Casa en la Playa

This morning as I was finishing my first coffee, one of my Deltares colleagues shared a video of a house on the beach in El Salvador. It was a surreal, post-apocalyptic, Planet of the Apes scene, made all the more mysterious by the accompanying news text (translated from Dutch): “Abandoned villa washes up entirely on the beach in El Salvador. The ruins of a villa have recently washed up on the beach of La Puntilla in El Salvador. It is a great mystery how the entire house ended up on the beach. Hurricane Elsa may have blown the building away.

Beach house at La Puntilla, on the Pacific coast of El Salvador [Source: La Presna Grafica]

Having spent a fair bit of time professionally contemplating the destructive power of storm-generated waves on tropical coastlines, I was immediately skeptical about this hypothesis. How could a massive concrete structure like that survive such a pummeling and then just “wash up” on a beach?

Being a bunch of sediment transport nerds, my colleagues made all kinds of jokes about building a house-transport module into XBeach or SedTRAILS, or estimating the critical shear threshold and porosity of this giant “particle”. However my curiosity was irrevocably piqued- how did the damn thing get there? It must have been there the whole time, with the coast eroding around that, but how or why?

The Dutch news site was not particularly helpful, so I snooped around on the internet until I found a promising lead in the El Salvadorian news. Apparently the house is located near the village of La Puntilla, on the Pacific coast of El Salvador. Apparently it was a former hotel built (unwisely) right next to the beach. Suffering significant damage during Hurricane Mitch in 1998, it later became a church. Numerous people died in the former hotel and more recently among the ruins, so the building is even spookier than it first seemed. Indeed, after looking in Google Earth, it became apparent that the hotel has been there the entire time:

Before-and-after satellite photos of the abandoned hotel in La Puntilla, showing about 150 m of beach erosion between 2003 and 2021. The dashed line indicates the approximate position of the 2003 shoreline, based on some quick-and-dirty georeferencing and not accounting for potential variations in shoreline position due to the stage of the tidal cycle at the time each photo was taken. The tidal range on 50 km up the coast is about 2 m, so if the beach is very flat, the 2003 shoreline may actually be much closer at high tide.

So the hotel didn’t just “wash up on the beach” one day. But why exactly did it erode so much? Was there a particularly devastating storm? Is climate change or some nefarious human meddling in the coastline to blame? By zooming out a little bit, we can put this coastal erosion into a larger context.

Google Earth is nearly always my first go-to tool for investigating a new site as a coastal engineer — you can learn so much by carefully observing how a given beach or estuary has changed through time, and by looking for evidence of the physical processes shaping the landscape. For instance, white foam usually indicates breaking waves, and by watching how sand piles up in certain places, we can estimate in which direction the sand is mostly moving. What can we learn about this site?

Tidal inlet at La Puntilla, El Salvador. The inlet channel connects the Pacific Ocean to a tidal lagoon, and is fronted by an underwater ebb-tidal delta. Large swell waves travel thousands of kilometers from the stormy south Pacific and break on the ebb-tidal delta (see the white foam), which makes greatly reduces their size. In this way, the ebb-tidal delta acts like a massive, sandy breakwater. Waves generate currents that push the sand from right to left along the coast, but the sand needs to cross the inlet. It makes this journey in regular spurts that lead to periodic accumulation and erosion of sand along the nearby coasts. This is the fate that befell our poor abandoned hotel.

It seems that the hotel was built on a type of coastal feature that is notorious for massive erosion world-wide: the coast adjacent to a tidal inlet. Tidal inlets connect lagoons with the ocean, the water rushing in and out of them several times a day. The water moves very fast and is stirred up by waves, so it can carry lots of sand and mud to and from the nearby coast. This means that this tidal inlets and the coastlines next to them have the potential to change shape dramatically over time. The inlet channel and smaller channels connected to it have a tendency to migrate back and forth across the underwater ebb-tidal delta like an out-of-control fire hose. If one of these channels is pushed up against the coast, it can cause massive erosion. This seems to be what happened to the abandoned hotel.

If you examine a timelapse in Google Earth (here), the incredibly dynamic nature of this system is apparent:

By a remarkable coincidence, this is the same process that I am investigating in my PhD: how does sediment move around at tidal inlets like this? It is not only the El Salvadorians who need to worry about this process — the topic is also a vital matter of investigation for Dutch coastal management authorities trying to protect their coast. Lesson learned: don’t mess with ebb-tidal deltas!

