The Altoona Water Authority and its contractor for the renovation of the Bellwood dam have finally managed to drain the reservoir behind the dam, clearing the way for the project.
Draining the reservoir and keeping it empty has been problematic, because the outlet pipe under the dam is small. The reservoir watershed is large and fills the basin quickly, and there have been periodic significant rains, according to officials.
“That was a major hurdle,” consulting engineer Mark Glenn of Gwin Dobson & Foreman said at an authority meeting Thursday.
The contractor has been: “Very patient” in dealing with it, Glenn added.
The next major task is: “Water management,” which includes channelizing Bells Gap Run along the edge of the farthest basin from Route 865, and partial demolition coupled with repairs on the concrete spillway, according to Glenn and authority General Manager Mark Perry.
Channelizing the stream will require excavation, then placement of a synthetic rubber liner to prevent erosion and leakage, Perry said.
That channelization will direct the stream to the spillway, which is also on the side of the dam farthest from the highway.
Workers will remove about a quarter of the spillway’s length, lowering its elevation at the crest of the dam so the stream can pass through.
Where workers remove the concrete and excavate, they’ll install rip rap rock, then cover the rock with sprayed-on concrete, to prevent erosion.
To ensure the integrity of the remainder of the spillway, workers will drill holes and pump in grout to fill voids that were previously identified with ground-penetrating radar, Glenn said.
They’ll also grout the joint.
Such work is necessary to prevent water that could be flowing fast over the concrete from getting underneath and lifting, according to Perry.
The contractor will be responsible for water management, although the authority will help, Perry said.
Even with the drained reservoir and Bells Gap Run channelized, the contractor will need to run pumps to remove groundwater when workers build forms and pour concrete for replacement of the intake tower, Perry said.
The authority is renovating the dam to meet the state Department of Environmental Protection’s safety requirements.
The new spillway is designed to handle the maximum probable flood for the area.
The old one was sized to handle only about half that, which led to: “High concern” at DEP, according to Perry.
The authority is hoping to remove all the silt that has collected in the reservoir basin over the years, but the extent of that removal isn’t definite yet.
One concern is potential contamination.
The contractor plans to take the silt to state game lands near Blandburg, but the Game Commission is asking the authority to take 20 more samples to check for substances, to complement six samples previously taken, said Perry, who is nevertheless confident the results will be OK.
The other concern is cost, as there are between 8,000 and 10,000 truckloads of silt in the basin, and fuel costs are rising, Perry said.
Mirror Staff Writer William Kibler is at 814-949-7038.