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Our response – Model Calibration: Using data collected during flowmetering in 2023, GHD calibrated the model The Town expressed several concerns with using a long-term simulation of the dataset. Results upcoming requirements of the 2012 Order, which of the I/I Study suggested that the relationship included updating the Long Term Control Plan in the between rainfall and I/I is linear, which indicated that required time. Further discussions with the NYSDEC the RTK unit hydrograph model was sufficient to led to developing a revised approach to I/I reduction achieve a satisfactory simulation. This was and SSO abatement and negotiating modifications to confirmed by satisfactory calibration metrics for ran the Order to include a phased approach to events in the simulation. The model is currently addressing I/I and SSOs moving forward. GHD was being used for planning and reporting. engaged with the Town throughout every step of this process, including preparing the first modified Order – Engineering Report: Results were presented in a deliverable, the Sewer System Inflow and Infiltration model calibration report that followed the outline in Work Plan that outlined the framework for the the approved Model Work Plan. The report proposed flow monitoring and system-wide I/I Study. described calibration methods, general statistics, The I/I Study was submitted to NYSDEC on and included storm hydrographs and scattergraphs April 17, 2024 and outlined flow and rain monitoring showing the correlation between model results and data collection for both the I/I Study and the sewer measured data. system model update/calibration requirements concurrently, quantified I/I in the study subbasins, The impact and validated flow data, which allowed the Town to Creation of the I/I Work Plan and calibrated model be more efficient with its limited resources. improved the Town’s ability to plan system assessment and rehabilitation. The improved model increased the The Sewer System Model Work Plan described a accuracy of overflow predictions and is anticipated to general plan for constructing a new sanitary sewer contribute to optimized sewer system upgrades. model, its proposed uses, and a schedule for submission of a model calibration report. The Model Work Plan was approved by the NYSDEC on September 27, 2023. While the Model Work Plan was under review by the NYSDEC, the Town was performing field investigations to update the sewer GIS dataset in anticipation of the recalibration work. This updated dataset was used as the basis for the updated model, which was due on November 1, 2024. Model calibration included the following tasks: – Model Update: GHD created an “all pipe” model using the updated GIS dataset provided by the Town. This dataset was imported directly into Infoworks ICM and evaluated for missing information, pipes sloping in the wrong direction, and other items that can lead to inaccurate output. Pump stations within the Town were modeled based on information provided in record drawings and operational data. ghd.com Page 179 of 301