Ulatis System Drainage Study
Solano County Water Agency, California

West Yost updated the flood control master plan for the Ulatis Creek Watershed. The objectives were to determine the City of Vacaville’s (City’s) existing level of flood protection along the major creeks draining through the City, evaluate flood control alternatives to increase the City’s level of flood protection, and identify preferred projects for implementation within each watershed.

The project included updating the hydrologic models (HEC-1) to current land use conditions, updating existing hydraulic models (HEC-RAS) with detailed surveyed cross-section data, and calibrating the models to stream gages located on several creeks using the 2002 and 2005 storms. The hydrologic modeling encompassed six large subsheds, totaling about 150 square miles; and hydraulic modeling of about 50 miles of open channels. West Yost presented the findings of the master plan to the public, City Council, and the Solano County Water Agency Board:

  • Updated the HEC-1)and HEC-RAS models for 150 square miles watershed with current land-use conditions and detention storage facilities (for 2002 and 2005)
  • Calibrated the updated models to produce flow and water surface elevation consistent with significant storms that occurred in December 2002 and December 2005
  • Evaluated the City’s level of flood protection and analyzed flood improvement alternatives to reduce or eliminate flooding within the City
  • Using HEC-RAS models, determined that the causes of the extensive flooding were under-capacity creek channels and increased rainfall over the last 30 years
  • Evaluated several flood protection alternatives for Ulatis and Alamo Creeks and determined construction and environmental feasibility and costs
  • The recommended alternative for Ulatis Creek included a 650 acre-ft regional detention basin, removal of wing walls on a drop structure to increase creek capacity, and construction of 2,000 linear feet of levees
    The recommended alternative for Alamo Creek included three regional detention storage basins totaling 1,610 acre-ft of storage, installing an additional box culvert at an-under capacity bridge, and sediment removal in flood-prone creek reaches
  • The estimated total cost for the recommended projects was $46 to $69 million
  • Developed and gave a PowerPoint presentation to the public detailing the causes of flooding, the recommended projects and their progress

Since the completion of this study, the City has built the 200 acre-ft Encinosa Creek Detention Basin, has received a $3.5 million grant from Solano County Water Agency (SCWA) and an additional $6 million from Prop. 84 funding and FEMA. The City is pursuing the construction of the remaining basins

Based on results of this study the City has proceeded with pre-design work for the Encinosa Creek, Alamo Creek, and Laguna Creek Regional Detention Basins and secured a $3.5 million grant from the SCWA and an additional $6 million from Prop. 84 funding and FEMA.