FLOOD08407 - Laserfiche WebLink

Home Browse Search

I I I I I I I I I I I I I I I I I I I 2. Concrete Box Culverts - Concrete box culverts are utilized at crossings where design discharges are moderate and where box culverts may provide less expensive alternatives to bridges. Riprap is included at all culvert inlets and outlets to minimize the effects of scour. Minimum velocities are to be limited to 2 feet per second for the two year event to lessen sediment deposition in culvert bottoms. 3. Drop Structures - The effect of a drop structure is to lessen the channel slope, thereby reducing channel velocities to acceptable limits. The concrete drop struc- tures proposed have sloped faces of 4:1, or 2:1 (Horizontal to Vertical) , as recommended in the Urban Storm Drainage Criteria Manual, to minimize turbulence and erosion. 4. Irrigation Ditches - 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 coordinated with the irrigation company or ditch owner(s). On grade flumes should be used where feasible to reduce silt and maintenance problems. The drawings indicate the locations of proposed ditch crossings, but design of the crossings should occur during the final design stages. 5. Reinforced Concrete Pipe - Concrete pipe is used primarily to convey flows col- lected by storm sewers and in detention pond outlet works. 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. 6. Storm Sewer Inlets - Storm sewer inlets are required at bridges and at the area east of the existing detention pond on the Shaw Heights Tributary. 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 P. Right-of-Way Generally, the pUblic land available for drainage improvements is nonexistant through the lower reaches of the mainstem of Little Dry Creek. Right-of-way acquisition then becomes an inherent aspect of the improvement plan. In Westminster, a narrow 50- foot drainage easement dictates channel shape for two separate stretches of channel in Reach 3. This requires the use of concrete walls approximately four feet in height on either side of the channel to limit channel width to that of the drainage easement. Such a design eliminates the need for the costly purchase of property adjacent to the easement and/or relocation of structures and occupants. v - 2 Operations and Maintenance Requirements So that the channel continuously provides the flood protection for which it was designed, regular maintenance of the channel and related structures is essential. The channel capacity in its present condition is restricted as a result of neglect and lack of proper maintenance. 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: 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, reseeding 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. Structural Repair: Apart from erosion, concrete and riprap structures should be inspected regularly to assure their integrity. Concrete channel lining and walls should be repaired where they become cracked, chipped or scoured. Where gabions are in use, the wire baskets should be examined and repaired as needed to maintain adequate containment of the inner riprap. Debris Removal: All channels, culverts, and related structures should be kept free from trash, uprooted vegetation and other material. The channel should be cleared on a regular basis during the flood season and after every major storm. Vegetation should be maintained and trimmed, with trees and other tall growing plants kept out of the channel. Maintenance Road: The construction of hiker/biker/maintenance trails will assist in providing economical maintenance for the Little Dry Creek Channel by furnishing suitable access where currently none exists. For a portion of the improved channel section on the main stem of Little Dry Creek, the maintenance road is shown situated on the channel bottom. This allows for a narrower channel right-of-way and a reduction of land acquisi- tion costs, since the area above the 10-foot wide maintenance road is utilized for flood where the existing drainage easement is very narrow. The actual location of the maintenance road should be determined during the final design but should be located out of the two year flood plain wherever possible.