2151002Princeton Hydro West Essex Stormwater Comments to Planning Board 20250310

Stormwater Management Review for Municipal Planning Board Approval West Highlands, Inc. Block No. 179, Lot No.32, West Orange Township, Essex County, New Jersey March 2025 Princeton Hydro, LLC: 2151.002 13 Figure 8: Existing Conditions Drainage Path for POA-2, demonstrating how the analysis stops approximately 700 feet short of the POA location This is similar to using the older TC calculation, it artificially shortens TC so it lessens the existing peak flow rates and potentially risks the proposed improvements increasing the flow rate leaving the site instead of decreasing. To understand the impact that the TC errors have on the design, Princeton Hydro calculated the TC for POA 2 using the correct method and entire flow path. The report has a total TC of 16.8 minutes. The Princeton Hydro calculations show a TC of 60.8 minutes, approximately 3.5 times longer. The resulting 2-year peak flow using the TC as calculated by PH is 7.52 cfs, whereas the applicant’s model states 14.76 cfs (page 22) and the report states 17.53 cfs (page 22). In addition to severely overestimating the existing peak outflows, the values as listed in the report do not match the flow rates in the model, which must be corrected. Note that Princeton Hydro evaluated the applicant’s calculations against the model results not what is in the body report. Based on the corrected time of concentration calculations, the maximum allowable peak flow rate for the 2 year storm is 3.76 cfs. Currently, the design’s peak flow rate is 7.24 cfs, almost twice what should be permitted. Princeton Hydro calculated the existing peak flow rate for the 100 year storm as 37.7cfs using the corrected TC value, but the applicant’s model results in an existing 100-year peak outflow of 72.79 cfs and a proposed peak outflow of 38.99 cfs. As such, the proposed site will actually generate a higher peak flow rate than what currently exists for the 100 year storm. This is particularly alarming, especially considering there are flood prone areas further downstream. To meet the peak flow rate requirements set forth by the NJDEP and the local ordinance would require the volume storage for the BMPs to dramatically increase. This would have a noticeable impact on the site layout in general.

Stormwater Management Review for Municipal Planning Board Approval West Highlands, Inc. Block No. 179, Lot No.32, West Orange Township, Essex County, New Jersey March 2025 Princeton Hydro, LLC: 2151.002 14 The analysis of the existing drainage area for POA 2 has yet another flaw. The topography within the area delineated for POA 2 has two clear discharge locations in the proposed condition, but it only assumes discharge location at Kuzik Road (see Figure 9). The design maintains both discharge locations, but the modeling assumes they are combined. The flows do eventually combine approximately 600 ft downstream into the adjacent neighborhood, not at the boundary of the site as currently assumed by the model. The drainage area analysis should be redone and the drainage area should be split into two separate drainage areas with two separate points of analysis to ensure the peak flow rate at both locations are compliant. It is difficult to predict how this will impact the site design, but it could be substantial. This re-evaluation is necessary to ensure that the peak flow rate of runoff from the site is compliant with the state and local regulations and to prevent downstream flooding. Figure 9: These are images of DA-1 and DA-2 from the Stormwater Report for the existing condition (left) and proposed condition (right) to demonstrate that the DA for POA 2 should be modeled as two separate DAs because there are two distinct points of discharge. 2.7 SEPARATION OF SANITARY INFRASTRUCTURE AND BMP The design has a sanitary pipe traveling through porous pavement system (Figure 11). While Princeton Hydro could not find a specific rule against a sanitary line traveling through a subsurface basin, there are rules dictating adequate separation between stormwater piping and sanitary piping. Stormwater pipes and sanitary pipes should maintain a minimum horizontal distance of 10 ft and a minimum vertical distance of 1 ft, and the sanitary line should be below the stormwater pipe. The purpose of the separation is to prevent leaking sanitary pipes from entering the stormwater system and potentially causing an illicit discharge of raw wastewater. The design should be modified to relocate the sanitary pipe out of the subsurface system. This may require the project to have additional Flood Hazard and/or Wetlands Permitting depending on how the layout is impacted by

modified to relocate the sanitary pipe out of the subsurface system. This may require the project to have additional Flood Hazard and/or Wetlands Permitting depending on how the layout is impacted by this modification.

Stormwater Management Review for Municipal Planning Board Approval West Highlands, Inc. Block No. 179, Lot No.32, West Orange Township, Essex County, New Jersey March 2025 Princeton Hydro, LLC: 2151.002 15 Figure 10: Profile of BMP 9 showing a sanitary line traveling through it. 3.0 SUMMARY In conclusion, our detailed review has found that the application is not in compliance with applicable State and local stormwater requirements (§25-29). It is my professional opinion that the application does not meet all three central performance requirements of the local stormwater requirements. Since the project has not demonstrated compliance with all three specific stormwater management requirements of the central goals of stormwater management planning as outlined in NJAC 7:8 and §25-29 of the local general ordinance, in my professional opinion, the application is non-approvable as designed. If the application is approved as designed, the development will increase the frequency and severity of damage expected during a flood in the adjacent and downstream areas. I appreciate the opportunity to provide you with these engineering services for you. Please do not hesitate to contact me with any questions regarding this review. Sincerely, Sean Walsh, PE Senior Project Manager