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 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.