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.