Orestimba Creek Feasibility Study - Stanislaus County

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Economics Appendix – Draft Report - Orestimba Creek Feasibility Study, Stanislaus County, California – September 2012 Table 5-4: Number of structures flooded by event – Urban EIA only Damage Category 10% (1/10) Annual Chance Exceedance Flood Event 5% (1/20) 2% (1/50) 1% (1/100) 0.5% (1/200) 0.2% (1/500) Commercial 51 60 61 62 62 62 Industrial 15 15 16 16 16 16 Public 5 7 7 7 7 7 Residential 409 675 748 828 860 885 Total 480 757 832 913 945 970 %increase 0% 58% 10% 10% 4% 3% 5.4 Existing Condition Expected Annual Damages Expected Annual Damages, under existing without project conditions, were estimated for each damage category for both impact areas. Results are summarized in Table 5-5 below. Table 5-5: Expected Annual Damages - Without Project Conditions October 2011 Prices ($1,000’s) Damage EAD by Impact Area Category Rural Urban TOTAL Agriculture $1,381 $0 $1,381 Auto $61 $50 $111 Cleanup $37 $183 $220 Emergency $148 $194 $341 Commercial $0 $520 $520 Industrial $0 $417 $417 Public $0 $33 $33 Residential $657 $1,733 $2,389 TOTAL $2,284 $3,130 $5,413 5.5 Effect of Risk and Uncertainty on Existing Condition Urban Expected Annual Damages in HEC-FDA Due to the alluvial fan nature of the Orestimba floodplains, the actual location of flooding and extent of flooding is inherently uncertain. Small changes to topography could impact spreading of the water significantly. Because the rating curves (flow vs. stage) are relatively flat for the Urban EIA area, uncertainty in the stage (0.6ft standard deviation) can have a large (increasing) effect on EAD when applied during Monte Carlo simulations. HEC-FDA is generally very sensitive to flat H&H curves. The index point (IP) used for reporting purposes in this document 26
Economics Appendix – Draft Report - Orestimba Creek Feasibility Study, Stanislaus County, California – September 2012 is located at the intersection near the Lundy Road and the Railroad, which is where initial floodwater enters the Urban impact area. An index location directly next to the channel was not deemed to be to be a good representation of the nature of flooding (stage increase) in Newman due to the spreading of the water that occurs when it leaves the channel. Because EAD results are sensitive to rating curve changes, other index locations were tested in HEC-FDA to test the impact on the EAD results in Newman. Sensitivity IP 1 is located on the CCID Canal at the point of initial overtopping into Newman. Sensitivity IP 2 is located in the center of Newman in an area that is representative of stage increases in the city itself. Sensitivity IP 3 was the location used for the rural area, at the creeks initial point of overtopping. The index locations of the sensitivity runs are shown along with the documentation location in Figure 5-2 below. The sensitivity runs show that the flatter the rating curve on the more frequent end, the more sensitive the results are to this stage uncertainty and the higher the EAD. These results can be found in Table 5-6 below. The index point used for this document appears to show a conservative EAD estimate compared to Sensitivity IP’s 1and 2, but is one that is more in line with historical flooding in the area. One last sensitivity run was done “without R&U” to see how much the overall results were being impacted in general and the result was an EAD of $2.8 million, which is about 20% less than the “with R&U” result. This 20% seems to be in line with a modest increase in EAD usually seen when R&U is used. 27
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Orestimba Creek Feasibility Study - Stanislaus County

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