In this project, Flow Science analyzed the flow patterns in an existing 8.8 million-gallon rectangular covered distribution storage reservoir located in San Diego County, California. The analysis was performed using computational fluid dynamics (CFD) and revealed the presence of a large flow recirculation (dead water) zone in the center of the tank. The presence of “dead water” was related to poor water quality characteristics. By adding an inlet manifold diffuser with 4 inlet ports, the volume of the dead water zone was greatly reduced, leading to better “plug flow” characteristics. The T10 ratio was increased from 0.09 to 0.36.