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11 II 11 11 II il :1 :1 :1 :1 :1 :1 :1 II :1 :1 ]1 JI JI Lower Big Dry Creek Hydrologic Study with comprehensive regional plans and should be coordinated, particularly with planning for land use, open space and transportation. Erosion and sediment control, flood control, site grading criteria, and regional water quality all closely interrelate with urban stormwater management. Any individual master plan or specific site plan should normally address all of these considerations. 3. Every Urban Alrea Has an Initial (i.e., Minor) and a Major Drainage System, Whether or Not They Are Actually Planned and Designed. The initial drainage system, sometimes referred to as a ."minor system, " is designed to provide public convenience and to accommodate moderate, frequently occurring flows. The major system carries more water and operates when the rate or volume of runoff exceeds the capacity of the minor system. Both systems should be carefully considered. 4. Runoff Routing Is Primarily a Space Allocation Problem. The volume of water present at a glven point in time in an urban region cannot be compressed or diminished. Channels and storm sewers serve both conveyance and storage functions. If adequate provision is not made for drainage space demands, stormwater runoff will conflict with other land uses, will result in damages, and will impair or even disrupt the functioning of other urban systems. 5. Planning and Design of Storm water Drainage Systems Generally Should Not Be Based on the Premise That Problems Can Be Transferred From One Location to Another. Urbanization tends to increase downstream peak flow by increasing runoff volumes and the speed of runoff. Storm water runoff can be stored in detention facilities, which can reduce the drainage capacity required immediately downstream. 6. An Urban St(J'rm Drainage Strategy Should Be a Multi-Objective and Multi- Means Effort. The many competing demands placed upon space and resources within an urban region argue for a drainage management strategy that meets a number of objectives, including water quality enhancement, groundwater recharge, recreation, wildlife habitat, wetland creation, protection of landmarks/amenities, control of erosion and sediment deposition, and creation of open spaces. 7. Design of the Storm water Drainage System Should Consider the Features and Functions of'he Existing Drainage System. Every site contains natural features that may contribute to the management of stormwater without significant modifications. Existing features such as natural drainageways, depressions, wetlands, floodplains, permeable soils, and vegetation provide for infiltration, help control the velocity of runoff, extend the time of concentration, filter sediments and other pollutants, and recycle nutrients. Each development plan should carefully map and identify the existing natural system. Techniques that preserve or protect and enhance the natural features are encouraged. Good 971-179.092 June 2005 Wright Water Engineers, Inc. Page 26