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 9 Table 2: Number of Soil Tests Performed for each BMP against the Minimum Number Required. BMP ID Infiltration Area, Plan (SF) Number of Tests Provided Number of Test Required Compliant (Y/N) 1 45,490 12 6 Y 2 25,830 4 3 Y 3 5,558 2 2 Y 4 12,369 3 3 Y 5 14,611 3 3 Y 6 37,473 4 5 N 7 13,405 1 3 N 8 15,900 2 3 N 9 17,981 3 3 Y 10 9,467 1 2 N 11 22,826 4 4 Y 12 1,004 2 2 Y If you look at the plan, it will appear that BMP 7 has more soil tests than is shown on the table above. While the applicant did perform more testing than what is recorded above, it was performed at the wrong time of the year to establish SHWT with the encountered groundwater elevation alone. Mottling to establish the SHWT is a reliable method that may be performed at any time of the year, but the encountered groundwater elevation can only be used to determine SHWT if the work was done between January and April per the BMP Manual. The test pits labeled with letters (not numbers) were performed after April and cannot be counted towards compliance. BMPs 6, 8, and 10 are short one soil test and BMPs 7 is short two test pits. 76,425 sf of the 221,914 sf of infiltration areas are not compliant with local ordinance. Though this may seem minor, but the SHWT can vary greatly as exemplified by BMP 6 and therefore it’s important to do enough testing to confirm the design conforms with standards. This is a vulnerability in the design that shows how the project is potentially not conforming to N.J.A.C. 7:8 5.4 (§25-29.4 P) 7:8 5.6 (§25-29.4 R) for groundwater recharge and stormwater quantity control standards. 2.4 GROUNDWATER MOUNDING CALCULATIONS The soil testing showed very low infiltration rates for most of the site. The applicant proposes to replace the existing soil from the bottom of the BMPs to the SHWT with a sandy soil that has much higher infiltration rate to help meet the requirement that all basins must empty in 72 hours. This is a commonly used practice that is accepted by the NJDEP, which works well if infiltration is not used in the stormwater runoff routing calculations. However, the applicant did use infiltration in the runoff routing calculations which means that the model assumes some

s well if infiltration is not used in the stormwater runoff routing calculations. However, the applicant did use infiltration in the runoff routing calculations which means that the model assumes some of the runoff is infiltrated and does not travel through the stormwater piping network. When one uses infiltration in their runoff routing calculations, then one must provide groundwater mounding calculations for the water quality and 100 year storm. During a rain storm, the groundwater will temporarily rise (mound) below the BMPs that infiltrate, thus calculations are necessary to confirm the mounding does not encroach on the storage volume of the BMP and impact its ability to perform as designed. The applicant provided calculations for both scenarios. Unfortunately, the groundwater mounding analysis for the 100 year storm has errors in it and upon further investigation, the groundwater mounding will reach into the storage volume for the following BMPs: • BMP 3 • BMP 4 • BMP10