Coastal storm forensics on Ediz Hook

Looking to the east from about here.  The area of overwash referenced in the text below is just out of the frame to the left.

With permission from the US Coast Guard I made my annual trip last week out to the end of Ediz Hook, to survey the shoreline. I was struck on this trip by the sign left beyond by what looked to be a pretty exciting storm that must have happened this winter, that both eroded and over-washed the berm on the northern shoreline of the hook, and flooded the large grassy field that used to host the lighhouse that sat at the end of the hook.

Overwash is driven by waves, typically coupled with a high still water level (there is some cool footage of overwash in action from North Carolina here...), and sort of by definition leads to flooding behind the berm.  That looks to be what happened here.  I surveyed in the extent of the overwash I could observe, and found it to be pretty extensive:

Mapped overwash deposit (the tan polygon) on Ediz Hook - surveyed 26 May 2021

The signs of the event are also readily observable in some of the beach profiles I collected from within the overwash zone - showing an eroded beach and berm as well as the overwash deposit behind the beach:

Profile data from inside the overwash zone from 2020 and 2021

   I also surveyed the debris line that would have roughly demarcated the area flooded by this storm:

Mapped flood extent based on the location of a debris line, surveyed 26 May 2021

So when did it happen?  That is a bit hard to say, but if I had to guess I would think that it must have been associated with one of the two storm events of note from this winter, one in November 2020 and the other in January 2021.  I wrote about both events in this blog (i.e. here for the January 12th event), but was in Friday Harbor for the November event, so didn't really get a first hand view of what it looked like in the Strait.  Between the two events the highest still water level, or the water level as recorded by the tide gauge, was just over 0.75 meters relative to MHHW, associated with the November 2020 event.  But that doesn't get us very far - the peak of the berm in the overwash zone I mapped on Ediz Hook, though, sits almost 2 meters above MHHW - so the still water level elevation would come nowhere near to over-topping the berm and flooding the land behind.  

My write-ups clearly suggest that both events were characterized by wind and waves, and the data suggest that both events featured wave heights of well over one meter in the Strait.  But if we look in more detail at the co-occurence of wind and waves with still water level the evidence seems to point clearly to the January 2021 event as the culprit.  Specifically, I calculated an estimate of total water level - or the elevation that water would reach on the beach during a storm under the influence of tides, storm surges and wave run-up - using data from the Port Angeles tide gauge coupled with wave data collected at the New Dungeness buoy.  Now first let me make it clear that this is far from a perfect way to estimate total water level...and acknowledge that waves striking Ediz Hook during a storm may be very different than those recorded some 30+ kilometers away.  But assuming that the New Dungeness buoy is somewhat representative of waves in the central Strait of Juan de Fuca my total water level model suggests that the peak total water level this past winter was reached on 12 January 2021, that it probably exceeded the elevation of the berm on Ediz Hook, and that it may have been the only event this winter that exceeded that critical berm elevation threshold:

Estimated total water level on Ediz Hook based on water level data from the Port Angeles tide gauge and wave records from the New Dungeness buoy.  The dashed line is the estimated elevation of the beach berm on the end of Ediz Hook.

Fortunately, in this case, there wasn't much in the way of the damage to infrastructure, and my subsequent follow-up with USCG personnel suggested they didn't even note this erosion or flooding.