Technical Corner: Sense and soakability—Filtering water on site
TEXT BY STEPHANIE SNOW, OALA
I’m always up for a good argument but recently, standing at the edge of a suburban California soccer field, my debating skills were pushed to the limit. My friend and I were looking at a recently deforested hillside, crisscrossed with concrete drainage channels. He was explaining to me why California needs to pave more to “protect us from the water.” This (colourful) conversation reminded me of what an important role landscape architects have to play in reducing infrastructure costs, helping to protect our water, and educating people about the most effective way (i.e., not paving!) to achieve water infiltration goals.
Rain gardens, bioswales, infiltration ditches, vegetated swales… call them what you like, they’re all planting beds intended to collect rainwater and reduce runoff. Incorporating them in landscapes wherever possible will filter water on site and return it directly to the water table, where it belongs. If done right, they can also lead to real financial rewards for developers, municipalities, and homeowners.
The value of rain gardens can be demonstrated in a recent project I worked on in which I was tasked with expanding an existing parking lot that had some real problems. The initial site visit identified an advantageous low point in the parking lot, a massive puddle doing a decent impression of a small lake. The majority of the trees were struggling in individual, undersized islands while the adjacent building sat well below the level of the asphalt, making it susceptible to flooding. After careful consideration, we adjusted the layout of the parking lot, improved traffic flow, and increased the total number of parking spaces. We reduced the amount of impervious surface by including a single large island with an infiltration area. This biofilter was planted with a combination of grasses, sedges, willow, and other attractive native plants. We were also able to replace some of the asphalt and underlying heavy fill material with permeable pavers and subsurface overflow drains connected to the existing storm system. Making use of what was already on site reduced the overall development costs and limited the negative impact on the surrounding ecosystem. Today, the trees are thriving and the bioswales (we included a second one to cut off the flow of water towards the building) do double duty as attractive landscape features.
The most important factors, for infiltration features, are location and site preparation. I always start by reviewing the site itself, as the existing topography often includes natural infiltration areas. A review of the subsoil conditions across the site will tell you if you have areas with naturally occurring high permeability.
Once the preferred locations have been established, it is critical that the areas be protected against sediment and erosion. If the subsoil is contaminated with silt and clay during construction, the rain garden planting will fail at its intended task.
Generally speaking, the smaller the site, the fewer options you have for the location of your infiltration areas. In many instances it is preferable to have several smaller infiltration areas as opposed to a single large stormwater management pond.
The appearance of your bioswales is up to you. A naturalized (i.e., not regularly mown) turf-lined swale works well in an out-of-the-way area, but a more visible location can be quite decorative with a host of perennials and woody vegetation.
I have found that when specifying or purchasing live plant material for infiltration areas, 1-gallon pots are the smallest you should go. Seed tends to wash away, and smaller plants are more susceptible to damage during inundation. Plugs or large flats of plants may appear cost effective but tend to either wash away or get buried in mulch during storm events. The best bet is to plant substantial plant material (in 1-gallon pots or larger) that can hold up to the weather during the establishment period.
The other important lesson I’ve learned is that you need to visit the site plenty of times during the installation and make sure everyone understands what you’re trying to achieve. On one project a couple of years ago, I had the contractor rip out the rain garden and rebuild it three times. In our initial meeting, we had reviewed the grading and layout with the project manager. He then passed the information on to his site supervisor, who had his crew install it. The problem was that the person actually doing the installation knew that planting beds should be higher in the middle, not lower. His reasoning made sense—just not for a rain garden!
The key elements of an infiltration area are the pooling zone, the detention/filtration zone, and the retention or recharge zone. The profile of the individual feature depends largely on the existing soil permeability. Sandy soil is ideal for the introduction of an infiltration area as it has excellent permeability, allowing water to dissipate quickly. Clay soil will need to be amended, and a retention or storage area should be included in the bottom of the pit where water can collect and infiltrate more slowly. Other ways to help a less-than-perfect situation include sub-surface drains and overflow structures.
The filtration zone should include rich organic material; this will allow water to percolate through the soil more efficiently. The pooling zone can be planted and mulched. Mulch may include stone or shredded hardwood as both resist floating and being washed away. A sod or stone mulch strip around the perimeter of the feature helps keep larger debris out and acts as a pre-treatment or filter strip.
Regardless of the size or location of your infiltration area, remember to plan for some degree of maintenance. Over time, sediment can build up and reduce the infiltration rate, leading to standing water. Facilities should be monitored and care should be taken to ensure nearby activities keep your bioswales in optimal health. Blocked openings into the swale or garden can cause damage and reduce the effectiveness of the system. Inorganic debris can also accumulate if left unchecked. This is particularly relevant in municipal park settings and adjacent parking lots where garbage may get washed into the bioswale.
If you would like to learn more or teach your clients about the benefits of including rain gardens, bioswales, and other infiltration systems in your designs, it is worth following up with your local Conservation Authority. Many have an abundance of resources specifically packaged for a variety of audiences. Good resources include www.sustainabletechnologies.ca and thelivingcitycampus.com.
BIO/ STEPHANIE SNOW, OALA, IS THE FOUNDER AND A PRINCIPAL OF SNOW LARC LANDSCAPE ARCHITECTURE LTD. IN TORONTO.