When land is converted from its natural state to other uses, especially urban land uses Such as roads, homes, and shopping centers, many impervious or paved surfaces are created.
Rainfall can no longer soak into the ground.
Instead it becomes stormwater or runoff'. As land is developed the volume, speed of flow, and pollutant loading of runoff increases. Swales are one of the most commonly used stormwater practices. For many years they have been used along rural highways and residential streets to convey runoff. Today, swales not only convey stormwater but also help to treat runoff to reduce pollutants.