Tanks, Pumps & Pipes Suggested for Pine Brook Flooding

by Judy Silberstein

(September 28, 2005) Plowing their way through one of the meatiest agendas in months, the Larchmont Village Board received recommendations from engineers on how to handle flooding along Pine Brook Drive; conducted a public hearing on a new ordinance to require more scrutiny by the Planning Board over partial demolitions and major additions to homes; and heard complaints about the condition of the tunnel leading from the train station to a parking lot in the Town of Mamaroneck. And that was only a few of the topics on the board's list.

This Week: Flood Recommendations
Next Week: Reining in Renovations + More on the Train Station Tunnel

Controlling floods in a low-lying area of Larchmont’s Pine Brook area could be accomplished with tanks, pumps and pipes, at a probable cost of more than $1 million dollars. That was the suggestion from consulting engineers Dolph Rotfeld and Brian Coyne, who presented their views on Monday, September 26 at a Larchmont Village Board meeting packed with residents from the impacted neighborhood. The engineers have been intensively studying the area since November of 2004 and presented an interim update in July. (See: Engineers Report on Larchmont Flooding: Interim Update.)

Mr. Rotfeld explained that the storm waters that gush from the drains on Pine Brook Drive under particular conditions are funneled from three separate watersheds. In a major storm, known as a “25-year event,” 120 cubic feet per second can come rushing down from the Beechmont section of New Rochelle. Another 600 cubic feet per second surge into the pipes from the Sheldrake area of New Rochelle and Town of Mamaroneck, and an additional 250 from the Chatsworth Avenue part of Larchmont. A small tributary contributes 30 to 50.

This amounts to “three trains on one track” said Mr. Rotfeld, describing the convergence of more than 1000 cubic feet of water, unfortunately, at a section of pipe that is both low-lying and increasingly narrow. Compounding the problem: during high tides, the pipe is often full even before any rainwater appears.

His recommendation called for two steps: diverting a portion of the water away from the choke point and building a storage and pump system to manage the rest. The “guestimated” price tag ranged from $1.3 to $1.5 million to handle a “two-year event” and up to 60% more for the larger storms of a “25-year event.” Mr. Rotfeld wryly commented that last year there were two “25-year storms” in the space of three weeks.

Just diverting the Chatsworth water into a separate pipe starting at Howard Street and Mayhew Avenue would be far cheaper – perhaps $350,000 to $400,000 - and might control much of the flooding in a typical storm. However, Mr. Rotfeld was skeptical that diversion would suffice, particularly in a major storm.

The pump approach would focus on Pine Brook Drive near the intersection of Beverly Place and would entail digging a large underground storage tank with three to six submersible pumps powered by an above ground generator. Three pumps could remove 5000 gallons of water per minute; six pumps would double the capacity.

Trustee Anne McAndrews asked the engineers about the potential impact of work at Beechmont Lake in New Rochelle, which for years has been viewed as a potential mechanism for retaining floodwaters that could be gradually released. Mr. Coyne rejected that approach. “Even if it were drained bone dry, it’s too small to make an impact,” he said.

“I was surprised to hear that,” said Trustee Mike Wiener in a later interview. He’s been working towards that solution since coming on the board in 2002. (See: Summer Storm Damages Area) And New Rochelle is starting to make progress on fixing Beechmont Lake (see: Engineers Report on Larchmont Flooding: Interim Update). He noted the major implication of the Rotfeld study is that: “The models proved that we are so tide-dependent. When the tide is up, nothing helps.”

tide chart

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 Rd. and near Beverly Place is almost filled - even before any rain water appears.

“”It’s a big project,” said Trustee Wiener. “A big part of the decision would be how to fund it.” Mr. Rotfeld indicated that one potential funding source, the Army Corps of Engineers, had already declined to become involved based on their cost/benefit analysis.

What is the next step and what is the timetable, asked Eric Berg, whose property backs onto Pine Brook Drive.

“I don’t have a step; I don’t have a timetable,” said Mayor Bialo. “It’s a lot to chew on.