Orestimba Creek Feasibility Study - Stanislaus County

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Economics Appendix – Draft Report - Orestimba Creek Feasibility Study, Stanislaus County, California – September 2012 furniture rentals, etc). Due to the relatively shallow alluvial fan type flooding seen from the Orestimba Creek, annual damages for emergency costs were very minimal. Damage Category Table 4-5: Emergency Costs Depth-Damage Curves Depth in Feet -1 0 0.5 1 1.5 2 3 4 5 6 7 8 9 10 % Damage to Structure/Content SFR1 0 17 28 38 49 57 74 89 100 100 100 100 100 100 SFR2 0 0 15 21 28 34 45 55 66 77 85 94 100 100 MFR1 0 17 28 38 49 57 74 89 100 100 100 100 100 100 MFR2 0 0 15 21 28 34 45 55 66 77 85 94 100 100 Mobile Home 0 0 81 85 85 87 100 100 100 100 100 100 100 100 Cleanup Costs Cleanup Costs mainly include costs associated with cleanup of debris resulting from flooding. Cleanup costs were derived using structure values and a depth damage curve used in other Corps studies, such as the Yuba General Re-evaluation Report, (normal distribution and 8% standard deviation). The cleanup costs depth damage curve is shown below in Table 4-6. Similar to emergency costs, because of the shallow flooding that we see with the Orestimba Creek, cleanup costs turn out to be minimal. Damage Category Table 4-6: Cleanup Cost Depth-Damage Curve Depth in Feet -1 0 0.5 1 1.5 2 3 4 5 6 7 8 9 10 % Damage for Cleanup Cleanup 1.00 1.50 1.75 2.00 2.25 2.50 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 For all three of these categories, elevation uncertainty was the same 0.5 feet as used in structure analysis. Agricultural Damages Agricultural Damages were estimated by event in a model outside of HEC-FDA. These event damages were then input into HEC-FDA in order to estimate EAD. For a more detailed writeup of this analysis and the assumptions used, please refer to Attachment B of this appendix. 20
Economics Appendix – Draft Report - Orestimba Creek Feasibility Study, Stanislaus County, California – September 2012 5.0 Expected Annual Damages –Without Project Conditions 5.1 HEC-FDA Model For the Orestimba Creek Feasibility Study, expected annual damages were estimated using the US Army Corps of Engineers, FRM-PCX certified risk-based Monte Carlo simulation program HEC-FDA v. 1.2.5a. Risk is a function of both probability and consequence, and the fact that risk inherently involves chance leads directly to a need to describe and plan for uncertainty. Corps policy has long been to acknowledge risk and uncertainty in anticipating floods and their impacts and to plan accordingly 4 . Historically, that planning relied on analysis of the expected long-term performance of flood-damage reduction measures, application of safety factors and freeboard, designing for worse case scenarios, and other indirect solutions (such as engineering judgment) to compensate for uncertainty. These indirect approaches were necessary because of the lack of technical knowledge of the complex interaction of uncertainties in estimating hydrologic, hydraulic, geotechnical, and economic factors due to the complexities of the mathematics required for doing otherwise. However, with advances in statistical hydrology and the availability of computerized analysis tools (such as HEC-FDA described below), it is now possible to improve the evaluation of uncertainties in the hydrologic, hydraulic, geotechnical, and economic functions. Through this risk analysis, and with careful communication of the results, the public can be better informed about what to expect from flood-damage reduction projects and thus can make more informed decisions. The determination of EAD for a flood reduction study must take into account complex and uncertain hydrologic, hydraulic, geotechnical, and economic information: Hydrologic - The discharge-frequency function describes the probability of floods equal to or greater than some discharge Q, Hydraulics - The stage-discharge function describes how high (stage) the flow of water in a river channel might be for a given volume of flow discharge, Geotechnical - The geotechnical levee failure function describes the levee failure probabilities vs. stages in channel with resultant stages in the floodplain, and Economics - The stage-damage function describes the amount of damage that might occur given certain floodplain stages. 5.2 Estimation of Expected Annual Damages Figure 5-1 illustrates the conceptual risk approach for Corps’ flood damage analyses. To find the damage for any given flood frequency, the discharge for that frequency is first located in the discharge-frequency panel (panel #1), then the river channel stage associated with that discharge value is determined in the stage-discharge panel (panel #2). For the Orestimba Creek study it 4 In a flood risk management study, risk is defined is the probability of failure during a flood event. Uncertainty is the measure of the imprecision of knowledge of variables in a project plan. 21
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Orestimba Creek Feasibility Study - Stanislaus County

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