Building city subdivisions up the side of a mountain is not without its challenges; steep roads, rocky soil, and deep ravines whose stream flows vary from a trickle to a raging torrent. In the case of rainy North Vancouver, turning the mountainside into a carpet of single-family homes calls for a bit of creative stormwater management. Not surprisingly, it's easy, cheap, and visually appealing to leave natural ravines in place to convey creeks and stormwater- and overall, this seems the strategy favored by the city. But on occasion, those natural ravines- whose banks have been encroached upon, and lower reaches culverted- will experience spring floods that exceed the ravines' capacity and put nearby property at risk. What is a smart city engineer to do on a limited budget? The solution is diversion! Instead of oversizing the ravine parklands or burying large pipes everywhere to deal with 1 in 20 year flood events, it is much easier to build a single huge pipe to intercept and divert this water before it becomes a flood. But that's just my guess.
Such is the Kilmer Diversion, which could also bear the name of "pick-up and drop-off". Designed to prevent flooding, the cutoff is simply a pipe that intercepts the creek at a certain depth and diverts excess water to a more acceptable outlet. In fact, the cutoff intercepts several creeks along its path and the pipe becomes larger at each inlet; quickly growing from three to five feet in diameter over four city blocks.
The Cutoff is a nice drain to visit, if a bit out of the way. The creek inlets are located along a certain road, and where each creek vanishes into a culvert there is also a larger, second opening covered by imposing metal bars. These dark, dry rabbit holes emit a cool breeze that smells of dirt and damp concrete. Inside, each intercept is a boxy junction room where rising creek water would join that of its neighbours and be directed into a round concrete pipe that increases in size with every additional flow. Patches of graffiti on the junction room walls are evidence of the local visitors from years past.
Shortly after the last creek junction, the main diversion becomes a square 2 x 2.5m duct, which allows for a nice easy stroll through the shallow nuisance flow. Random patches of rust-red minerals seep through joints in the walls, and judging from the impression of plywood forms in the cement the pipe looks to be made of cast-in-place sections, not pre-cast duct. The diversion curves in a jagged path to follow an even grade on its was to the outfall. Blurbs of graffit continue to mark the walls as we walk on. Overhead a galaxy of water droplets clings to the grey ceiling, sparkling when hit with our lights. The tunnel widens to a 2m tall by 3m wide box conduit and we keep walking, coming closer to a faint roaring noise that I assume will be some sort of waterfall.
At last, a classicaly helpful bit of graffit warns us to "Watch out for the..." and we come to the edge of a slide that slopes several meters down to a narrowed opening below. Luckily there isn't much water flowing and it's an easy feat to walk down, but I can't help imagining what a roaring flume this would become during a spring flood. The narrower pipe at the bottom is a mystery- why constrict the water just as it gains energy from flowing down a slide? Considering the pipe is probably buried in solid rock at this point, perhaps a narrower duct was the cheaper option.
A short way past the slide, the duct curves slightly and small rectangle of bright white light can be seen in the distance. This portal quickly grows and takes on a greenish hue, and then you're at the end.
We stood at the edge of the outfall, and looked out into a densely forested valley. Water flowing past my boots spilled out and disappeared down a rocky cliff face to an unseen space below. The valley was densely filled with green trees, and the warm forest air was amazing. The outfall was much more exhilarating during the day; previously we'd only seen it at night, when the pipe ended abruptly but the water sputtered out into a bottomless black void. We joked about bringing down some lawnchairs and drinks during a future warm summer afternoon to enjoy the cool breeze and green view; I wonder if we'd be the first to do so?