While the catchbasins under sewer grates work well on swift-flowing surfaces like asphalt and concrete, heavy storm water flow on grass lawns or other open areas will pool in low areas if there is no outlet. An infiltration gallery serves this purpose in two ways.
Primarily, upright plastic pipes capped with simple grates are placed every 5-8 metres along the low point of a slope, to handle heavy surface runoff. The pipes proceed straight down, about two metres, to a horizontal cross-pipe; this pipe is the secondary system.
The horizontal pipe is then perforated slightly (about 10% of its surface area) and surrounded by gravel. Initially, runoff will exit the pipe and infiltrate the gravel to the soil beyond, dissipating naturally. As flow increases, the water will eventually fill the pipe and need to be dissipated more quickly. Thus, a catchbasin is placed at the lowest point of the sloping ground, which is connected to the storm sewer system at large.
Such galleries are a relatively new development in urban planning, and are thus found in newer housing developments.