Wednesday, November 18, 2020

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.



Wednesday, August 5, 2020

Dungeness Spit: Shockingly stable, but also maybe shrinking?

Every year since 2013 I've been able to get out and collect shoreline profile data on Dungeness Spit, along roughly 35 cross-shore oriented transect lines spread out evenly along the length of the spit:
Dungeness Spit is an awesome place to study the dynamics of spit morphology - its long and largely un-modified, with un-modified bluffs to the west that presumably supply sediment for its on-going growth.  And grow it has.  Schwartz et al. (1987) estimated an average annual rate of progradation of the spit of 4.4. m/yr, based on an analysis of 4 data-sets (either survey maps or aerial photos) collected between 1855 and 1985.  I walked into this effort thinking that what I would learn about spit progradation would be how much that average rate of positive growth varied annually.  Indeed, after my first year of surveying (2013) I could see that the end of the spit grew by roughly 5 meters or so based on a comparison to an aerial LiDAR dataset collected in 2012:
Intertidal profile data along a transect bisecting the very tip of Dungeness Spit for 2012, 2013 and 2014.  2012 data are from aerial LiDAR, 2013 and 2014 are from topographic GNSS surveys.

 Exactly as expected.  Since then, though...things have been a little different, with an average annualized erosion rate on the end of the spit of roughly 8 meters/yr.  That is ~25 feet per year of erosion on average:
Same location and data as are shown above, but for 2013-2020, all GNSS surveys.
  Instead of finding myself analyzing the annual variability in spit growth, I'm trying to interpret the annual variability in erosion...and it does vary.  Some years there is more...and some years virtually none.  But the trend here is unmistakable, and fascinating, and vexing.  What is going on? 

Its worth noting that this erosion at the very end of the spit is NOT representative of the spit as a whole.  Largely the rest of the spit, especially the long skinny "strand" connected to land, is astonishingly stable.  I started out this study hypothesizing that I might see a very slow rate of migration of this part of the spit, associated with storm-driven overwash.   But I don't.  Year after year the beach and crest are in an almost identical position despite being regularly battered in the winter:
GNSS survey-based profile data along a transect at roughly mile 2.5 on Dungeness Spit. 
  So what is going on?  I'm not totally sure, and in fact I don't really have too many ideas that I can even use to formulate a proposed mechanism.  And there don't appear to be too many analogous observations that I can find from the limited number of papers out there describing spit dynamics.  There is at least one exception, though - this paper describes a spit on the shoreline of the North Sea that, based on sedimentary evidence, has grown in fits and starts, with periods of growth alternating with periods of erosion.  In a conversation with Maury Schwartz some years ago, he described to me a model of spit growth that required that the spit build out a sub-tidal platform before it could grow sub-aerially...which would manifest in a data set like mine as periods of no- or low-growth punctuated with periods of rapid growth.  That's not really what I see in my data...but Maury thought hard about these things and I'm still trying to fit my observations into that model.

Any other ideas out there?

Thursday, May 28, 2020

Low tide on Tongue Point

I wanted to try to visit Tongue Point during a low tide this spring.  This basalt outcrop (part of the Crescent Formation) is generally a pretty popular and heavily impacted tide-pooling area, but I figured that since the adjacent campground has been shut down, it might have reduced some of the pressure on the intertidal community.  I made the trip with McHenry and Theo as an alternative to the usual at-home schooling we are doing, and we ended up spending most of our time here.  This is a bit of what we were able to see in an hour or so of rooting around. 

Carnivorous Nucella sp. (Dog Whelk) and eggs

Adult Pisaster ochraceous, the only one we observed

Halosaccion grandiforme (Sea Sac!), a super widely distributed red algae

A rosette of Pollicipes polymerus (Goose barnacles)

Anthopleura xanthogrammica (Giant Green Sea Anenome)

Tectura scutum (most likely; Plate Limpet), grazing an encrusting coralline algae (maybe Lithothamnion sp.?)

A Mopalia sp. (probably muscosa; Hairy Chiton)

A pot of gold!  Egregia menziesii (Feather Boa kelp) and Hedophyllum sessile (I think; Sea Cabbage kelp)

Anthopleura sp., likely Anthopleura sola.  My field team does a lot of "feeding" of anemones on these trips, but this one came by this meal all on its own.  Cool to see.

Acanthodoris nanaimoensis, the Nanaimo dorid.  Our only nudi on this trip.  

Eudystilia sp., probably vancoverii.  A Pacific coast feather duster worm.  One of my favorites 

Haliclona sp. I think (This one doesn't appear to have a common name...but it needs one).  Super cool sponge
 
Diodora aspera (Keyhole Limpet).  This one was in a group of maybe six individuals.  Haven't seen a congregation like that before


Juvenile Henricia sp. (Mottled Henricia; probably sanguinolenta?)  I can't keep track of the Henricia's :)

Thursday, May 14, 2020

Diving into the Oligocene ocean

A few weeks back we headed out for a hike along the shoreline near the West Twin River, on a beautiful night just before Washington's stay at home order kicked in.  As we were poking along, we came upon some of the usual bivalve fossils that are quite common in the area:

but also ran into some quite nice examples of fossils of a type I'd seen before, but less frequently...and I'd never quite been able to figure out what I was looking at.  To me they look sort of kelpy:

But as I understand it kelp fossils are pretty rare.  Also, if you look closely, many of these fossils have a lot of internal structure going on, in a way that kelp don't:
So I reached out to Liz Nesbitt at the University of Washington, who I had the good fortune to meet doing some field work at Discovery Bay a few years ago.  Liz has to be the foremost expert on the paleontology of Washington's coast, and so I was very delighted that she replied to my email quickly, and not at all surprised that she could explain what I had seen.  

