Orestimba Creek Feasibility Study - Stanislaus County

More documents

Recommendations

Info

Economics Appendix – Draft Report - Orestimba Creek Feasibility Study, Stanislaus County, California – September 2012 was assumed that levees would only overtop and would not fail, so only non-damage levee heights were put into the FDA model and the resulting residual floodplains were analyzed. Once the banks are overtopped and water enters the floodplain, the stages (water depths) in the floodplain inundate structures and crops and cause damage (panel #4, left side). 5 By plotting this damage and repeating for process many times, the damage-frequency curve is determined (panel #4, right side). 6 EAD is then computed by finding the area under the flood damage-frequency curve by integration for both without and with project conditions. Reductions in EAD attributable to projects are flood reduction benefits. Figure 5-1: Uncertainty in Discharge, Stage and Damage in Determination of Expected Annual Damages Source: San Joaquin and Sacramento River Basins Comprehensive Study (2002) Uncertainties are present for each of the functions discussed above and these are carried forth from one panel to the next, ultimately accumulating in the EAD. These uncertainties are shown in Figure 5-1 as “error bands” located above and below the hydrologic, hydraulic and economics curves. 7 Some of the important uncertainties specific to the Orestimba Creek Study include: 5 For reaches with no levees, the stage in the channel and overbank areas is used to determine damage. 6 The HEC-FDA model uses Monte Carlo analysis to repeat this “sampling” process thousands of times. Mathematically, HEC-FDA computes EAD in a different manner than illustrated by this figure. 7 Uncertainty in the geotechnical levee probability of failure curves are multitude in character and the resultant curve used in the analysis reflects how well that levee can be expected to perform during random periods of high flows for a particular reach length. Typically, the greater the length of the levee reach, the less reliable that reach will perform during a flood event. 22
Economics Appendix – Draft Report - Orestimba Creek Feasibility Study, Stanislaus County, California – September 2012 Hydrologic - Uncertainty factors include hydrologic data record lengths that are often short or do not exist, precipitation-runoff computational methods that are not precisely known, and imprecise knowledge of the effectiveness of flow regulation. 8 Hydraulics - Uncertainty arising from the use of simplified models to describe complex hydraulic phenomena, including the lack of detailed geometric data, misalignments of hydraulic structures, debris load, infiltration rates, embankment failures, material variability, and from errors in estimating slope and roughness factors. For the Urban EIA as standard deviation in stage of 0.6 feet was used, while 1.5 feet was used for the Rural EIA (EM-1110-2-1619 guidance for minimum uncertainty standards alluvial fan vs. channel stage). Geotechnical – Under without project conditions, we have assumed the CCID canal fails when overtopped for hydraulic modeling purposes. This is based on input from the SPK geotechnical section and historical performance. Economics - Uncertainty concerning land uses, depth/damage relationships, structure/content values, structure locations, first floor elevations, the amount of debris and mud, flood duration, and warning time and response of floodplain inhabitants (flood fighting). 5.3 Probability-Stage-Damage Functions Damages were estimated by each category, by impact area and by event based on varying depths within the flood plain relative to individual structures. These damages were then linked to a stage within the channel (or the floodplain) at a common index point for each impact area by exceedance probability. For the Urban EIA, an index point in the floodplain was used at the first point flooding within the area at the Railroad and Lundy Rd intersection. Index locations used for can be found in Figure 5-2. These stage-damage functions were calculated within HEC-FDA and they represent the economic relationships between the hydrology and hydraulics. These functions can be found by event, damage category and impact area in the tables below. These damage values differ from the calculated damages by event shown in the HEC-FDA calculated stage-damage curves due to uncertainties in each relationship. Damages within FDA are aggregated back to the chosen index location. It is noted that there is a difference between the event damages calculated within HEC-FDA in the tables below and the actual damages reported from past flood events (Section 1.3), but this is to be expected. Historical record is not believed to be a good reference for actual damages in the area. A 2002 national study 9 of local flood damage estimates showed that “individual damage estimates for small floods or for local jurisdictions within a larger flood area tend to be extremely inaccurate. It is rare to have actual cost data to compare with damage estimates. The […] analysis of one large flood disaster indicates that, in cases where actual costs are less than $30 8 The hydrologic data record lengths (period of record) are the number of years of a systematic record of peak discharges at a stream gage. This parameter directly influences the uncertainty associated with the frequency-discharge function shown in Figure 5-1 and consequently the project performance statistics. In general, a longer period of record implies less uncertainty associated with this function. The period of record for Orestimba Creek Hydrology is 45 years. 9 Pielke, Jr., R.A., M.W. Downton, and J.Z. Barnard Miller, 2002: Flood Damage in the United States, 1926-2000: A Reanalysis of National Weather Service Estimates. Boulder, CO: UCAR. 23
Page 1 and 2: Economics Appendix - Draft Report -
Page 45: Economics Appendix - Draft Report -
Page 97 and 98:
Economics Appendix - Draft Report -
Page 99 and 100:
Page 101 and 102:
Page 103 and 104:
Page 105 and 106:
Page 107 and 108:
Page 109 and 110:
Page 111 and 112:
Page 113 and 114:
Page 115 and 116:
Page 117 and 118:
Page 119 and 120:
Page 121 and 122:
Page 123 and 124:
Page 125 and 126:
Page 127 and 128:
Page 129 and 130:
Page 131 and 132:
Page 133:
show all

Orestimba Creek Feasibility Study - Stanislaus County

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?