Orestimba Creek Feasibility Study - Stanislaus County

Economics Appendix – Draft Report - Orestimba Creek Feasibility Study, Stanislaus County, California – September 2012
Table 5-3: Without Project Probability-Stage-Damage Functions – TOTAL STUDY
AREA
October 2011 Prices ($1,000’s)
Damage
Category
Annual Chance Exceedance of Event
99% 20% 10% 5% 2% 1% 0.50% 0.20%
AUTO $0 $101 $241 $311 $380 $451 $508 $581
Clean $0 $66 $1,266 $1,920 $2,115 $2,295 $2,365 $2,461
Emergency $0 $386 $895 $1,532 $1,788 $2,181 $2,367 $2,554
Commercial $0 $39 $2,266 $3,210 $3,386 $3,699 $3,798 $3,919
Industrial $0 $0 $2,232 $2,414 $2,476 $2,547 $2,586 $2,685
Public $0 $0 $78 $142 $151 $177 $182 $189
Residential $0 $824 $6,912 $14,381 $16,819 $19,463 $20,555 $21,917
Agricultural $0 $3,173 $5,751 $7,211 $8,474 $9,283 $9,978 $10,696
TOTAL $0 $4,590 $19,641 $31,120 $35,589 $40,095 $42,341 $45,003
5.3.1 Stage-damage curves and alluvial fan (FLO-2D) floodplain modeling
Table 5-2 above shows a significant increase (83%) in urban damages between the 10% (1/10)
annual chance event and the 5% (1/20) annual chance event, while only showing a 0.1 foot
increase in WSEL at the index location. While this large increase in damages given a small
increase in WSEL may not make intuitive sense for traditional “storage area” HEC-RAS
floodplain modeling, it does make sense when looking at alluvial fan floodplain spreading FLO-
2D modeling. The increase in damages vs. stage can be summarized for the Orestimba Study
area by the following two points:
1. A very small change in stage causes a large increase in flooding extent as the
water spreads over the alluvial fan. The number of structures within the
floodplain increases by 58% between the 10% (1/10) and 5% (1/20) annual
chance events (see Table 5-4). In addition to these newly flooded structures,
those that were already flooded are now slightly deeper causing additional
damages.
2. HEC-FDA uses stage-aggregated damages, meaning that all damages within an
Economic Impact Area must be aggregated back to the stage a single
representative index location. While the stage at this chosen index location may
be generally representative of the area, it does not mean that the WSEL over the
entire area only increases by 0.1 feet when modeled in FLO-2D. There are many
homes and smaller pockets that see a more significant 0.5 -1 foot increase in
flooding between these particular events.
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