Take yourself back to your sixth grade science class where you were taught the hydrologic cycle. Your teacher told you that surface water evaporates changing water from a liquid state to a gaseous state. Condensation occurs when water vapor condenses onto small airborne particles to form clouds. This vapor and clouds are then carried by winds aloft. Sufficient condensation results in precipitation falling from the atmosphere to the ground. The falling or melting water may adhere to objects on or near the surface. The water may be carried over and through the land into stream channels, or the water can percolate into the soil.
Water percolates though the soil horizons by gravity and capillary forces. Rivers and streams are formed when precipitation exceeds the soil’s ability to percolate water at the same rate as precipitation falls or melts or water percolating through the soil encounters an impermeable layer. This water then flows to lakes and eventually to the ocean where the cycle repeats itself.
When we cover the soil with a non-permeable substance or alter the soil structure, we interrupt the hydrologic cycle. Altered or compacted soil may no longer absorb precipitation at a rate that will sustain plants or in the case of a parking lot, absorb any moisture at all.
When the soil in an urban area can no longer accept precipitation or there is insufficient soil to absorb the moisture, stormwater results. Stormwater disposal in urban areas is expensive. Stormwater systems are constructed and maintained at taxpayer expense to collect, transport and eventually dispose of stormwater. Stormwater systems are also used to capture run-off from landscapes, car washing and when we clean our driveways with the hose.
“Unfortunately, water is not the only substance a stormwater system captures. Urban stormwater pollution carries sediment, oils, grease, petroleum byproducts, materials that wear off brakes and tires, asphalt, metals, pesticides and fertilizers from park and lawn management, and toxic contaminants from industrial facilities.
Stormwater pollution is not just an urban problem. Residential and commercial development has led to polluted runoff problems in suburbs. Even in rural areas, stormwater carries sediment off dirt roads, pesticides and fertilizers off lawns, parks and agricultural land, and fluids and solvents from poorly maintained vehicles and machinery.
There is a high correlation between the area of impervious surface in a watershed and the adverse impacts on receiving waters. The more asphalt, the more pollution and the greater the volume of water discharged into the stream.
Pollutants carried into water bodies by stormwater have negative effects on many uses such as aquatic life, recreation and public water supplies. Sediment is known to be one of the pollutants causing the most damage in aquatic environments, for example, by carrying chemical substances into the water, clogging spawning and feeding areas, causing damage to fish gills, and leading to changes in fish communities.
Unnatural high flows, caused by runoff over increasing amounts of impervious surface, result in significant changes to hydrology and stream channels. These high flows scour the stream banks, remove vegetation (which leads to increased temperatures), carry away large debris critical for fish survival, and reduce the opportunity for groundwater recharge.
The long-term effects of poor stormwater management are very site specific and “are related to habitat degradation, deposition and accumulation of toxic sediments, or the inability of the aquatic organisms to adjust to repeated exposures to high concentrations of toxic materials or high flow rates.” (Pitt, Robert, Ph.D., “Effects of Stormwater Runoff from Development,” River Voices, vol.14, no.3)
The problem is not just from pollution or natural precipitation. Over 25% of all domestic water used by a homeowner ends up in the gutter and into the stormwater system. Irrigation practices, landscape design and leakage all contribute to water waste.
One of the most common tools used in managing the impacts of development in our environment is called “mitigation”. Mitigation can be defined as measures taken to reduce adverse effects on the environment. Mitigation efforts can be site specific or non-site specific.
Let’s say a developer wants to put in a parking lot over the top of a wetland area. To encourage development in a community, a local government may allow the developer to mitigate the impact on the overall environment by having the developer construct a wetland at another location (non-site specific) or create a similar wetland next to the development (site specific).
In recent years progressive communities have mitigated the problem of reduced groundwater recharging, polluted stormwater and wasted landscape water with the use of bioswales.
A bioswale is an urban landform used to convey surface water in order to enhance infiltration and reduce surface runoff. Bioswales are typically moderate gradient devices (approximately one to five percent in channel slope) and may be covered by grasses, landscape fabric, mulch or other vegetation or leaf litter. These landforms are typically integrated into an urban landscape design to enhance the visual appearance, but also may be used in agricultural settings as drain-ways to intercept runoff containing silt, pesticides or nutrients. In urban settings the bioswale may serve to reduce sediment load and other water pollutants from reaching natural watercourses. A typical bioswale has gently sloping or curved sides to emulate the appearance of a natural watercourse; in fact, if established with appropriate native vegetation, the bioswale may become a riparian corridor or wetland restoration element of the natural landscape.
Bioswales are specially designed troughs that offer a path for water to run through. They are typically shallow and wide, and packed with a specifically selected type of vegetation or plant life that allows for water to travel through them slowly. Bioswales can be designed for various purposes and situations. The first design is a wet swale, which is predominantly used on commercial properties because they actually pool storm water in order to allow it to infiltrate the ground water at an acceptable rate. Wet swales can be filled with grass and other organic filters that remove pollutants and contaminants (see photos I and II).
Another variation is a dry swale, which prevents water from pooling on the surface. These are popular in residential areas because they remove puddles that can collect during heavy rain fall. Dry swales are often outfitted with a bottom layer of soil and sand, which are placed on top of an under-drain that consists of gravel encasing a perforated pipe that directs water in a desired direction (see photo III).
A third variation useful in arid climates is designing the slopes to actually capture water and slow the flow to enable percolation (see photo IV). Bioswales are an excellent, eco-friendly drainage solution that can allow you to replenish ground water without sacrificing integrity. The major benefit of a bioswale is that it can absorb low levels of overflow easily, while properly redirecting large volumes of water at the same time.
Photo I- Bioswale designed for capturing street and parking runoff.
Photo II- Bioswale designed to intercept stormwater.
Photo III- Dry bioswale used to carry off and deposit storm water to be recaptured.
Photo IV- Bioswale designed to slow water runoff and capture stormwater.
Bioswales intercept large quantities of surface runoff from low permeability man-made surfaces, such as parking lots, roadways and roofs. Bioswales are useful in industrial parks, office complexes, retail centers and high density apartment projects. Bioswales next to parking lots and roads gathers runoff that often contains vehicular fluids, lead, sediment, etc.
In addition to water pollution control discussed above, the bioswale can benefit in flood control since greater infiltration and reduced surface runoff are products of the bioswale landform. In addition, the bioswale traps may allow actual decomposition or destruction of certain pollutants. For example, nitrogen and phosphorus pollutant loads may be consumed in vegetation growth within the bioswale.
There are communities nationwide that are moving forward with the use of bioswales at both the commercial and residential level. Governments and individual citizens alike need to examine the effect of our interruption of the hydrologic cycle.