RES 06035 - Laserfiche WebLink

Chapter 4 Environmental Setting, Impacts, and Mitigation Measures H. Hydrology, Drainage. and Water Quality development. However facilities that control peak flow rates usually allow the cumulative duration of peak flows to increase over time, which can still cause increased erosion of the downstream system. Protecting these stream systems from hydromodification requires management measures that control the duration of a range of geomorphically significant flows - those capable of moving sediment and eroding stream bank material. These processes are most affected by small, frequent storms that become much more frequent when development occurs in the watershed. In many parts of North America, observations that peak flow controls alone are not adequate for erosion control have initiated revisions to stormwater requirements to incorporate a flow duration standard, where pre. and post- development flow duration curves must be matched according to specified criteria. Flow Duration Control requires that the increase in surface runoff created by the installation of impervious surfaces be retained onsite with gradual discharge either to groundwater through infiltration, reduction by evapotranspiration, and/or discharge to the downstream watercourse at a level less than the critical flow for bed and bank mobility of the stream. Pleasanton recognizes that impacts related to flooding and hydromodification could occur as a result of the type of development proposed. Therefore, the Specific Plan specifies that all projects covered under the C.3 NPDES Permit and implemented under the Specific Plan will fully comply with permit requirements. The detention basins have been sized to accom- modate anticipated development and designed to be operated in a manner that the post- development storm hydrograph closely resembles the predevelopment storm hydrograph. In addition, as required by City ordinance, all drainage components will be designed in compliance with City of Pleasanton standards. The City of Pleasanton Department of Public Works will, under existing requirements, review the grading and drainage plans for compliance with these requirements. No adverse environmental impacts relating to flooding and hydromodification would occur because the Specific Plan contains adequate provisions to mitigate these potential impacts. (2) Stormwater Quality Degradation Construction Period Impacts Construction and grading within the project site would require temporary disturbance of surface soils and vegetative cover and could result in exposure of soil to runoff, potentially causing erosion and entrainment of sediment in the runoff. Soil stockpiles and excavated areas would be exposed to runoff and, if not managed properly, the runoff could cause erosion and increased sedimentation in downstream creeks and stormwater conveyance facilities of the City of Pleasanton. The potential for chemical releases is present at most construction sites. Once released, substances such as fuels, oils, paints, and solvents could be transported to surface waters and/or groundwater in stormwater runoff, wash water, and dust control water, potentially reducing the quality of the receiving waters. A potential impact on runoff water quality during construction is a potentially significant impact. 89