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i 1 . TR-20 Model Parameters: o Rainfall Distribution: SCS type 1 o Antecedent Moisture Conditions: Average 2 o Rainfall Intensity: Approximately 3.2 inches for the 100-year 24 hour flood (NOAA Atlas 2, Precipitation Frequency Atlas of the Western United States, Volume XI - California) Current Conditions Results I The estimated 100-year peak flow from the site to Little Johns Creek is 134 cfs. The TR- 20 results for current conditions are included as Attachment C. 2b. Step 2 - Post-Development Conditions: General Site Conditions The proposed landfill area topography is illustrated on Figure B. The site conditions represent the ultimate development of the landfill after the entire landfill has been capped. Elevations range from approximately 35 feet MSL to approximately 150 feet MSL The area planned for the detention pond is located along.the southwesterly portion of the existing landfill. Open channel ditches will cant' runoff from the landfill to the detention pond. The site development plan calls for the relocation of Little Johns Creek to be located along the north and west property lines. As a part of the creek relocation, the County will require compensatory flood plain storage to ensure that there is no increase in the water surface elevation downstream or upstream of the landfill as a result of the relocation. As discussed earlier, the detention pond will include a pump station to discharge to Little Johns Creek during major flood events. However, during low flow conditions, it is preferable for the pond to gravity discharge to the creek. To maintain gravity discharge during low flow periods, the pond must be raised a minimum of one foot above the normal water elevations of Little Johns Creek. The normal water elevations of the creek should be monitored and established prior to final design of the pond. However, at this conceptual phase, it will be assumed that the normal water level of the creek is approximately 27 feet MSL at the westerly property line;therefore, the proposed pond bottom elevation is proposed to be 28 feet MSL. The pond would gravity discharge to the creek until the elevation in the pond reaches some design elevation, e.g., 30 feet MSL, at which time the pumps would engage. The pond would be designed to limit the peak discharge into Little Johns Creek to below the current predicted flow of 125 cfs. For the preliminary analysis, it was assumed that two primary pumps and one back-up pump would be used, as follows: r Pump 1 - 25 cfs capacity Pump 2 - 75 cfs capacity Pump 3 - 75 cfs capacity, (used only if pump 1 or 2 is out of service