Water Quality Design Storms for Stormwater Hydrodynamic Separators

When measure of the device performance is based on laboratory data, NJCAT calculates the average annual removal efficiency using the NJDEP-specified weighting factors. In this study, ten years of precipitation records were used to quantify frequency distributions of runoff volumes/depths as well as runoff peak flow rates. The SWMM model was used to simulate the runoff events produced by the given precipitation events. The model results indicate that the weighting factors, based on frequency distribution of the peak runoff rates normalized by the peak runoff rate generated by the uniform-intensity water quality design storm, are close to the NJDEP-specified weighting factors. The impact of using two different water quality design storms, uniform vs. non-uniform intensity distribution, to size an interim-certified hydrodynamic separator was also evaluated. The SWMM model was used to continuously simulate solids loading to the treatment device. The lab-measured relationship between flow rate and removal efficiency was used to specify the removal rate of solids within the storm event. The model results also indicate that sizing with the uniform-intensity design storm would achieve a removal efficiency close to the one verified. Sizing with the non-uniform intensity design storm, although could be taken as a conservative approach, would achieve the removal efficiency considerably higher than the one verified.