Orestimba Creek Feasibility Study - Stanislaus County

Economics Appendix – Draft Report - Orestimba Creek Feasibility Study, Stanislaus County, California – September 2012
6.1.1 Project Conditions- Model Simulations
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.
6.1.2 Average Annual Equivalent Damages –With Project Conditions
All benefit values in the remaining tables of this report included average annual equivalents,
which because of our assumptions, equal Expected Annual Damages for the Orestimba Study.
6.1.3 Alternatives Evaluated – Preliminary Screening Net Benefit Analysis
Incremental benefit evaluation to determine which plans to move forward for optimization into
the final array was formulated based on reasonable separable project features and sizing.
Features were identified and categorized based on potential flood reduction and magnitude of
cost. Levee Design Height (height based on depth of water for the given flood event at levee
location) was used to select reasonable increments for benefit evaluation. 2%, 1%, 0.5% and
0.2% annual chance design heights were considered. As mentioned earlier, details of the project
components and selection can be found in the main report. With project residual damages and
benefits were modeled for all of the increments/alternatives listed in Table 6-1. In total,
seventeen different project increment combinations were modeled and analyzed economically
during this screening process. Annual benefits in the table, represent the difference between the
without and with project equivalent annual damages for each alternative. Costs are based upon
preliminary reconnaissance costs, but are all at the same level of detail for this screening
purpose. Because this analysis was done in 2009 and only serves as a screening to in order to
choose plans to further optimize, price levels were not updated from October 2008.
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