A runnel redirects stormwater where erosion, flooding or damage to building and wall foundations is possible. Because a runnel is such a narrow drainage way, it allows for flexibility in the selection of sites for a rain garden or bioswale. These vegetated depressions can be fed by transported water from areas that receive excessive amounts of runoff. Installing a runnel serves as a way to control stormwater in order to filter runoff into the ground or direct it to areas where infiltration is possible. Runnels seek to lessen the impact of stormwater.