Exfiltration and Dry Wells

Difference between infiltration and exfiltration

Exfiltration trenches store water that is fed by pipe, whereas infilatration trenches are fed from the top surface and percolates into the trench.

Engineers will design a stormawater drainage system for a project and calculate all the dimensions needed for an exfiltration area.

The size and elvations of pipe and the top of exfil elevation are extremely important for the drainage to work properly.

The exfiltration areas' elevations are important so that all the other drainage flows to the correct areas. If elevations do not match the design then the system will not perform as designed.

Below is a plan of an exfil. It was designed by an engineer and the highlighted exfil specs are what we use to install a correct exfil.

engineers plan of an exfiltration trench
Engineered exfil specifies all dimensions needed for this fairly large exfiltration area
exfiltration trench with washed #4 rock and perforated pipe
The Exfil area is wrapped in cloth to keep small particles out of the rock area, a perforated pipe is run into the rock to allow water to seep in.

Engineered Elements of an Exfiltration Trench

  • Washed Rock, in Florida usually Washed #4 Rock
  • Perforated Pipe (size & length of runs)
  • Filtration Cloth to wrap the rock
  • Invert elevation of the pipe
  • Dimensions of the trench/pit (determines water storage)
  • Top of Rock Elevation

What is a dry well ?

A dry well is almost the same as an exfil trench, but is usually a square or round hole rather than a long trench.

What is a detention pit?

Some exfilatration trenches or pits can also be detention systems. The only difference is that a detention system also has an outflow as well as in inflow. This will enable the exfil to overflow to another area if it ever fills up to the maximum.