Orestimba Creek Feasibility Study - Stanislaus County

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Economics Appendix – Draft Report - Orestimba Creek Feasibility Study, Stanislaus County, California – September 2012 The HEC-FDA model results (Expected Annual Damages) for contents, structures, automobiles, cleanup costs and emergency costs for each economic impact area (EIA) under the without project condition from are found in Table E- 4. Total study area without project expected annual damages are equal to approximately $5.4 million. Table E- 4: Expected Annual Damages - Without Project Conditions October 2011 Prices ($1,000’s) Damage EAD by Impact Area Category Rural Urban TOTAL Agriculture $1,381 $0 $1,381 Auto $61 $50 $111 Cleanup $37 $183 $220 Emergency $148 $194 $341 Commercial $0 $520 $520 Industrial $0 $417 $417 Public $0 $33 $33 Residential $657 $1,733 $2,389 TOTAL $2,284 $3,130 $5,413 ES.5 With-Project Damages and Benefits This section will describe how benefits of flood damage reduction of the final array of alternatives were estimated. Non-monetary outputs such as environmental measures, which may vary for the final array of alternatives, are not included but may factor in the plan formulation decision process. Benefits were determined by incorporating residual with-project floodplains and rating curves into the FDA model that represent various with project improvements. Flood risk management benefits equal the difference between the without project damage conditions and the with-project residual damages. The reduction in project flood plains in both extent and depth from the larger without project flood plains accounts for the decrease in damages of the given alternative. Optimization of Chevron Levee Height The chevron levee height was optimized by inserting incrementally higher levees into HEC-FDA and comparing the increased benefits to the estimated incremental costs. Costs were estimated by Cost Engineering for a levee equal to the mean 2% (1/50) WSEL and the mean 0.5% (1/200) WSEL + 3ft. These two values were then used to create a linear interpolation between the two points in order to estimate the cost of incrementally higher levees. These costs include only ES-viii
Economics Appendix – Draft Report - Orestimba Creek Feasibility Study, Stanislaus County, California – September 2012 those costs that are variable depending on the height of the levee (mostly fill material). The index point used for this height optimization was at the Stuhr Rd. and CCID canal intersection, so the benefits here appear higher than those reported in the without project chapter of this report. The optimization of the levee height must use an index point at a location where the levee would be located to come up with a proper levee elevation. The particular index point location for the optimization was chosen because it was the first point that flooding overtopped the existing CCID canal (near Stuhr Rd), which is adjacent to the chevron levee’s location. This was the original location for the economic evaluation as well, but the index was changed later based on rating curve sensitivity for high probability events (see section 5.5 of the main Economic Appendix) and likely overestimation of EAD. Because the CCID Canal location and the eventual urban economic analysis index location (intersection of Lundy Rd. and the Railroad) have rating curves with nearly identical slopes for lower probability events (2% (1/50) annual chance event and above), the difference in an optimization height was assumed negligible. To confirm this assumption, a sensitivity analysis was performed using the Lundy Rd. index location and the optimal heights were found to be within 0.2 ft of each other and well within error bands in stage/ground elevations. The final benefits used for economic justification will be based on the without project conditions as summarized in Table E- 6. In addition, costs reported here do not reflect Total Project First Costs; instead these costs represent only those variable costs in relation to levee height. Total Project First Costs can be found in Table E- 10. This analysis is solely used for optimization of the levee height in order for costs and residual benefits to be determined in more detail on only one plan. As shown in Table E- 5 below, the optimal elevation for the top of levee at this index point is determined to be around 112.75 feet, which equates to a levee 5.5 to 8 feet tall depending on the ground elevation changes along the levee alignment. It is noted here that this height is higher than the mean 0.2% (1/500) WSEL, but because of the alluvial fan type of flooding, the mean 0.2% (1/500) WSEL is only 9 inches higher than the mean 2% (1/50) mean WSEL. ES-ix
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