Anatomy of a coastal storm: 17 November 2020

I'm sitting in Friday Harbor at the moment, teaching at Friday Harbor Labs for the quarter.  Yesterday we were hit by a little storm, characterized mostly by rain and some strong-ish south winds - the video above was shot at South Beach on San Juan Island in the afternoon), but what made this storm notable from a coastal stand-point is its co-occurrence with a higher-than-usual astronomical tide.  This led to higher-than-average water levels, something with an average annual return frequency of approximately 3 years on the coast, and maybe 1-2 years in Puget Sound.  However, in some places water levels on the shoreline were exacerbated further by run-up associated with waves.  

Some visuals of the event: here on San Juan Island the ocean was interacting with large wood on the very upper part of the shoreline for most of the day.  Here is a time-lapse of the shoreline on South Beach covering most of the day, in which you can make out large wood getting pulled into the swash zone and moved rapidly alongshore:

On the more protected shores on the other side of Cattle Point, where wood tends to accumulate lower on the shoreline, large wood was afloat, but not moving anywhere as rapidly:

So lets look at some numbers.  First off here are the magnitudes (in meters relative to Mean Higher High Water) and times of the peak still water level, as measured at tide gauges in coastal Washington:

Peak still water level measured at tide gauges in coastal Washington on 17 November 2020.  The time of the peak is given on top of each bar.

So a few things jump out.  First, the peak water levels, relative to each stations local MHHW, were much higher on the coast than in Puget Sound (addendum:  Check out this footage from Westport, Washington!).  Next, the time of that peak, perhaps not surprisingly, was generally associated with the peak of the largest high tide of the day...on the coast and in the Strait the highest astronomical tide occurred in the afternoon, whereas in Puget Sound it was in the morning.  The one exception was in Port Townsend, where the highest astronomical tide occurred in the morning around 7am, but the peak water level occurred in the afternoon.  This happened because of the extra push provided by the non-tidal residual...which brings us to the second big process that occurred yesterday, a substantial non-tidal residual associated with low pressure and wind, that elevated water level well above predicted:

Peak non-tidal residual (NTR) measured at tide gauges in coastal Washington on 17 November 2020.  The time of the peak is given on top of each bar.

So again, the NTRs varied between the coast and Puget Sound, and were relatively large - approaching 1 meter on the coast, and 0.6 meters in Puget Sound.  The timing of the peaks was also quite interesting...generally the largest NTRs occurred on the coast in the morning, and in the early afternoon in Puget Sound.  This turned out to be really important, as the peak NTRs were out of phase with the highest tides in each location.  This is good, as it helped to reduce the peak water levels for the day.  If they had been in phase we would have definitely seen some water level records broken in Washington State yesterday.

To assess the influence of waves on south-facing shorelines I was also able to survey water level elevations on South Beach right around 8am...and found that the run-up elevations were ~0.81 meters relative to MHHW.  At the same time the tide gauge in Port Townsend was reading a still water level of 0.45 m MHHW, and in Friday Harbor 0.64 m MHHW...suggesting that wave run-up was responsible for elevating water level on the shoreline an additional ~0.2 to 0.4 m.  I suspect that this component was a bit higher later, as the wind picked up quite a bit.