I've blogged in the past about rain gardens and runoff. Plus, some of my columns in Friday AM have been on the same subject. Bear with me; it's an important topic!
The best rain gardens are nature's intact ecosystems, created over decades, with minimal human intervention.
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| Natural area in the Shuswap, in the BC interior |
A rain garden is an area that has the ability to absorb rainfall, and then filter it before it gradually is discharged either as surface runoff, or into groundwater.
As leaves and woody debris decompose over the years, the organic material is converted to humus-rich soil able to absorb water. The deeper the humus-containing soil layer, the more water which can be absorbed.
When rain falls over an undisturbed area that has not been cleared, much of the water stays in place. Wetlands often hold overflow, and the amount of overland runoff is minimal.
The image above, from the Shuswap, shows a natural area that would have a good ability to keep rainwater in place. Here are two more images.
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| Rainforest, Victoria |
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| Garry Oak meadow, Victoria. Victoria's dry summers lead to brown grasses by the early fall. The soil underlying Garry Oak meadows is deep and filled with humus, helping retain water so that the trees -- which have adapted to summer droughts -- can survive. |
Urban Development
Urban development usually results in the clearing of native soil and vegetation, and replacing these natural areas with hard surfaces. Creating hard surfaces (think paved roads and parking lots, and the roofs of buildings) increases runoff. Here are some impacts:
- The more hard surfaces, the greater the runoff when it rains.
- Increased runoff means less water is absorbed by the ground.
- With increased runoff comes the potential for floods.
- Runoff picks up pollutants, and through the storm drainage system polluted water may end up in streams or ocean inlets.
This diagram illustrates the impact of urban development.
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| The diagram shows the increase in runoff in an urban area, compared with that of a forested area. Also note the differences in the permeable layer of topsoil, and the depth of the water table. The word "transpiration" means the direct evaporation of water from a plant -- e.g. through its leaves. Source: http://www.melbournewater.com.au/planning-and-building/stormwater-management/wsud-intro/pages/default.aspx (Melbourne, Australia) |
In new suburbs, "stormwater retention ponds" are a common way of handling runoff. They help stormwater drainage systems from being overloaded, but they don't mimic nature as well as the rain garden.
Some years ago I worked with the City of Edmonton to prepare a manual for living beside stormwater ponds. The City has embarked on a program to "naturalize" vegetation around some of its stormwater facilities. I was happy to see that perhaps some of my work had an impact.
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| Naturalized stormwater pond in Edmonton. Source: https://transforming.edmonton.ca/naturalization-at-stormwater-ponds-in-edmonton-a-good-use-of-resources-and-birds-like-it/ |
Creating Human-Made Rain Gardens
Rain gardens are aimed at offsetting the impact of urban development. I first learned about them many years ago, when I attended a lecture about a pilot project in Seattle. This pilot project had a goal of zero runoff -- in rainy Seattle! The idea was to copy nature's way of handling rain.
Now, there is a goal to create 12,000 rain gardens in the Puget Sound area! See https://www.12000raingardens.org/
I researched rain gardens in Victoria, and learned that the City is promoting them here. It has a website devoted to rain gardens and lists several examples of places where residents can find rain gardens (https://www.victoria.ca/EN/main/residents/water-sewer-stormwater/stormwater/managing-rain-as-a-resource/rain_gardens_in_Victoria.html)
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| Rain garden at Fishermen's Wharf Park, during winter. Notice the diversity, and thickness, of vegetation which has grown around this wet area. It was full of birds when I visited it. |
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| The rain garden from a different angle (also in winter), showing overflow drainage. |
This Fishermen's Wharf rain garden is quite large, and as the photo shows it is also at a lower grade than the surrounding area, with a retaining wall surrounding it.
Benefits of the Rain Garden
Rain gardens are slightly depressed areas of deep porous soil, which can absorb rainfall runoff from surrounding land. The idea is to keep water near the place where it fell as rain, rather than culverting it away in storm sewers, or even to stormwater retention ponds.
If we had rain gardens dotting our neighbourhoods, we'd see reduced flood risk in heavy rainfall times. I'd like to see every property have an area where water can be absorbed into the ground. The more widespread are rain gardens, the more they can do their job and keep water near where it fell, as nature would have done.
With climate change, and the risk of extreme rainfall events, rain gardens are going to become essential to reduce flood risk.
There are other benefits. A rain garden can protect the foundation of a house during rainfalls, by receiving roof runoff diverted away from downspouts. And, during drought conditions, the water that has been absorbed may be available for nearby vegetation, thus reducing the stress of the drought. The rain garden helps filter and purify water, so when it is discharged, it does not pollute creeks.
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| Small rain garden along Blanshard Street |
Victoria's Incentive Program for Rain Gardens
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| The way the rock has been used here suggests it might be a planned rain garden. |
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| This yard has porous paving (to the left). Again, might it be planned to receive rainfall? |
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| One more example of a yard that has an area of drain rock; sometimes, a downspout from a nearby building is a clue that it's a rain garden. |