The Beach: A River of Sand

It’s February, which means it’s Coastal Dynamics season again!  5 years ago (time flies!), I  first arrived in the Netherlands as part of my master’s program.  I walked into the classroom for Judith Bosboom’s Coastal Dynamics 1 course, and it really changed everything for me.  CD1 felt like the course I had been waiting for all my life, combining geology, geography, physics, and practical engineering all in one package.  The course was also so well-taught and structured that it seemed like an IV drip of knowledge, pulsing straight to your brain.  I felt like I was truly in the right place and coastal engineering was the field for me.  It cranked my enthusiasm for all things coastal to 11.

After I became a PhD student, I began TAing the course and learned what it was like on the other side of the classroom.  This revealed a hitherto unsuspected enthusiasm for teaching (although perhaps it shouldn’t have been a big surprise, given that I come from a large family of teachers), and has been a big factor in my interest to stay in academia.

Every year, we start the course by showing students the video “River of Sand”, which explains coastal sediment transport in an easily understandable way.  This video is 55 years old now, but it still does a better job of explaining how beaches work than almost anything else I’ve seen.  I hope you enjoy it as much as we do!

Sand: Xerokambos Beach

My friend Claudia responded with great zeal to my call for sand from different beaches around the world.  In addition to her samples from Sword Beach and Dunkirk, she also brought back sand from her holiday to the Greek island of Crete.  The sand from Xerokambos Beach is interesting compared to those two French beaches, since it is much more diverse- there are many different colours and likely different mineral origins for the sand grains that we see there.

xerocambos_000002

That being said, when I see pictures of how lovely Crete looks, I have the feeling that I would not be too focused on the finer details of local sand composition if I went on holiday there!

Sand: Archipel Glenans

Ga je naar het strand? Mag ik
als je terug komt het zand
uit je schoenen voor
de bodem van mijn aquarium?

Are you going to the beach?
when you come back, may I have the sand
from your shoes for
the bottom of my aquarium?

– K. Schippers

I have had that Dutch poem on a postcard on my bedroom wall for a few years now, but it unexpectedly came to life a few weeks ago.  I mentioned to some friends that I was taking pictures of sand from different beaches with a microscope and wanted to expand my collection.  My colleague Silke enthusiastically responded- she had just returned from a holiday in France and still had sand in her shoes!  “Should I bring it to the office tomorrow?” she asked.  How could I say no?

archipelGlenans_000002

Her holidays had taken her to the beautiful Glenans Archipelago off the coast of Brittany, not too far from where I am living right now in Brest. Unlike a lot of the sand I have looked at so far (which was mainly quartz), this beach appears to be quite shelly.  The islands are famous for their maerl beds, a sort of coral algae rich in limestone.  That may account for some of the interesting shapes and colours we see, but if you look closely, it seems there are also some threads and bits of lint from Silke’s socks!  It might not be a scientifically valid sample, but I’ll take it!

archipelGlenans_000006

Sand: Dunkirk Beach

Here is another sample brought back by my friend Claudia, from Dunkirk Beach in northern France.  Dunkirk is famous from the Second World War, when the Nazis had cornered Allied troops there and forced a major evacuation across the English Channel.

dunkirk_000003

This is where my inner history nerd and my inner sand nerd collided to ask an interesting question: is the sand on that beach now (and in the photograph below) the same sand that was on the beach during the famous evacuation?  There’s no easy answer to that question, but as it so closely relates to the main research questions of my PhD, I can’t resist indulging in such a thought experiment.  Shall we try together?

To answer this question, let’s ask ourselves a few things:

