Before gabbing about how I did this and why, I’ll put the answer first (this is for a development area about 500m long and 500m wide):
I’ve been watching the rain collect on my narrow streets and run down the drain into fish habitat.
Every time it rains, I wonder how development has affected the amount of road runoff that ends up in rivers and streams.
I did some basic analyses to get a general idea. I chose a recently-developed area, about 256,000 square meters (thanks COSMOS mapping program!), and highlighted the roads (not rooftops or driveways…let’s say that developers have used sub-soil gravel diversions and other techniques to divert that runoff into soil).
I looked at climate data from Environment Canada from a weather station in White Rock, and saw that on rainy days in winter, 10mm was pretty normal, while 30mm was a higher rainfall day. I used Photoshop to calculate square meters of roads, then estimated liters of runoff water. There are no swales here, so road runoff goes right into fish habitat.
About 8 times more road runoff is running into fish habitat after development compared to before. By the way, the detention ponds you see in the picture might hold about 1 million liters each, from my estimates (if they average about 1.2m deep). They’re basically to catch overflow from the drainage pipes, when more water falls than can fit in the pipes. I don’t know what the capacities of the drainage pipes are…but Surrey’s website says it all goes into the rivers without being treated.
I think it’s a bit optimistic to think that 1,000,000L/day won’t have a significant impact on river water quality.
You long-term residents will think I’m astonishingly ignorant, but I’ll plunge on anyway. After visiting a nearby hatchery, I learned that the rivers around here are used by salmon! Maybe the runoff from this development goes to a different area somehow, but the signs on every drainage grate, and the City of Surrey’s own information, says otherwise. “It is important to know that any road run-off flows directly into local creeks and watercourses.”
This Canada government website highlights the dangers of development such as we see here.
There are alternatives – ways to mitigate road runoff. Portland’s Green Streets initiative comes to mind. They use curbside swales, pervious pavement, rain gardens, gutter cut-outs, things like that, to divert water from rivers and streams. Another Oregon website has excellent pictures that show how to do it. And here’s another document, from California, with marvelous pictures and information.
I’ve seen lots of roadside plantings and curb extensions here that could very easily be modified to soak up some of the road runoff. Maybe that sort of thing doesn’t work here; I admit I didn’t grow up here, and there’s a lot I don’t understand. But I think we should consider doing something to reduce road runoff. It’s worth a shot.