Sustainable Stormwater Management

Get Creative in the Garden with Bryn — By on May 2, 2008 12:10 PM

For many years the preferred approach to stormwater management has been to treat stormwater as a problem to be controlled-get that stormwater into drains and offsite as fast as possible. After all, rain feeds the streams and we’re just helping out by getting it there faster, right? Slowly we are learned that we’ve been causing a lot of problems with this approach.

The water cycle, or hydrologic cycle, is the complex movement of all the water on our planet. Water rises into the atmosphere through evaporation (the process of water changing from a liquid to a gas, typically due to increased temperatures) and transpiration (the water given off by plant’s leaves as part of the photosynthesis process). It falls to the ground again through the processes of condensation (the opposite process from evaporation) and precipitation (when the condensation falls back to earth). In between the rise and fall of water vapor, it is held in the atmosphere and frequently travels a distance from its original location. Once the water falls to the ground, some of it runs off into streams but much of it infiltrates into the ground. The infiltrated water may emerge at another location as a spring or be drawn up by plant roots. As springs flow down hill they create streams, which merge to create larger streams then rivers, eventually heading as far down hill as possible, where they end up in the ocean. Water may be stored for a time as snow packs or glaciers, in lakes or oceans, or underground in aquifers, but it will eventually move on to a different part of the hydrologic cycle. To see a nice diagram of the hydrologic cycle, as well as more detailed explanations of the various terms/phases involved, see the USGS’ website.

As a site becomes more developed, it becomes less permeable-water isn’t able to permeate into the ground. Any rainfall that lands on buildings, parking lots, roads, sidewalks, or any of the other impermeable surfaces that make up our urban environments is not able to soak into the ground, but instead runs off. This runoff is typically collected in drains that connect to storm sewer systems. Most of the storm sewer systems in the US flow directly to streams and other bodies of water without any treatment to remove contaminants. There are many problems with this system. First, stormwater is not able to percolate into the ground and therefore isn’t available to plants or able to recharge the groundwater aquifers. There is a nice diagram here that compares the hydrologic cycle of urban and non-urban areas. Second, the rainfall is rushed to streams which causes highly variable water flows in the stream leading to cycles of erosion-causing flooding followed by very low flows. Third, faster moving water is able to carry more pollutants and sediment than slower water. Seeing as the runoff comes from buildings, streets and parking lots, there are plenty of contaminants available. (This is referred to as “nonpoint source pollution.” An outflow pipe from a factory would be an example of “point source pollution.”) Fourth, the runoff frequently flows over hot surfaces and warms up more than the stream water. Warm water is able to hold less oxygen, causing problems for many fish and other stream dwellers.

So what can the average person do to remedy the situation? First, reduce the amount of pollution you create. Keep your car tuned up so it doesn’t drip oil and take it to a car wash, where the wash water is treated. If you really prefer to wash it yourself, wash it on the lawn where the water has a chance to soak into the ground. Most definitely don’t dump your used motor oil down a storm drain. Pick up after your dog. (Yes, I know dog poop is natural, but if large quantities are swept from the curb and into a stream it can be a big problem.) If you feel the need to fertilize your yard, be sure to follow the directions and only put down as much fertilizer as the plants can use. If you use pesticides and herbicides, do so sparingly and again follow the directions.

There are two general ways to manage runoff more effectively. The first is to collect it in a holding tank of some sort. An example of this would be to connect your roof downspout to a rain barrel. A large scale example of this would be a retention basin created as part of land development. Rain barrels can be very useful because you can use the water at a later point to water plants. However, the typical rain barrel holds about 55 gallons and it doesn’t take much rainfall to fill it up. Linking multiple barrels together gives you more storage capacity but remember, a full rain barrel that overflows during a storm isn’t helping control runoff at all.

The second general concept for runoff management is to slow it down. Slowing the water down allows it to permeate into the ground when it flows over an appropriate surface. Slower moving water also will carry fewer contaminates and soil particles. It also means that the stormwater will flow into a creek at a lower volume over a longer period of time, causing less erosion and less dramatic changes in the stream volume. A nice hydrograph that illustrates the difference in streamflow after a storm, between a largely impermeable site versus a more natural site, can be found here. (To return to the article that the image comes from, click the link at the top of the webpage.) I also saw a nice hands-on illustration of this concept at a museum once. It involved a marble, a smooth ramp, and a ramp with lots of pegs. On the smooth ramp, the marble (raindrop) easily and quickly made it to the bottom (stream). On the pegged ramp, it took much longer and occasionally even was stopped before reaching the bottom.

The typical way to slow runoff is to reduce impermeable surfaces. For example, instead of creating your new patio by pouring a concrete slab, use brick set in a gravel bed or even non-compacted decorative gravel. Vegetated roofs, or green roofs, are a wonderful way of counteracting many of the environmental impacts of buildings. Their benefits extend beyond stormwater management and include noise control, insulation, habitat, and mitigating the urban heat island effect. Green roofs are a large topic and are a good subject for a later article. The Green Roofs for Healthy Cities website is a good place to start exploring the topic and a web search will return lots of sources, as well, if you are looking for more information now.

It is also worth mentioning that lawns are fairly impermeable. Turf grass has a very short root system and lawns become easily compacted. A very nice diagram of different perennials’ root systems compared to that of turf grass can be found here. After looking at this diagram, you can easily see how more water will infiltrate into a meadow than a lawn. It is especially important to have a planted buffer against a stream bank instead of having lawn right up to the edge. In addition to improving infiltration, the longer roots also do a better job of stabilizing the bank.

There are also several methods that combine the techniques of both slowing and holding the stormwater. For example, you might set up your rain barrel to feed a soaker hose-the collected water will slowly drain out of the barrel over the next day or so. Rain gardens are another good technique for managing stormwater that utilize both the slow and retain methods. Typically a rain garden is designed to hold the rainfall from a one inch storm. If the storm drops greater than an inch of rain, the extra will overflow. For a more detailed discussion of rain gardens, see my article in the July/August ’07 issue of Yoga Bean Magazine.

Reduce impervious surfaces, replace lawn with planted beds, set up rain barrels, create rain gardens, install green roofs, disconnect downspouts from storm sewer Hopefully, I’ve given you a few ideas to try at home. Let’s get rainwater to infiltrate and recharge our aquifers and create healthier streams. Treat it as the valuable resource it is instead of something to be disposed of. Remember, every little action helps.

Happy gardening!

Bryn Richard is a licensed landscape architect with a strong interest in sustainable design. She can be reached at Bryn@BlueTrillium.net and welcomes your questions and suggestions for further articles.

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