Rain gardens and other streetside stormwater retention systems are designed to infiltrate street runoff, retain pollutants, and incorporate attractive plantings. That is the vision, but the reality is that they are hard to get right, particularly in a cold climate like Minnesota. What makes a curb-cut rain garden project sucessful?
The move to install decentralized stormwater management infrastructure in our streets makes a lot of sense. In a typical suburban residential area anywhere from 20%-35% of the developed landscape is a “connected impervious surface.” This category includes streets, parking lots, driveways and rooftops that drain in concentrated flows directly to a stormwater pond or water body rather than towards open space. It is these hardened areas that contribute the largest volumes of runoff and concentrations of pollutants to local water resources. So if we are looking to make retrofits to the built environment, streets and their drainage networks are a logical place to start. Especially since stormwater detention ponds, the traditional approach required by regulatory agencies, have come into question regarding their tendency to export phosphorus pollution and accumulate toxic sediments over time. The reasons are there, but standing in the way is a whole host of thorny nuts and bolts issues that can turn the most well planned project into a waste of money and a liability in the landscape. As a designer who has worked on this kind of project, I have seen it go both ways and feel that in order to be successful there needs to be careful attention to detail.
The Burnsville Rain Garden: Before and After, Image Courtesy of Barr Engineering
For this reason, it makes sense to start with an example from a successful and well known project. If you’ve ever been to a rain garden workshop or seminar on low impact development in Minnesota, you are probably familiar with the Burnsville Raingarden Project. It was a paired watershed study conducted by the City of Burnsville and Barr Engineering that used monitoring to track the effectiveness of curb cut rain gardens as a low impact development technique. The results indicated that the gardens achieved about a 90% reduction in runoff volumes during typical small storm rain events. In addition, the gardens all looked really good. This project has been the poster child for rain gardens and stormwater retrofits that I and others have presented as a case study to many different audiences for years. But until yesterday, I had never actually been out there, even though it is right here in the Twin Cities. So I threw the kids in the car (sorry kids) and went out to take a look at the gardens now that it has been almost ten years since they were installed to see how well they are holding up and figure out what makes this project stand out.
Lo and behold, the Burnsville Gardens still look really good. One reason why these gardens likely are still doing so well is that there are so many of them in a concentrated area. Undersizing or overloading the capacity of a rain garden is a surefire way to ruin it. Because 80% of the residents on the street installed rain gardens, the runoff never has a chance to gain too much momentum or pick up large quantities of sediment. This point underscores the interrelationship between the stormwater analysis and the outreach activities. It involves placing the gardens correctly and getting enough people on board for a decentralized system to work effectively in the first place. Since the Burnsville project there have been major advances in how this kind of retrofit project is planned. The Sub-Watershed Analysis is an approach advocated for by the Center for Watershed Protection and practiced locally by Soil and Water Conservation Districts. The goal of a sub-watershed analysis is to understand the characteristics of the area contributing runoff to the water body in question. This stage in the process is the fundamental basis of good decision making for the project. Using field reconnaissance, GIS systems, and stormwater modeling software, the design team can identify the extents of the drainage area, typical runoff volumes/pollutant loading for different rain events, the soil suitability for infiltration, and determine suitable sizes and locations for different stormwater management practices. Above all, this stage must consider the regional context to determine if the project will have meaningful results. For this reason, many of the models incorporate a cost-benefit analysis and rank implementation activities to guide decision making. These are the technical considerations that regulatory agencies and water resources professionals are concerned about, but for the people agreeing to have this piece of infrastructure installed in their yard, they care about how it looks.
The Burnsville Rain Gardens are one of the premier case studies on the subject because the plantings really shine (hats off to the designer, Fred Rozumalski). The before and after images of the rain gardens never failed to get some oohs and ahhs in every rain garden workshop that I ever presented. The gardens have a robust and colorful plant palette but also utilize the tactful placement of shrubs and grasses to streamline maintenance. One feature of the design process was that each resident was able to calibrate their own level of maintenance in collaboration with the landscape architect. Ten years later, you can see how each approach has evolved. The perennial heavy gardens have the most issues with weeds, but those that are well maintained have the most striking appearance. The shrub heavy gardens are a bit more mundane looking, but are nearly weed free and have much greater mass in the landscape.
The shrubs, which are barely noticeable in the initial photographs have come into their own as low maintenance structural elements that have performed admirably in keeping weeds from establishing around the edges of the gardens.
Likewise, the grasses and perennials are also are carefully located at the inlet to the gardens to act as an additional barrier against sediment intrusion and seed establishment. This kind of pragmatic planting design is critical to any kind of living green infrastructure because it extends its practical life to a point where it may actually be worth the money we spend on it.
Aside from the fact that they all seemed pretty well maintained, you could tell that there had been some professional refurbishment as well. The grassed inlets had been replaced somewhat recently error to prevent sedimentation of the garden basins and landscape edging or pavers served as an additional sediment trap and energy dissipation system. Without this kind of protection, the fine particles carried in the runoff will rapidly plug up the basin, killing the plants and preventing infiltration. In addition, you could see that for the newer gardens, the curb cut inlet had been replaced with a sump catch basin that could be cleaned out by a Vactor Truck. This move results in a more reliable maintenance regime than expecting the homeowner to regularly rake sediment out of the turf strip as it accumulates.
The new and improved sump catch basin inlet
The Burnsville rain gardens are still thriving after almost a decade because they are functionally designed and we can intuitively recognize them as infrastructure. This visibility and care in engineering is the key to earning green infrastructure a real seat at the infrastructure table. The water captured by these rain gardens is a drop in the bucket compared to the ecological services performed by say, an intact wetland or an agricultural buffer strip. However, they present a compelling vision for a green street with layered functions that people can appreciate and wrap their minds around.