Wednesday, October 26, 2016

The coastal anatomy of an October storm

Looking down at the Elwha River mouth on Friday October 14th at about mid-day.
Its not too often that you are in the right place and right time to sample during an extreme event, and while the series of storms that hit the Washington Coast starting on the evening of Thursday October 14th turned out to be not quite as severe as expected, they still represented a reasonably strong wallop to the coast in general.  I happened to be working through that entire series of stormy days on the beach of the Elwha River delta, primarily supporting a group of students from a University of Washington course that I am co-teaching.  But the intensive field time gave me the the chance to observe and measure various aspects of the storm response of the beach to the storm. 

As a starting point lets look at some of the forcing:

Surface pressure and wind magnitude recorded at the Port Angeles tide station
The pressure record from the Port Angeles tide station clearly shows the passage of the two major low pressure systems that passed over Washington - the first on Thursday evening and into Friday, and the second in the afternoon of Saturday.  The wind largely tracks those two pressure systems, though the wind speeds associated with the Saturday storm were about twice as strong.

Particularly important relative to the coastal impacts of a storm is water level, and during this storm water level peaked on Sunday mid-day at 2.74 meters above MLLW, which is more or less the average annual extreme water level at this station, and no where near the record high of 3.2 meters set in 2003:

Predicted tidal water level (red) and measured water level (blue) from the Port Angeles tide station.
But these relatively unexciting water levels occurred in part because this storm happened to coincide with a neap tide period, when the astronomical tides were relatively low, because the non-tidal residual, or 'storm surge', were pretty healthy for our area:

Sea level pressure (red) and non-tidal residual (the difference between the predicted tidal water level and the actual measured water level) from the Port Angeles tide station
For the storm surge the peak occurred around 7pm on Saturday, and coincided nearly perfectly with the peak low pressure.  It also (thankfully) happened to co-occur with a relatively low tide at this site, which again helped to keep the peak coastal water levels associated with this series of storms relatively low.  Amongst other things, this is another clear demonstration that in our area coastal flooding/erosion and other storm impacts is often a game of chance. 

The other beautiful thing about this plot is the obvious tight relationship between pressure and storm surge - the changing pressure essentially influencing the sea surface.  Here is another perspective on that:

Relationship between pressure and non-tidal residual between 12 and 18 October 16 based on data collected at the Port Angeles tide station
In this case the linear trend here is equivalent to roughly 13 mm of water level change per mb of pressure change, which is roughly similar to the other trends I've seen estimated for this relationship.  What is interesting here though is the suggestion that there are essentially two relationships, one that I am guessing is associated with the approach of the storm (i.e pressure lowering), and the other that I am guessing is related to the passed storm (i.e. pressure going back up).

Lets get back to the beach, though, and look at some of the morphology changes that occurred during this series of storms along this same set of cross-shore transects that I've discussed previously:

and lets start over towards the east side of the delta and move west.  Here are the profiles from a subset of the transects above, moving from east to west, showing typically profiles from 12 Oct, 14 Oct and 16 Oct, plus in most cases a profile from my most recent pre-storm survey in September.  At the bottom of each panel is a time-series of shoreline position over the last 5 years.

 In general the theme during this event seems to be overwash of the recently formed backshore driving accretion at the very upper part of the intertidal profile.  Here, for example is a photo taken on Friday October 14th, taken near Line 198, of an approximately 20-30 cm thick overwash deposit.  Based on the differences between the 14 October and 16 October profiles in the figure above, this deposit continued to grow over the next two days.

Interestingly, at most locations the accretion on the upper beach seemed to occur without much in the way of erosion of the beach face (except at Line 198), suggesting that the sediment building the upper beach was derived either from lower down the intertidal, or elsewhere (i.e. alongshore).  Regardless, it also suggests that the Elwha beach is in a state of plentiful sediment supply at the moment...not a huge surprise.

The exceptions here are Line 180, which was erosional on the upper beach during this storm (there always has to be one), and Line 156 right near the river mouth, which surprisingly didn't change much, especially because during the peak of the storm on Friday it was almost fully overwashed:

Photo taken Friday October 14th at 1:40 pm looking west from approximately Line 164 towards the river mouth and Line 156

The other interesting thing to note here is that in general the accretion on the shoreline was occurring relatively high on the beach face, above 3.0 m relative to MLLW in most cases (I use NAVD88 in the plots above, which is ~0.12 m offset from MLLW here), and well above the measured tidal water levels.  Clearly the other big process at play during this storm event on the Elwha River delta were waves.  Here is a video of waves breaking on the delta taken on Friday October 14 around 2pm:

So I don't have local wave data available for the Elwha for this period (though we did have sensors in the water - excited to check that out), but here are the significant wave height time-series (in orange) and wind (in blue) from the Hein Bank buoy, out in the middle of the Strait:

So here, the tight coupling between wind and wave height suggest that these are primarily locally generated wind waves...and big ones at that - 1.8 m significant wave heights in the Strait are not unheard of, but not extraordinarily common.  Another way of looking at the role of local wind as the wave generating force is by partitioning the significant wave height into wind wave (in blue) and swell wave (in orange) components:

Clearly most of the wave energy we saw on the beach during this storm was comprised of locally generated wind waves.

What was also clear from working on the beach was that during this time period there were often waves coming from both directions (east and west), and while I didn't take the time to analyze the full wave spectra, just in looking at the mean wave directions supplied by NOAA its evident that there was quite a bit of switching between east and west over the time period.  In the plot below the orange line is the significant wave heights at Hein Bank buoy during this time period, and the blue dots are the mean wave direction (this is the direction from which waves are coming):

Its also clear that the largest wave sizes during this period were associated with waves propagating from east to west, not west to east as is the case under most "normal" conditions.

and while its not a perfect way of thinking about it we can try to integrate the measured water level with the wave time-series from Hein Bank to at least start to think about the potential total water level (including the tidal, non-tidal and wave-driven components of water level) by looking at the measured water level (in orange) and the significant wave height estimates (m):

There are a few periods that pop out here that were the time periods when large waves coincided with high(ish) tides, and likely drove shoreline change high up on the beach face, and into the back shore, including mid-day on Oct 13th and Oct 14th and again probably during the night early on the 16th.  The influence of waves on water level, especially mid day on the 13th and 14th, on the delta is apparent in this timelapse looking down at the river mouth from the east:


1 comment:

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