Downtown Overlay Zone

New Rochelle, NY

H2M was retained to analyze the existing sanitary sewer system within the downtown area of the City of New Rochelle as part of a zoning plan.

The City of New Rochelle has approved a zoning plan, known as the Downtown Overlay Zone (DOZ), that creates a path for developers to incorporate housing, retail, commercial, office, hotel, and community facilities in and around its downtown area and transit center. This zoning has allowed for unprecedented development opportunities. In response, the City needed an evaluation of its existing sanitary sewer capacity to identify sections that would be impacted by additional sanitary wastewater flow resulting from development within the DOZ. H2M was retained to analyze the existing sanitary sewer system within the downtown area.

Our ongoing engineering efforts include:

• Topographic survey of horizontal coordinates and all manhole rim and invert elevations of sewer subsystems that would be impacted by proposed development projects. The manhole locations identified during the topographic survey were correlated and labeled to match the numbering system used in the current GIS map provided by the City. A total of 141 sewer and drainage manholes, and approximately 27,500 linear feet (5.2 miles) of sewer was surveyed.
• Using the collecting survey data, a model was developed to calculate the maximum hydraulic flow capacity in each sewer segment. Critical system sections were identified. A total of 119 sewer manholes and 25,228 linear feet of sewer pipe were selected to be used to model the eight zones in the sewer system required to evaluate the existing hydraulic capacity and associated impacts anticipated from the connection of each proposed development project.
• Seventeen flow monitoring devices were installed at critical locations to provide actual current flow rates and values. The average daily flow (ADF), peak hourly flow (PF), and peak hourly flow with inflow removed (PF less inflow) were determined from the data during collection period. Rainfall data collected via rain gauge was correlated to the flow monitoring data to identify potential inflow and infiltration contributions to the sanitary sewer system by comparing dry weather peak flows to wet weather peaks at each flow monitoring location.
• Sanitary sewer loads were estimated for twenty-five proposed developments. New York State Department of Environmental Conservation (NYSDEC) Standards for Intermediate Sized Wastewater Treatment Systems (March 5, 2014 Edition) were utilized to estimate the average daily design flow for each proposed development. The peak hourly flows were calculated using the formula in the Recommended Standards for Wastewater Facilities (10 State Standards). Geographic Information System (GIS) mapping of the existing sewer system, made available by the City, was used to identify the location of each proposed development project and the flow input relative to the existing sanitary sewer system.
• The hydraulic capacity of the existing sanitary sewer system model was evaluated using the following six flow scenarios:

1. 1. Existing average daily flow (ADF)
   2. Existing peak hourly flow (PF)
   3. Existing peak hourly flow with inflow & infiltration removed (PF less I&I)
   4. Proposed average daily flow (ADF including average daily development flows)
   5. Proposed peak hourly flow (PF including peak hourly development flows)
   6. Proposed peak hourly flow with inflow and infiltration removed (PF including peak hourly development flows less I&I)

•  The hydraulic capacity analysis identified the sewer system to be sufficiently sized to accommodate the existing ADF while maintaining 50% or greater pipe capacity within all sections of pipe evaluated. To accommodate the additional ADF and PF from the proposed development sites, sewer enhancement projects were recommended for four of the eight drainage zones to eliminate capacity issues and potential surcharges. Additional sewer enhancement projects to bring existing six inch diameter sewers into compliance with current Westchester County Health Department requirements. An engineer’s cost opinion was developed for the sewer improvements in each zone.

H2M has prepared plans and specifications for multiple construction phases during this ongoing project to replace undersized sewers. New sewers range from 8- to 30-inches in diameter and the length of pipe in a phase range from hundreds to thousands of feet. H2M provides services during construction.