This project is situated on a 440,000 sf lot. The existing lot was as follows: 135,000 sf vegetation; 90,000 sf existing building/asphalt; 215,000 sf compacted/contaminated dirt [the site was previously a junk yard and is being remediated]. Therefore, it is our determination that the existing imperviousness of the site was greater than 50%. The site is located on Long Island in New York, which is essentially a glacial sandbar. Rainfall is absorbed into the ground, and there are no gutters, pipes, sewers or other infrastructure that transport the water off-site. Regardless, there are no receiving streams or waterways. The stormwater management system for the project consists of 57,000 sf of landscaped area and six, concrete, underground drainage sheds. The size of each drainage shed has been calculated based on the runoff properties of the surrounding site features and located appropriately. The site and drainage system are designed so that 100% of the rainfall on the site hardscape will be directed into these sheds and infiltrated into the ground below, eventually recharging the local aquifer. Therefore, exceeding the requrements for stormwater quantity, the post-development peak discharge rates and quantities will be zero. Soil borings reveal that rainfall will be filtered by dirt, gravel and large amounts of sand as it infiltrates the ground. Per the requirements for stormwater quality, rainfall that is infiltrated on-site is assumed to be 100% treated for the purposes of that credit. In addition, rainfall percolates filters built into each drainage shed before penetrating the ground. This project is located on a previously developed site that had no stormwater management and was largely impervious. The new system consists of landscaping to reduce impervious cover and eliminates stormwater runoff completely by ensuring it is infiltrated on site. The entirety of the infiltrated volumes will be filtered through vegetation or the sand below. Please advise if the stormwater system for this project has met the intent for both quantity and quality of stormwater systems.
In order to qualify for SSc6.1, Stormwater Management: Quantity Control, the project would need to compare pre-development discharge rates and quantities with post-development discharge rates and quantities, based on the one and two year 24 hour storm for that area. The above CIR does not provide these calculations, although the proposed system does appear to achieve zero percent stormwater runoff. Stormwater runoff is defined in the NCv2.2 Reference Guide as "All precipitation waters that leave the site boundaries on the surface are to be considered stormwater runoff volumes." In order to achieve SSc6.2, Stormwater Management: Quality Control, the site systems would need to treat the stormwater runoff from 90% of the average annual rainfall using acceptable best management practices (BMPs). Your project submittal would need to show that the proposed filtration systems are considered to be BMPs to meet these requirements and that they remove 80% of the average annual post-development TSSs (Total Suspended Solids). Applicable Internationally.