This report presents a preliminary analysis to determine the effect that setback
levees would have along a specified reach of the Sacramento River. This was done
using a three-scenario strategy such that recommendations could arise from the
analysis of several setback width options. The project reach ranges from river mile
143, just south of Colusa, to river mile 84, near the Sutter and Yolo bypasses.
Three setback scenarios were analyzed, and the inter-levee distance was 3000ft,
6000ft, and 9000ft, respectively. Each scenario was analyzed in terms of
hydrology, ecology, and economics. The floodplain inundation depth and the
change in channel velocity were determined for each scenario at several cross
sections using a number of standard flood recurrence intervals. An overview of
river meandering processes was given in order to help explain the potential
ecological benefits of setback levees. The change in riparian habitat was predicted
by comparing the study reach to ecologically similar reaches having a wider interlevee
distance. To determine the effect of setback levees on riparian biota, the
habitat requirements of indicator bird species were examined. In addition, changes
in the hydraulic parameters were used to describe the ecological consequences to
fish habitat. The cost estimates of implementing the scenarios were compared to
the benefits of creating increased area for riparian habitat, which were estimated
using a willingness to pay report for wetland habitat.
The analysis of the three scenarios indicates that benefits increase with increased
inter-levee distance, and scenario three was found to provide the greatest benefits.
For the aquatic ecosystem, this scenario establishes the most desirable conditions
for improving habitat because channel velocity is decreased and there is great
potential for backwater habitat formation. In terms of the terrestrial ecosystem, the
area of willow, cottonwood and mixed riparian communities (the most common
communities found in Sacramento Valley riparian habitat) is maximized under this
scenario. This scenario also allows the most freedom for channel migration to occur
over time, potentially establishing a more diverse range of aquatic and terrestrial
habitats. Furthermore, economic analysis shows this scenario to be the most
attractive, as the calculated cost/benefit ($238 million/$1.1 billion) ratio is at a
minimum (0.23).