  1. What kind of sand is on the beach?
    The size of the sand grains will determine how easily it is moved around by the waves and tides.  Bigger particles require more energy to move, and are thus more likely to stay where they are.  In general, smaller sand grains are more likely to get picked up and transported far away*.  Based on the photo above, let’s assume that most of the sand grains are about 200 μm in diameter (that’s 0.0002 m).
    The sand also seems to be mainly made of clear or white-brownish grains, so we can probably make a safe guess that they are mainly made of quartz.  This will come in handy later if we need to make an assumption about how dense the particles are. Most of this sand comes from large sand banks offshore, which is moved to shore by waves during large storms [1].
  2. How do waves and tides shape the coastline here?
    To predict how sand moves around on a beach, we need to understand the behaviour of the water there.  The tidal range on this part of the French coast is quite large, between 5-8 m [1]. That large range means that a correspondingly massive volume of water is moved back and forth past the beach twice a day, which generates powerful tidal currents.  Waves here mainly come from the English Channel to the west or the North Sea to the northeast, and are generally at their strongest during occasional winter storms.
  3. In which direction does the sand usually move?
    There are several possible fates for our 1940 sand: (a) staying where it is, (b) moving offshore into the English channel, (c) moving westward towards Calais, (d) moving eastward to Belgium, or (e) moving onshore to build up the sand dunes there.
    At these beaches, the tidal currents moving eastward towards Belgium are slightly stronger than the ones moving westward back towards England [1].  This is eastward motion is reinforced by waves and wind-driven currents, which also tend to move eastward on average [2].  As a result, the sediment effectively takes two steps forward and one step back, gradually moving in an eastward direction (i.e. (d) rather than (c)).
    We also know that there is a regular supply of sand from offshore [2], so let’s rule out (b) for simplicity. The dunes in that area are also relatively stable [2], so let’s rule (e) out, too.  If most of the sand is then either moving east (d) or staying put (a), what is the likelihood that our 1940 sand is still there?
  4. Have humans intervened with the coast there?
    In 2014, the French government created the largest sand nourishment in the history of France on the beach at Dunkirk [3].  This is visible in Google Earth as the giant pile of sand near the red pin (below).  If there was still 1940 sand on the beach there, it is now likely buried underneath the nourishment.  Depending on where my friend collected her sand, there is a good chance that it is made up of this sand that was dredged from the nearby harbour, rather than sand that was on the beach in 1940.

    I had a similar issue with my tracer study: several months after our investigation, the Dutch government placed a huge nourishment right on top of our study site.  That means that even if some of our tracer sand is still out there, it is likely buried deep beneath a giant pile of sand, which means that we can’t go back there to take more samples.
  5. What is the likelihood of sand leaving the beach?
    After placing the nourishment at Dunkirk in 2014, scientists monitored how the beach changed, and found that it lost 9% of its volume in 2 years [3].  Most of this sand appeared to migrate eastward, as predicted by those other studies.  If we had similar data about how much the volume of the beach has changed in the past 80 years, we could estimate the rate at which sand is leaving, and hence how likely it is to still be there.  From that, we could come up with a sort of “residence time”: how long we expect sand to remain on the beach given the volumes that are coming in from offshore sandbars and leaving down the coast.  That would at least give us a ballpark idea of what to expect.  We could also use computer simulations to more precisely predict this transport, but that’s a lot of work for our little thought experiment!

Given all of this information, I would guess that most of the sand that was on the beach in 1940 is somewhere on its way to Belgium, or is still there but buried beneath the new nourishment.  Based on the assumptions that we made about this being quartz sand about 200 μm in diameter, we can estimate that in a handful of sand (say, 250-300 mL), there will be about 5 million individual grains!** If we scale this up to an entire beach, then I think the odds are good that at least a few grains have stuck around since then.

There are lots of different ways that you could go about this, though- how would you try to tackle it? Am I missing anything important?


* This “smaller-particles are more likely to get picked up by the waves and currents” rule only works for sand grains that are all more-or-less the same size. If your sand has both large and small particles, you can also have “hiding” effects where little grains of sand hide behind big grains and are harder to move. And don’t even get me started on mud! Mud particles (usually 10-100 times smaller than sand) obey a whole other set of complicated rules that are frankly a little absurd sometimes. But these are discussions for another time…

** Even though the grains in that picture are clearly a bit irregular in shape, we can pretend that they are spheres and calculate their volume Vgrain = 4/3π(0.0002/2)3 = 3.3×10-11 m3.  The volume of your hand Vhand is 300 mL = 3×10-4 m3, so we can calculate the number of grains as Vhand /Vgrain, which is about 9 million. But wait! We have to account for all the spaces in between the sand grains, since we’re not dealing with a solid block of quartz. This is usually about 40% for sand, so this is how we get our final number of about 5 million.


[1] Sabatier, F., Anthony, E. J., Héquette, A., Suanez, S., Musereau, J., Ruz, M. H., & Régnauld, H. (2009). Morphodynamics of beach/dune systems: examples from the coast of France. Géomorphologie: relief, processus, environnement15(1), 3-22.

[2] Anthony, E. J., Vanhee, S., & Ruz, M. H. (2006). Short-term beach–dune sand budgets on the north sea coast of France: Sand supply from shoreface to dunes, and the role of wind and fetch. Geomorphology81(3-4), 316-329.

[3] Spodar, A., Héquette, A., Ruz, M. H., Cartier, A., Grégoire, P., Sipka, V., & Forain, N. (2018). Evolution of a beach nourishment project using dredged sand from navigation channel, Dunkirk, northern France. Journal of Coastal Conservation22(3), 457-474.