When it rains, the water falling from the sky runs down the roofs of our houses, across our lawns and yards, across sidewalks and down streets to disappear into the little grates that dot the roads and parking lots of our towns and cities. What happens after that may be a mystery to most, but, as a civil engineer, managing that rainfall run off properly is one of the main focuses of my career.
For most of the modern age, urban and sub-urban rainwater management has consisted of capturing and channeling rainwater to catch basins to keep our homes, business, streets and sidewalks free from flooding. After the water enters the catch basins, it flows into pipes that increase in size as more areas are connected until, eventually, the rain water discharges into a natural watercourse. The pipes were sized to contain storms up to a certain size, with the water from larger storms intended to run overland to the nearest watercourse.
While the above methodology sounds reasonable, it has some inherent flaws. For example, when you take an area of land that was originally forested, and build roads, houses and sidewalks on it, you greatly reduce the amount of rainfall that was naturally absorbed by the landscape. This can lead to flooding and erosion issues at the point of discharge of the stormwater system. For comparison, a forested area will absorb 80% of all rain that falls, whereas a residential development might absorb 20% of the rainfall.
Furthermore, when rain falls on a forest, or vegetated area, it tends to stay spread out in sheet flow or small rivulets. The traditional stormwater system concentrates the flow, and the pollutants that end up on our sidewalks, parking lots and streets get washed off into the stormwater system.
Fortunately, new techniques, standards and legislation are being developed and implemented all the time to remedy this situation. For example, detention and retention of a certain amount of rainfall on a developed site has become common practice in much of North America. Oil-water separators are more or less a standard requirement, and the technology available improves all the time. While these measures can increase the up front costs to develop land, the will help ensure a more sustainable future for everyone.
Andrew Gower is a partner and Courtenay branch manager of Wedler Engineering. He volunteers with several local non-profits and is passionate about the Comox Valley’s sustainable future. He can be reached at 250.334.3263 or www.wedler.com.Andrew Gower