9. Landscape Sustainability: Stormwater
I’ve already gotten into what sustainability means to me and now it is time to talk about specific methods of landscape sustainability, with a focus on simple things that can be done in residential yards. Let’s start by looking at all that fresh, clean water that falls from the sky, typically referred to by engineers not as rain, but stormwater. When it falls on a paved surface, or a building roof, it is not able to immediately go where it would if us humans weren’t around – into the ground. The traditional approach has been to get the stormwater from these impermeable surfaces into pipes as quickly as possible, but this has led to increased erosion, flooding, water pollution, and habitat degradation.
The sustainable approach is to take care of all the rain that falls on your land right there. If you can, collect the runoff from your roof and store it for use, something I will talk about in depth separately. Otherwise you are trying to do three things with your stormwater, nicely put by the Sonoma Valley Groundwater Management Program: Slow it. spread it. sink it. By slowing the water down you take energy out of it that would cause erosion, and by temporarily holding it on your property, you keep the local streams from getting overwhelmed and flooding. Spreading the stormwater allows dirt and other particles to settle out before the water gets into our streams, rivers and lakes. Sinking the runoff into your soil follows the natural cycle and replenishes groundwater. What are the ways you can do these things in your yard?
Pave Less & Make It Permeable
I can’t put it any simpler than the authors of Sustainable Landscape Construction: pave less. Take away every square foot of concrete and asphalt that you can, and use permeable paving surfaces as much as possible. There are many good material options to make functional but permeable pathways. The most common are loose gravel or stepping stones that allow water to enter the soil in the gaps between – but these are for foot traffic or light vehicle use only. For regular vehicle traffic one of the best options is a precast permeable paver. This type of paver has built in spacers which create gaps for water flow while maintaining the necessary interlocking strength. Other materials have been recently added to the list of possibilities including: permeable concrete (looks like rice crispy treats), permeable asphalt, soil reinforcing grid systems and even synthetic fibers directly in the soil that reinforce it enough to support vehicles.
With any permeable paving surface, it is critical to consider what is underneath it. If you have clay soil, only a limited amount of rain water will be quickly absorbed. In those cases a layer of drain rock can be placed under the pavers. The gaps between the rocks store the water, giving it more time to sink into the ground. This can require a pretty thick layer to do much good, and may become construction intensive and costly. So, you’re probably better off turning to the other category of sustainable stormwater solutions: on-site infiltration in planting areas.
Rain Gardens and Bioswales
For a few square feet of impermeable paving, simply sloping to a planting area may be all you need to do. However, for larger areas you want to be careful not to concentrate too much water into a single point in your garden; you will start to create erosion and even pollution problems. Any surface where cars travel will collect the fluid drippings, particles from brake pads, etc, so the water that washes across these surfaces needs to be treated accordingly.
The Rain Garden or Bioswale is the typical approach to sustainably take care of your stormwater. In my mind these two things are pretty much the same: you are creating a swale (a.k.a. ditch) or other depression designed to accept the stormwater without eroding. The stormwater gently flows into, then slowly across the swale, allowing for surface filtration by the plants as well as absorption by the soil below. In addition to the particles that settle out of the slowed down water, plants and their associated soil organisms can break down pollutants, making the water cleaner (known as bio-remediation).
Again, with compacted or clay soils, you may not get much absorption, but you are still slowing and spreading out the water temporarily. For larger scale projects, the clay soil is often removed and replaced with a special well-draining soil mix that provides additional filtration, but is connected to the storm drain piping downstream. In all cases you need to be careful not to create a ponding situation where you have standing water longer than 48 hours – mosquito breeding becomes an issue – so always include an overflow outlet for bigger storms.
These bioswales can be made into interesting design features that earn it the more evocative Rain Garden title. The extra moisture of the swale will create a nano-climate for unique plant types that will make it noticeable in your landscape. Round river pebbles are also typically used as a mulch in place of bark chips, which will tend to float away. This is a great opportunity to give visible expression to the natural process of water flow, an important part of sustainable landscape design, connecting us back to the natural world.
The control of stormwater runoff is a major issue in all urbanized areas and the State of California now has a set of stormwater management rules to address it, known as the C.3 Regulations. New construction projects beyond a certain size require bioswales and other measures be installed to take care of stormwater within the property’s boundaries. You have probably seen these required bioswales in the parking lot of your office building or shopping center without realizing what they were. By capturing and controlling the rain that falls on your property you are not only making use of a valuable resource, you are improving your neighborhood’s ecology and keeping the regional watershed cleaner. The only real question is why wouldn’t you do it?
References:
Sustainable Landscape Construction, by J. William Thompson and Kim Sorvig
Water Management, Occidental Arts and Ecology Center workshop led by Brock Dolman, 2012
Water Harvesting in California: Obstacles and Opportunities, published by The Water Institute at OAEC, https://oaec.org/our-work/projects-and-partnerships/water-institute/publications/
Slow it. Spread it. Sink it., published by the Southern Sonoma County Resource Conservation District, https://sonomarcd.org/resources/
Sustainable Sites Initiative Guidelines 2009, American Society of Landscape Architects, http://www.sustainablesites.org/report/