Use of Outlet Controls to Improve CSO Reductions Using LID SCMs

William C. Lucas and David J. Sample


This presentation examines the potential deployment of LID stormwater control measures (SCMs) for reducing CSOs in a small 17.4 acre CSOshed in Richmond VA. Using a GIS to obtain source area data, a total of 68 different LID SCMs were modeled. 5 Green Roofs, 13 porous pavements, 5 bioretention and 21 planter-trench arrays were located in such a manner that the entire catchment could be intercepted.

One LID alternative examined the results using typical LID designs in which there are no outlet controls on the underdrains from these SCMs. Another LID alternative installed controls on the underdrains to provide extended detention and improve infiltration volumes. This array of distributed LID SCMs was compared to a Grey alternative using a detention vault to intercept enough runoff to control CSO exceedances. These three alternatives were compared to existing conditions in terms of both runoff responses and water quality implications. A typical year was compared to the intense rainfall patterns expected due to global climate change.

SWMM 5.0.022 was used to model these four scenarios. Due to the high degree of baseflow in a highly impervious watershed (>81%), it was apparent that a considerable amount of runoff was able to infiltrate into the soils. Therefore, aquifers were used to model these flows with a reasonable degree of accuracy. This meant that runoff volume reductions due to infiltration were much lower than normally anticipated.

While the free discharge LID controls were able to provide substantial reductions in CSO exceedances, they were not as effective as the Grey infrastructure alternative. However, by installing outlet controls to promote infiltration and reduce underdrain flows, the controlled discharge LID SCMs were able to match the performance of the grey alternative in the typical year. In the extreme year, the controlled LID systems outperformed the grey alternative, indicating the LID SCMs are more resilient in terms of meeting the challenges of future global climate change.

Permanent link: