Past conversations have said that the volume stored in the soil mix and the dead storage above the soil mix (below the overflow) in bio-retention basins can be used to offset the runoff volume increase from pre to post development conditions. We have clay soils with low permeability, so we install underdrains in our bio-retention cells, a few inches above the bottom of the stone. There is a 24 hour design draw down time. Can the dead storage above the soil mix be used to reduce the post development volume? Can the soil mix, with a 0.17 porosity, be used as offsetting volume if there is an underdrain?

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Michael DeVuono

Regional Stormwater Leader

Arcadis North America

LEEDuser Expert

144 thumbs up

Fri, 03/17/2017 - 16:30

If I were doing this, I would route the under drain, and the infiltration. You aren't going to get credit for whatever bleeds out of the underdrain, but even the worst of soils infiltrate something. Simply providing the volume required to manage the delta does not meet this credit, it needs to infiltrate in this situation.

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