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XII-3 Natural Type Waterway In this alternative the river would follow an existing flow route with local improvements to control erosion or to improve the boatability or recreation potentials of the river. This alternative was considered only in Reaches 1 and 3. In Reach 1 the natural type waterway alternative would include main- taining the existing channel alignment with riprap erosion protection pro- vided in the area of the gravel lakes. An additional 2,000 feet of natural vegetation would be planted on the left bank to provide protection of the gravel mining area. On the right bank, about 4,500 feet of bank would be protected by bank shaping and planting willows and other natural vegetation. This would provide erosion protection for the Littleton Floodplain Park as well as maintaining a natural appearance of the park and river in this reach. The existing sewer line downstream of C470 would be protected to prevent erosion damage. In Reach 3 the natural channel would be protected against erosion for a total of approximately 2,000 feet at the outside of bends and in locations where the 10-year flood channel velocities exceed 7 feet per second. Lined channels were considered for Reaches 4, 5, 6, 7 and 8. In Reach 2 the Corps of Engineers project which includes lined channel is the recommended alternative. In Reaches 1 and 3 the lined channel alternative would not be appropri ate. An engineered stable channel was evaluated for Reach 8. The channel would consist of an engineered stabilized channel following an alignment as defined in the Stevens report. The general alignment of the channel is sinusoidal, with a bottom width of 250 feet, and a stabilized bank height as necessary to convey the design capacity. The three subalternatives studied for this alternate are the 10-year, 50-year and lOa-year conveyance capaci- ties. No immediate attempt would be made to stabilize the channel invert. Degradation will be allowed until the channel invert approaches that desired for the design flow. The channel invert would then be stabilized to prevent further degradation. In Reach 2 the Corps of Engineers project is the recommended alternative, in Reaches 4, 5, 6, and 7 the natural channel no longer exists and there is very little likelihood of re-creating a natural channel for the urban areas. In Reach 8 the natural channel is not stable and in places it has been altered by gravel mining operations. The lined channel alternative can be used in combination with the limited structural improvements alternative to provide erosion protection and lower flood depths in critical areas. Lined Channels The purpose of a lined channel is to provide protection against erosion and to make the channel more efficient for the conveyance of water thereby reducing the size of the channel required to carry flood flows or reduce flood elevations. The channel could be lined with concrete, rock riprap, or natural planting. Riprap lining would involve a layer of rocks, in most places about 3 to 4 feet thick, on top of a 6 inch layer of filter material. In areas natural vegetation could be pl anted on a 1 ayer of topsoil pl aced over rubble riprap. The banks could also be lined using grass with no riprap protection where the erosion potential is minimal. Limited Structural Improvements This alternative involves the removal or modification of identified obstruc- tions to the flow and other localized improvements to reduce floodwater sur- face elevations and prevent erosion. Erosion protection is provided where the 10-year flood velocities in the channel are more than 7 ft/sec. For greater floods erosion is assumed to occur. It was determined that existing river banks could generally withstand velocities up to 7 ft/sec. This al- ternative was studied in detail for Reaches 4, 5, 6 and 7. Reach 4. In this reach the limited structural improvements alternative would consist of about 1,100 feet of channel lining and buried riprap with natural plantings for erosion protection. The Evans Avenue Bridge and the 6th Avenue Bridge would be improved. The Evans Avenue Bridge causes about a 2 foot rise in the lOa-year water surface elevation as the result of ineffi- cient piers and an inadequate flow area under the bridge. This bridge would be modified with improved piers and a reshaping of the channel section below the bridge.