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I I I I I I I I I I I I I I I I I I I When a hi stori c dra i nageway receives i ncreas i ng fl ows due to urbani zat ion, the erosion process may accellerate causing slope stability problems. Channel and bank eros i on may encroach and undermi ne exi sti ng structures causing extensive damage to public and private property. Historic drainageways in a natural state must be adequately stabilized to prevent bank and channel erosion. The investigations necessary to insure that the natural channel will be adequate are different for every waterway. However, the designer should prepare cross sections of the channel for the major design runoff, investigate the bed and bank material as to the particle size cl ass ificati on, and generally study the slope stabil ity of the channel under future conditions of flow. The extent of stabilization will depend on channel slope, soil conditions and hydraulic conveyance requirements. When possible, soft channel linings should be utilized in conjunction with grade stabilization structures to provide a natural channel. Hard channel linings are required in steep areas and/or with erosive soils. In all cases, the historic drainageways should be utilized wherever possible. Chapter contains greater detail on channel design criteria and specifications. -- 2.2.2 Channel Storage Drainageways can provide significant storage capacity. This storage may be beneficial to the downstream community in that it reduces downstream peak runoff rates and provides the opportunity for local ground water recharge. Also, wide, natural channels can provide urban open space for park settings or aesthetic purposes. 2.3 Street Flows Streets are an integral part of the urban drainage system and can be used for transporti ng s tonn runoff up to reasonab 1 e 1 imits. However, the engi neer should recognize that the primary purpose of streets is for traffic, and therefore reasonable limits for the use of streets for storm runoff must be followed. The City/County policy is to permit the use of streets for the initial drainage system within the following limitations: All structures classified as bridges shall not allow overtopping of any roadway classification for within the initial or major storm. A bridge for the purposes of this manual is defined as a structure with a clear opening of 20 feet or a clear opening of 200 square feet or a capacity of 1600 cfs. The minimum clearance between the bri dge low chord and the water surface profil e shall be the more restricted whether 6" from the energy grade line or 2 feet from the hydraulic grade line. 2-2