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ie APPENDIX D STORMWATER DETENTION CALCULATIONS 1. Introduction i The purpose of this investigation was to develop a conceptual plan and perform the preliminary sizing of the Austin Road Landfill stormwater detention facilities to meet San Joaquin County drainage standards. In accordance with County requirements,the SCS TR-20 hydrologic computer model was used for the analysis. Applicable County stormwater standards which apply to the proposed stormwater detention facilities are included as Attachment A. The major assumption was that the detention pond must be sized to prevent any increase in peak run-off rates to Little Johns Creek between the current landfill conditions and the ultimate development of the landfill. The detention pond sizing done for this analysis was based upon ultimate development of the landfill. i County standards specify that detention ponds with gravity outlets need to operate without 3 backwater conditions. For the Austin Road Landfill to meet this requirement, the bottom of the pond must be above elevation 36 +/- to be above the FEMA flood elevation of 35. Based upon preliminary sizing estimates, the pond size needs to provide approximately 15-20 acre-feet of storage. Providing this much storage above elevation 36 and yet draining the entire site would be difficult. For example, a pond of approximately 200' x 700' would require a depth of approximately 7 feet, up to elevation 43. Because much of the site is lower than elevation 43, this approach was rejected. As a result, we recommend that the detention pond be constructed at a lower elevation and employ a pump discharge. Lowering the pond elevation will allow gravity drainage to the pond from the landfill site. Another method to provide the required on-site storage that was not considered, yet could be an alternative approach, includes combining the on-site detention storage with the compensatory flood storage required to relocate Little Johns Creek. This could be done in such a manner as to prevent any increases in the creek peak flow rates or in the water surface elevations as a result of relocating the creek and developing the landfill. This alternative was not considered because analyzing the required storage volume would require a detailed hydraulic analysis of Little Johns Creek which is beyond the scope of this investigation. 2. Analysis s The methodology used in this investigation included two major steps. The first step involved a determination of the peak flow rate discharging to Little Johns Creek under current conditions. The second step involved calculating the detention pond's storage volume in order to maintain the current peak run-off rates once the entire site is developed into a landfill and capped. 1 2a. Step 1 - Current Conditions General Site Conditions The landfill area topography is illustrated on Figure A. Elevations are generally flat ranging from approximately 20 feet mean sea level (MSL) in the excavated borrow areas to approximately 84 feet MSL at the highest point on the waste mass. The area planned for