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.

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

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 10 Princeton Hydro shall use PP3 as an example of the analysis that was performed on each of the BMPs to determine the mounding issue under the 100 year storm scenario. Princeton Hydro observed that the duration of infiltration in the groundwater mounding analysis for the water quality design storm (WQDS) and the 100 year storm were nearly identical for both storms (0.21 hours for water quality and 0.2 for 100 year). The analysis for the water quality storm can be found on page 821 of 946 in the Drainage Report and the analysis for the 100 year storm on page 831 of 946 in the Drainage Report. The duration of these two storms should be very different from one another. The water quality storm is only 1.25 inches whereas the 100-year storm is 8.33 inches of rain. The 100 year storm rainfall total is 6.64 times more than that of the WQDS; the sheer difference in volume would make it nearly impossible to have identical durations. It was also noted that the recharge rate is also different for both storms, which should be constant across any storm. The design proposes to replace the existing soil from the bottom of the BMP to the SHWT with a sandy soil that has a higher infiltration rate to allow the runoff to infiltrate quicker through the BMP storage area. The soil at the SHWT still has a very low infiltration rate so the “infiltrated runoff” from the BMP will back up in the replacement soil while it infiltrates below the elevation of the SHWT at a much slower rate. Typically, the stormwater report would provide calculations demonstrating that there was enough storage in the replacement soil to retain the “infiltrated runoff” to allow it to infiltrate below the SHWT at a much slower rate than would be acceptable during the storm event. The calculations would show that the “infiltrated runoff” would be held in the replacement soil and not mound into the storage volume of the BMP. If it did mound into the storage volume, it would impact the runoff peak flow rate calculations and the groundwater recharge calculations by reducing the amount of volume available to hold runoff in the BMP. During the water quality storm, BMP 3 infiltrates 880 cf of runoff. The area of

ulations and the groundwater recharge calculations by reducing the amount of volume available to hold runoff in the BMP. During the water quality storm, BMP 3 infiltrates 880 cf of runoff. The area of the infiltration basin is 5,558 sf. 880 cf of water is equivalent to about 2 inches of water over the area of the basin. That means that two vertical inches of water will be held above the SHWT and slowly infiltrate. Assuming a void space factor of 0.4 for the sand layer, the 2 inches of vertical water realistically equates to about 5 inches, meaning that there is 5 inches of water in the sand layer between the SHWT and the bottom of the storage for PP3. The system works under this scenario because the soil replacement area is 2 feet deep. Under the 100 year storm, the model assumes that 10,971 cf of water will infiltrate through the system (page 529 of 946 in Drainage Report). That is because the model is assuming the runoff is infiltrating through the soil replacement area at a constant rate of 10 inches per hour. The reality is the at the surface of the basin, the runoff will infiltrate at that rate but once it reaches the SHWT, it will infiltrate at the much slower rate of 0.25 inches/hour (assuming a factor of safety of 2, based on the soil testing results) and will cause water to back up into the sand layer. There needs to be thick enough layer of sand to prevent that runoff from backing up into the storage area of the BMP 3. 10,971 cf of water over the area of the basin is equal to 1.97 ft of water. As noted above when you apply the 1.97 feet of water to the void space ratio of sand (0.4), the height of the sand and the water is 4.93 feet (59.16 inches) from the SHWT. BMP 3 only provides 2 feet of sand storage and an additional 26 inches of stone storage (see Figure 6). The system will become overwhelmed during a 100 year storm and the peak flow rates will not be reduced. Instead of infiltrating, a large portion of the 10,971 cf that the model says will infiltrate will discharge through the stormwater system. The design will fail and potentially create higher peak flow runoff rates than currently exist. Furthermore, this issue will compound elsewhere on the site. When BMP 3 is overwhelmed, the excess flow will be captured by inlets further downstream, potentially exceeding their capacity, thus resulting in uncontrolled discharge from the site and risking

site. When BMP 3 is overwhelmed, the excess flow will be captured by inlets further downstream, potentially exceeding their capacity, thus resulting in uncontrolled discharge from the site and risking flooding downstream. If the excess runoff is captured by the inlets, the runoff would then enter either BMP 1 or 2, and similarly could overwhelm those systems and increase the peak flow rates from those BMPs as well.

