Route I-91 Eel Ladder

The construction of Route I-91 in Middletown, Connecticut in 1963 inadvertently disrupted an environmental pathway for eels that had to travel from their spawning grounds to their habitats upstream. When a culvert failure in the highway’s drainage system required emergency repairs, state officials took the opportunity to create a precast concrete “eel ladder” that restored easy access to an upstream habitat for the creatures.

The ladder was created after a steel culvert conveying water from a brook began to collapse, requiring an aluminum liner to be added. The liner reduced the culvert diameter, causing the water to flow faster at maximum runoff times. This required further adjustments in the channel to slow the water and dissipate energy. When the Connecticut Department of Transportation (CTDOT) began planning those corrections, the Fisheries Division of the Department of Energy and Environmental Protection made a suggestion.

At the outlet of the culvert, water dropped 9 nine feet down a concrete ramp at a 30-degree slope between two retaining walls. A plunge pool at the bottom slowed the water and kept it from scouring the natural stream-bed material, explains Jon Hagert, transportation supervising engineer of the Connecticut Department of Transportation (CTDOT).

“The pool was originally designed to dissipate the water’s energy and reduce the stream velocity to prevent scour in the downstream channel,” he explains. “Because the repair liner reduced the size of the culvert and increased the speed of the water at the outlet, we wanted to add some roughness to the channel to slow the water.”

That’s when Fisheries suggested an alternative. A variety of eels spawned in the salty, tidal waters and then swam upstream to their habitat. They had had to find alternative routes when I-91 was built, cutting off this waterway and disrupting their natural path. Fisheries had earlier created an “eel ladder” for another project to aid migration near a dam by embedding stones into a shallow concrete ramp as it was cast, and they asked CTDOT to investigate a similar option.

“It seemed more effective to use precast concrete panels,” Hagert says. CTDOT created a design that Fisheries tweaked, and they finalized the details. Precaster Coreslab Structures (Conn) Inc., was awarded the fabrication contract, which consisted of six panels: two panels on a flat shelf just outside the culvert outlet, and four panels, each about 9 feet long and 4.5 feet wide along the slope.

The trapezoidal-shaped cross section of each panel allows the panels to sit flat upon the concrete floor of the drop-structure while creating a V-shaped design on top that channels the water to the center during low-flow periods. Natural stones were embedded in the concrete and were exposed at the surface to simulate a natural stream bottom. The panels on the left side feature larger aggregates spaced further apart, while the right side features smaller aggregates closely packed together. This accommodates eels of any size, which can range in length from 4 inches to 3 feet. Several mockups were produced to find the appropriate combination of stone size and spacing.

Leveling bolts were used to raise and lower the panels to align with each other and with the outlet invert of the culvert liner. The precast panels were grouted in place, and anchors were drilled into the concrete drop-structure floor through prefabricated holes to secure them. Keyways cast in the edges of the precast panels were filled with mortar to connect the panels and additional aggregate was set into the mortar to provide a smooth pathway from one panel to the other for the eels. After placing mortar and aggregate, the joints between the panels became less apparent, making the floor of the channel look more natural.

The eels now have a fast and easy path back to their habitat without disrupting the flow of the water or the roadway. “This was an easy and cost-effective way to restore the eels to their habitat,” says Hagert. “I expect we’ll use this concept again in the future when the need arises to help eels reach their habitat safely.”

Project Details






Connecticut Department of Transportation, Newington, CT


Arborio Construction, Cromwell, CT

Architectural Precast Elements:

• Water channel for runoff and drainage for I-91.
• Sluiceway features 30-degree slope to plunge pool and continues downhill.
• Six 9-foot long, 4.5-foot wide precast concrete panels with varying sizes of aggregate and spacings.
• Top two panels installed flat, with other four installed on slope, allowing eels to crawl up them to reach their habitat previously cut off by drainage system on highway.
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