O30 Princeton Hydro Report Dated 9 3 25

West Essex Highlands Review ? West Orange, NJ Permit No. 722-04-0001.1 LUP230001 April 2025 occurs after sheet flow and the depths range from 0.1 to 0.5 ft. lo 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 POA1, the designer stated that it would take 7.2 minutes for the runoff to travel 1,625 ft via shallow concentrated flow. The average slope for that length is 5.5%. According to Chapter 5 of the New Jersey BMP manual, the velocity of runoff traveling through a forested area with heavy ground liter is 0.55 fps when the slope is 5.5% (Figure 5-13 of New Jersey BMP Manual). The travel length of 1,625 ft divided by 0.55 fps is equal to 49 minutes. The revised TC is almost seven times longer than 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 New Jersey BMP Manual states the velocity of runoff at a slope of 9.2% for wooded area is 0.75 fps (Figure 5- 13). Using that velocity the TC is 12.5 minutes. Again, the revised TC is almost seven times longer than what is shown on the stormwater report. Establishing the existing TC and peak flow rates are a crucial part of a compliant design. The TC has a significant impact on the existing peak flow rate. All development projects are required to have their peak flow rates leaving the site be less than the existing peak flow rate. In general, a shorter existing condition TC translates to a higher peak flow rate. The higher your peak flow rate, the easier it is to apparently meet your peak flow rate reduction requirements. An over- representation of the existing peak flow rate allows for a reduction in the size of your stormwater basins. The problem with this approach is that it does not reflect reality or site conditions and instead of reducing the peak flow rate generated by the project it will increase the rate in comparison to the existing conditions. The flow path shown on the Drainage area Map for POA 2s 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

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 where the analysis ends (as the crow flies, see Figure 9). This analysis is incorrect and incomplete because it does not capture the entire flow path. This is similar to using the older TC calculation, as it artificially shortens TC so it shortens the peak flow rates for the existing conditions and potentially risks the proposed improvements increasing the flow rate leaving a site instead of decreasing as the regulations require. To understand the impact that the TC errors have on the design, Princeton Hydro calculated the TC for POA 2 using the correct method and included the entire flow path. The applicant's report has a total TC of 16.8 minutes. Princeton Hydro’s calculations show a TC of 60.8. That is about 3.5 times longer than that calculated by ACS. For the two-year storm, PH calculated a peak flow Princeton Hydro, LLC Page | 19