Saturday, January 21, 2017

Semiahmoo Spit

Semiahmoo Spit reaches north across the mouth of Drayton Harbor - with a long narrow neck and a very broad bulb at its distal end. This large expanse was created when the marina was dredged and the dredge "spoil" was used to greatly increase the size and elevation of the spit.


The perimeter of the spit consists of an irregular pattern of modest banks eroded into the dredged material and low depositional segments likely formed by the gradual redistribution of material along the shoreline. That means much of this shoreline is constructed, yet behaves much as a natural beach. And other than at the neck of the spit itself (see below), there's been no need to armor the beach.  

After numerous delays, a new phase of development is occurring on the outer edge of the spit, south of the existing resort. Fortunately, the development maintains a public trail system along the shore and has placed the new buildings fairly far back from the edge. Rather than try to fight the chronic erosion, the designers pulled the abrupt edge landward, creating more beach-like area. I say beach-like, because it's still fairly high and steep compared to a natural upper beach.

It looks great, but bears watching. The underlying reasons some segments of this shoreline were receding have not changed by reducing the height of the bank (any more than armoring an eroding bluff prevents the ongoing erosion of the beach itself). So we may find that the new beach continues to erode, probably as a low scarp. And ironically, by reducing the height of the eroding "bluff" we may have reduced the supply of sediment to the beach, which could result in more rapid erosion.

Again, this bears watching. If erosion continues, this might be a good place to consider a localized feed source - a periodically restocked pile of gravel that can meter out sediment to adjacent shorelines as it erodes.  On the other hand, maybe this won't be necessary. And regardless, kudos to Blaine and the development and their designers for what they've done here. In another era, this would have all been riprap.

Speaking of riprap, the real problem at Semiahmoo is the neck of the spit (aside from the clever idea to build a destination resort on a sand spit). Narrow and low - armored on both sides - the roadway is increasingly a causeway. And increasingly vulnerable - even at today's sea levels. (No pictures this trip - maybe next time). There have been some efforts to rebuild a berm on the inside (low energy, marshy), but they may need to be more ambitious. And the outside may require some serious thinking - maybe a coarse gravel cobble revetment of some sort?  It probably won't be easy or cheap, though maybe throwing some coarse gravel at the south end would be a useful experiment?

Previous posts from Semiahmoo:
Tongue Point: March 2009
Semiahmoo: March 2009

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