The STEM Sessions Podcast – Episode 1. Bioswales: More than a Ditch
Bioswales are a valuable tool in water ecology and urban design. They’re found in parks and landscaping and ecological reclamation areas. They often blend in with their surroundings, so you’ve likely seen them in action without realizing what was in front of you.
Let’s first look at the root of the word – swale. Typical definitions include: 1) “a low place in a tract of land, usually moister and often having ranker vegetation than the adjacent higher land”, and 2) “a valley-like intersection of two slopes in a piece of land”. Its origins are the Old Norse word svalr meaning cool, or svalir meaning a covered porch.
Etymology aside, a swale is a natural ditch or depression that collects and retains enough water to support a lusher ecosystem than that found in the surrounding area. When the prefix “bio” is attached, the definition becomes a low lying section of land that has been enhanced, or engineered and constructed, to specifically mitigate water runoff from the surrounding land.
Mitigating water runoff in human inhabited urban landscapes is traditionally handled by a system of interconnected storm drains. During a rain event, water collects on non-permeable surfaces such as sidewalks, paved parking lots, and streets. These surfaces are graded to funnel that water into storm drains. Smaller storm drains merge into bigger storm drains, which eventually empty into nearby bodies of surface water. Along the coasts, that body of water is usually the ocean, but in inland regions, storm drains may empty into rivers or reservoirs or human-built collection basins.
Water in storm drains tends to move quickly, because all surfaces are relatively smooth, and there are no features impeding the water’s progress. In fact, storm drains are designed to move water quickly to prevent flooding. Further, water picks up and carries away nearly any substance sitting on those surfaces. These substances like dirt, oil, fertilizers, animal waste, and pesticides are ultimately deposited into the final bodies of surface water where their concentrations can increase to hazardous levels.
In the case of fertilizer runoff, high concentrations can cause destructive algal blooms in the surface waters – a process called eutrophication. This algal bloom can be so thick or opaque that it prevents sunlight from penetrating into the water, causing die off of any plant life requiring sunlight for photosynthesis. This in-turn can lead to die-off of the animals who would have fed on those plants, and ultimately results in explosions of bacteria as they consume the dead bio matter.
Uncontrolled runoff not only negatively impacts bodies of water, it also negatively impacts the land. If the rate of precipitation is more than the ground can absorb, the landscape can quickly flood and erode. Channels may be carved in the ground, trees may uproot and fall over, hills may become unstable resulting in mudslides.
At best you’re left with a few trenches that need to be filled in. At worst, you’re left with very serious and costly property damage. Additionally, much of that disturbed soil is swept away by the flood, polluting the downstream water system with silt, rocks, and other particulates. A properly engineered bioswale prevents much of the above damage and pollution by mimicking, or repairing, the natural hydrology of the landscape, and allowing much of the rainfall to be retained on site.
Erosion is the result of fast running water, so to prevent erosion, a bioswale slows the water by providing a gentler grade, winding paths, and impediments such as rocks and plants. As the water slows, erosion decreases. The slower current also prevents the particulates from pollution downstream water by allowing them time to settle in the bioswale before leaving the area.
In a similar fashion, slowing the water allows time for the ground to absorb chemicals like phosphates and nitrates – the key components in fertilizers – preventing them from collecting in the surface water. If the bioswale is expected to see high quantities of fertilizers, specific soils are selected for their ability to efficiently absorb those compounds. For example, iron and aluminum added to the soil will bind the phosphates, and wood particles will absorb the nitrates. Plants will also be selected for their phosphorus and nitrogen fixing abilities, not only locking away the chemicals, but ultimately converting them to more advantageous compounds.
Heavy metals such as lead, mercury, and cadmium will also settle in bioswales, becoming trapped in the soil with some soil recipes trapping more efficiently. There are even a plant species that will absorb the heavy metals, but unlike fertilizers, the metals are not transformed into anything usable. Ultimately, concentrations of heavy metal will reach a level where they pose a danger to the resident plants and animals. In those cases, the soil will be removed before this concentration is reached, and replaced with clean soil allowing the process to begin anew.
Bioswales are also customized for the overall climate in which they’re placed – accounting for the amount and timing of rainfall, temperature ranges, sunlight, and the general look of the area.
Grass bioswales, for example, tend to be manicured and found in parks, golf courses, and commercial and private landscaping. These bioswales are primarily designed to capture runoff from artificial irrigation with natural precipitation being a secondary concern. They’re also designed to retain some fertilizers, but they typically lack the variety of plants and soils needed to absorb a full range of chemicals.
On the other hand, vegetated bioswales provide terrain features and a high variety of plants to increase their effectiveness in chemical retention, water capture, and erosion prevention. These bioswales are typically found in climates with large amounts of precipitation. They also look more natural than their grass counterparts – a look that lends itself to large plots of land and can be integrated with natural landscapes like wetlands and rivers.
Bioswales can also be designed for xeriscaping (low water use). These are found in hot climates, which receive little to no rainfall through the vast majority of the year – deserts with monsoon seasons, for example. Plants and physical features in these bioswales must survive eleven months of the year with little to no water, AND then also survive one month of extremely heavy rainfall.
Conversely, there are bioswales designed to be continuously wet, often times with some degree of flowing water. These may be found in areas with rainfall year round or in areas with constant irrigation runoff or a high water table. In these bioswales, the selected plants must thrive in permanently, or near permanently, wet soils.
After this deeper look, we can see a bioswale is much more than a “ditch with plants”. Optimizing one for its location is a non-trivial design problem. The designer needs to understand the area’s hydrology, climate, weather patterns, wildlife, and pollutants. Soil types and the stack-up of soil layers must be properly selected for drainage and pollutant absorption. Plants must be properly selected to match the climate, the selected soil, the amount of water present, the wildlife it needs to support, and the available nutrients and pollutants.
Each variable and decision impacts several other variables and decisions, resulting in complex relationships and varying levels of effectivity. But when designed and built correctly, a bioswale will complement the surrounding landscape aesthetically and ecologically, while offering a sustainable means of controlling water runoff, preventing erosion, and reducing the amounts of pollutants that enter the local water system.
References
https://en.wikipedia.org/wiki/Bioswale
https://en.wikipedia.org/wiki/Eutrophication