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 11 Figure 6: Cross Section of BMP 3. 2.5 IMPACTS TO WETLANDS In the existing conditions, the stormwater runoff generated from the existing topography travels over the surface and reaches the upgradient wetlands, providing valuable water to the wetland ecosystem. In the proposed condition, the discharge locations for each of the drainage areas are at the bottom of the project area, as illustrated in Figure 7. The blue arrows indicate the existing flow paths and the red arrows indicate proposed. The proposed stormwater system intends to route the stormwater runoff through a pipe network to down-gradient basins where all of the surface runoff will discharge much further downstream. The wetlands uphill of the discharge locations will therefore collect less stormwater runoff due to their reduced contributory drainage areas. Princeton Hydro included two attachments that show the existing drainage areas for eastern and western wetlands. The western wetlands have a drainage area of approximately 32 acres. The proposed improvements to this area will reduce the drainage area by 9 acres, or 28% of the total existing drainage area. The eastern wetlands have a drainage area of 48.4 acres. The proposed improvements to this area will reduce the drainage area by 16 acres, or 33% of the total existing drainage area. Figure 7: Proposed Grading Plan with Blue Arrows showing the existing drainage path and the red arrows showing the proposed drainage paths and how the runoff is being diverted from the wetlands.

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 12 2.6 PEAK FLOW CALCULATIONS Based on our review of the existing conditions mapping and modeling, we believe the time of concentration computations were done incorrectly for this project. The time of concentration (TC) is the time it takes for runoff to travel through the watershed on the most hydraulically restrictive path. Imagine taking a bucket of water and pouring it out at the highest part of the drainage area. The time it takes for that water to travel to the discharge location at the bottom of the drainage area is the time of concentration. It appears that the designer used an old method to determine the TC for drainage areas POA 1 and 2. Due to the very small size of the drainage area for POA 3, it was not included in this analysis. There is a clue that an older method was used because the plans describe the land use for DA-1 as “unpaved,” which is an older land use classification that is no longer used. Princeton Hydro would note that the NJDEP ruling on this project means that the latest (2023) Stormwater Rule update does not apply. However, the TC calculation methodology referenced above was updated with the previous rule change in 2021. TC for this system was broken down into two different flows: sheet flow and shallow concentrated flow. Sheet flow is how runoff initially travels overland. Sheet flow typically occurs at depths less than or equal to 0.1 ft. Runoff transitions from sheet flow to shallow concentrated flow. Shallow concentrated flow occurs after sheet flow and the depths range from 0.1 to 0.5 ft. To demonstrate the magnitude of how incorrect TC calculations are, Princeton Hydro recalculated the shallow concentrated flow portion of the TC for both POA 1 and 2. For POA 1, the designer stated that it took 7.2 minutes for the runoff travel 1,625 ft via shallow concentrated flow. The average slope for that length is 5.5%. According to chapter 5 of the BMP manual, the velocity of runoff traveling through a forested area with heavy ground liter is 0.55 ft/s when the slope is 5.5% (Figure 5-13 of NJDEP BMP Manual Chapter 5). 1625 ft divided by 0.55 f/s is equal to 2954 seconds or 49 minutes. The revised TC is almost 7 times longer than the

er is 0.55 ft/s when the slope is 5.5% (Figure 5-13 of NJDEP BMP Manual Chapter 5). 1625 ft divided by 0.55 f/s is equal to 2954 seconds or 49 minutes. The revised TC is almost 7 times longer than the what is shown on the stormwater report. For POA 2, the report states that it takes 1.9 minutes to travel 564 ft via shallow concentrated flow. The average slope for that length is 9.2%. That is an average velocity of 5 fps. The NJDEP manual states the velocity of runoff at a slope of 9.2% for wooded area is 0.75 fps (Figure 5-13 of NJDEP BMP Manual Chapter 5). Using that velocity, the TC is 12.5 minutes. Again, the revised TC is almost 7 times longer than what is shown on the stormwater report. Establishing the existing TC and peak flow rates are crucial to a robust design. The TC has a significant impact on the existing peak flow rate for a proposed site. All development projects are required to have their peak flow rates leaving the site be less than the existing peak flow rate. The shorter the TC, the higher your peak flow rate is. The higher your peak flow rate is, the easier it is to meet your peak flow rate reduction requirements. Keeping the peak flow rate as high as possible allows you to reduce the size of your stormwater basins. The problem is that it doesn't reflect reality and instead of reducing the peak flow rate coming out of the project you could potentially be increasing the flow rate and risking the stormwater runoff from overwhelming the stormwater system downstream and causing a flooding event. That is why the miscalculation of the existing conditions TC is alarming. An easier way to think about this is lowering the height of a basketball net below the regulated height so you can dunk but still telling people you can dunk at regulation height. The flow path shown on the drainage area map for POA 2 is only 664 feet long and that matches what is shown on the model. The TC is meant to measure the time it takes for the water to travel the entire flow path. The actual flow path for POA 2 is greater than 664 ft. The discharge location is approximately 700 feet away from the end of where the analysis ends (as the crow flies, see Figure 8). This analysis is incorrect and incomplete because it does not capture the entire length of the flow path.