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The original item was published from 9/22/2023 9:30:11 AM to 9/22/2023 10:43:46 AM.

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News & Town Board Reports (gblist)

Posted on: September 22, 2023

[ARCHIVED] Consultant highlights difficulty solving Troublesome Brook flooding problems

town has been trying to reduce flooding. We hired a consultant to study and make recommendations to address flooding issues at Troublesome Brook (Edgemont) and Manhattan Brook in Fairview

CONSULTANT TO TOWN HIGHLIGHTS WHY IT IS SO DIFFICULT TO SOLVE SOME FLOODING PROBLEMS
THEY HAVE BEEN STUDYING TROUBLESOME BROOK, MANHATTAN BROOK
  NEXT STEP: a survey needed to complete their predictive models

 
 
The consultancy hired to study flood impacts and provide mitigation recommendations for two historically problematic areas of Greenburgh reported to the Board during Tuesday’s work session on their progress. You can watch the full presentation here: https://youtu.be/pTEICro2uVg
 
The town’s most flood-prone areas have generally been in the basins surrounding Troublesome and Manhattan brooks, where residents were hit hardest from Hurricane Ida, and continually experience problems in the increasing amount of major storms since. Firm Woodard & Curran, who are working with a number of municipalities in the region dealing with similar issues in face of climate change, are midway through a study that started in the spring of 2022, when half of the study cost ($440,250) was approved for funding from the Westchester County Stormwater Advisory Board. 
 
Having completed data collection, field observations, and establishing baseline conditions, the firm now needs further specific data from an extensive survey in order to fill out the computer models they’ve created for the Troublesome and Manhattan basins and their drainage behavior for any scenario. The models combine hydrologic development with hydraulic–that is, looking at how much water enters the system (rainfall amounts etc.) and then where/how does the water go (where does it flow, to what depth). 
 
The study, thus far, showcases how complicated these issues are with so many factors to consider, and how costly to fix. The infrastructure is often aging out and/or damaged, and built to different standards insufficient for the 100-year storms we face more regularly now. DPW Deputy Director Brian Simmons noted the town code considers 25-year storms the standard we require new development to meet; but perhaps this merits the Land Use Committee investigating if the threshold should be raised. However, as board members noted, you can’t just replace a 25-year level pipe with a wider 100-year one in one area that will likely feed into another area that has only 25-year pipes that can’t handle the increased load; everything interconnects.
 
While study investigation has progressed, the W&C firm has also provided technical support for the Town in smaller mitigation efforts we conducted along the way, such as clearing the culvert of Manhattan Brook under route 119 and widening and armoring the channel of Troublesome, which has helped the rainwater situation for both since. Continuing these specific localized projects all help contribute to the bigger picture in what is a “cumulative and combined” solution plan, said flood mitigation expert Joe Kirby. 
 
Other possible solutions include drainage improvements (repairing/upsizing), adding storage facilities wherever possible, using FEMA funds to buy out properties for demolition, and elevating others (as the Town is trying to do with a grant application pending for some Babbitt Court residents along the Saw Mill River). 
 
Any recommendations ultimately made have to be mindful of the “do no harm” rule. For instance, anything we change to improve Troublesome Brook conditions in Greenburgh can’t have negative impacts downstream where the system ends in Yonkers. 
 
The DPW suggests putting out a Request for Proposals (RFP) forn the survey soon, with town funding freed up from capital bonding in Mid-October. When a survey is completed, the elaborate models will provide much clearer maps for the best paths forward, and can be flexible to allow various inputs of new developmental impacts along the way, such for the draft proposed Fairview neighbor low income housing rebuild.
 
 Paul Feiner
Greenburgh Town Supervisor

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