Anatomy of a coastal storm: December 20

December 20, 2018 photo of flooding in the Three Crabs area.  Photo from the Peninsula Daily News

The storm on December 20, 2018 brought some coastal flooding with it...notably in the Three Crabs area on the Dungeness River delta, and locations in Port Townsend.  I'm going to focus this post on the Three Crabs flooding.  Shortly after the event a set of aerial photos shot by John Gussman of Doubleclick Productions began to make the rounds, and a few things caught my attention.  First, this was a still water flood - there was little wave action associated with this event at this location (in contrast to areas exposed to south wind, like in Port Townsend).  As a result, we might be able to learn something interesting about the processes driving this event from tide gauge data.  Second, it was very localized near the mouth of Meadowbrook Creek, which suggested to me the possibility of a river component to the event.  I grabbed those photos, and a few published by the Peninsula Daily News (above) and headed out with an interest in seeing if I could get some flood elevation data.  I had a particular question in mind - was this flood driven entirely by coastal processes, or was there a river component to it?

Debris line left by flooding on December 20, 2018, Three Crabs area on the Dungeness River delta.  Photo collect 18 January 2019

So first, the flood elevation data.  I didn't really need the photos...there were plenty of places where a debris line was still evident, a month after the flood (photo above).  I ended up collecting 52 estimates of the elevation of the flood waters using Peninsula College's ProMark 200.

Location of 52 flood elevation estimates from the Three Crabs area

I'm going to report elevations here in meters relative to NAVD88, to facilitate comparison to tide gauge data collected in Port Angeles (at this location, based on VDatum, NAVD88 is 2.09 m BELOW Mean Higher High Water).  So here are two presentations of the flood elevation data:

Flood elevation estimates based on surveys of debris lines, or water lines from photos, in meters relative to NAVD88, for the December 20, 2018 flood at Three Crabs.

The top panel show the data plotted against longitude, since I was interested in whether there was any elevation variation in the flood that might suggest a fluvial component to the flooding.  There is a bunch of scatter, and its too hard to say.  The points at larger longitudes do seem to be a bit higher in elevation, but that is the opposite that I would expect if Meadowbrook Creek was a source, since Meadowbrook Creek is in the opposite direction.  I'm going to call this inconclusive.

The histogram in the bottom panel provides a sense for the range in the elevations I collected.  The mean of these values is 2.94 m NAVD88, which equates to 0.85 m (2.8 ft) above Mean Higher High Water at this location.  There is a total range in the values of 0.35 m, or just over a foot...with is most likely a reflection of trying to estimate a flood surface elevation from a pile of debris on the side of a road.  

So lets move on to comparing the flood elevations at Three Crabs to tide gauge data collect in Port Angeles.  For this purpose I'm going to compare to the mean flood elevation value from the data I collected, 2.94 meters NAVD88.  Here are the data collected at the Port Angeles tide gauge that day:

The tide gauge in Port Angeles recorded a maximum water level of 2.98 m NAVD88 at 11:36 am that day, driven by the combination of a high tide (predicted to be 2.32 m NAVD88, just above Mean Higher High Water), and a 0.66 m non-tidal residual, or "storm surge".  In this case the "storm surge" was probably driven mostly by low atmospheric pressure.  My conclusion though?  The flooding at Three Crabs very well could have been entirely driven by coastal processes - specifically the combination of a relatively large non-tidal residual co-occurring with a high tide.  

Lets dig a bit further to address two other questions.  First, where does this water level rank in the record collected in Port Angeles dating back to 1975?  Turns out that this was indeed a pretty high water level...but not the highest.  Three other events have driven coastal water level higher in Port Angeles.  The record for that tide gauge was set on 2 January 2003 by an event that was very similar - a high tide co-occurring with a storm surge of 0.66 m or so.  The big difference was in the timing of the tide...the predicted tide on 2 January 2003 was almost 0.10 m higher, and hence the record water level on that date was 3.07 m.  Two other events, one on 31 December 2005 and another on 27 January 1983 also both exceeded this December 20 event, if only barely.  Both of these events were characterized by lower non-tidal residuals, but higher astronomical tides than the event on December 20.  I'm curious about what happened in the Three Crabs area during those events.  

The next question...how bad could it be?  Coastal flooding is a game of chance, with the worst events happening when the astronomical tide is high, and the non-tidal residual is also large.  It gets even worse when we add in winds from the right direction...but lets ignore that here.  So the highest astronomical tide on record for Port Angeles was 2.63 m NAVD88 on 30 December 1986.  Pretty darn impressive astronomical tide!  The maximum non-tidal residual?  That occurred on 1 January 1997 when water level hit 0.9 meters above the predicted water level associated with a notable winter storm.  Fortunately on that day two things happened that kept water level relatively un-exciting (the tide gauge peaked out at a maximum water level of 2.79 meters NAVD88); first, the predicted high tide wasn't that high, and second the peak of the non-tidal residual didn't correspond with the peak of the high tide. 

So how high could it be?  Well what if that highest astronomical tide of 2.63 m NAVD88 corresponded with that maximum non-tidal residual of 0.9 meters?  A peak water level of 3.53 meters, fully 0.55 meters, or almost 2 feet higher than the December 20 event.  Now that would have been unpleasant in the Three Crabs neighborhood.  Fortunately for us the odds of those two things co-occurring are pretty darn low...but we are certain to get higher than we did on December 20...its only a matter of when.