Wednesday, May 09, 2012

Cama Beach

There is a small perennial stream at the south end of Cama Beach.  It emerges from a gully and flows across the beach.  Some years it just ponds behind the berm, seeping out through the gravel.  Other years, it opens a channel through the berm and flows openly.  This is what it's been doing this winter.

Few streams arrive at the beach unimpacted by human activities and this one is no exception.  The historic condition is unknown - perhaps it actually flowed into the old lagoon where the resort is now and exited through the barrier farther north.  Regardless, it's outlet is now fixed with the stream pushed against the adjacent concrete bulkhead.  In addition, at some point, the concrete planks of the old boat ramp were dumped at the outlet (presumably to prevent some historic erosion problem?).  Interestingly, the planks are often buried, but this week they are more exposed than I've seen them in the 16-17 years I've been visiting.

There's always been a small delta on the intertidal beach here, but currently the stream is busy building a fresh gravel fan across it.  People often point to these deltas as evidence of a fluvial source of sediment.  While streams no doubt deliver sediment to the beach, or at least have historically done so, the presence of this kind of feature does not require upstream sediment. Much of the sediment may simply be upper beach sediment, relocated by stream flow to the lower beach.  You could probably form a small delta simply by running a fire hose across the beach for a few weeks.

One consequence of the stream eroding the upper beach is that in areas with significant drift the back beach is often narrower downdrift of the stream mouth.  In this case, the beach widens northward toward the stream mouth, then narrows distinctly on the north side.
 This is not a function of the bulkhead -- although that may complicate the dynamics a little bit.  Basically, the drift is diverted by the stream, starving the downdrift beach.  I like to think of this as a kind of negative groin.

Other examples, although each bears its own complications, include Seahurst in Burien and Narrows Park south of Gig Harbor.


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