Pertintent hydrological characteristics for each of the five subbasins are summarized below in Table 11-1: SUBBASIN 5 (Wadsworth Blvd. to confluence with Lakewood Gulch) This subbasin must contend with locally-generated storm runoff, as well as those flows generated upstream. The area is totally developed, with a com- bination of residential and commercial areas, as well as a country club golf course at the lower end. Overall imperviousness is about 38 percent, with a stream length of 1.25 miles and an average slope of about 2 percent. Except for the piped area in the upper reach of the subbasin, and a short section just upstream of the golf course, the surface channel is well-defined. Major problems in this subbasin include the overland flow from across Wadsworth Boulevard and onto the large commercial auto sales area, sharp channel turns, the disappearance of the channel just before the golf course, and the Meadow Creek subdivision below the golf course. TABLE 11-1 SUBBASIN CHARACTERISTICS Area Stream Length Slope Percent Impervious (Acres) (Feet) (Mi les) (Feet/Feet) Present Future Subbasin 170 6,500 1. 23 0.04 31 32 350 4,000 0.76 0.04 25 37 2 3,500 0.66 0.025 33 40 3 170 2,200 0.42 0.02 40 40 4 150 200 6,600 1.25 0.02 38 38 5 TOTAL 1,040 22, il'lO &.32 'l.03" 32" 37" . * Weighted average for entire basin. 11-2