Thursday, April 21, 2016

Kitsilano Beach

Most of Vancouver's shorelines are oriented obliquely to the large western fetch across Georgia Strait, which means that beaches (natural or constructed) tend to do best when the orientation of shoreline allows west-facing swash aligned beaches to form. This is best seen by looking at aerial views of West Vancouver, Third Beach in Stanley Park, English Bay, and Kitsilano.


The main beach at Kitsilano, on the south side of English Bay, is controlled by a bouldery headland that makes the swash-aligned beach possible. I suspect sediment has been added periodically to maintain the broad recreational beach, but other than a couple of small rock groins, this beach doesn't depend on larger groins like we've seen at Ambleside or on the north shore of English Bay.

There's a second beach at Kitsilano, just around the corner northeast of the main beach. But it appears to have been created by the construction of a rock jetty at the entrance to False Creek (see aerial). It's oriented a little differently than the main beach, suggesting (to me, at least) that waves may refract around the headland between the beaches and approach this one at a slightly sharper angle.

I've always been intrigued by these rippled patterns on Salish Sea beaches. They're often pretty subtle, but they raise questions about their formation. These are not the same as ripples you might see in other types of beach environments. They typically form perpendicular to the beach and may extend through a significant tidal range. They tend to have wavelengths of 30-60 cms, but sometimes more. It seems like they would have to form on a falling tide to be preserved. But once the pattern is established at high tide, it is propagated down the beach as the tide falls. They may be somehow related to larger scale horns and cusps on other beaches, where the emergent bedform influences subsequent swash patterns, further reinforcing the original undulations. If that makes any sense at all! This phenomenon is probably well documented by others, but I've never seen it described. There is an abundant literature on cusps (still controversial) and on ripples, but I think this is something a little different. Or maybe in between.

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