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08/22/95 TUE 09:38 FAX WOODWARD CLYDE a002 <br /> CITY OF EUGENE <br /> STORMWATER MANAGEMENT PROGRAM <br /> STANDARDS FOR NEW DEVELOPMENT <br /> PUBLIC WORKS ENGINEERING DIVISION August 11, 1995 <br /> TECHNICAL MEMORANDUM #5 <br /> WATER QUALITY BASED DRAINAGE AND DESIGN STANDARDS FOR NEW <br /> DEVELOPMENT <br /> Issue: The City of Eugene has committed in their NPDES permit, and in their <br /> Comprehensive Stormwater Management Plan (CSWMP) to develop water quality based <br /> drainage and design standards for new development (and significant re- development). <br /> The first policy issue for consideration is whether the City should use and/or require on- <br /> site facilities, regional facilities, or a combination of the two for stormwater quality and <br /> quantity control. <br /> 1.0 INTRODUCTION <br /> As development occurs, changes to the natural hydrology of an area are inevitable. <br /> Hydrologic and hydraulic changes occur in response to site clearing, grading, the addition <br /> of impervious surfaces (e.g., paving and roof tops) and maintained landscapes: These <br /> activities disturb the natural pre - developed environment resulting in the following <br /> impacts: <br /> • an increase in the volume of stormwater runoff, <br /> • an increase in the peak discharge rates during runoff events, <br /> • a decrease in the time for runoff to reach the creek or receiving stream (particularly <br /> when pipes are installed to carry runoff), <br /> • an increase in the frequency and severity of flooding, <br /> • an increase in streambank (and in some cases stream bottom) erosion due to increased <br /> flow volumes and rates, <br /> • increased deposition of newly eroded debris and sediment in some locations, which <br /> reduces flood conveyance capacity, <br /> • damage to riparian habitat, <br /> • a decrease in vegetation which provides pollutant of stormwater, <br /> • reduced levels of infiltration in the watershed (and hence reduced groundtater <br /> "recharge), and in some cases reduced stream flows during the dry season <br /> • impaired water quality as a result of increased pollutants associated with urban <br /> stormwater runoff, <br /> • increased maintenance needs and liabilities, <br /> • negative impacts to the natural environment due to some operations and maintenance <br /> activities (e.g., dredging to maintain capacity), <br /> • increased infrastructure- related costs. <br />