We humans are fascinated with the coast. It’s not just that the sea facilitates commerce and travel. It’s not only because it’s fun to swim in the water and lie in the sun on the beach. There’s something inherently interesting about seeing the place where two things meet; where the vast expanse of ocean touches the land on which we live. Just like campfires, we are naturally drawn to the coast, even just to simply watch and hear the waves crash ashore. It might not seem like it, but there’s an endless battle going on between land and sea along every coastline in the world (and just a hint: the sea is almost always winning). They may look static and unmoving on a map, but coastlines are some of the most dynamic areas in the world. Hey, I’m Grady and today, we’re talking about coastal erosion and the ways we fight against it.

The position of the coastline over time is highly variable. Tides create fluctuations in the level of the sea moving the shore in and out, sometimes hundreds of meters, over the course of a day. But, it’s not just the level of the ocean that influences the shape and topography of the shore, that infinitesimal line between land and sea. The material that makes up the land, soil and rock, is in constant flux, largely due to the interminable power enacted by seawater over time. Although the currents sometimes deposit more sediment than was there already, usually things work the other way around. Rock and sediment are carried out to sea in a process we all know as erosion. The big difference between coastal erosion and other types is the timescale. The sea steals away land so much quicker than other forces on inland areas for many reasons.

Ocean currents move beaches constantly, but the biggest component of coastal erosion is waves. If you’ve ever played in the ocean or even in a wave pool, you’ve probably been surprised at the power behind them. Just like waves wash around swimmers with no hesitation, they can also wash away the coastline. Simply put: waves are destructive because water is heavy. This isn’t exactly a precise law of physics, but it is a good rule of thumb in engineering: when you bash heavy stuff against something, it’s liable to break. When you combine this helpful hint with the fact that a good proportion of coastlines are made of not-very-erosion-resistant loose sandy beaches, you get a recipe for serious erosion.

What happens along coastlines across the world is mostly a physical process where the relentless crashing of water exerts pressure that can separate soil particles and even splinter and remove pieces of rock. A single wave can smash tons of force into a small area, easily washing away loose sediment or wearing away at rocks. Waves also carry sand and sediment from the seabed which gets bashed against the rocks, grinding, scraping, and chipping them over time. In some cases, the seawater can actually dissolve the rocks themselves, a process called chemical weathering. This destructive environment certainly creates some serious erosion, but it gets even worse. All of these processes are amplified during storm events like hurricanes and typhoons which produce some of the fastest sustained winds on earth. That high wind leads to high waves, which accelerate erosion way beyond normal levels.

That would be fine if the coast wasn’t such a popular place to put stuff - and by stuff I mean houses, commercial buildings, apartments, condos, etc. - basically cities and all the expensive infrastructure that comes with them. Erosion literally steals land away from the shore, carrying it piece by piece out to sea or to be deposited somewhere else along the shore. That means development nearest to the coast is constantly at risk of being claimed by the sea. In addition to that, beaches support massive local economies, providing millions of jobs and billions of dollars of economic activity. As I mentioned before, people love the beach, and they’ll spend lots of money to see and hear and swim in those waves. So, just by adding humans to the mix, what was this perfectly natural geologic process of coastal erosion is now a certified hazard in many places, threatening structures along the shore and the livelihood of huge portions of coastal populations. That’s bad and we don’t want it to happen. So, over time, we’ve developed some solutions to try to mitigate these adverse impacts.

A lot of engineers' solutions to coastal erosion involve armoring the shore with structures like seawalls, bulkheads, and revetments. These involve building some kind of hardened structure that can withstand the continued impacts from waves. Some seawalls even include a recurve to make sure waves don’t crash over the top and erode the area beyond the wall. Another protective structure, called a groin, protrudes into the sea to reduce the currents directly along the shore and retain the soil and sand. Finally breakwaters are structures built parallel to shorelines to break up waves before they make it to the shore. Hard armoring often provides a more long-term solution to erosion, but it also creates a lot of unintended consequences. Smooth seawalls like concrete reflect waves rather than absorbing them. This is not ideal because waves can be sent towards other parts of the coast, worsening erosion at the edges of walls or further downshore. Improperly designed groins can also worsen erosion on the downdrift side. These structures can also affect the quality of habitat in the sea, creating environmental challenges.

So, when possible, we look toward softer solutions to erosion. These might not last as long, but they have fewer unintended consequences. One of those solutions is planting of mangrove forests. These are trees and shrubs that grow in tidal zones along coasts. They can’t grow everywhere, but where they can, they provide a natural stabilization of the coastline, reducing erosion from tides, waves, and storm surge. The other soft solution is simply to reverse the process of erosion by replacing the material that has been lost. This is commonly known as beach nourishment. Beaches are not only important recreation areas and economic drivers, they also serve as buffers between development and the sea. Replenishing lost sand by dredging it from the seafloor and pumping back to the shore protects coastal structures and creates important areas for recreation. It’s not without its own environmental impacts, and it’s certainly not a permanent solution, but beach nourishment is one of the primary tools for addressing coast erosion.

Just like with riverine flooding, sometimes the cheapest option to protect development from erosion is for it not to be there in the first place. For coastal structures, this strategy is called “retreat”: either purchase property and condemn it to serve as a buffer or relocate housing and infrastructure further from the shore. The National Oceanic and Atmospheric Administration projects that, in 50 years, the global mean sea level will be at least a foot higher than it was in the year 2000 and potentially more than 3 feet higher (that’s about a meter). Higher sea levels mean more inundation, more exposure to tides, waves, and storm surge, and ultimately more erosion. This is a real threat that is already affecting coastal areas and will only continue to worsen over time. It’s not necessarily something to panic over, but it is an ongoing challenge for property owners, government officials, politicians, and in some cases, even for engineers. We have to be thoughtful about our relationship to the sea and what solutions are appropriate to manage its constant battle with the land. In many cases, the best option is simply to let nature do what it does best, maintaining the coastline as the vibrant and dynamic place that draws humans to it in the first place.