Water Supply Planning and Conjunctive Use Integration
City of Modesto, California

Over the past decade, West Yost has completed the water system Hydraulic Model Update Project and numerous comprehensive water supply planning studies for the City, including the South Modesto Interim Water Supply Study, the 2005 and 2010 Urban Water Management Plans, water system capacity studies, as well as preliminary design/design of critical distribution system improvement projects. The primary focus of these projects was the improved integration of existing groundwater and new treated surface water supply systems.

The Water System Hydraulic Model Update Project developed an analytical tool to evaluate the City’s existing and future water systems, and identified specific system improvement projects required to meet the anticipated future water demands of new customers, while continuing to meet the demands of existing users. The hydraulic model was calibrated to within +/-5 psi of actual observed field measurements by conducting several days of hydrant flow testing, measuring residual system pressures, and estimating pipeline “C-factors” in the hydraulic model by pipeline material type and age. The project included integration of extensive new service areas acquired by the City (former Del Este service areas), and integration of groundwater and surface water supply sources. The water system planning also accounted for the loss of groundwater production wells due to the migration of groundwater contaminant plumes and regulatory issues.

Regarding phasing for a new surface water supply to the existing groundwater supply system, the hydraulic model defined the radius of influence of chlorinated surface water as it was slowly integrated into the City’s distribution system. This technique allowed the City’s operational staff to isolate and integrate sections of the City on a predetermined transition schedule. This work was performed in conjunction with an aggressive public information/relations plan to minimize customer comments regarding water taste and odor.

West Yost analyzed water demand and supply data to identify new transmission/distribution options available to the City when new surface water supplies become available, developed recommendations, and prepared a prioritized capital improvement program associated with the water supply recommendation.