Landfill or Stormwater Detention? The Choice is Obvious According to This P. E.
The stormwater management system consists of four beds of under - ground detention chambers to handle 330,000 CF of storage. More than 1,700 ADS StormTech® MC-4500 chambers were utilized to improve water quality and for storage. For the removal of total sus - pended solids (TSS), the StormTech Isolator® Row was incorporated into each unit. “StormTech with the Isolator row gives us a leg up on meeting the quality and quantity requirement,” Shelton stated. For pretreatment, each of the units in the system is equipped with an ADS Barracuda® Hydrodynamic Separator in either four, six or eight-foot diameter. The fourth unit, a 3K ADS BaySeparator® water quality unit with a 60-inch diameter and an eight-foot depth and two 60-inch manholes, was placed upstream for additional treatment due to the volume of the water entering that particular chamber unit. The BaySeparator utilizes three flow paths to route stormwater between the precast structures and trap pollutants, keeping them separate until they are removed during routine maintenance. The Barracuda is a high-performance vortex hydrodynamic separator designed to remove 80 percent of total suspended solids to protect the public waterways. It does this by using density differences and grav - ity to remove suspended solids and floatables from stormwater runoff. Named for its “teeth”, the Barracuda mitigates turbulence in the sump area to prevent resuspension of captured contaminants. The chamber units drain to a common point using ADS N-12 cor - rugated HDPE pipe for conveyance of the water to the pond. ADS Nyloplast® Water Control Structures were used for the stormwater basins. A key feature is the ability for the installer to make changes “on the fly”, which was important given the terrain. For example, if the inlet is sitting above grade, the PVC riser can easily be cut flush to grade. Or if the riser ends up sitting below grade, the inlet can be raised up using extension pieces. The project utilized 30 ADS Nyloplast® PVC curb/drop inlets. Because mostly in-fill material was used and there were concerns of infiltration of fines into the conveyance pipeline, Shelton specified 9,000 linear feet of ADS N-12® corrugated high-density polyethylene (HDPE) pipe with watertight connections. The storm water system used InsertaTee® for lateral connections.
Many times, a site will need to be drastically excavated and sculptured to meet the building requirements. This naturally includes leveling but also importing soil to fill voids. Grooming the site can be used to great advantage to meet the continuing importance of controlling water flow and quality according to Bill Shelton, P.E. Senior Civil Engineer at ZMMArchitects & Engineers (Blacksburg, VA). Primarily focused on projects in Virginia, Shelton works to help his clients meet the Virginia Department of Environmental Quality’s regu - lations for stormwater. “The Department of Environmental Quality has some pretty stringent guidelines for water quality and quantity,” he said. In early 2021, Shelton and his team completed a project that required a stormwater management system that could handle up to nearly 2.5 million gallons of water. “This system is one of the largest we’ve designed. It can manage a 100-year storm with the volume capacity it has.” “According to the plans, this new construction would increase the amount of impervious area to nearly 25 acres, which included all the associated loading docks and employee parking lots and road paving, plus there would be a 350,000 square foot building on the property. This all added up to a significant impervious area and meant that we had quite a bit of water to control and store. It’s a pretty substantially sloped area which would require a lot of cut and fill. I figured that since the area would need to be filled in, we might as well use the storm water detention system to help bring it up to grade.” “The way our system works is that we collect all the water, run it though a water filtration units for pre-treatment and then into the cham - ber units that have an added section to trap additional sediment.”
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csengineermag.com
DECEMBER 2021
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