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3-10 Forward Composting Facility SWT Engineering Report of Composting Site Information - January 2022 Specific precipitation intensities are based on precipitation data from the Stockton weather station number B00-8560 located approximately 8 miles northwest of t he site at latitude 38°00', longitude 121 °316', and an elevation of 12 feet National Geodetic Vertical Datum (NGVD) (California Department o f Water Resources, 1981). This precipitation station was selected due to its long and reliable period of record and its proximity to the project site. The precipitation depth-duration-frequency data for the site was obtained from the Stockton Weather Station Number B00-8560 (except where noted): ♦ 100-year, 24-hour storm intensity of 3.57 inches* ♦ 1,000-year, 24-hour st orm intensity of 4.92 inches* ♦ 100-year annual precipitation of 25.42 inches ♦ Annual mean precipitation of 13.31 to 14.35 inches *Source: Non-Water Corrective Action Plan for Forward Landfill, GeoLogic Associates, November 2011 ; Updated November 2017 . From the lat e 1980s to the early 1990s as well as recently, California is experiencing a severe drought. Although the precipitation data available during this period may not represent the statistical minimum precipitation that could occur at the project site, the lik ely minimum precipitation range anticipated during the service life and post -closure period of the landfill can be estimated. Table 9 presents monthly average total precipitation recorded for the Stockton Area, for a period of 62 years, between 1950 and 2012. During this period, the average annual precipitation recorded for the Stockton area was approximately 17.87 inches. Considering the drought conditions experienced during that period, a minimum annual average precipitation of 17.87 inches during the service life and post -closure period of the landfill site was estimated. As shown in Figure 8, the Forward site is located between the North Fork and the South Fork of the South Littlejohn’s Creek. Based on the Federal Emergency Management Agency (FEMA) Flood Insurance Rate Maps (FIRMs) for the San Joaquin Valley, Forward Landfill is located within four FIRM Panels: 0490F; 0495 F; 0630F; and, 0635F. From the FIRM Legend, the majority of the landfill (including the RRF and Composting Facility) is located within “Zone X”, which are areas determined to be outside the 0.2 percent annual chance of being in a floodplain. The South Fork of South Littlejohn’s Creek runs south of and adjacent to the southern perimeter of the refuse footprint of the landfill. From the FIRM map (Map No. 06077C Panels 0635F and 0630F), the existing So uth Fork of South Littlejohn’s Creek is labeled as “Zone AO, (Depth 1)”, indicating depths of one foot, and Zone AE, indicating b ase flood elevations within the creek. These current flows are very slow and non -erosive and as a result, will not inundate or potentially erode the adjacent creek or inundate the landfill. The predicted elevations from the 500 -year flood event range fro m 40.5 feet on the southwest edge to 43 feet on the east edge of the landfill along Austin Road and this level is only one to t wo feet higher than the toe of existing landfill slopes. The existing South Fork of South Littlejohn’s Creek was designed to channel the 100 -year flood flow of 1,550 cfs within its banks and an additional one foot of freeboard. In the event that a 500 -year flood occurs (1,700 cfs, according to FEMA Flood Flows), the excess flow will spill over the southerly bank and into the adjacent area, rather than impact the Forward Landfill. The South Fork of