A swale is a long trench or channel excavated along level survey lines (contour), which can vary in width and depth according to the soil type, the excavated soil is then mounded on the down hill side of the channel, called a bund wall. The purpose of a swale is to catch over land flow, infiltrate the soil and recharge the ground water and root systems, vegetation is planted in stages on either side of the swale banks (Bill Mollison 1988).
Swales can be connected to dams and interconnected between dams on contour. Swales collect runoff water and passively move water into a dam along contour, until the dam is full and back floods into the swale, potentially filling other dams connected on the same contour line, via the swales.
While swales have evolved to vary in size, vegetation and purpose, there are a few definitive characterstics:
- The channel is built on level contour lines
- If there is more than one metre of fall over one thousand metres of channel, then it is not a swale and more like a diversion drain, as it will result in movement of sediment.
- Swales and bund walls are not compacted, (as it is a water infiltration system not a water storage system).
- “The distance between swales” (including the bund) “can be from three to twenty times the average swale width (depending on rainfall). Given a useful swale base of 1-2 m (4-6 feet), the interswale space should be 3-18m” (Bill Mollison 1988).
Swales are not to be confused with bio-swales which are dry or wet channel courses for stormwater management, the channels are planted with vegetation, which aids filtration of silt and pollutants, bio-swales are similar in design to retention and detention ponds, but longer in length (Encyclopedia of Earth 2011).
Bill Mollison 1988, Permaculture: A designer’s manual, Tagari Publication, Tyalgum Australia.
Encyclopedia of Earth 2011, National Council for Science and the Environment, United States of America viewed 10 January 2014, <http://www.eoearth.org/view/article/150668/>.
Geoff Lawton 2008, DVD, Harvesting Water-The Permaculture Way, The Permaculture Research Institute & Flashtoonz Films, Australia.