There is often less space available in urban areas for infiltration and these areas get more wear and tear from traffic and pedestrians. A system of concrete containers has been developed for the highly-urbanized areas in Portland, which are open at the bottom, filled with gravel and soil, and planted. Such structures can of course be made of other materials.
In Portland rainwater is transported from buildings and streets through above-ground gutters to these infiltration strips. The infiltration strips buffer the rainwater and release it slowly into the ground. The water is partially filtered by the gravel and vegetation before it infiltrates. The ground must of course be suited for infiltration. [Environmental Services, 2006]