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.