2022 08 Drainage Report

X. STANDARDS FOR OFF-SITE STABILITY This project has been designed to satisfy the Off-Site Stability standards set forth in the Standards for Soil Erosion and Sediment Control in New Jersey. Conditions Where Practices Applies For purposes of analysis, two areas of concern shall be addressed: (1) at the point of discharge and (2) downstream of the discharge point (which may require a watershed-based analysis). Technical criteria for demonstrating off-site stability include consideration of proximity to a defined waterway, site topography (slope), soil texture, vegetative cover and other factors. Where the potential for erosive forces from stormwater runoff exceeds the threshold of acceptability as defined below, the plan shall provide for the construction of a stabilized channel, installation of a conduit to a stable condition or other types of hydraulic improvements to the channel. Additional design criteria or restrictions from the New Jersey Department of Environmental Protection or the New Jersey Pinelands Commission may also apply. Compliance with this Standard will be provided for both areas of concern: (1) at the point of discharge and (2) downstream of the discharge point. Design Criteria This Standard involves two areas of analysis: (1) at the point(s) of storm sewer discharge and (2) beyond the site boundaries, typically a receiving channel or waterway. Stability documentation for each area is outlined in detail below. Generally, the analyses involve the manipulation of peak rates of discharge for the 2 and 10 year, 24-hour storm events such that peak flow rate values and velocities are below established thresholds. Discharges shall be located in areas with low gradient topography and covered in perennial erosion resistant vegetation as noted in table 21-1 below. The proposed discharges have been designed to be located in areas with low gradient topography to the greatest extent feasible due to the exceptional topographic and physical features uniquely affecting the property, and will covered in perennial erosion resistant vegetation as noted in table 21-1. Special consideration shall be given to the use of infiltration for peak flow modifications as follows: POINT OF DISCHARGE STABILITY ANALYSIS When infiltration practices are proposed, an alternate analysis (failure analysis) must be provided which ignores infiltration (no dead

odifications as follows: POINT OF DISCHARGE STABILITY ANALYSIS When infiltration practices are proposed, an alternate analysis (failure analysis) must be provided which ignores infiltration (no dead storage volume available, no static or dynamic infiltration loss rates in the routing calculations, etc.) and demonstrates that no erosion will occur at the point of discharge if infiltration fails to function. Flow rates based solely upon basin inlet and outlet hydraulics must be used in comparison to Table 21-1 (below) to document a stable outlet. No infiltration is proposed as part of the proposed development to reduce flow rates. SHI Builders August 2022 DEC# 4266-99-001 -7-