$4M Pipe Proposed to Reduce Pine Brook Flooding

by Judy Silberstein

(August 10, 2007) $4 million and a few asterisks. That was the consulting engineers’ cost estimate for expanding a storm pipe to relieve flooding along the lowest-lying stretch of Larchmont’s Pine Brook Drive. The new pipe would increase capacity from 250 cubic feet per second (cfs) to 750, more than enough to contain the 500 cfs associated with a “two-year storm” – an event predicted on the basis of long-range weather data to occur once every two years.

Ken Pritchard, the Dvirka and Bartilucci engineer heading up the study for Larchmont, and his associate, Jason Tonne, presented their latest findings on Thursday, August 9 in the conference room at Village Hall, which was filled with Larchmont Village Board members and residents from the Pine Brook neighborhood.

Consulting engineer Ken Pritchard traces the course of a section of culvert he proposes to replace to relieve flooding along Pine Brook Drive.

The Data

Results of hydrology studies presented by Mr. Tonne update those from 2004. (See: Tanks, Pumps & Pipes Suggested for Pine Brook Flooding.) By Mr. Tonne's calculations, the watershed that drains through the Pine Brook culvert encompasses 1025 acres that include the Beechmont Lake section of New Rochelle, parts of Town of Mamaroneck and the Chatsworth and Pine Brook sections of Larchmont Village. The pipe empties into the Premium River near the Boston Post Road, where it is 7-8 feet above sea-level. With high tide often above 8 feet, the storm water frequently encounters incoming tidal water that impedes the outflow. Even without a high tide, stormwater often backs up spewing out of storm drains and flooding Pine Brook Drive and nearby homes. (See: Summer Storm Damages Area.)

The bad news from the recent study: more water is flowing into the Pine Brook culvert than previously estimated – and much more than the culvert can handle. The 2004 studies, conducted by different engineers, estimated around 300 cfs at the “design point” on Pine Brook Drive in a 2-year storm. However, Mr. Tonne’s estimates were closer to 500. Under optimal conditions (at low tide), the culvert can handle only 250 cfs.

Further bad news: an increase in flooding experienced in recent years cannot be attributed to drainage improvements made in 1995 to the toll plaza along I-95, as was recently suggested. Hence, financial or other support from the Thruway Authority is unlikely.

Alternative Solutions: Beechmont, Ballfields & Chatsworth Diversion

Mr. Tonne calculated the amount of water contributed by various sectors of the watershed, which includes areas of New Rochelle, Town of Mamaroneck and Interstate-95, in order to assess the relative effectiveness of a number of potential flood control solutions. By his calculations, dredging, building a dam and installing flood control measures at Beechmont Lake would remove around 50 cfs. That would help relieve flooding around the lake, but Mr. Pritchard said, “this doesn’t buy enough,” at the Larchmont end.

Creating a water retaining pond along Fifth Avenue in New Rochelle would divert an equivalent amount, around 50 cfs, but would require use of the entire 10-acre park now dedicated to ball fields.

Installing a new pipe to divert storm water from the Chatsworth Avenue sector of the watershed would remove an additional 50 to 65 cfs, but at an estimated cost of $2.5M.

Even combining all three solutions would still not divert enough water to relieve the undercapacity at the design point.

Mr. Pritchard did not recommend another option suggested by some of the residents, building a new smaller culvert just for the lower Pine Brook area and forcing overflow from upstream water to backup. He said “pressurizing” the pipes this way could be dangerous and associated costs would be high. He was also opposed to the installation of a tank and water pump, as had been proposed in 2004. The tank would require too much space, and the pump would require continual maintenance along with its own generator, he said.

Recommendation: New Bigger Pipe at $4M*

What Mr. Pritchard did recommend was reconstructing the last two blocks of the culvert at a cost of about $4M. Mayor Liz Feld characterized the suggestion as “positive” and “doable” and “feasible.”

As proposed, the current pipe, installed in the 1930s, would be completely replaced along the stretch that runs from above Kilmer Road all the way to the outflow into the Premium River. Where the old pipe is 8 feet wide and 6 feet high, the new one would be 14 by 6. Larchmont would need to acquire a wider easement to accommodate the greater width of the new pipe, which in places runs between two homes

The new capacity of 750 cfs would be sufficient to handle a two-year event, even if future improvements to the conduit upstream were to increase the flow of water rushing down the culvert. Mr. Pritchard said there would also be a “vast improvement” in flooding associated with less frequent but more intense storms.

The estimated cost of $4M came with a few asterisks – additional costs that are yet to be estimated for a number of smaller projects that might be required. These include shoring up walls, arches and other infrastructure along the Pine Brook and Premium River to handle the increased rush of water the new culvert would produce.

How Fast Could the Pipe Be Installed?

Additional flooding this week only added to the urgency of the questioning by Pine Brook residents who were again drying out their basements. Mr. Pritchard estimated it would require a minimum of 14 to18 months for design, approval, bid, fabrication and construction of the new culvert. However, required approvals from the Department of Environmental Conservation could take much longer. Because the new pipe would go under the Boston Post Road, dealing with issues of traffic and underground utilities might increase both time and cost.

Also required is a multi-step process at the level of the Village Board. Mayor Feld warned that the entire Larchmont community needs time to learn about the project, its details and its costs. She said she was persuaded of the necessity of undertaking the project, but that might not be true for residents of other neighborhoods.

That point was brought home by questions and comments from the one resident at the meeting who was there to address flooding outside of Pine Brook. According to Syrette Dym Grant, in recent years, she and her Wendt Avenue neighbors have begun to experience flooding that had been previously unknown in their area. “Something has changed,” she said, and that something will also need attention, she suggested.

How to Pay?

Mayor Feld was not hopeful about obtaining county, state or federal funding for a stand-alone Larchmont Village project. Following the massive flooding earlier this year, especially in Mamaroneck Village, competition for available funds will be stiff. She was more optimistic about collaborating with New Rochelle and Town of Mamaroneck to qualify for grants, especially since Town Supervisor Valerie O’Keeffe has recently been appointed to a Westchester County task force that will be influential in the disbursement of $10M per year in grants for flood amelioration projects.

“We have to be realistic about the money,” she said. She expects that Larchmont would borrow most of the money, (i.e. sell municipal bonds) to pay for a Pine Brook project, if it is approved.

Next Steps?

Bob Cumella, a Pine Brook resident, recommended investing in further design studies in order to have a “75%” confidence level in the estimated costs. Mr. Pritchard thought it would cost approximately $50K for the next round of studies, which Trustee Jim Millstein said “I would support.”

The Pine Brook project will be on the agenda at upcoming board meetings, perhaps as early as August 20.

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