Ecosystem services are arguably the most important and most undervalued function of landscapes. These functions are often taken for granted. Take for example, the air that we breathe. How many times during the day do you think about the cleanliness of your air. Probably only when you’re parked behind a car that’s expelling poisonous gasses at a traffic light. And the reason that you don’t think about the air that you breathe? It is simply because trees, shrubs, perennials and grasses are performing an ecosystem service that enables you to live and breathe comfortably.
Ecosystem Service Functions of Landscape
But the ecosystem service functions of landscapes go well beyond this knowledge that you learned in kindergarten about trees making oxygen. Landscapes also manage stormwater, control air temperatures, and create habitat for animals and insects. Because these functions are complex, I’m going to break them into separate blog posts. In this one, we will focus on the ecosystem service of stormwater management.
When Landscapes Aren’t Allowed to Perform Ecosystem Services
When I was a kid, the only thing I liked about doing the dishes was that I could play in a sink full of water (until my mom noticed that I wasn’t making much progress on cleaning). I loved going to the beach and swimming at the lake. Water was captivating to me and any chance I got, I played in it.
Fast forward 25 years, and some things haven’t changed. I still love water and am fascinated by it. I am intrigued by how water moves, especially during a good hard rain. Part of me loves to see water rushing along the curb – the sheer power is impressive. And yet, another part of me knows the danger of such power and force.
With the creation of urban development came an abundance of impervious surfaces. Where rain previously fell on open fields and soaked into the ground, it now falls on asphalt and concrete. It then collects into concentrated streams along the curb and is deposited into a grate in the ground. Once in the storm sewer, poof! The water is gone, like magic.
Hundreds of sites, miles of roadways, acres of parking lots manage their stormwater (unwanted rainwater) this same way. As quick as possible stormwater gets ushered into pipes and moved off-site. But where does it go?
A Stormwater Journey Through the Storm Sewer
The simple answer is that it goes to the nearest stream, lake or other body of water. Which on the surface, doesn’t seem like such a bad thing.
However, when stormwater flows along a paved surface where there is little friction, it picks up speed. When it picks up speed, it also picks up whatever else is laying on the paved surface – anything from car oils to soil particles to that potato chip bag that flew out of your car when you had the windows down. This water also takes heat out of the pavement that it falls on. Once the water reaches the curb, it gets collected into a concentrated flow and picks up more speed. At the catch basin, the grate where the water enters the storm sewer, the water changes direction. As with a natural river, water slows down at a change in direction and some debris is deposited. Some debris. The rest is carried along with the stormwater to the nearest body of water.
As the stormwater continues its journey through the storm sewer pipes, it continues to pick up speed as more water is added to the flow. At the outlet, fast moving stormwater bursts into an unsuspecting lake or stream polluting the water and creating a less than ideal environment for fish and other wildlife.
But, pollution isn’t the only problem with stormwater going straight into lakes and streams. The fast moving stormwater also erodes soil in the streambed and banks near its outlet. Stormwater is generally warmer than the body of water that it is flowing into and will heat up the lake or stream unnaturally. The timing of the stormwater’s arrival in the waterbody is also worth noting. Water in a storm sewer is headed straight for the nearest body of water. It does not pass go, and does not collect $200. It’s fast. There is little delay between a rain storm and increased lake and stream water levels; it’s the perfect recipe for flooding. Isn’t there some other way?
Stormwater Management Through Ecosystem Services
Indeed there is. What’s neat is that by emulating the systems found in nature, we can design ecosystem services into landscapes that can mitigate the issues of pollution and flooding. Three of these design tools that I want to discuss are bioswales, detention basins, and permeable pavement.
Bioswales in Stormwater Management
A bioswale is a fancy name for a ditch that is filled with plants. Similar to a wetland system, its function is to provide a temporary place for stormwater to go after a rain event.
By daylighting the stormwater runoff (initially keeping it on the surface and not in a pipe), the water encounters more barriers, such as rocks, plant stems and plant roots, which slows the water down. When the water is moving slower, it has a chance to infiltrate the ground. This is advantageous for the plants that are growing in the bioswale because once the water is in the ground their roots can take it up.
Slower moving water also allows the soil and pollutants that the stormwater picked up in the parking lot to fall out in the bioswale. Again, the plants and soil in a bioswale do their work. They take the once harmful pollutants (gas, oil, phosphorus, metals) and uptake/degrade them so that they aren’t polluting the environment anymore. The slower moving water has now received a quick cleansing and some of it has soaked into the ground. What is left now enters the storm sewer or potentially our second system of stormwater management – the detention basin.
Detention Basins in Stormwater Management
A detention basin is a large depression in the ground where stormwater can be held for a brief period of time. A detention basin has two main purposes though it functions similarly to a bioswale, just on a larger scale.
The first main function of a detention basin is to hold water and release it slowly into the storm sewer system. The basin is typically sized by a civil engineer to hold stormwater from a 100-year storm (a storm with a 1% statistical chance of occurring each year). During a 100-year storm, the basin fills and excess water overflows into a catch basin that is within the detention basin.
On more typical storm events, when the detention basin doesn’t entirely fill up, water can slowly flow through weep holes in the sides of the catch basin. By requiring the stormwater to enter the catch basin at a slower rate, the storm sewer is less burdened. There is less water in the system during the storm event. When water moves more slowly through the system, it creates less downstream problems like we discussed earlier (read: less flooding, less erosion).
The second main function of a detention basin is the opportunity for infiltration. Because water is held within the basin for a longer period of time (usually 24-48 hours after the rain event), water can infiltrate into the ground. This is beneficial because when water infiltrates into the ground it recharges the aquifers (underground lakes) that we use for drinking water wells. Moving stormwater directly into lakes and streams bypasses this critical step of the water cycle and doesn’t allow for essential infiltration.
Permeable Pavement in Stormwater Management
Permeable pavement is pavement that allows water to infiltrate where it falls. This requires leaving air voids within the pavement where the water can go during a rain event. As an example, with our home landscape renovation, we installed a permeable paver sidewalk. There is nothing special about the pavers themselves other than the fact that they have been designed to leave ½” gaps between each paver. Below and in-between each paver we installed open graded aggregate where water can gather before it soaks into the ground. Open graded aggregate are rounded pebbles that can’t lock together no matter how they are arranged. Think of a jar of marbles. There are a lot of air voids that can fill with water during a rain storm.
The advantage of using permeable pavement is that rain is infiltrated where it falls. This water never has a chance to pick up speed, collect pollutants or enter the storm sewer. It never becomes runoff nor does it become stormwater. Managing water close to where it falls is the simplest place to manage it.
Growing the Design Toolbox
The stormwater management ecosystem service function of landscapes creates livable places for people by cleaning polluted runoff, slowing stormwater and infiltrating rain water as close to where it falls as possible. These are simple systems that have profound impacts on how our environments function.