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2.0 DRAINAGE CRITERIA 2.1 Design Storm Water Runoff Determination The City/County drainage policy allows use of the Rational Method and the Soil Conservation Service (SCS) Hydrograph Method as models to be used by designers in estimating storm water runoff for project purposes. The Rational Method will be required for drainage basins il30t acres or less in size. The SCS Hydrograph Method will always be used for basins greater thanliiO,acres. For large complex Drainage Basin Planning Studies, computer models may be utilized upon review and acceptance by City/County personnel. All drainage systems must plan for, perform design and implement storm drainage systems to handle runoff that results from both initial and major design storms. The initial design storm shall be the 10 year event. TliIelhjor4es.fgn stOl'lll shall be the 100 year ..... t Detailed procedures for using these models are discussed in Section II. The necessary data requirements and inherent assumptions are also discussed. 2.2 Open Channels and Roadside Ditches 2.2.1 Channelization Historic routing of drainageways shall be used for the conveyance of storm water runoff wherever possible. Major consideration must also be given to the flood plain and open space requirements in the area. A stable natural channel reaches "equilibrium" over many years. Minimum modifications to such channels should be made. However, in the City/County jurisdictional land area many of the historical channels are in a dynamic unstable condition and will require stabilization. When a channel improvement is necessary, the historic route should be maintained to the best extent possible. Good channel design includes the proper determination of: Carrying capacity, including freeboard Alignment Erosion resistence Aes thet i cs The hydraul i c properti es of natural channel s are generally i rregul ar. A comprehensive study of the behavior of flow in natural channels requires knowledge of other fields, such as sediment transportation, river morphology, etc. The field constitutes, in fact, a subject of its own known as river mechanics. The hydraulic properties of artificial channels can be ei th er cont ro 11 ed to the extent des i red or des i gned to meet gi ven requirements. The design and choice of open channels depend upon good hydraulic practice, environmental and sociological impact, and basic project requirements. The basic choice to be made initially is whether or not the channel is to be lined for higher velocities and in what manner. 2-1 I I I I I I I I I I I I I I I I I I I