Orestimba Creek Feasibility Study - Stanislaus County

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Economics Appendix – Draft Report - Orestimba Creek Feasibility Study, Stanislaus County, California – September 2012 Table 6-2: Optimization of the Chevron Levee height at Stuhr Rd and CCID Canal Crossing Return Period Levee Elevation (ft.) *Annual Benefits ($1,000's) *Annual Costs ($1,000's) *Net Benefits ($1,000's) 1/50 111.21 6,079 749 5,330 111.25 6,188 753 5,435 111.5 6,412 777 5,636 111.75 6,612 800 5,812 1/500 112 6,785 823 5,962 112.25 6,910 847 6,063 112.5 6,990 870 6,120 112.75 7,035 893 6,142 113 7,056 917 6,139 113.25 7,064 940 6,124 113.5 7,066 964 6,103 113.75 7,067 987 6,080 114 7,067 1,010 6,057 114.25 7,067 1,034 6,034 1/200+3ft 114.8 7,067 1,085 5,982 *The Benefits and Costs shown here are different than those presented in Chapter 3 of the main report, because these show the actual values used for the sensitivity, while the main report has adjusted those values to reflect total project benefits and costs. Annual Benefits, Annual Costs and Net Benefits shown here do not reflect actual benefits and total costs used for final net benefit and BCR calculations. The index point used for this height optimization exercise is different from the one used for economic justification. Benefits and costs used for economic justification can be found in Table 7-1. More information about different index points and the effect on damages in this study area is shown in Section 5.5. 6.1.4.1 Sensitivity of Optimization Height to Index Location 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) 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 36
Economics Appendix – Draft Report - Orestimba Creek Feasibility Study, Stanislaus County, California – September 2012 optimized height is approximately 1.5ft above the existing mean 2% (1/50) WSEL, which seems reasonable for an alluvial fan floodplain given uncertainty in ground elevations and stages. 6.1.5 Incremental Analysis of the Channel Modifications Although the channel modification (and associated “Hybrid” Plans) appeared to be incrementally justified during the preliminary screening of alternatives, more detailed cost estimates showed the increment to be significantly more costly. Because the channel modifications are a second increment to the project after the chevron levee, the benefits for this increment come from the Rural EIA and the ability of the modified creek to better contain some of the more frequent event flows which cause more significant flooding under existing conditions. See Figure 6-1 for the location of the Channel Modifications and see the Hydraulics Appendix for floodplains and more detailed information about this residual flooding. Updated incremental benefits for the Channel Mods as part of the “Hybrid” plan are shown in the table below. Table 6-3: Incremental Benefits of Channel Modifications October 2011 Prices ($1,000’s), 4% Interest Rate Alternative Annual Damages Annual Benefits Incremental Benefits Without Project 5,413 0 0 1. Chevron Levee 2,285 3,128 3,128 2. Chevron Levee + Channel Mods 3. Local Plan (Taller Levee + Mods) 1,800 3,613 484 1,800 3,613 0 Updated floodplains and economics showed the incremental annual benefits for the channel modifications to be around $500 thousand. Updated (but not completed) costs for the channel modifications were estimated to be around $1.3 million in annual costs ($28 million first costs) on the lower end. These costs are on the low end because they did not yet include environmental mitigation, OMRR&R or real estate costs, which could add another $1 million in annual costs to the increment. Before these additional costs are added to the channel modifications, it has an incremental benefit-to-cost ratio of 0.39 to 1, and thus “The Hybrid” plan was dropped from the federal interest as being economically infeasible. Development of more detailed MCACES level costs for this increment for the final array of alternatives was forgone. 6.2 The Final Array of Alternatives Economic efficiency is measured based on the maximization of project net benefits. Net benefits are determined as the difference between the annual benefits and the annual costs of an alternative. While net benefits were very similar for many of the preliminary plans, it was 37
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