By Carol P. Bartold, Senior Reporter     

Oct. 24, 2018:  An extreme and unprecedented rainfall event on September 25 put Bronxville’s flood mitigation system to its most rigorous test yet. Installed at The Bronxville School and designed to collect excess storm water in wet wells and discharge it into the Bronx River, the system is intended to attenuate the severe flooding of the kind that occurred at the school during storms in 2007 and 2011. The system currently operates with two of the five pumps in the original plan. The additional pumps are scheduled to be installed in November.

Flooding caused by the two-hour September storm, mainly on Meadow Avenue, reached the school and affected the C Wing, causing the closure of both the main gym and the blue gym, as well as displacement of several classes. It also rendered numerous student lockers inaccessible.

“As designed, the system worked,” stated Village Administrator Jim Palmer. However, he noted, the amount of rain that fell during the two hours of the September storm was far greater than any rainfall during the same amount of time during either the 2007 or the 2011 storm. “We went back over fifty-two years of weather data,” he said, “and there wasn’t anything comparable that fell in that two-hour period.”

Palmer also stated that a third pump would have helped in keeping the water level down. He said that, at times, during the rain and as the pumps were running, the water level in the wet wells began to rise. “That told us that the two pumps were not keeping up with the amount of water that was coming in through the diversion structures.”

The flood mitigation system, designed by J. Robert Folchetti and Associates, contains two flow diversion structures, a 36-inch pipe and a 72-inch pipe, installed as additions to the existing water trunk lines. Those gravity-based trunk lines carried excess storm water to the Bronx River. During severe storms, such as those in 2007 and 2011, the river can rise to an elevation higher than that system, at which point it shuts off.

The new flow diversion pipes catch storm water, divert it away from the trunk lines, and deposit it into wet wells installed below Hayes Field on the school campus. The pumps then send the water through a dedicated force main pipe that runs underneath the elementary school parking lot, across Midland Avenue and under the Bronxville Public Library campus, and across Pondfield Road and under the village hall property to Palumbo Place, where it makes a 90-degree turn and runs to Gramatan Avenue. The excess water pumped out of the wet wells is then discharged into Laurel Brook near the corner of Gramatan Avenue and Palumbo Place.

Paul Pelusio of J. Robert Folchetti and Associates explained that the Meadow Avenue side of The Bronxville School drains through a 24-inch pipe that extends across the campus underneath the building and ties into a junction manhole in the elementary school parking lot and drains into the 36-inch diversion structure. Past flooding, he said, has resulted from the old trunk lines becoming overwhelmed with water, which renders the 24-inch line from Meadow Avenue unable to push water through to the junction, resulting in ponding and flooding on Meadow Avenue. “It basically becomes a balancing act between how much water is accumulating on Meadow versus how much is accumulating at that junction under the parking lot,” he said.

The village and school district are working on solutions to the recent Meadow Avenue flooding. Palmer stated that they are evaluating activating the pumps earlier during storms to allow for more storm water capacity in the wet wells. “The system was designed for a long-term duration event,” he said. “Now what we’re looking at are the effects of these extreme shorter burst events as well.” Palmer also stated that the village has already lowered the trigger point in the wet wells at which the pumps will activate. That level has been lowered by eighteen inches so that the water can be pumped out more quickly.

The village and Folchetti engineers are also considering expanding the size of the pipe from Meadow Avenue or installing a secondary pipe from there around the school building to the junction underneath the elementary school parking lot.

Village officials and consulting engineers continue to study what occurred during the September 25 rainfall event to prevent it happening again. Paul Pelusio stated that they expect to have a good understanding of the situation within a month.

“You might not have seen water like this in your backyard before,” said Jim Palmer, “because no data available shows anything like this happening.”

Photos by Staff