The City of Edmonton has parks all over the place. For most of the year, these parks are dry and open for all sorts of recreational activities. Some parks are designated as dry ponds, though, and occasionally serve a very important role in protecting the city from storms.
Municipal engineering often involves making educated choices between benefits and costs; it is hypothetically possible to build a pipe system that could handle any amount of rain falling over the city, but it certainly isn’t cost effective to do so. Instead, engineers look at historical data for rainfall over the region, and pick an acceptable frequency of failure for the system.
When it comes to rainfall, storms are often measured based on their anticipated return period. A 10-year storm, for instance, is a storm that you could expect to happen once every 10 years, on average. This also means that, on any given year, it has only a 10% chance of occurring. Of course, this should not imply any regularity to the occurrence of storms, but in the long term it would average out. Based on the risk that the city is willing to accept, a storm water system can be designed to handle a certain size of storm, with the implicit acknowledgement that it will fail with a certain frequency.
The City of Edmonton’s storm water system is designed to handle a 1-in-5 year storm – that is, on average, in any given year there is a 20% chance of the system not being able to handle a given storm. The stormwater system includes of all of the catch basins, pumphouses, and pipes strewn about the city, and has the task of taking all the water that’s poured onto the city and getting it into the river before it floods roads and houses.
Dry ponds and other stormwater lakes are crucial for when the amount of water pouring onto the city is more than the pumps and pipes can handle. Excess stormwater is diverted to the lakes to act as storage until the storm is over and the pumps have extra capacity again, at which point the ponds are drained. By controlling which parts of the city are flooded, and the rate that water is dumped into the stormwater system, the city’s engineers can minimize the effects of massive storms.
These have proven so effective that the city has designated 60 dry ponds and 80 stormwater lakes. The dry ponds can handle several meters of water at their deepest points before risking any damage to neighboring houses, allowing them to store massive amounts of water. They have also proven to be much better than storm water storage tanks (some of which are located under the Whitemud and have a tendency to flood the highway).
Apart from occasionally providing people with short-term waterfront property, water retention ponds also have a role to play in reducing the effects of river erosion. Before Edmonton was founded, rain that fell over the region would get absorbed by grass, trees, dirt, et cetera, and only slowly work its way to the river. A storm would certainly cause a river’s flow rate to rise, but it would only happen slowly. Cutting down the vegetation and covering up all of Edmonton’s footprint with pavement and buildings would normally have the effect of taking all the water that would have taken days to get to the river, and dumping it in the river as fast as possible (if one wants to avoid flooding the streets).
This understandably causes a huge surge in the river flow, and huge surges are terrible for causing erosion. A large storm last June, for instance, dumped 43 mm of rain over the 680 km2 area of the city. If all of that was dumped into the river at once, it could easily double the average flow rate of the North Saskatchewan River – this sudden surge in river levels would certainly cause damage to the river banks, and even runs the risk of flooding houses located too close to the river. A 100% efficient storm water system could manage to quickly deal with all the water from a storm, only to dump it into the river and cause damage that never would have happened otherwise.
Water retention ponds can play a big role in alleviating this problem – by holding back the water (even if the system isn’t overwhelmed yet) and releasing it at a steady rate, the water surges in the city’s creeks and river can be reduced, thereby hopefully minimizing erosion damage.
Dry ponds are great as both public park spaces, and for reducing the effects that storms can have on our city. Next time you see a park flooding, instead of being alarmed you should be happy that city engineers are doing the best they can to stop your basement from flooding.