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CDS® Design Basics Using patented continuous deflective separation technology,the There are three primary methods of sizing a CDS system.The CDS system screens, separates and traps debris,sediment, and Water Quality flow Rate Method determines which model size oil and grease from stormwater runoff.The indirect screening provides the desired removal efficiency at a given flow rate for capability of the system allows for 100%removal of floatables a defined particle size.The Rational Rainfall MethodIM and and neutrally buoyant material without blinding. How and Probabalistic Method are used when a specific removal efficiency screening controls physically separate captured solids,and of the net annual sediment load is required. minimize the re-suspension and release of previously trapped pollutants, W Typically in the Unites States, CDS systems are designed to ine units can treat up to 6 cfs,and internally bypass achieve an 80%annual solids load reduction based on lab flows in excess of 50 cis. Available precast or cast-in-place,offline generated perform ace curves for a gradation with an average units can treat flows from 1 to 300 cfs.The pollutant removal n particle size(d50)of 125-microns(jim). For some regulatory capacity of the CDS system has been proven in lab and field environments, CDS systems can also be designed to achieve an testing. 80%annual solids load reduction based on an average particle Operation Overview size(d50)of 75-microns(urn). Stormwater enters the diversion chamber where the diversion Water Quality Flow Rate Method weir guides the flow into the unit's separation chamber and In ma ny cases, regulations require that a specific flow rate,often pollutants are removed from the flow.All flows up to the referred to as the water quality design flow(VVQQ), be treated. system's treatment design capacity enter the separation chamber This WQQ represents the peak flow rate from either an event and are treated. with a specific recurrence interval (i.e.the six-month storm)or a water quality depth (i.e. 172-inch of rainfall). Swirl concentration and screen deflection force floatables and solids to the center of the separation chamber where 100%of The CDS is designed to treat all flows up to the WQQ.At influent floatables and neutrally buoyant debris larger than the screen rates higher than the WQQ,the diversion weir will direct most apertures are trapped, flow exceeding the treatment flow rate around the separation chamber.This allows removal efficiency to remain relatively Stormwater then moves through the separation screen, under constant in the separation chamber and reduces the risk of the oil baffle and exits the system.The separation screen remains washout during bypass flows regardless of influent flow rates. dog free due to continuous deflection. Treatment flow rates are defined as the rate at which the CDS During the flow events exceeding the design capacity,the will remove a specific gradation of sediment at a specific removal diversion weir bypasses excessive flows around the separation in th chamber, so captured pollutants are retained the separation efficiency.Therefore they are variable based on the gradation and removal efficiency specified by the design engineer. cylinder. Rational Rainfall Method"' Differences in local climate,topography and scale make every site hydraulically unique. It is important to take these factors into consideration when estimating the long-term performance of any stormwater treatment system. The Rational Rainfall Method combines site-specific information with laboratory generated - performance data,and local historical precipitation records to estimate removal efficiencies as accurately as possible. Short duration rain gauge records from across the United States At. 1.4 and Canada were analyzed to determine the percent of the total -442 annual rainfall that fell at a range of intensities. US stations' depths were totaled every 15 minutes,or hourly,and recorded in 0,01-inch increments, Depths were recorded hourly with 1-mm resolution at Canadian stations.One trend was consistent at all sites;the vast majority of precipitation fell at low intensities and high intensity storms contributed relatively little to the total ..141annual depth. •, These intensities, along with the total drainage area and runoff coefficient for each specific site are translated into flow rates using the Rational Rainfall Method. Since most sites are relatively small and highly impervious,the Rational Rainfall Method is appropriate. Based on the runoff flow rates calculated for each intensity,operating rates within a proposed CDS system are determined. Performance efficiency curve determined from full scale laboratory tests on defined sediment PSDs is applied to 2