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 8 may also affect water quantity compliance. As such, the BMPs need to be revised. These revisions could require the BMPs to be widened, shortened, or relocated in general. The bottom of the BMPs need to maintain a minimum separation of two feet to prevent groundwater from mounding into the storage system, reducing the system’s storage capacity and increasing the peak flow rate from each BMP. 2.3 SOIL TESTING REQUIREMENTS FOR BMPS The BMP Manual provides guidance on how many test pits are required based on the size of the basin in Subsection 2a of Chapter 12 of the NJDEP BMP Manual. The soil testing allows a designer to understand how the soil will respond to rainfall, determine the elevation of the SHWT, and measure the infiltration rate of the soil at the elevation where infiltration will occur. It is a crucial part of the design process. Typically, each BMP is required to have two test pits, but more can be required based on how large the proposed BMP is. The SHWT can vary based on its location and it’s important to take measurements at different locations to better understand that variation. If an insufficient number of soil tests are taken, the engineer runs the risk of not understanding the elevation of the SHWT across the BMP and could design a BMP that is below the SHWT. A good example of this is PP 4 and 6. If you look at only some of the test pit data, the BMP seems properly designed. However, when considering the 2021 data, you can see how widely the SHWT varies by location. Princeton Hydro compared the number of test pits for each BMP to the number required by the NJDEP Manual to confirm the applicant performed the minimum number of soil tests. Princeton Hydro compared the infiltration area of each BMP on the plan to what was entered into the model. Table 1 is a comparison of the infiltration area found in the model calculations and the infiltration area shown on the plan. The infiltration area is the bottom of the basin, not the side slopes. Table 1: Comparison of Infiltration Areas Shown in the Plan and Model BMP ID- BMP Type Infiltration Area, Model (SF) Infiltration Area, Plan (SF) Difference, Model – Plan (SF) 1 – Large Scale Infiltration

e 1: Comparison of Infiltration Areas Shown in the Plan and Model BMP ID- BMP Type Infiltration Area, Model (SF) Infiltration Area, Plan (SF) Difference, Model – Plan (SF) 1 – Large Scale Infiltration Basin 42,000 45,490 -3,490 2 – Small Scale Infiltration Basin 26,023 25,830 193 3 – Porous Pavement 5,227 5,558 -331 4 – Porous Pavement 12,197 12,369 -172 5 – Porous Pavement 14,810 14,611 199 6 – Porous Pavement 35,719 37,473 -1,754 7 – Small Scale Infiltration Basin 14,375 13,405 970 8 – Porous Pavement 10,019 15,900 -5,881 9 – Porous Pavement 6,534 17,981 -11,447 10 – Small Scale Infiltration Basin 9,583 9,467 116 11 – Porous Pavement 13,504 22,826 -9,322 12 – Small Scale Infiltration Basin 900 1,004 -104 As demonstrated, there are differences between what was modeled and what was shown on the plan. A majority of these differences are not consequential, but the differences at BMP 1, 8, 9 and 11 are much more significant as the infiltration areas as modeled are much larger than the final BMP footprints as proposed on the plans. The modeling must be revised accurately analyze the proposed conditions. Table 2 provides a comparison of the number of test pits required by the NJDEP BMP Manual against those performed by the applicant. The BMP footprint as shown on the plan was used as the reference to determine the required number of test pits.