Orestimba Creek Feasibility Study - Stanislaus County

Economics Appendix – Draft Report - Orestimba Creek Feasibility Study, Stanislaus County, California – September 2012
6.5 Project Performance – With Project Conditions
Table 6-10 and Table 6-11 below show a comparison of project performance under both with
and without project conditions by economic impact area.
The annual exceedance probability measures the chance of having a damaging flood in any given
year. As larger increments are analyzed, the annual exceedance probability (AEP) drops.
The long-term risk numbers measure the chance of having one or more damaging flood over a
giver period of time. As shown in Table 6-11 building the chevron levee reduces the chance of
getting damaged (within the Urban EIA) over the next 30 years from 100% under the without
project condition to only 1% with the project.
The assurance probability measures the probability of not being damaged if a given event were
to occur. As with the other measures, project conditions reduce the risk and larger projects have
a greater reduction in risk than small projects. The Assurance for the 1% annual chance event is
often targeted to determine if a project meets Corps criteria for levee certification. It is important
to note the relationship between AEP and Assurance in determining project accomplishment.
For example, in the rural impact area, adding the channel modifications takes the area from a 1%
annual chance of non-damage to a 5% chance of non-damage due to a 1% annual chance flood
event. To be 90% confident that the 1% annual chance event can pass without causing damage
in the rural impact area, a larger project with a smaller AEP must be constructed. This often
causes confusion in how to identify the performance of a project in a single traditional term such
as “100-year level of protection,” and as per the guidance ER 1105-2-101, the Corps has dropped
all reference to describing level of protection.
It is noted that the Assurance for the chevron levee in the urban area is extremely high even for
low frequency events. This is due to the relatively small change in stages going from frequent to
non-frequent events in an alluvial fan floodplain. While Assurance appears high, the federal
interest chevron levee still does not meet FEMA levee certification criteria as it falls about 1.5
feet short of the FEMA certification criteria. According to EC-1110-2-6067, FEMA levee
certification requires a levee to be either: a) 3ft above the mean 1% annual chance WSEL and
has 90% Assurance for the 1% annual chance event OR b) 2ft above the mean 1% annual chance
WSEL and 95% Assurance for the 1% annual chance event. The levee for Alternative 3 (Local
Plan) would meet FEMA levee certification criteria.
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