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Void Ratios for stormwater retention facilities

Void Ratios for stormwater retention facilities

Void Ratios for stormwater retention facilities

(OP)
I've proposed a bioretention area for a project which is sized based on the runoff volume. This is in California and is based on the C3 Technical Guidance Manual. I used a void ratio for biotreatment soil and for Caltrans class 2 permeable material of 0.35. However, the County Planning Dept reviewer said the void ratio for both is 0.01 to 0.05, which is ridiculously low. I tried to find void ratios for both of these materials on line, but could find nothing. Does anyone know of a reliable data source for these two materials?

RE: Void Ratios for stormwater retention facilities

Uniform (graded) stone typically has 35-40% voids. Any kind of "biotreatment soil" is going to be much less.

Peter Smart
HydroCAD Software
www.hydrocad.net

RE: Void Ratios for stormwater retention facilities

Interesting in that the void ratio for pervious concrete is in the range of 30%...

Mike McCann, PE, SE (WA)


RE: Void Ratios for stormwater retention facilities

We should be careful here with terminology: "Voids" or "porosity" refers to the fraction of the overall stone volume that is empty space, and is often expressed as a percentage such as 25%.

"Void ratio" is the ratio of voids to solids, so 25% porosity would be a void ratio of 25:75 or 1:3

Peter Smart
HydroCAD Software
www.hydrocad.net

RE: Void Ratios for stormwater retention facilities

20% void ratio for bioretention soil is quite common for design purposes here in the NE.

RE: Void Ratios for stormwater retention facilities

30% is common in the NW for infiltration trenches filled with washed, uncrushed gravel.

Mike McCann, PE, SE (WA)


RE: Void Ratios for stormwater retention facilities

I never use a void ratio for the bio soil, and 30-40% is typical for the perm rock. If you need to cite a source, look at Contra Costa's C3 guide:
V1 is the floodable volume above the soil layer (that is, the total volume of surface storage when the facility just begins to overflow). V2 is the storage volume below the soil layer. If gravel fill is used, multiply the volume of gravel by an assumed porosity of 0.4 to obtain the subsurface volume.

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