Given the nature of very intense storms, it’s not always possible to increase natural absorption to the point where there’s not substantial runoff. In developed settings, this runoff can cause more widespread damage, sometimes catastrophic destruction. So, it’s critical, in urban settings, to establish strategies to redirect runoff to areas where it is safe to handle.
Traditional stormwater methods use smooth concrete pipes designed to encourage rapid, unimpeded flow to their ultimate outlet. In fact, they’re designed to maintain a minimum water velocity in order to ensure sediment remains suspended in the water column. When this high velocity water (traveling up to 50 feet per second) enters local waterways, the erosive force is devastating.
In order to protect drinking water supplies and critical ecosystems, while preventing problems like toxic algae blooms, alternate methods for diverting stormwater from impervious surfaces need to be implemented across the country. Fortunately, this problem is currently being studied and a variety of best practices have been identified as a part of climate sensitive urban design and low impact development. They’re developed using research and development of integrated stormwater management and green infrastructure, much of which is sponsored by the EPA.
Traditional vs Green Stormwater Infrastructure
Traditional stormwater infrastructure typically involves directing precipitation to street level curb openings, grates and sump inlets or similar structures to manage drainage from ditches, etc. As we’ve mentioned, this water is gathered through a series of concrete pipes to convey water at high velocity to its final destination. Aside from the negative environmental effects, these systems are still vulnerable to overflow and flooding when the volume of precipitation overwhelms the system or there are failures due to clogging, sedimentation, or deferred maintenance.
Green infrastructure methods, in contrast, work by slowing down runoff, rather than accelerating it, and spreading it out over land where it can slowly soak into the ground rather than concentrating it to a single point of discharge. Green infrastructure practices also include methods for reusing water onsite rather than dumping it as troublesome waste. Another key benefit to green infrastructure (also known as low impact development) is that, by using vegetated areas for absorption and filtering, plants can remove significant amounts of pollutants from runoff. Nutrient pollution can be directly used by the plants, and many other types of pollutants are naturally absorbed and sequestered.
Rain gardens and green roofs are increasingly popular elements of green infrastructure. They can be employed to reduce runoff, using it immediately to nourish plantings in locations throughout the city, in residential neighborhoods and even individual homes. Rain barrels are another popular element added to home gardens, patios and restaurant terraces.
Diverting runoff into green stormwater management areas presents opportunities to slow and move water beyond the problem areas, whether those are low lying areas in a yard with poor drainage, or low lying areas in the city where water pools and floods streets, sidewalks, or parks. It’s important to match diversion techniques with constructed tools to manage and disperse the flow, such as catch basins, French drains, and swales. In each instance, runoff will need to be managed in route using these tools or directed to further points with effective capture and containment. Diverting stormwater runoff, without providing effective and suitable endpoints, will not solve the problem.
Grassed Swales
Swales are wide, shallow ditches which guide water to the next management stage. Grassed Swales are planted with grass and other densely rooted plantings that protect the underlying soil from erosion and can absorb smaller amounts of received water during light or moderate rains. In some cases, plantings are unsuitable due to the slope of the swale or growing conditions, like dense shade, prevent plantings from thriving. In others, where the volume and velocity of water is expected to be consistently high, some swales may be lined instead with stone, from gravel to rip rap, or a combination thereof. In these cases, it is typically a good idea to line the swale with a permeable woven geotextile liner to further protect the underlying soil from erosion while still permitting water to flow through.
French Drains
French drains are drainage devices buried below ground. They’re common in residential situations where the foundation or basement suffers from excess moisture, and they’re effective in commercial developments as well. A French drain consists of a perforated pipe that’s buried underground in a low-lying area and surrounded by gravel, lined with a durable, permeable geotextile fabric. The drain collects excess water and directs it to an area that can more readily absorb it. If the surrounding area presents no suitable candidates, French drains can connect to a catch basin for longer-term retention.
Since French Drains serve to capture, slow, and encourage infiltration of stormwater into the soil, they’re considered part of the arsenal of green infrastructure used for stormwater management. They’re typically employed when the area to be drained is relatively small and more complex engineered solutions aren’t appropriate. They serve to slow and reduce runoff and mitigate its effect on nearby development. French drains may not be constructed on a scale to handle very large storm events, but they are invaluable when used in concert with other efforts to minimize and moderate runoff.
