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bottom of the channel conveniently by foot. Additional items of criteria include, 2. Natural Channels Flood plain zoning of the natural waterway is utilized wherever such zoning will not have a detrimental effect on the overall drainage system. The unstable characteris- tics of the channel due to excessive slopes means that only certain areas may be left in this state without the danger of major erosion and subsequent plugging of downstream facilities. A. Maximum velocity of flow in channels during the 100 year storm occurrence of 8 feet per second. Pesign storm velocity (10 year frequency) will normally be less than 8 feet per second. B. Design depths dependent on available right-of-way and existing channel invert elevation. C. When space allows, side slopes shall be 4 to 1. In areas with extreme encroachment, 3 to 1 slopes may be used with grass lining. A. Channel and overbank capacities must be adequate for 100 year runoff. D. Centerline curvatures of twice the top width, but not less than 100 feet. B. Area must be in public ownership or designated as a flood plain area under an enforceable flood plain ordinance. E. Where right-of-way is available, freeboard of 1 to 2 feet shall be maintained for the design storm. In areas of Channel encroach~cnt freeboard may not exist and areas subject to flooding shall be flood plain zoned. c. When a particular design is recommended for future development of the area the design shall be indicated on the master plan although the area is flood plain zoned. F. Paved inverts Shall be used in areas where maintenance roads cannot be accommodated in the channel cross section or where particular ground water problems are evident. 3. Channels with Riprap, Gabion, Concrete, or Other Lining In areas where channel encroachmcnt prevents the use of shallow side slopes, or where high vclocity flow is necessary, artificially lined channels may be used. The type of lining material shall be that most economical while maintaining necessary maintenance characteristics. A. Riprap size, design velocity, and On" values to be based on Figures 19, 20, and 21 of Bureau of Public Ro~ds puhlication, Desiqn of Roadside Drainage Ditches and the Urban Storm Draina e Cr~teria Manual. Use of loose r~prap s ~scourage ue to ma~ntenance problems associated with it. G. Hydraulic roughness factors in ;!ecordance with L'rban Storm Drainage Criteria Manual, Il. Blue grass sod shall 10e installed adjacent to low flow channel where sufficient ground water is available to establish the grass. Above this point, pneumatically applied dry land grasses shall be seeded with proper mulch covers. Irrigation of these grasses during initial growth period should be included as part of the con- struction contract to assure their establishment. I. Low flow channel shall conform to the historic channel invert wherever possible. B. Gabion encased riprap may be used on side slopes from 3:1 to 1:1. J. A maintenance road and or hiker/biker path shall be in- stalled in all portions of channel where right-Of-way allows. This road should be constructed to allow use under virtually all weather conditions and might consist of a first phase gravel base with future asphalting as funds become available. C. Steeper side slopes may be obtained by using concrete retaining walls or gab ion baskets stacked at a l(2:l slopc. D. The channel cross section must generally be lined if thc vclocity exceeds 8 feet por second or the Froud", number exceeds 0.80. K. Fruede number (turbulence factor I shall be less than 0.80 whenever possible. E. Riprapping of low flow channelS in areas of erosion may be necessary as a maintenance operation along the channel. Low flow channels should be stable if the recommended blue grass sodding can b~ established in the arca immediatE:ly surrounding the low flow channel. -$6- -87-