Engineers Report on Larchmont Flooding: Interim Update

by Judy Silberstein

(July 21, 2005) Larchmont’s water has been studied from the air, land and underground since last November as part of an extensive effort by the Village of Larchmont to get a better understanding of causes and possible solutions to a decades-old problem of flooding in low-lying areas.

Dolf Rotfeld and Ryan Coynes, consultant engineers, were at the Larchmont board meeting on July 13 with interim findings of particular concern to residents on Pinebrook Drive who were most impacted during last year’s severe storms. (See: Larchmont Launching More Study of Pine Brook Floods) Mayor Ken Bialo noted that talks with county, state and federal officials have convinced him that before Larchmont can get help from elsewhere, it needed to first conduct its own scientific study.

Engineers provided new details on: the amount of water entering culverts under Pine Brook Drive; the size and configuration of the culverts; and some of the impacts of tidal water on the outflow from the culvert. Meanwhile there was mixed news on upgrades at New Rochelle’s Beechmont Lake, whose condition is thought to impact flooding downstream in Larchmont.

Beechmont News

Although engineers do not consider fixing Beechmont Lake to be a total solution to Larchmont’s problem, a retrofitted lake still might help retain some of the storm water that gushes downstream. (For example, see Summer Storm Damages Area.)

Last week the New York Department of Environmental Conservation (DEC) approved permits allowing two projects to proceed at the lake, and New Rochelle plans to begin the work in September, according to Jeff Coleman, Commissioner of Public Works. The projects entail: removal of sediment in the forebay (a small pond that keeps sediment from filling the lake), and excavating the lake to expose its valve system for repairs. In past years, the valves allowed water to be drawn down in anticipation of a storm. That left room to store the extra water, which could then be slowly released to avert flooding.

One critical step remains: dredging and removing sediment from the lake so there’s room to hold more water. A grant application to pay for this step was rejected and further plans are on hold until funding can be found.

The “Beechmont solution” is only part of the complex picture to Larchmont’s water woes, as is clear from the engineering studies.

More and More Water Coming into a Narrower and Narrower Culvert

The engineers showed the board maps drawn with data from a number of overhead “flybys” depicting the approximately 870 acres of land in New Rochelle and the Town of Mamaroneck that send water into Larchmont, through the Pine Brook area and to an outflow near the Boston Post Road near Tony’s Nursery. This stream is sent through a concrete culvert that starts at 8 feet wide and 6 feet high, but shrinks to 5 feet high as it crosses into the Village of Larchmont under Palmer Avenue. By the time it gets to the Pine Brook area, the culvert has irregular walls and is continuing to narrow. There are a number of additional pipes intersecting the culvert and bringing water from elsewhere, including an additional 50 acres in Larchmont.

Additionally, there are a number of old drains emptying into the culvert. One ties into the bottom of the culvert at a low point near the site of some of the worst flooding on Pine Brook Drive. If the culvert is already full (for example when the tide sends water up the channel), the water coming in through the bottom would have nowhere to go. This scenario might explain water that occasionally gushes out of the storm sewers – so the engineers will be focusing attention on this spot.

Storm water gushes from a Pine Brook Drive drain in 2002. See: Pine Brook Floods Again

Mr. Rotfeld also showed a diagram of how much water is in the culvert at various points under mean high tide conditions throughout the month. At certain points water fills the culvert even when there is no rain. However, it was not clear that tides were as significant factor in the flooding as they were once thought to be.

The engineer's chart indicates how much water (blue) is in the pipe at different locations at high tide. At the far right, the pipe near Kilmer Road and near Beverly Place is almost filled - even before any rain water appears.

Next Steps

They are past the midpoint of the study, reported the engineers. Next they will be gathering more information on the topography, on water flowing from the surrounding communities under more conditions and on tidal influences at different times of the month.

In addition to learning more about current conditions, the aim is to have sufficient information to be able to simulate how potential interventions might impact the water flow. That will allow the engineers to develop and test potential solutions in theory before specific (and possibly expensive) solutions are recommended.

“We’re very interested in learning what the recommendations will be,” said Jay Fischer, a long-term resident of Pine Brook Drive.

The results, so far, confirm his view and that of his neighbors that this is a regional issue. “While progress has been made from time-to-time, it’s often a partial response to immediate problems instead of a technically validated, intermunicipal response to the larger problem - and that’s what is needed,” he stressed.

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