The Drainage Infiltration Strata is an artificially devised soil structure comprised of artificial turf, permeable pavement, a layer of gravel, a layer of sand and a drainage pipe with a watertight sheet. When impermeable pavement on grounds or in other public spaces is replaced with the drainage infiltration strata, the volume and the rate of stormwater runoff is reduced. This is due to the fact that the water on the surface of the drainage infiltration strata passes into the layer of gravel and is retained. The drainage pipe with the watertight sheet at the bottom of the strata removes water retained within the layer of gravel. This causes runoff from the strata, but the capacity within the layer of gravel is maintained. Data on a full scale model experiment at the baseball field of the University of Tokyo showed that

  • the average volume of runoff for sixty rainfalls was reduced to 42% of that of rainfall;

  • the average rate of peak runoff for 12 especially heavy rainfalls was reduced to 22.3% of that of peak rainfall.

A numerical model to reproduce the behaviour of stormwater within the strata was formulated using the two-dimensional Richard's equation. With this model, the effect of the drainage infiltration strata on reducing the volume and rate of stormwater surface runoff, during a design storm, was predicted.