Flow routing with bottom withdrawal to improve water quality in Walnut Canyon Reservoir, California

Abstract

Walnut Canyon Reservoir is a small source-water reservoir for the City of Anaheim, California. The reservoir receives raw water from the Metropolitan Water District of Southern California and is used for source water storage and emergency storage for the Lenain Filtration Plant. The reservoir naturally stratifies through the summer, with anoxia and elevated concentrations of phosphorus (P), nitrogen (N), manganese (Mn), and sulfide present in the hypolimnion. Water quality modeling using the 1-D DYRESM-CAEDYM model was conducted to evaluate different management strategies for the reservoir. Simulations evaluated the water column conditions in 2010 with (1) existing conditions, with operation of a SolarBee; (2) diffused aeration; (3) hypolimnetic oxygenation; and (4) flow routing with bottom withdrawal. Meteorological data from the nearby California Irrigation Management Information System stations were used to drive the hydrodynamic component of the model; the model was validated using available profile measurements and discrete sampling from April to July 2010. Diffused aeration, hypolimnetic oxygenation, and flow routing with bottom withdrawal were all predicted to improve dissolved oxygen conditions and lower concentrations of P, Mn, and other elements, although flow routing with bottom withdrawal could be implemented without incurring significant capital and operating costs; therefore, the flow routing strategy was implemented in spring 2011. Water column sampling demonstrated improved water quality in line with model predictions and indicated that flow routing with bottom withdrawal can serve as a practical and effective alternative to other engineered solutions under favorable hydraulic and source water quality conditions.