Sediment Pathways on Ebb-Tidal Deltas

After 5 years of blood, sweat, and tears, I present to you my PhD thesis: Sediment Pathways on Ebb-Tidal Deltas: New Tools and Techniques for Analysis! I will defend my PhD on March 8th.

How do sand and mud move around on our coasts?  This is a question that we need to answer in order to sustainably manage coastlines in the face of sea level rise and climate change. To do so, we use a combination of field measurements and computer simulations at Ameland Inlet in the Netherlands. In the course of my PhD we developed several new methods, including morphodynamic mapping techniques, a sediment composition index (SCI) derived from optical and acoustic measurements, techniques for sediment tracing, the sediment connectivity framework, and a Lagrangian sediment transport model (SedTRAILS). Together, these approaches reveal new knowledge about our coasts which can be used for managing these complex natural systems.

That’s a bit of a mouthful, so let’s break it down and try to explain what I have been doing with sand for the last half-decade…

Continue reading Sediment Pathways on Ebb-Tidal Deltas

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 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 beach 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!