Indian Trail Improvement District Engineer Jay Foy gave a presentation last week to the ITID Board of Supervisors on a new, larger bypass line recently completed on the M-1 Canal to add to the drainage of an old 84-inch pipe built in 1993.
The Sept. 14 presentation was at the request of Supervisor Michelle Damone, who said residents had asked her what the construction activity was for and how much it cost, which was about $1 million.
The new bypass at 71st Place North and the M-1 Canal will allow the reversal of stormwater flow to the south rather than the north, as had been conceived years ago.
The second bypass project involved the installation of more than 350 feet of a 7-foot-high by 9-foot-wide concrete box culvert, which, under certain conditions, will allow for the gravity flow of stormwater past Pump Station No. 2. The pump station pushes water north into the upper basin when needed.
“We realized that the drainage was inadequate and only a small benefit to allow water to flow from the upper basin to the lower basin,” Foy said. “We changed the entire direction of flow. In order to get water to Pump Station 2, we had to help it.”
After the rainfall from Tropical Storm Isaac in 2012, water levels fell after a week in the lower basin, but the water in the upper basin didn’t come down for 14 days, and the existing Pump Station 2 bypass was a major restriction for post-storm drawdown of the upper basin.
“What we have constructed is a much bigger bypass,” Foy said, explaining that the second bypass will allow the transfer of three times as much water as the old bypass.
The construction required a coffer dam to be built in order to dewater the trench where the bypass was to be built.
The rectangular duct was built with 7-foot by 9-foot by 8-foot precast concrete sections that were set in place in the trench with a crane and aligned so they could be pulled together. Sections that had angles had rebar installed and concrete poured onsite, Foy said. The finished pipe was then backfilled and compacted.
The gated structure will provide an improvement to conveyance after storm events. Also included in the scope of the project was bank stabilization, canal grading, a canal gate and actuator, fencing and roadway reconstruction.
“The bypass is complete,” Foy told the Town-Crier on Wednesday. “It’s functional if needed, but I would say there’s no real need to use it right now.”
He said the old bypass will remain in operation, and the new bypass will enhance the drainage capacity when needed.
Foy said the next weakest link in conveying runoff through the lower basin from the upper basin will be construction of a new inverted siphon under the M Canal owned by the City of West Palm Beach.
One of the existing inverted siphon heads is within the M Canal easement and will need to be completely rebuilt, he said, explaining that the design is 80 percent complete and the district would need a permit from West Palm Beach to construct it.
Foy added that ITID is trying to time the construction with the city’s plan to widen the canal.
“We don’t know when they’re going to widen the canal,” he said. “You might as well do them both at once and you can save construction money.”
Foy noted that ITID has two inverted siphons under the M Canal. The other is east of the inverted siphon to be rebuilt.
“We did the other one when they widened the canal over there,” he said. “Because the project was let all at once, they didn’t share the cost, but they cooperated and we saved money. We would hope to have that relationship here and save money again. The city would probably save money, too, on the canal widening.”
Foy said the inverted siphons are necessary because the city holds its water at a higher level than ITID does.
“We’re for drainage, and they’re for water supply,” he explained.