DrainsofmyCity
Mission: When it Rains
July, 2004
We've all heard the popular adage. If you're like me, you try and abide by its inherent wisdom as much as possible. But there have been times when I've been caught underground during small rainstorms, and I've always wondered how far to take this little proverb. When walking in a massive storm sewer trunk line, it's difficult to visualize the amount of water it would take to fill the pipe.
My current rule is to not go draining when there is a serious threat of rain, or when there is water 'ponding' on the streets. I've discovered, by observing storm sewer outfalls and drainage structures shortly after large storm events, that being underground when such weather hits would be at the very least life-threatning- if not suicidal.
Recently, Edmonton has been hit by several major thunderstorms. Judged to be 1:25 to 1:50 year storm events, these dumped over 5mm of rain onto the city in the space of less than an hour. The amount of water was sufficient to overwhelm the city's drainage system. Freeway underpasses flooded, manhole covers were blown off by the pressure, and roads became waterways. Combined Sewer Overflows (CSOs) lived up to their names, as millions of gallons of combined sewage spewed untreated from outfalls, directly into the river.
As an Urban Explorer, it fascinates me to watch the sewer system at work, especially so if that system is overwhelmed. What better time to learn, and watch, the potential dangers of draining? If I'd had more foresight, I would have been better prepared to record these events. I would like to have gotten footage of manholes geysering, or popping off, or footage of a CSO gushing. As is, I managed to gather photos and video of a particular location being overwhelmed by the water.
This location is at a point mid-way through Mill Creek ravine. The ravine carries a large creek (Mill Creek) several kilometers through the city. It emerges from a storm sewer, flows through the ravine, and eventually goes underground again through a long tunnel, where it outfalls directly into the river. At the mid-point, the city engineers have built a dam of sorts, that limits the creek's flow. At normal, low water levels, the creek flows harmlessly through a pair of small (1350mm dia?) pipes, and carries on. At high water levels, the water begins to back up behind these small pipes, which limit the flow. As the water rises behind the dam, it eventually reaches the level of a huge bypass pipe. The water flows into the bypass, plunging down several stories, then traveling about a kilometer before reaching the main outfall tunnel mentioned above, and going into the river. At very high water levels, the water goes into the bypass, but also flows right over the dam into the creek.
The recent storms were enough to make the water rise to its highest, and flow directly over the dam. I've explored both the drain that carries the creek to the river and the connecting bypass tunnel, and the flow after the storm was frightening to watch. If you're a drainer, I highly recommed watching your city's outfalls after a major storm. It explains all the strange things I see in drains that would require insane amounts of force to move or do: displaced manholes, broken steel ladders & structures, boulders in the drain, damaged outfall grates, large tree branches, etc.
It's unfortunate that most of Edmonton's storm sewers aren't capable of handling large storms without overflowing- to say nothing of the combined sewer system and its shortcomings. This rain teaches us all the importance of drainage to a large, sprawling city like Edmonton; and it's the reason why we say; "When it rains, no drains!". See below for photographic and video evidence.
Some of the photos and videos taken of the flooding are blurry, because I had my camera encased in a Ziploc bag to protect it from the rain.
The mid-point dam a couple days after the storm, and during. The large concrete wall on the left is the bypass grate structure. The small outfall in the center acts as a flow restriction device, allowing water to build up behind the earthen dam. When there's too much water, it just overflows. Check video #1 below.
The bypass grate, as seen from above. On the left is during the storm, on the right is a couple days afterwards. During the storm, the water flowed down into the pipe, forming a (very cool) huge circular waterfall. Unfortunately, I couldn't photograph it well.
This is the bypass grate in normal weather (taken last summer). The grate is covering a vertical shaft 3m in diameter. Note the absence of trees, sticks and mud- the city seems to clean it periodically. I'd guess they also clean out the pit at the bottom. Check video #2 below.
Looking up at the bypass grate from far below. That's a big pipe, about 3m dia. This was safely taken during the winter, that's ice creeping down the walls.
The small, flow-restricting outfall. Top is a couple days after the storm (note the flattened grass) and below is during (note water washing over).
The end of the creek, where it goes underground and is directed to the river. During the rain, there's a good 3ft of water rushing in- that "Do Not Enter" sign posted makes so much sense, now. Check video #3 below.
The Mill Creek Ravine outfall. This 4000mm dia pipe comes out in a riverbank cliff face. Inset is a pic of the outfall during normal low flow- compared to the high flow waterfall.
Video 1
From across the bank, you can see the creek flowing over the dam designed to channel it into the bypass tunnel (that slanted, metal grille).
Video 2
Up close to the rushing water. At normal levels, the creek flows placidly through the dam via that outfall. And this is an hour or so after the heavy storm.
Video 3
Here we have the end of the creek, rushing into the drain tunnel that carries it to the river. I've explored this pipe many times, but now I finally understand that (Do Not Enter by Order of City Comissioners) sign posted above the mouth. That water is about 3-4 feet deep, and movin'.