Swale
Swales or ditches (swale in English) are closed ditches, equipped with an overflow that temporarily stores runoff water at the top of plots, channels the water and allows infiltration of water into the soil[1]. They are increasingly used to limit the risks of flooding, soil erosion and drought.
Description
A swale is a kind of shallow, wide ditch, vegetated, with gently sloping banks, which temporarily collects runoff water, either to evacuate it via an overflow, or to let it evaporate (evapotranspiration) and/or infiltrate on site thus allowing replenishment of groundwater tables.
This shallow system has gentle slopes and thus allows integrated hydraulic flow management within a project. Swales have the advantage of limiting runoff which causes erosion, pollution concentration, and too rapid water increase downstream.
In some swales, plantings of trees and hydrophilic plants can be observed. These will allow better water infiltration thanks to their roots. In these retention systems, it is preferable to plant species adapted to wet environments. Swales are a privileged place that allows the development of biodiversity.
Types of swales
There are several types of swales depending on soil infiltrability conditions: infiltrating swale, draining swale, mixed swale.
- The swale can result from a natural relief (examples: old oxbow, elongated depression) or be a man-made structure, generally as part of urban or agricultural runoff control devices.
- The swale can be simply grassed and periodically mowed (late mowing if the device includes a objective of biodiversity protection) or grazed. It is sometimes grazed.
- It can be individualized or discreetly blended into a landscape or eco-landscape design. It is then generally subject to ecological management and thus differentiated. Its main objective being water management, or even restorative water management when planted with helophytes to add a natural lagooning role, thus avoiding any use of fertilizers and pesticides.
- Depending on the quality of water directed to it (pollution risk), the desired functions for the swale, and the vulnerability and proximity of the groundwater table, it may contain a draining substrate or conversely be built on a clay layer (e.g. bentonite), a geotextile or a completely impermeable liner.
Utility
Hydraulic interest
Swales are one of many alternative techniques for runoff water management. They have the advantage of slowly evacuating rainwater, which allows infiltration and recharge of local aquifers while reducing the risks of drought. They allow water management as close as possible to its point of fall, with passive solutions (not dependent on pumps, valves, and pipes that may clog, etc.). If often supplied with water, they contribute to the restoration of wetland networks (with a possible natural lagooning function). They prevent or limit runoff, which is a powerful factor of water pollution and rapid pollutant transfer downstream.
Ecological potential
Swales are primarily a buffer zone for rain or flood water, but they can also be part of a landscaping or agricultural project.
They allow integration within and at the edges of plots of corridors rich in vegetation and wildlife adapted to soil and climate to reduce the impact of bio-aggressors.
Advantages
- Swales locally combat both flooding and drought.
- They contribute to the replenishment of groundwater.
- They have a low cost and a very low ecological footprint compared to conventional solutions (artificial basins, pipes, pumps, mechanical filters).
- They allow reduced pollutant discharge into the natural environment thanks to the purifying power of plants, bacteria, fungi (provided the water contains only biodegradable pollutants).
- They are reliable, as a passive system and eco-technical solution largely self-maintained by natural ecosystem processes.
- They produce wood (heating, mulch, crafts…), fruits, vegetables…
Implementation
Like keylines, swales follow the contour lines of the plot; they are closed ditches. The swale will become a super keyline that will accumulate much more water. This ditch of 80 to 120cm wide and 30 to 40cm deep, will be downstream of the slope, topped by a small mound increasing water storage capacity. Various forest species and fruit trees will be planted on this mound; this first hedge will be doubled by a hedge of small fruits selected according to climate and soil to complement fruit production. At the base of the mound (an area regularly richer in water), a third hedge of perennial vegetable crops (asparagus, artichoke, or others requiring little maintenance) can also find its place.
The swale will distribute, infiltrate, and store runoff water. Each swale will be equipped at one end with an "overflow" that evacuates excess water to various ditches which can feed one or more ponds.
Swales, which often cross plots entirely, quickly become corridors where wildlife can move and settle (birds, game, insects and multiple beneficial insects). They also become a source of wood supply (heating, mulch) as well as fruits, small fruits, herbs for teas, vegetables, and medicinal plants. They become the "nerve center" of the plot's biodiversity.
Depending on the slope, size, and number of key points of the plot, one or more swales can be installed. Most often, the vine which also follows contour lines, will run along the swales. It is along the swales, upstream, that paths crossing the plots will be placed.
Swale calculator
Here is a tool to calculate/define, based on the number and length of desired swales, their minimum width, the volume of water capable of being captured/distributed/infiltrated, their position and spacing between them: Swale calculator[2].
It takes into account:
- The desired number of swales.
- The distance between the top of the slope (hill) and the bottom of the plot in meters (Distance from bottom to top of hill (m)).
- The volume of the largest intense rainfall events in mm (Volume of rainfall in a large event (mm)).
- The length of each swale (Length of swale (m)).
- The estimated slope in degrees (Estimated slope above swale in degrees). For safety and efficiency, no swale should be made if the slope exceeds 22° or 40% due to risk of landslide, overflow...
- The estimated runoff percentage (Select estimated percent of runoff). Take into account soil composition + cover + cultivation method.
Once these parameters are entered, the calculator indicates for a chosen number and length of swale:
- The distance in meters where the swale should be positioned relative to the bottom of the plot.
- The volume of water captured in m3 for each swale during large rainfall events.
- The minimum useful surface area in m² per linear meter to capture runoff (knowing that a "standard" swale has a depth between 30 and 40cm). This can be translated into the minimum width in meters the swale must have.
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Valorization of swales
Swales can become a new source of semi-wild production, requiring very little maintenance, while encouraging at least annual care.
The idea is not to encompass production chains, but to make agreements with professionals from each sector, who will handle processing, packaging, and possibly marketing these productions. It will remain possible to have finished products to market at the estate.
These productions can be multiple depending on soil and local climate and be located as double or triple rows of hedges:
- Blackberries, blueberries or Raspberry: for teas (leaves) and jams (fruits).
- Pears and Apples: perry and cider.
- Tea plants: Teas.
- Bees and flowering trees with staggered blooming: multiple honeys.
- Olive trees: oils, table olives or tapenades and soaps.
- Lavender and Thyme: essential oils, herbs or jellies.
- Artichoke and/or Asparagus: fresh vegetables,...
It is also possible to plant various forest species such as dogwoods, willows, lindens, ashes, ... preferably from seed or pits germination, because from there, they will produce a main taproot, like fruit trees, which can penetrate deeply into the soil. Secondary roots will spread much less horizontally and trees will better tolerate the presence of vines and other plants and vice versa.
Under these conditions of absolute biodiversity, soil moisture without excess (and residual even in summer), and favorable soils, these productions will only need to be harvested and for some just topped and lightly pruned.
Regulations
Swales are not subject to declaration or regulation. However, during design and construction, existing ditches and all water crossings cannot be modified or diverted. New ditches can be created under the same conditions as swales. Ponds are subject to specific regulations.
Sources
- Wikipedia page on swales
- Why not swales? - Oxyria
- Malard Alain. Vines, wines and permaculture. Editions France Agricole. 2021.
- Page on swales in Permaculture Design: https://www.permaculturedesign.fr/la-baissiere/
- ↑ Malard Alain. Vines, wines and permaculture. Editions France Agricole. 2021.
- ↑ See this article explaining the use of the "Swale calculator": Swales from A to Z part 2/2
