PLT23 - Off Channel Reservoirs: Unique Challenges and Lessons Learned

Abstract Description: Off channel reservoirs provide resiliency to a water supply system by providing the required storage capacity to transfer water from a separate location, over long distances in some cases, to an area of need. These reservoirs are referred to by a variety of names including terminal storage reservoirs, balancing reservoirs, suction reservoirs, and off channel reservoirs. They also serve multiple purposes including providing backup storage, surge protection, energy savings, water quality improvements, and they help facilitate maintenance. These structures can be constructed linearly, similar to traditional dam alignments, but can also be designed for a variety of geometries with circular, rectangular, and square shapes being most common.

Despite the variety of names, functions and geometries, these structures are dams and share common dam design considerations (embankment design, seepage and slope stability, spillway design and sizing). However, off channel reservoirs also require unique design considerations such as water quality mixing, future maintenance, balancing cut and fill, and compatibility with system hydraulics. We will discuss the unique challenges and lessons learned from various stages of an off channel reservoir project life cycle based on the design of a dozen off channel reservoirs totaling over 1.5 billion gallons of storage over the last 20 years. We will cover:

• Conceptual level siting, sizing and design considerations.
• The importance of understanding future demands and incorporating flexibility for future phases or cells of the reservoir during the initial design.
• Assessment of multiple breach locations for emergency planning and avoiding simplifying assumptions. The final inundation extents should consider multiple breach locations around the reservoir, not just the one producing the greatest discharge.
• Construction challenges and lessons learned from previously constructed off channel storage reservoirs.