Bronxville's Flood Mitigation Project Explained

Written by Bronxville Mayor Mary Marvin

Now that the FEMA flood mitigation grant project is reaching fruition, more residents are focusing on it and interested in learning some of the nuts and bolts of the plan.

As a recap, the Village of Bronxville was awarded a hazard mitigation grant in 2012 for stormwater drainage system improvements. Though the project greatly benefits the Bronxville school campus, it also aids neighbors on Meadow, Willow, Midland Avenue and even Bolton Gardens.

Based on a cost benefit analysis, factoring in the storm events of 2008 and 2011 in which the school alone had losses upwards of $28 million, FEMA determined the project to be cost effective at a dollar value of $6,889,905. The village then entered into a funding agreement with FEMA whereby FEMA would cover an unprecedented 75% or $5,167,430 of the cost with the Village and School district evenly splitting the remaining 25% or $1,722,475 as the local match.

Following the financial approvals, the Village contracted with Folchetti and Associates to do the engineering work and the school also hired an engineering firm to review and tweak plans where necessary.

The engineers from FEMA (along with the Army Corps of Engineers) who specialize in flood mitigation projects nationwide further refined the very detailed plans, (engineering costs alone topped $800,000), to produce a multi-faceted system with four primary components:

Two Flow Diversion Structures – a new flow diversion structure will be installed in the alignment of each of the two existing primary water conveyance trunk lines to divert excess runoff (that otherwise accumulates as flood waters) to the proposed stormwater pump station.

Stormwater Pump Station – a new system will be constructed in the parking lot on the school campus to deliver the excess runoff diverted from the existing trunk lines to the Bronx River.

• Five diesel powered pumping units will be installed with a pumping capacity of 100,000 gallons per minute.
• A below grade concrete wet well will be built to receive the pump suction lines and to provide temporary runoff storage during pumping.
• Construction of an architectural wall enclosure around the pumping units for aesthetics and noise attenuation.

Below Grade Piping Network - A new below grade storage piping network will be constructed below the existing natural turf field, (Hayes Field), on the school campus. The piping network will provide peak runoff attenuation allowing pumping units to operate at a more uniform and efficient rate. The stormwater storage capacity will be approximately 45,000 cubic feet.

New Conveyance System Pipe – A new system will be installed from the pump station location to the Scout Field directing the excess runoff to the Bronx River via Laurel Brook. The new forcemain will cross Midland Avenue extending across Library and Village Hall property to the area near the intersection of Palumbo Place and Gramatan Avenue. A new stabilized drainage channel will also be constructed on Scout Field to direct excess runoff from the forcemain pipe.

Specifically, the pump station will capture and deliver excess stormwater runoff to the Bronx River before flooding occurs under most storm event conditions and do so because it will utilize a dedicated forcemain pipe (not the existing gravity based system) so that stormwater can reach the Bronx River. The existing gravity system (although equipped with valves to stop water from flowing from the Bronx River into the line) is unable to function under severe storm events because the flood stage of the Bronx River becomes considerably higher than the lowest catch basin elevation (near the school) thereby reducing any ability for water to exit the pipe.

Construction is expected to begin in January 2015 with a completion date of August/early September 2015.

As a final point of clarification, the FEMA funding is separate from the funding for the school field project. My understanding is that they will be voted on at the same time to make it easier for residents to vote on only one day and also save money on a duplicate voting procedure.

Patch file photo.