ARCHIVED REPORTS_1993_1 - Laserfiche WebLink

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III. Environmental Setting, Impacts,and Mitigations K. Hydrology, Geohydrology,and Water Quality Impact K.3. Stormwater runoff from landfill surfaces,if not properly controlled,could contribute to increased peak flows downstream,or degrade surface water in receiving waters. (SIGNIFICANT) During the active life of the landfill,clean storm water would be routed and collected separately from the leachate system by a network of ditches and a detention basin,prior to discharge to Little Johns Creek. Stormwater runoff from the active landfill side slopes would be collected and treated as leachate. Stormwater that has come in contact with the landfill surface would be prevented from mingling with stormwater from other portions of the site. The landfill surface water would be routed to a detention facility located in the southwest comer of the site. The detention facility would be designed and constructed to meet the requirements of the San Joaquin County storm drainage standards and Title 23 regulations. Sizing of the facility is based on a 100-year, 24-hour storm, for the full closure area of both the existing and expansion areas.3 Peak flows from the site for ultimate development of the landfill, after the entire landfill has been capped have been modeled using the SCS TR-20 computer model. The post-project peak flows as shown in Table III.K.3 includes the proposed detention basin design(see Table III.K.3). The detention pond would be excavated below the existing grade and discharge would be achieved by gravity or pumping. The pump would be capable of emptying the detention basin within 48 hours. The detention pond would gravity discharge to the creek until the elevation in the pond drops to a designated elevation,at which time the pumps engage.Two primary pumps and one back-up pump would be installed,with a maximum pump capacity of 100 cubic feet per second(cfs). In the event of a pump failure/power failure coincident with a high precipitation event,the back-up pump and generator power system could be used to discharge from the pond. The major components of the stormwater system: ditches, pipes/culverts,detention basins,berms and oil/water separators would be inspected during major storms to assure proper functioning of the system. 3 Since portions of the storm runoff may not be routed throught the detention basin for certain interim periods,the final full closure situation represents the worst-case site runoff generation conditions for the life of the project. III.K.13