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Model recalibration will incorporate modeling methods that have been introduced over the past decade, including taking antecedent moisture into account when modeling groundwater infiltration and its effect on sewer flows and expanding model resolution to include smaller sewers. These improvements will help the Town evaluate new sewer connections as part of the construction permitting process. Flowmeters will be installed at 20 locations as shown on Figure 1. GHD proposes to retain a subconsultant, ADS Environmental Services, Inc. (ADS) to perform the flowmetering. The subconsultant will assist in site selection, visiting the sites to service the meters and host data on a web service for real-time monitoring. We recommend that flowmetering begin in March to capture snowmelt and continue through July when groundwater infiltration tends to be lowest. Additionally, three (3) rain gauges will be installed at key locations in the Town so that measured rainfall response will reasonably reflect the local conditions. Collected flow data will be analyzed to produce I/I metrics for the metered subbasins. The evaluation will generally follow the guidelines of the Water Environment Federation (WEF) Manual of Practice FD-6, Existing Sewer Evaluation and Rehabilitation (MOP FD-6). Information from the I/I metrics calculation will inform model calibration by providing an initial “guess” for calibration parameters and include information that can help choose the hydrologic model that will be used to simulate I/I in the sewer system. Advances in computing power have allowed models to be calibrated using multiple rain events that can extend up to multiple years of simulation time, so the intent is to calibrate the model to the entire flow dataset. A series of evaluation storms will be used to identify how long-term sewer behavior affects overflows. Once the model is calibrated and validated, GHD will perform an alternatives analysis on the system to identify an overflow mitigation approach. Prior to performing the analysis, GHD will meet with the Town to discuss model calibration results, findings, and workshop potential rehabilitation options. It is anticipated that the alternatives analysis will include the projects identified using the Genetic Algorithm along with other projects identified in the workshop. Results of the calibration and alternatives analysis will be documented in an engineering report that complies with the latest version of the New York State Environmental Facilities Corporation Outline so that it may be used for grant applications. Approach and Scope of Services GHD proposes the following approach and scope of services for the defined work and the subsequent actions to be taken hereafter. Our scope and proposed fee are based on conversations with the Town and GHD’s experience with the sewer system. The project will include the following tasks: Task 1 – Flow Metering 1. Install and maintain 20 flow meters, two level sensors at overflow weirs, and three rain gauges for up to 4 months. If sufficient data are not collected in the base 4-month period, additional flow metering will be required. 2. Monthly visits to Plant 5 to download flow data from permanent flowmeters. 3. Periodically review flow data for accuracy and repeatability during the flow monitoring period. Task 2 – Model Recalibration 1. Evaluate the flow balance and calculate I/I metrics for multiple rain events for each sewershed. Identify sewersheds with high I/I that could benefit from additional investigations. Metrics will include: a. R-Value b. Relative sewershed contribution to peak flows 12681649 | Proposal | Sewer Model Recalibration and SSES Re-Evaluation 2 Page 64 of 192