CONCRETE PIPE “Apples to Apples” Sizing Comparison Generally, each Stormceptor system is sized using PCSWMM for Stormceptor, a continuous simulation model based on US EPA SWMM. As described in previous sections, this program calculates hydrology from up-to-date local historical rainfall data and specified site parameters. With US EPA SWMM’s precision, every Stormceptor unit is designed to achieve a defined water quality objective. However, in cases of competitiveness an ‘apples-to-apples’ comparison is often sought by the designing engineer. As such, Stormceptor is able to provide an alternative sizing based on the same principles used by our competitors to size their units. We feel this ignores many important factors associated with stormwater quality, but provides a more direct comparison. Some of the major differences include: Lab Evaluation Based Sizing • Performance based on controlled laboratory settings using clean units, and clean sediments. • Testing is often completed without sediment pre-loaded into the sediment chamber. These tests do not account for scour, (high-flow flushing) of the units. This lab evaluation conducted for the Stormceptor unit tested removal performance at both 50% and 100% pre-loaded sediment capacity without any measurable amount of scour well beyond the nominal flow rates of the unit. • The sediments used in these lab tests are generally clean quartz sand as purchased by a foundry supplier. These sediments do not account for the changes in density and settling velocity that can occur with the addition of organics and other pollutants that happens in situ. • These performance calculations often include ambiguous performance ‘adjustments’ to modify the final performance efficiency. However, the application of these adjustments seems to be site specific without much justification. Calibrated, Continuous Simulation • Model performance is calibrated to a number of actual insitu field tests that take into account real world flow rates, sediment loads, and particle sizes and densities. The correlation between modeled and actual is 0.94. • The model is calibrated to units that have been installed in the field and allowed to collect sediment over time. Installing a unit that is only going to lose a portion, or all, of it’s sediment load during the next high flow completely undermines the purpose and value of these devices. Stormceptor has conducted rigorous third party tests to ensure the Stormceptor unit will not scour. • The settling velocities are calculated based on the generally accepted Stoke’s law, which are then modified for shape and density. These calibrations allow us to more accurately model sediment settling velocities that would occur in-situ over that of foundry sand. • Each value used in the hydrologic model is a default setting used by US EPA SWMM. These values are based on calibrated models that have been improved upon for the last 30 years. The justification of these values can be found from any number of sources including “User’s guide to SWMM”, James, William et al. CHI. Toronto (2003). PAGE 28 LAFARGE PIPE eMANUAL