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XII-4 The 6th Avenue Bridge is overtopped in the 100-year flood. The inadequate capacity of this bridge to pass floodwaters causes an increase in water sur- face elevation which results in upstream flooding of the left, or west, bank area. The recommended improvement for this bridge is to include reconstruc- tion to raise the lowest part of the bridge, as well as enlarge the area under the bridge. The channel under the bridge would be lowered and the foundations of the piers and abutments would be lowered and strengthened. The Colorado Highway Department is currently developing plans for the rebuilding of the Interstate 25/6th Avenue interchange. However, none of the alternatives being seriously considered by the Highway D~partment con- template rebuilding of the 6th Avenue Bridge to improve its hydraulic capac- ity. The pedestrian bridge at Interstate 25 would be modified with a new bridge to remove the center pier and widen the distance between the left and right abutments. A new bridge closer to the Childt'en's Museum could also be con- sidered. The current bridge, although it is a breakaway type bridge, re- sults in an obstruction to flood flows due to the presence of a pier in the channel and excessive fill at the right abutment. The Interstate 25 bridge hydraulic cross section is constricted with fill. This fill should be removed and replaced with rock riprap at a steeper slope to increase the hy- draulic capacity of this bridge opening. Although not necessary from a flood control standpoint, removal of the rub- ble dam upstream of Florida Avenue could reduce the magnitude of the modifi- cations required to the Evans Avenue Bridge to prevent flooding. From the Third Avenue Dam to Colfax the capacity of the channel is exceeded by the 100-year flood. To increase the capacity of the channel this alter- native would include deepening the channel by 3 feet and reShaping the banks. In order to lower the channel by 3 feet, utilities crossing the channel would have to be relocated. These include a 3~-inch water line at 4th Avenue, a 48-inch water line at 11th Avenue, a 54-inch water line at 12th Avenue, an 8-inch gas line at 13th Street, and a l2-inch water line at 14th Avenue. From approximately 12th Avenue to 9th Avenue, it will be necessary to replace the existing channel with a deepened channel, about 150 feet wide with vertical walled sides 13.5 feet high. From 3rd Avenue to Colfax, the banks of the river would have buried riprap with topsoil and natural vegation on top. The bridges at 13th Avenue and 14th Avenue would be modified to provide additional capacity and hydraulic efficiency. Recon- struction of the 13th Avenue Bridge should include raising of the low chord above the design flood water surface. The Public Service Company's inflat- able dam downstream of 13th Street might also be rebuilt at a lower eleva- tion to accommodate the channel improvements, though this will be studied in greater detail in Phase B work to derive the optimum balance. Reach 5 - Colfax to Cherry Creek. The limited structural improvement alter- native for this reach includes modifying the channel generally according to the cross section shown in Figure XII-I. This cross section allows suffi- cient channel capacity to contain the 100-year flood when the existing chan- nel bottom is lowered. To accommodate the lowered channel the 18-inch sewer 1 ine at 7th Street would be reconstructed or relocated. In addition, the 15th Street Dam (Farmers and Gardeners Oi tch Divers ion Dam) may al so be replaced with an inflatable dam that could be lowered in times of flood. The existing boat chute would remain as it currently exists. The need to replace the Farmers and Gardeners diversion dam will be considered further in Phase B. The other improvements listed may be adequate without replacing the dam. Consideration has been given to constructing a flood channel from Interstate 25 to a point in Cherry Creek about 1,000 feet above its mouth in lieu of replacing the Farmers and Gardeners Ditch Diversion Dam. This flood channel would be about 1 mile long, 100 feet wide and 10 feet deep with vertical sides to reduce right-of-way requirements. This channel could be designed in such a way that it would provide a focal point for restaurants, shops and walks. During times of low flow, water could be maintained in this channel by pumping groundwater near the head of the channel. The disadvantage of this subalternative is that it may require modifications to the Cherry Creek channel through Confluence Park to increase its capacity. Under existing conditions, if a 100-year flood were to occur on Cherry Creek simulatane-