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I I I) I 3. Flow Separation Structures - Where an irrigation ditch crosses a drainage channel section, a flow separation structure should be provided. The final design of any irrigation ditch or lateral crossing, however, is to be coordi- nated with the irrigation company or ditch owner(s). On grade flumes should be used where feasible to reduce silt and maintenance problems. Where runoff design flows are moderate, culverts that pass runoff beneath canals have been shown. The drawings indicate the locations of proposed ditch crossings, but design of the crossings should occur during the final design stages. I I I I 1\ I' I I I I I II I' I I 4. Reinforced Concrete Pipe - Underground concrete pipes have been proposed where on-grade land uses, such as for streets or residences, or safety con- siderations have justified the added expense involved. In addition, the flows in the Middle and Northern Branches of Direct Flow Area 0054 are sufficiently small to allow the use of reinforced concrete pipe to convey flow beneath roadways and certain irrigation ditches. Generally, minimum velocities of 2 feet per second are required to lessen sediment deposition in pipes and manholes. Stilling basins should be included at all outlets to control scour. 5. Check dams - Check dams or check drops are recommended in natural channels to safeguard against erosion by lim i ting eff ecti ve eroded channel slopes to 0.5%. The check dams are usually made from concrete, 2 to 3 feet deep and extending across the channel, constructed by pouring concrete directly into a trench formed by a narrow bucket backhoe. 6. Storm Sewer Inlets - Storm sewer inlets are required at all box culverts, along Cherry Drive in Reach 4, and along Holly Street in the Middle Branch of Direct Flow Area 0054. These inlets should be "Type R" as designated by the Colorado Department of Highways and should be designed using the Standard Curb-Opening Inlet Chart as it appears in the June, 1977 issue of "Flood Hazard News", Reference S. Right-of-Way Except for existing public right-of-ways along the Holly Street, Riverdale Road and 112th Avenue corridors, all of the improvements in Reaches 1 and 2 in Adams County fall totally within private property limits. This necessitates land acquisition to build and have '1-2 Reaches. The tributary drainage ways, the Middle and North Branches of Direct Flow Area 0054, are not to undergo formal channelization above Riverdale and permanent right-of- way requirements are nominal. However, construction and maintenance easements most likely will be necessary to construct the cut-off walls, flow separation structures, and other minor drainageway improvements recommended in these tributaries. Above Holly Street, within the City of Thornton in Reaches 3 and 4, the necessary drainage right-of-way either exists or is in the process of being platted over to the City. Existing right-of-way controlled the extent of pond enlargements and led directly to the underground culvert and overflow channel configuration recommended in Reach 3. Operations and Maintenance Requirements So that the channel continuously provides the flood protection for which it was designed, regular maintenance of the channel and associated drainageway structures is essential. Consequently, one of the major considerations acknowledged in the design of the flood control plan was the operations and maintenance aspect of the improvements, including the proper inspection and repair of these improvements. Particular attention was given to the following: 1. Erosion Control - The design of the channel minimizes erosion by reducing the channel slope and providing lined sections and drop structures for steeper channel portions. After the channel has been subjected to several storm flows, the erosional tendencies of the channel can be determined, and suitable corrections can be made. Where grass-lined channels are in use, re- seeding may be necessary in some areas to establish a good stand of grass. The use of wild grasses in the channel will minimize the need for irrigation and mowing. Rock riprap or gabion revetments have been shown on the drawings in areas considered to be SUbject to erosion. Additional riprap may be required at drops, transitions, low flow channels, or other such structures to more fully protect the banks and channel bottom. In areas not reinforced with riprap and where erosion appears to be endangering channel structures, riprap or gabion protection should be placed. The check dams placed in the low flow channel area along the steep, unimproved tributaries will reduce potential erosion problems that would result from increased runoff from upstream urbanization. As channel grades seek equilibrium and erosion