Soscol Gateway Interior Drainage Improvement
City of Napa, California
West Yost reviewed previously prepared technical studies and schematic-level designs of two design alternatives to handle overland drainage in the Soscol Gateway area of Napa. Following this review, West Yost developed an alternative drainage plan that had an estimated cost of about $6.4 million, nearly a 50% cost savings from the previously developed project (at about $12 million). In addition, the performance of this alternative, low cost approach was verified as better than the performance of the previous plan.
The project includes hydrologic/hydraulic modeling of the
existing floodplain and alternative improvement projects,
coordination with private land owners, coordination with the
CEQA consultant, and regulatory agencies (California Department
of Fish and Wildlife, etc.). The engineering evaluations for this
project included confirmation of the sizing of a proposed US Army
Corps of Engineers detention basin/pump station, evaluation of
existing storm drains (sized for a 10-year storm) and channels to
partially convey the 100-year storm flows, sizing of a proposed
new pump station, and determination of the residual flooding
depths, flow directions, and velocities through 2-dimensional
flood modeling using XP-SWMM.
The 2-dimensional flood modeling results were presented in easily
understandable maps of floodwater depths and flow vectors (arrows
depicting the direction and velocity) for existing conditions.
From these existing condition maps, improvements were developed
that reduce the 100-year flooding to a level that will include
only minor ponding in the public streets. Flooding depth and
vector maps were prepared for the improvement alternatives.