O30 Princeton Hydro Report Dated 9 3 25

West Essex Highlands Review 4 West Orange, NJ Permit No. 722-04-0001.1 LUP230001 April 2025 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 two 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 porosity of 0.4 for the sand layer, the two inches of vertical water turns into five inches. That means that there is five 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 two feet deep. However, during 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 at a constant rate of five inches per hour. The reality is that at the surface of the basin at the beginning of the storm, the runoff will infiltrate at that rate but once it hits 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) that will cause water to back up into the sand layer. Once the system is under the influence of a groundwater mound the rate of infiltration could be expected to decrease by orders of magnitude as the system will rely on horizontal groundwater flow. There needs to be thick enough layer of sand to prevent that runoff from backing up into the storage area of the BMP3. 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 anticipated porosity of sand (0.4), the height of the sand and the water is 4.9 feet (59 inches) from the SHWT. BMP 3 (see Figure 4) only provides two feet of sand storage and an additional 26 inches of stone storage (see Figure 7). 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 design issue is not isolated to BMP 3 since any unanticipated

h the stormwater system. The design will fail and potentially create higher peak flow runoff rates than currently exist. Furthermore, this design issue is not isolated to BMP 3 since any unanticipated surcharge from the BMP will be captured by inlets further downstream and will potentially overwhelm those inlets. If the excess runoff is captured by the inlets, the runoff will enter either BMP 1 or 2, it could overwhelm those systems and increase the peak flow rates from those BMPs as well. Princeton Hydro, LLC Page | 14