ARCHIVED REPORTS_2013_4 - Laserfiche WebLink

Home Browse Search

FORWARD LANDFILL NON-WATER CORRECTIVE ACTION PLAN is 4.4.2 North Fork of South Littlejohns Creek The North Fork of South Littlejohns Creek was relocated to allow further expansion of the northern portion (Austin Unit) of the landfill, (Concept Desi?n Report o the North Branch of the South Fork ofLittlejohns Creek Relocation Project, Questa Engineering,July 2000). The 7,600-foot reach creek relocation was designed to channel the 100-year flood-flow of 1,025 cubic feet/second (cfs) (see the excerpt from this Report in Appendix B)within its banks with an additional one foot of freeboard. In the event that a 500-year flood occurs (1,110 cfs, according to FEMA Flood Flows), the excess flow will spill over the northern bank and into the adjacent farm land, rather than affecting the Forward Landfill. Based on the realignment of the North Fork of South Littlejohns Creek, the landfill is not located within the predicted elevation of the 500-year flood zone. Therefore, the existing realignment does not require an assessment for potential erosion or saturation of the landfill. 4.4.3 South Fork of South Littlejohns Creek The South Fork of South Littlejohns Creek runs south of and adjacent to the Forward Unit of the landfill. From the FIRM map (Map No. 06077C Panels 0635F and 0630F), the existing South Fork is labeled as"Zone AO, (Depth 1)", indicating depths of one foot, and Zone AE, indicating base 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 cover the landfill. The predicted elevations from the 500-year flood event range from 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 1 to 2 feet higher than the toe of the existing landfill slopes. The existing South Fork was designed to channel the 100-year flood flow of 1,550 cfs (see excerpt from Concept Design Report for the South Branch of the South Fork of Littlejohns Creek Relocation Project, Questa Engineering,November 2007 in Appendix C) within its banks with 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 alignment is also a very flat gradient, 0.0006 feet per foot; therefore, erosion is not a significant concern. Based on this analysis, the predicted elevations for both forks of South Littlejohns Creek will not impact the landfill; therefore, do not require an assessment for potential erosion or saturation of the landfill. M:\2011-012MWFA REPORT.DOCX 18 Geo-Logic Associates