Thoughts on Storm Water Drainage Using a Drywell

July 17th, 2008 - I found out today that we are not connecting our sump-pump and downspouts to the city storm sewer as I'd thought. Apparently, the city decided not to allow the connection and instead specified a drywell be installed in the front yard.

The problem is they did not specify either the size of the drywell, nor the leaching capacity, nor the expected gallons / day dissipation rate. It is, it seems up to me and the contractor to arrive at some solution that minimizes run off onto our neighbors lawns!

I got on the internet to do a bit of researching and here's what I've learned:

  • Rule of thumb is to limit each drywell to supporting no more than 1000 square feet of impervious surface.
  • 2" of rain produces about 1.25 gallons / sf
  • Select the volume of the well-container to handle the maximum expected surge.
  • Design the area of the walls /floor or the well surfaces to handle the maximum gallons / day that you will be leaching into your yard (the sponge). At normal soils depths you need 32 cubic feet of well for each 100 sf of impervious area drainage.
  • Use gravel around the well-container sides and bottom to enhance leaching into surrounding soil. 1' of gravel quadruples the leaching capacity of the container (100 gallons becomes 400 gallons) per day.
Our drywell is required to be positioned in the front-south side of our property adjacent to our driveway (civil engineering spec). This limits us to a 5' width of the drywell in one dimension, but allows for 10-15' in the other dimension.

In our case we plan to connect our sump-pump to this drywell as well as a backyard low-spot drain pipe, and gutter downspouts on the south side of the house. Total area of impervious surface is calculated as:

  • 160 sf (home perimeter x 1ft footing depth) for the sump area;
  • 100 sf (low spot in backyard);
  • 600 sf (downspout south drain).
Based on 860 sf of equivalent impervious area, the drywell needs to have a volume of at least 275 cubic feet; or dimensions of 15 x 5 x 4.

A drywell container costs around $250 (dimension of 2' x 4' cylindrical) with a 2' stand-pipe above. It holds 100 gallons.

I may need to install two of these to get the dispersion I need. They will be connected to a 6" PVC pipe running below the frost-line to the well.

If the flow-rate exceeds the capacity of the drywell, the standpipes allow water to rise to the surface and run out (not a desirable characteristic, but likely during really heavy storms).

I'm hoping to get this work done for under $2500 (including the 110' run of 6" PVC from the back of the house toward the front of my drive).

I will be running this by my General Contractor and our Civil Engineer at CES to see if they concur that this is acceptable sizing for the drywell excavation.

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