J_2024 05 09_West Orange Stormwater Management Plan

PSE&G West Orange Switching Station West Orange, NJ Stormwater Management Report Table III-2 Precipitation Frequency Estimates with 90% Confidence Intervals (inches)1 Duration 24-hours 1) 2 year 3.42 inches Average Recurrence Interval Vears) 10 year 100 year 5.21 inches 8.72 inches NOAA Atlas 14, Volume 2, Version 3, West Orange, NJ For each drainage area, time of concentration (Tc) was determined using the hydraulically longest flow path. The flow path for the drainage area was extended to “Analysis Point A,” where drainage from the site reaches an existing catch basin, which then carries flow to an unnamed tributary of the Rahway River. The Tc flow paths are identified on the PreDevelopment Drainage Area Plan. Curve numbers (CNs) were generated based on hydrologic soil group and land use cover type. The yard stone surface of the switching station pad, which will consist of a 4-inch thick layer of loosely packed 3/4-inch crushed stone, was assigned a CN value of 85 to reflect available storage capacity within the yard stone prior to the initiation of surface runo ff. HydroCad output for pre-development conditions for the 2-year, 10-year, and 100-year, 24hour storm events is included in Appendix C. Pre-development runoff rates and volumes are summarized in Table III-3. Table III-3 Pre-Development Runoff Rates and Volumes at Analysis Point A Storm Event 100-year, 24-hour Runoff Volume (acre-ft) 2.684 4.777 9.133 Peak Runoff Rate (cfs) 23.25 41.95 79.82 NJDEP 0.541 10.16 2-year, 24-hour 10-year, 24-hour 3. Analysis of Post-Development Stormwater Runoff Post-development runoff rates were determined by subdividing the drainage area into nine smaller drainage areas by land use. The Post-Development Drainage Area Plan is included in Appendix D. The new secondary containment areas for the transformers were determined to be a non­ contributory area during a storm event, since they will be operated on and inspect and release basis. As such, it was excluded from the post development runoff calculations. Thus, the total drainage area under post-development conditions is 689,600 square feet. Proposed land use within the drainage area is summarized in Table III-4. TRC Engineers, Inc. II March 2012