New Homes Today

Tuesday, November 20, 2012

When it Rains, It Pours.

I attended a sustainability building conference a couple of weeks ago, and one of the more interesting sessions was about how municipalities are changing the way they deal with storm water runoff, and how significant further changes are right around the corner.
Think of a heavy rain falling on and undisturbed natural area.  The first thing that happens is that the ground will absorb the water.  This water will either continue sinking through the earth and replenish subterranean aquifers, or will slowly move across the surface, following gravity, forming tiny currents, and eventually becoming part of our streams, lakes, and rivers.
Now think of heavy rain falling on a typical suburban subdivision.  Over half of the surface area is impervious, hard substances, such as roads, driveways, or roofs.  There is way less earth absorb the excess water, and there are far fewer access points for the water to drain into a subterranean aquifer.  Instead, most of the water races across the hard surfaces, and enters the civic storm water system either through a street drain, or a backyard catch basin.
20 years ago civic authorities had two primary concerns with respect to storm water runoff: keep the water out of basements, and get it to drain into rivers or lakes as soon as possible.  The engineers made sure the sewer pipes were big enough to drain away water quickly.

At the conference I learned that this method of storm water management is not sustainable from an environmental point of view.  There are two problems: the first is that the sudden flow of large amounts of water after a rain creates a deluge of silt and surface pollutants into the rivers.  The second is that during extreme rain events the storm sewer system inevitably backs up and overflows into the sewage system.  The sewage system then overflows, and large amounts of raw sewage enter our waterways.  We all become aware of this when beaches close after heavy rains due to elevated E. coli counts.

During the last decade or so there have been considerable changes in the way storm water systems are designed.  The idea is to avoid a large sudden influx of rainwater.   One method is to construct larger storm water retention ponds.  This is an intermediate holding area of the rainwater as it drains away from the streets and backyards, is held in the storm water retention system, and slowly introduced into the downstream storm water system.   The second method is to temporarily retain the rainwater on the surface areas in the subdivision, and allowing it to slowly enter the drains and catch basins over a period of hours.  The most common way to achieve this is to construct small swales and valleys in the backyards, which will cause the excess surface water to slowly meander toward the designated catch basin. This subtle grade design often has surface water traversing numerous property boundaries on its way to the catch basin.  It is a delicate balance: we don’t want the water to enter the basements, but we also don’t want to overwhelm the storm pond or storm water system as a whole.

The City of Ottawa spends a great deal of time studying and approving the grading plans.  A City engineer comes to inspect the completed grade roughly one year after final installation of the landscaping.  If they find the installed landscaping does not match the grading plan, they compel the builder to make alterations.  It is for this reason that new home owners are prohibited from installing fences or pools within the first year of occupancy.

If you live in a subdivision that is 10 years old or less, you might notice that there is more surface ponding after a rainstorm.  This might occur in the lower areas of backyards, or in the streets.  Immediately after a rainstorm it might appear that a storm sewer is backed up because of the ponding all around it.  More than likely it is working as it should, and over the ensuing hours the water will slowly drain away.

Likewise, you might also notice areas of a boulevard or lawn being spongy with moisture immediately after a rain event.  This is the storm water system working as designed, and using yards and surface areas to withhold the storm water runoff for a brief period of time following a storm.

The conference presentation discussed numerous trends in reducing the size of storm ponds, and retaining the storm water on the surface areas for a longer period of time.  In some cases, entire storm ponds are eliminated.  In all cases, the notion is that if the water sits on unsealed surface areas, such as lawns, boulevards, and interlock brick surfaces, it will slowly drain into the earth, and will not enter the storm water system at all.

The benefits are less silt and surface pollutants entering our waterways, and an elimination of sewage overflows that might result from heavy rains.  Another benefit is the retention of moisture in the soil which will mitigate against drought.

These trends in suburban development are known as “Low Impact Development”, or LID.  Google will provide a lot of information about this.  Here are a couple of noteworthy links:

For those of us moving into subdivisions now, we can expect to see some changes in the landscaping.  There will be more landscaped swales, more temporary ponding and soil sponginess, and more ponding around street sewer drains. 

As we know, Ottawa is not a city that eagerly embraces change.  To quote the comedian, our town is a “hotbed of social rest”.  Other communities in Ontario have already started implementing innovative storm water treatment features, such as slightly depressed rainwater garden areas in boulevards and small parks, the mandatory use of open-drain pavers for driveways, a larger number of thirsty trees in the front and backyards, and rainwater gardens on private lawns.  These elements are part of the landscape plan and must be installed by the builder.  We can expect to see these things in Ottawa in the coming years.

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