DRY WELL SIZE, STORAGE VOLUME OF INFILTRATION TRENCH 2

[popeye77]

I AM TRYING TO SIZE A DRYWELL FOR A TEN YEAR STORM OF 4" WITH AN IMPERVIOUS SURFACE COVER OF 4000 SQ FT. IF I REMEMBER CORRECTLY TH TOTAL RAIN RUNOFF WILL BE APPROX. 10,000 GALLONS. WHAT SIZE DRYWELL? ALSO, IF THE INFILTRATION TRENCH IS 4'X2'X75' THAT GIVES ME AN INFILTRATION VOLUME OF 6000. WOULD THE RAIN STORAGE CAPACITY BE 60% OF THAT GIVEN THE FACT IT WILL BE FILLED WITH #2 STONE. ITS BEEN AWHILE SINCE IVE HAD TO CALCULATE THIS STUFF SO I WOULD APPRECIATE THE HELP. THANKS

 

[onyeuko]

Convert 10,000 gallons to cubic feet(using the freely available unit converters. You can google and download one from the web). If you have decided to use a surface area of 4000 SQ FT(given some engineering considerations and your constraints), then simply divide the volume resultant from your conversion by 4000 SQ FT as to get the depth of the Drywell. Then subtract the height of the #2 STONE(depending on the number of layers of the stone that you are going to employ-thus the restricted height).That will give you the effective depth.Recalculate the volume of the Drywell, then divide with the volume of THE INFILTRATION TRENCH (which you gave as 4'X2'X75' THAT GIVES MEAN INFILTRATION VOLUME OF 6000cubic feet). This will give you the ratio of the volume of drywell to the trench, then the percentage.
Teddy

 

[KatieTX]

Is the 4 inches of rainfall falling on a drainage area with 4000 sq. ft. impervious cover, or is 4000 sq. ft. the area of your dry well? How does the infiltration trench come into play? Is it the "outlet" for the dry well?

4'x2'x75' = 600 cubic feet, not 6000. And the infiltration area would be 300 square feet (assuming the trench is 4 feet wide and 2 feet high).

When I am working with gravel, I usually assume that 40% of the volume is available (rather than 60%), but I'm not really sure how "gravel" compares to No. 2 stone, or where my 40% number comes from.