Home-scale Stormwater Runoff Solutions: An Overview

Get Creative in the Garden with Bryn — By on May 1, 2009 12:10 PM

water system

There is a finite amount of water on the earth.  Think about it. The same water that exists today was around when dinosaurs roamed and was there a million years before them.  This water may exist frozen solid in a glacier, drifting as a gas in the sky or tumbling as liquid down the side of a mountain. Any single molecule of water may change form or location innumerable times through the millennia.  This is called the hydrologic cycle.  Like any proper cycle, the hydrologic cycle has no beginning or end, but scientists generally recognize a number of distinct phases-evaporation (and transpiration), condensation, precipitation, infiltration, groundwater recharge, and runoff.  (For a simple animation of the hydrologic cycle see here.)  
 
As humankind has moved towards large-scale agriculture and urban/suburban living, we have dramatically altered our environment and similarly impacted the hydrologic cycle.  By cutting down trees, we have reduced the amount of transpiration that occurs (not to mention trees’ temperature modulating affects.)  By paving everywhere and building solid structures, we have reduced the ability of water to infiltrate the soil and similarly recharge groundwater sources.  These solid surfaces also frequently absorb and retain heat, changing the surrounding air temperature and also affecting the hydrologic cycle.  In essence, the changes we have wrought on the environment have served to cause the hydrologic cycle to favor the faster phases (evaporation, precipitation and runoff) over the slower (transpiration, infiltration and groundwater recharge.)
 
With all of these impermeable (solid) surfaces, rainfall has fewer places to go during a storm.  So a complex system of drains and pipes was developed in each populated area to deal with the problem.  The solution, it was believed, was to get the stormwater from where it fell to the nearest stream as fast as possible.  This, actually, has caused additional problems.  This fast moving runoff is able to carry much of the trash and pollutants that it comes across (“non-point source pollution”) and dump it directly in the stream.  The runoff also tends to be warmer and warmer water carries less oxygen than cold water.  The Center for Watershed Protection has a nice slideshow on the “Impacts of Urbanization“.  Of particular interest note the 2 hydrographs-the post-development stream has a much greater range of flows than the pre-development stream, leading to flood-induced erosion followed by periods of very low-flows.  Very difficult living conditions, to say the least.  Remember, we aren’t just talking about wildlife either.  We depend on water directly (for drinking) and indirectly (to grow our food, keep things clean, etc.)
 
So faced with such a large problem, what can a single person do?  This problem wasn’t caused overnight by one massive change, nor will it be fixed the same way.  An action everyone can take is to reduce your contributions to non-point source pollution.  Don’t litter.  Keep your car tuned up so it doesn’t leak or wear down the tires any faster than necessary.  Pick up after your dog (yes, I know it’s “natural” but we are talking unnatural quantities for the existing environment.)  Use no more fertilizer or pesticides than recommended on the label, or better yet use organic methods, and don’t apply them before rain is predicted.  As commercial car washes are required to filter and recycle their water, this method is preferable to washing your car yourself.  I hope I don’t even need to say it, but absolutely don’t pour any waste down a storm drain.
 
A step up from the basics would be to start trying to reverse the effects of urbanization, namely by increasing tree cover (and other plants) and decreasing impervious surfaces.  Not all plants have an equal ability to absorb water and later give it off through evapo-transpiration.  Root depth and surface area of a plant will both affect these traits.  Roots will not just affect a plant’s ability to absorb water but will also affect the permeability of the soil.  Check out this diagram of the root systems of some native prairie plants compared to turf grass.  Which soil do you think has better permeability-a lawn or a native prairie?  (A compacted lawn can have nearly the same permeability coefficient as pavement.)  In addition to helping infiltrate water, a tree will also help moderate the temperature of the surrounding area through both shade and evapo-transpiration.
 
When you look to build a driveway, a walkway or a patio, try to minimize the impervious surfaces you create.  This can be done in a variety of ways.  Reduce the area you use.  Does the whole driveway need to be paved or can just the tire tracks be paved?  Will you notice if the walk is 6″ narrower?  Second, try to use a more permeable method.  This used to be limited to just gravel (difficult to shovel in the winter, has a tendency to compact, gets tracked to places you don’t want it, etc.) and dry-laid brick or stone (laid on a gravel bed instead of cement; not stable enough for vehicular traffic), there are many more options now.  Both pervious asphalt and concrete have been developed.  For each, some of the smaller particles have been removed from the mix without affecting the overall integrity.  This has created voids that allow water to pass through (when constructed over an appropriate sub-base.)  There are also several manufacturers (here’s one) of plastic mesh systems that can be part of a lawn or gravel area and provide stability while prohibiting compaction.  They can even be strong enough to support the weight of a fire engine!  A number of manufacturers have also created pavers that are designed to be laid with gaps between each unit for permeability.
 
Installing a rain barrel is a simple way to help with runoff.  Rain barrels come in a number of sizes and shapes, or you can even make your own (make sure you use food-grade plastic).  In addition to the barrel itself, it should have a spigot (for accessing the water), an overflow, a lid or guard (to keep children or thirsty animals from falling in), and a screen that allows water in but keeps debris and mosquitoes out.  (Despite the screen on mine, I still get mosquitoes and have to use “mosquito dunks” –a product which contains the bacteria Bt– to keep them from hatching.)  The overflow can either point away from the house or be linked to another rain barrel.  Rain barrels are typically connected to house downspouts.  There are a number of products designed to help you do this, including just a flexible downspout.  Remember, although a full rain barrel will give you a free supply of water, it will not help manage stormwater should it rain.  The spigot can be connected to a drip irrigation system (not a soaker hose-they tend to clog) or can be used with a watering can or hose.
 
The rain barrel could also overflow to a “rain garden” (as could the downspout.)  A rain garden is, in essence, a depression that is planted with perennials, trees and shrubs that are tolerant of both wet and dry conditions. They are not intended to have standing water, but should drain after a few hours, if sized properly.  A rain garden will look similar to any other planted bed to the casual observer.  For more information on rain gardens, an online search should turn up plenty of resources or see my article in the July/August 2007 issue of Yoga Bean.  Remember that a rain garden should be at least 10 feet from the house because you don’t want water to soak in right next to the foundations.
 
For the more ambitious, a green roof can have very beneficial effects on stormwater issues (as well as mitigating air pollution and the heat island effect, providing habitat, adding to the life expectancy of a roof, increasing the insulation value, reducing noise transference, etc.)  Green roofs can be separated into two categories: extensive and intensive.  An extensive green roof will only have a few inches of growing medium and will be planted with sedums and other drought tolerant plants.  It is typically not irrigated.  An intensive green roof could also be called a “roof garden.”  It may have growing medium up to a couple feet in depth and be planted with trees and shrubs as well as perennials.  It will need to be irrigated and is typically designed to be walked on.  An extensive green roof has a much higher ROI (return on investment) than an intensive green roof, as it both costs less and as greater or equal payback on all levels.  Extensive green roofs are very good at absorbing the first inch or more of a storm event.  (The majority of storms are an inch or less.)For more on green roofs, good starting points are Green Roofs for Healthy Cities and GreenRoofs.com. Scott Arboretum at Swarthmore College also created a nice series of videos when they installed their latest green roof.
 
When trying to mitigate stormwater runoff remember, in the words of EPA video (9 minutes, nicely produced), “Slow It Down, Spread It Out, Soak It In”.  Never forget, water shouldn’t be considered a problem to be gotten rid of but a valuable natural resource that we depend on for life. 

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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