AWWA ACE63088

This presentation describes the development of a comprehensive program for controlling turbidity in New York City's 600 mgd Catskill supply system. Though the watershed is relatively pristine (~75% forested) and generally provides very high quality water, the system faces geological challenges that are difficult to control with traditional land management BMPs. In particular, stream banks and channels in the region are often minimally armored and underlain by natural glacial clay deposits. These deposits are mobilized during storm events and contribute to periodically elevated turbidity levels in Schoharie and Ashokan Reservoirs. Elevated turbidity is of concern with respect to both the overall quality of NYC's unfiltered drinking water supply, and to aquatic habitat and recreational issues in receiving waters. In order to protect and improve the quality of its unfiltered drinking water supply, NYC developed a program to identify feasible, effective, and cost-effective measures for providing reliable turbidity control in the Catskill System. The alternatives under consideration move beyond traditional watershed BMPs, and include in-reservoir structural measures such as impermeable baffle walls, permeable turbidity curtains, depth-selective withdrawal systems, clarification facilities, adjustable weirs and waste discharge structures. Operational measures for optimizing management of the multi-reservoir system to meet both water supply and water quality objectives are also considered. The performance, cost, and feasibility of each of these alternatives were evaluated for the purpose of eliminating infeasible or unreliable alternatives. Turbidity control performance was evaluated using two-and three-dimensional reservoir water quality models, bench-testing, and in-reservoir pilot-testing. Further detailed performance evaluation of surviving alternatives is ongoing based on long-term probabilistic modeling using 2-D and 3-D models of Schoharie, Ashokan, and Kensico Reservoirs. Non-structural alternatives are evaluated using a reservoir systems operation model that relies on linear programming to identify optimum diversion strategies and operating rules for turbidity control. This presentation summarizes the development and evaluation of structural and operational alternatives for improving turbidity control in the Catskill System, and provides an example for utilities considering active source water quality management measures as an alternative to treatment process modifications. Includes abstract only.