Orestimba Creek Feasibility Study - Stanislaus County

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Economics Appendix – Draft Report - Orestimba Creek Feasibility Study, Stanislaus County, California – September 2012 In addition to damages estimates, HEC-FDA reports flood risk in terms of project performance. Three statistical measures are provided, in accordance with ER 1105-2-101, to describe performance risk in probabilistic terms. These include annual exceedance probability, long-term risk, and assurance by event. Annual exceedance probability measures the chance of having a damaging flood in any given year. Long-term risk provides the probability of having one or more damaging floods over a period of time. Assurance is the probability that a target stage will not be exceeded during the occurrence of a specified flood. Project performance for the Rural and Urban Impact areas were calculated in two different ways within HEC-FDA. The Rural project performance statistics are based upon the top of channel elevation at the first point that water leaves the creek during a flood event. For the Urban impact area a different methodology was used, instead basing project performance on the stage when damages reach 5% of the 1% event damages. The reason that the urban model did not use the top of channel methodology is because the town is miles away from the channel and the index point used is along the railroad where water first enters the urban impact area. Using the 5% residual damages for the 1% annual chance event seemed to give us reasonable results for AEP (15%) in the urban area as flooding begins between the 1/5 (20%) and the 1/10 (10%) annual chance event. Two different methods were used here, because the flooding patterns are different. Table 5-7 is attempting to communicate the existing flooding problem and how it differs between impact areas. Economic Impact Area Annual Exceedance Probability Median Table 5-7: Project Performance by EIA Without Project Conditions Expected Long-Term Risk 10 Year Period 30 Year Period 50 Year Period Assurance by Events 10% 2% 1% 0.20% Rural 24% 24% 94% 99% 99% 6% 2% 1% 1% Urban 14% 15% 81% 98% 99% 13% 0% 0% 0% 30
Economics Appendix – Draft Report - Orestimba Creek Feasibility Study, Stanislaus County, California – September 2012 6.0 With Project Conditions –Flood Damage Reduction Benefits Project Benefits – The Role of Economics in the Plan Formulation Process This section will describe how benefits of flood damage reduction of various potential alternatives were estimated. In this section, benefits and project performance outputs will be limited to flood damage reduction components. 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. 6.1 2009 Preliminary Incremental Alternatives Analysis An economic analysis was performed in 2009 which served as a screening to get to the final array of alternatives. After 2009, the Hydraulic and Economic models were updated and re-run for the final array. The results provided in this section summarize the screening results from the old models and therefore, the without project condition damages will not be the same as those provided in section 5.0 of this report. The 2009 analysis has not been updated for price level, so it remains in October 2008 prices and interest rates. On Orestimba Creek, flood damage reduction measures have been considered and screened during several phases of the study. In Table 6-1 below, you’ll find a matrix of most of the different measures that have been evaluated throughout the screening process and variations of the alternatives that have been screened. In this section of the economic appendix, flood damage reduction benefits have been explicitly calculated for the measures that might be feasible and have been carried forward in the plan formulation. Benefits were calculated on incremental basis, and these increments are described in more detail in the main report. In summary, there were 3 different categories of increments being considered for Orestimba. First were the pieces of the project which included a chevron levee, channel modification, interceptor channel, or some combination therein. Second was the location of chevron levee to be placed either north along the CCID Canal, then east along Stuhr Rd; or further north along the CCID Canal and then east along Lundy Rd. The third increment category was the design level of the project/levee to the mean WSEL of a specific event (i.e. 1%, 0.5%, 0.2% annual chance event design). The goal of this incremental benefit analysis is to answer three simple questions: what, where and how big should the project be Is there a federal interest to construct a continuous project providing flood damage reduction to all impact areas And what could be the optimal size of project for these areas For a more detailed description of these preliminary alternatives and other conceptual alternatives, please refer to the plan formulation portion of the main report. 31
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