I remember as a child working furiously with my brother to erect a sand castle fast enough during low tide so we could enjoy it before the high tide began its work of destroying our youthful attempts at engineering. Even as children we had a respect for the power of the ocean. We knew our sand castle was a bit of fantasy that was temporary fun, but I am not so sure we have the same notion when it comes to seawalls or other structures erected along sand, gravel or cobble shorelines. Hurricane Sandy reminds us just how powerful the ocean is and how vulnerable nearly any structure is that we put within its reach. With its reach now expanding, what can be done?
Respect for wave power
This post is part of a series on Hurricane Sandy: Confronting the Realities of Climate Change.
Before we jump into seawalls, let’s take a look at wave power, which is proportional to the wavelength, the square of wave height and the weight of water. An example that research engineer Willard Bascom chose in his classic “Waves and Beaches” illustrates the force of waves. He compares three waves with different heights but each with the same wavelength. They each have the same wave period of 10 seconds (i.e. the time in seconds for a wave crest to traverse the distance equal to one wave length). The wave heights vary between 4 feet high, 8 feet high and 12 feet high, corresponding to around 65,600 foot-pounds, 262,000 foot-pounds and 590,000 foot-pounds respectively of energy per foot of wave crest! Foot-pound is used in engineering applications to describe a unit of energy transferred by a one-pound force moving a distance of one foot. Remember that 590,000 pounds is similar to stacking on top of each other over seven fully loaded legal weight limit trucks. So, any structure put up to defend against the power of the sea needs to be able to withstand that kind of impact, repeatedly.
Figure 1: Waves batter a seawall at Woods Hole, MA during a hurricane in 1938. Photo credit: NOAA Central Library Historical Collections.
I learned firsthand about the power of waves when I was repeatedly measuring beach profiles over a couple of years when I studied an eight-kilometer (~ five-mile) section of beach. These field measurements, as well as others I helped fellow graduate students gather in their coastal field research projects extending from New Jersey to Massachusetts, taught me to respect the power of the sea. Each month, I measured from the sand dune down to the lowest point along several profiles to capture recovery from a huge storm that punched through the beach to create a new inlet.
The difference in the winter beach profile, when the typical wave energy was much higher, and the summer beach profile taught me that nothing is permanent when it comes to that critical boundary between the sea and land. Indeed, one might not recognize their favorite summer beach if they visited in January. When that interface involves loose cobbles, sand, bits of shells or coral, the waves and wind can shape the sediment into a form that is best suited to handle the wave energy that occurs along the shoreline for the season. It is a give and take that occurs along natural shorelines. Coastal erosion is happening in many places along the United States coastline. Maintaining or restoring natural defenses has proven to be, in most cases, a robust way to protect coastal communities.
The struggle to keep pace with sea level rise
Over the course of that research I also learned that some storms are so powerful that they reshape barrier islands. This redistribution of sediment can allow those barrier islands, typical of those along the eastern seaboard of the United States, a chance to keep pace with sea level rise. To see how this works, look at Figure 2, which shows how a major storm can pluck sand from the barrier island beach and deposit it in the back bay. Over time the shoreline moves inland and upland. Yet if that natural redistribution of sediment is interrupted, the barrier islands can become stranded in rising seas and starved of sediment.
Figure 2. National Park Service figure and caption: Generalized summary of harrier island dynamics and migration (vertical scale exaggerated). Stage 1 is a hypothetical barrier island with a well-developed dune line, or series of dunes, and a forest behind. In stage 2, the sea level has risen slightly and storms have knocked the dune barrier back into the woodlands. By stage 3, much of the barrier island has been overwashed and the dunes pushed back. The marsh has grown vertically and been somewhat eroded, and some former uplands are now salt marsh as a result of sea level rise. In stage 4, the barrier has retreated considerably from its original position. Dune and overwash sand has moved completely over the old forest, which is now exposed on the ocean side. Marshes near the island interior have been covered as well. Further retreat places sand completely over the original marsh surface and into the lagoon behind, where new marshes form. At stage 6 an inlet has opened and a typical tidal delta has appeared behind it. The temporary inlet has closed in stage 7, and the tidal delta now supports salt marsh and low dunes. Overwashes have tied the marsh islands to the main barrier and have filled in the old channels in stage 8. The salt marshes are now well developed on the old tidal delta, woods have grown upon the low dunes on these marsh islands, the salt marsh fringe behind the barrier is expanding, and on the barrier itself new dune lines and woodlands have formed where only a time ago there was water.
For example, this interruption of sediment redistribution occurred on a small scale for those profiles I surveyed over parking lots or building structures that stood where dunes used to be. The damage to those structures and the loss of naturally protective landforms was accelerated, leaving the built environment even more exposed to the next storm.
Many communities along the path of Hurricane Sandy have learned these lessons as well, and have opted to place new construction behind dunes or create new dunes in front of homes, buildings and roads. A challenge remains because many buildings were built before these natural defensive options were fully appreciated. Expensive beach replenishment – over a million dollars a mile – is often the cost of living along the coast in many parts of the world, including North America. Especially for coastlines where most people live, the shoreline is prevented from migrating upland and inland. This makes the choices even harder for how to best protect lives and property under climate change that accelerates sea level rise, which in turn can enhance storm surge heights.
The bottom line is that many of the decisions we made about building along our coasts were made before we fully appreciated the vulnerability of those shores to storms, and today our options are more limited because of those choices and because of accelerating sea-level rise. This means we have to leap-frog ahead to create coastal communities that are resilient to the storms of the coming century.
Many communities have relied on seawalls. But they can give a false sense of security. As I think about the 12 foot high wave of 10 second period that can pummel around 590,000 foot-pounds of energy per foot of wave crest, seawalls starts to seem more like my sandcastle I built as a child. Natural barriers that can move with the ocean and its waves – not hardened structures – are sometimes the better protection for people and property further inland. We all like to live near or visit the coast, and collectively we all have a stake in figuring out the best way we can protect properties and lives.