Orestimba Creek Feasibility Study - Stanislaus County

Economics Appendix – Draft Report - Orestimba Creek Feasibility Study, Stanislaus County, California – September 2012
6.1.4 Optimization of the Chevron Levee Height
The chevron levee height was optimized by inserting incrementally higher levees into HEC-FDA
and comparing the increased benefits to the estimated incremental costs. Costs were estimated
by Cost Engineering for a levee equal to the mean 2% (1/50) annual chance WSEL and the mean
0.5% (1/200) annual chance WSEL + 3ft. These two values were then used to create a linear
interpolation between the two points in order to estimate the cost of incrementally higher levees.
These costs include only those costs that are variable depending on the height of the levee
(mostly fill material). The index point used for this incremental analysis was the Stuhr Rd. and
CCID canal intersection, so the benefits here appear higher than those reported in the without
project chapter of this report, but this is due to the rating curve and stage uncertainty as discussed
above in section 5.5. The final benefits used for economic justification will be based on the
without project conditions as summarized in Table 5-5. In addition, costs reported here do not
reflect Total Project First Costs, instead these costs represent only those variable costs in relation
to levee height. Total Project First Costs can be found in Table 7-1. This analysis is solely used
for optimization of the levee height in order for costs and residual benefits to be determined in
more detail on only one plan. As shown in Table 6-2 below, the optimal elevation for the top of
levee at this index point is determined to be around 112.75 feet, which equates to a levee 5.5 to 8
feet tall depending on the ground elevation changes along the levee alignment. It is noted here
that this height is higher than the mean 0.2% (1/500) WSEL, but because of the alluvial fan type
of flooding, the mean 0.2% (1/500) WSEL is only 9 inches higher than the mean 2% (1/50) mean
WSEL.
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