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 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.