Sept. 12 - Project enabling site work began VIDEO
Sept. 18 - Site work continues
Sept. 30 - Concrete entrance structure set in place
Oct. 9 - Concrete poured for base of the two resting pools, rip rap placement begins
Oct. 14 - Excavation for turning pool begins
Oct. 16 - Walls poured for base of the steep passes
Oct. 24 - Turning pool walls poured
Oct. 25 - First steep pass section installed
Nov. 1 - 55-foot, long run steep pass installed
Nov. 4 - Third steep pass section installed
Nov. 7 - Coffer dam constructed at spillway, rip rap placement continues
Nov. 8 - Prep work for the spillway notch for exit structure begins, fourth steep pass section installed
Nov. 11 - Spillway notch work continues with hydraulic concrete breaker
Nov. 14 - Exit structure footing installed
Nov. 18 - Exit structure poured, rip rap placement continues
Nov. 20 - Fifth (curved with knife gate) steep pass section installed
Nov. 21 - Turning pool viewing window installed
Nov. 26 - Exit structure infill poured
Anatomy of a Fishway
The fishway entrance is located at the downstream end enabling access to upstream habitats for fish migrating into freshwater from Long Island Sound. The entrance is narrow to create higher velocity flow to attract them. Sloped sections, called steeppasses, are broken up by level turning pools – like landings on a staircase – that allow fish to rest before ascending further. The elevation gain between tailwater (at the entrance) and the headpond (the level of water above the dam) is achieved through steeppasses, typically set at 20 percent slope, and resting pools that are set between the sloped sections at each 8 foot elevation gain.
Preparing the Site
Work begins to create construction access to the site, which is complicated by the 18-foot drop from dam top to bottom. An earthen access ramp capable of handling heavy machinery is constructed with erosion control to protect the waterway. Other enabling work includes shielding underground utilities – a sluiceway and piping for a pond-source geothermal system – with sturdy metal plates and removing part of an existing concrete pad to make way for the steeppasses.
Installing the Entrance Structure
The pre-cast entrance structure is located on the downstream end of the fishway. It is configured to attract fish with high velocity water flow. A watertight cofferdam was specified to create a dry work environment during installation. However, stream flows were low, so the team and the permitting agencies agreed to omit it to minimize streambed disruption. The entrance structure is supported by a layer of stone beneath it. Fisheries staff from the Department of Energy & Environmental Protection assisted with placing a band of stones in the stream to help direct fish to the entrance in the spring.
Pouring the Footings
The three turning pools rest upon poured concrete footings reinforced with rebar. After the areas are excavated and leveled with gravel, wooden forms are erected to form the shape of the footing. Concrete is delivered in a concrete truck and directed to the pour location by a driver-controlled chute. While being poured, the concrete is agitated with a probe-like internal vibrator, which is submerged into the wet concrete to eliminate air bubbles. Surfaces are smoothed and leveled with a trowel. The largest footing weighs approximately 23 tons.
Pouring the Turning & Resting Pools
The turning and resting pools allow the fish a break as they ascend through the steep passes. They’re constructed using the same methods as the footings, with rebar to add strength and form work to create the shapes. The concrete is tested to verify its strength and uniformity. Because the pools have a void inside for water, the form work has inner and outer walls into which the concrete is poured. To reduce turbulence, gentle curves are specified, which are constructed using custom bent plywood forms faced with Masonite to create a smooth surface. One pool makes a 180-degree turn. Its form is constructed to make an opening for a viewing window.
Installing the Steeppasses
Constructed by Sheepscot Machine Works in Newcastle, Maine, the steeppass is made up of individual 10-foot pre-fabricated aluminum units that can be bolted together. Internal welded baffles in each unit create turbulence slowing the flow so that fish can ascend through the chute. The slope of the fishway and the length of each sloped ascent between the pools is set according to the fishes swimming ability. Alewife, our target fish, are non-jumpers and do best with fishway slopes no greater than 20 percent and with no more than 8 feet of elevation gain between the pools.
Installing the Exit Structure
The exit structure, located on the upstream end of the fishway on the dam spillway, is fitted with a fish counter and an eel pass to allow upstream migration of young American eel. The curved steeppass connected to it incorporates a gate to stop water flow during the non-migratory period. A pre-cast concrete footing is installed after modifications are made to the spillway to accept it. The exit structure is poured in the same manner as the turning pools. A temporary coffer dam constructed of 21 large sandbags creates a dry work area.