Sunday, April 17, 2016

Lawson Creek

I visited this shoreline four and a half years ago (also at the end of a long day at the Salish Sea Conference) and made a note to come back at a lower tide. I've been hearing about this project from a number of directions and wanted to see it for myself. I dragged a few other folks along, too.

Previous Post: West Vancouver 2011

West Vancouver's shoreline is subject to highly oblique wave action from the west, which moves whatever sediment is available eastward. Prior to development, most of this material probably ended up stacked against the delta of the Capilano River (under the Lion's Gate Bridge), in a more natural version of today's engineered Ambleside Beach (Ambleside 2011). Now much of it is trapped against groins or other obstructions at Dundarave and Ambleside.

I don't know this area well, but I suspect most of the sediment comes from the streams that flow to the beach off the hills above West Vancouver. And I don't know how that budget has changed with development - is it increased, or is it trapped in detention ponds? The problem is keeping it on the steep foreshore when it gets there.

At Lawson Creek just west of Ambleside, there have been efforts to add large boulders to the beach face in order to retain, or trap, sand and gravel. The result is a beach covered with (almost) randomly placed clusters of rocks. I suppose there were already scattered glacial boulders on the beach, but the result here was one of the weirder beaches I've seen. This approach has also been tried, or is planned, at other West Vancouver creek mouths.

(Now that I've looked at the aerials again, I'm wishing we'd gone a little farther west to see the next creek, where this basic idea has been taken to a another level)


I admit to being a little skeptical about this approach. It's not that it may not work locally, but it seems like it depends on how much sediment is really available. And whether the volumes are sufficient to make a substantial difference in the profile on these steep, narrow beaches.

Engineers have been proposing placing objects of some sort on the foreshore to trap sediment and prevent erosion forever (only a minor exaggeration). Groins, artificial seagrass, submerged sills and breakwaters, and a whole series of patented concrete devices sold for their promise of building back a beach where one has been lost. They may work, in some places, for a while, but they often do not. When sediment is successfully retained, the consequence can adversely impact down-drift beaches (much as with traditional groins). This may not be an issue here where the beach sediment is already largely confined to the big pockets against the projecting groins and piers.

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