Lake Williams Dam Rehabilitation

After over a century of faithful service, the Lake Williams Dam remained structurally sound. However, the Pennsylvania Department of Environmental Protection (DEP) had recently modified its regulations, rendering the dam’s spillway undersized. Notably, the DEP required the spillway to pass a probable maximum flood (PMF) greater than 107,000 cubic feet per second (CFS). This new standard exceeded the dam’s design capacity of 38,000 CFS and the storm of record, Hurricane Agnes, which brought 17,000 CFS in 1972.

While changing regulations was the primary project driver, the proactive York Water Company recognized additional opportunities to optimize the dam’s performance, safety, and resilience. Among them, its original spillway did not include underdrainage beneath the slabs. The spillway had also been poured on embankment fill material – not uncommon in the early 1900s – rather than on today’s modern bedrock standard. Also, while the concrete core wall’s continuance through the spillway was ingenious, the bedrock foundation dipped below the bottom of the core wall and could compromise the spillway’s integrity. The absence of filters downstream of the spillway also presented an opportunity to improve seepage performance during high-flow events.

Aging bascule gates, added to the dam in the 1950s to increase water storage capacity, were an additional prospect for improvement. York Water desired to transition from a mechanical system with intensive operation and maintenance (O&M) requirements to a more passive, automated system for increased reliability, reduced risk, and lessened O&M demands. Finally, a Pennsylvania Department of Transportation-owned road traversed the original spillway and embankment crest. To alleviate security and public safety concerns, York Water sought to have the road decommissioned.

GFT developed a design approach to rehabilitate the dam; it exemplified innovation and collaboration while addressing complex dam safety challenges, efficient functionality, aesthetics, and environmental considerations. Conceptual alternatives were developed using hydraulic modeling to assess various strategies to address the spillway’s inadequacies while remedying other identified issues. GFT presented three alternatives:

• A modern, reinforced concrete chute – similar to the existing dam – featuring a weir wall instead of bascule gates.
• A roller-compacted concrete (RCC) ogee gravity spillway for improved flow dynamics.
• Demolition of the existing dam and the addition of a new replacement dam downstream.

The selected solution included a labyrinth spillway founded on bedrock and other significant structural enhancements, including:

• Adding a left abutment non-overflow gravity section to confine floodwaters to the spillway.
• Designing a stair-stepped spillway and a flattened downstream embankment slope.
• Installing filtered toe and blanket drains to enhance the dam’s resilience.
• Adding a parapet wall to the embankment crest to confine floodwaters to the spillway up to the 1,000-year storm event.
• Replacing the outlet works to improve water management and control.

The design retained the original dam’s concrete core wall from 1912 within the embankment as the primary seepage barrier. This decision was backed by the wall’s proven performance and excellent condition and validated by pre-construction borings. Incorporating the original core wall honored the quality work of the original constructors and optimized the rehabilitation process by leveraging the dam structure’s existing strengths.

In addition to robust provision for dam safety, the rehabilitation also demonstrated a profound commitment to ecological conservation and enhancement. Central to the project’s environmental initiatives was creating additional wildlife habitat, catering to various turtles and other reptiles, the American bald eagle, and various migratory bird species.

To enhance aquatic habitats, the project introduced a 1-foot elevation increase in the pool, generating three additional acres of open-water habitat. Nestled amid picturesque rolling hills and upland forests, the expansion was meticulously planned to ensure no adverse impacts on wetlands, streams, or riparian corridors, protecting the area’s ecological integrity. This initiative was complemented by creating two 0.4-acre habitat islands from excess fill material, designed to provide sanctuary for wildlife, including nesting and feeding grounds for turtle species, wading birds, and migratory waterfowl, some of which are protected.

Further ecological enhancements included establishing fish spawning and nursery areas at strategic depth intervals around the islands and constructing shoals and deep-water habitat structures from recycled natural materials. These structures offer vital refuge and foraging areas for diverse fish species, bolstering the aquatic ecosystem’s richness.

To ensure the well-being of certain turtle populations, turtles were carefully relocated under authorization from the Pennsylvania Fish and Boat Commission (PFBC). The project team also coordinated with PFBC to remove invasive species, underscoring York Water’s commitment to native species protection.

The York Water Company and GFT also undertook meticulous measures to ensure the newly rehabilitated dam blended seamlessly within its setting. Though not required for the dam’s satisfactory performance, to facilitate aesthetic harmony, the project team concealed the exposed RCC dam armoring with 2 feet of locally sourced topsoil, seeding it to integrate the structure with the surrounding park landscape.

The design and construction of the rehabilitated Lake Williams Dam reflected thoughtfulness, innovation, and adaptability. By addressing each aspect of the dam’s deficiencies and enhancement opportunities, GFT delivered a solution that significantly improves the dam’s safety, functionality, and longevity, setting a national benchmark for the design of future dam rehabilitation projects.