Turns out that these are teredolites (here is a nice example of some similar fossils from Wyoming) or fossilized wood that has been bored by clams of the genus Teredinidae, commonly known as shipworms.  

Liz Nesbitt is also an expert on the chronology of the strata that these fossils are associated with, and she places these in the Oligocene, a time in which what are now the Olympic Mountains were just emerging from the ocean fringing the North American continent.  Where I found these fossils was presumably a warm (the Oligocene was substantially warmer than today's world) fringing sea.  In fact, one of the oldest known whale fossils emerged from the rock near where we found these teredolites.  



Wednesday, May 6, 2020

Dive below the surface near the Elwha River delta



I am incredibly fortunate to be one of the few people that actually got to observe, with my own eyes, the incredible changes to the marine ecosystem that happened as a result of the Elwha dam removals.  Every year for 10 years I spent 30 to 40 hours underwater visiting the same sites, getting to know (and counting/measuring) their residents and their contours.  Of course we did what scientists do, and published our findings as a scientific paper.  However, some of the changes we observed are much more viscerally relatable when you can, well, see them.  So last year we put the finishing touches on a new website and interactive map that is designed to tell the story of the Elwha underwater.  In particular it features videos that we collected at all of our sites, that allow you to watch our sites change as we did, year after year.  Click on the map and enjoy!




Thursday, April 30, 2020

A blog about my new blog

Its been a while!  I'm not sure why really...other stuff going on I suppose.  This will be a short one...but I wanted to use this platform to announce the launch of a new blog, Washington Shorelines Now and Then.  This is a partnership blog, started with Rob Casey of Salmon Bay Paddle and Shanon Dell, in Sequim.  Both of these characters spend lots of time on the water, are excellent photographers and communicators, and share an interest with me in the history of Washington's shoreline.  Rob and I, in fact, have been talking about doing this project for years...so its immensely satisfying to see it come to something.

 

The idea is really simple.  We try to collect and replicate historic views of Washington's shoreline, in order to provide a visual summary for the how Washington's shorelines have changed through time...either due to natural or anthropogenic forces.  The key feature is a side-by-side comparison of the two photos - the historic and the modern:

Check out the post with this photo, and the photo credits, here.
And then we add just a bit of text wrapping.

Enjoy!



Monday, December 2, 2019

Storm impacts on the beach: 27 November 2019



I had a chance to poke around Ediz Hook and Port Angeles Harbor a bit around high tide on 27 November 2019, during a strong northeast wind that coincided with high tide.  Waves were breaking over the coastal defenses on Ediz Hook (video above shot from the Coho ferry), as well as on to the Olympic Discovery Trail:



and not surprisingly, led to a bit of damage along the trail:



Beaches exposed to the northeast were also impacted.  I just happened to collected a few beach profiles on the east side of the Elwha River delta the day before this storm, so went back out afterwards to re-occupy those transects, one collected about here:

 and another a bit further east here:

Both of these beach profiles definitely show the impact of that event on the beach.  In both cases the upper beach eroded landward by anywhere between a fraction of a meter (a few feet), and up to roughly 3 meters (~10 feet).  I don't typically have the opportunity to capture this kind of event-driven change, and in fact these sorts of quantitative characterizations of event-driven change on the shorelines of Puget Sound and the Strait of Juan de Fuca are pretty rare...so I'm glad the opportunity came up. 

The anatomy of this particular storm was interesting to me.  The tide itself wasn't particularly high.  The tide gauge in Port Angeles maxed out at about 0.3 m (~ 1 foot) above MHHW:

and there was no storm surge associated with this event.  In fact, the high tide was suppressed a little bit, probably by the outward flow of air in the Strait (since the air pressure was low-ish during the high tide).  A water level of 0.3 m (~1 foot) above MHHW is nothing - we typically hit 0.3 m above MHHW multiple times a year.  What really made this event tick was wind, and in particular the strong flow of air out of the Strait, that led to sustained winds measured in Port Angeles harbor of 20 to 25 knots from the northeast.  The wind kicked up waves with significant wave heights exceeding 1.5 meters at the NOAA buoy in the Strait of Juan de Fuca, which is big, but not huge, for the Strait in November.  And this is really where we get to what made this event so interesting...it was the direction of the wind and waves...from the northeast...directed straight into Port Angeles Harbor, and straight at the end of Ediz Hook and the east side of the Elwha.

The waves breaking over the rip-rap on Ediz Hook (in the video at the start at this post) also provide an important bit of context.  I know from my survey work out there that the crest of the rip-rap sits at an elevation of roughly 2.5 meters (~8 feet) to 3.0 meters (~9.5 feet)  above MHHW.  Since we know that the water level at the time, as measured at the tide gauge, was 0.3 meters (~1 foot), we also know that water was being pushed 7 feet or more above the water level at the time, up and over the crest of the rip-rap.  So wave-related process, like wave run-up and set-up, were really important in making this event exciting.  Furthermore, we can actually use the event to characterize the magnitudes of those processes during an extreme event...and those sorts of observations are also relatively rare in Puget Sound and the Salish Sea.