The Dynamic Seashore: The Shoreline is a Constant Battle Between Mother Nature and Man
Undoubtedly, the love of our area is focused on the beaches of Seven Mile Beach. They are the cornerstone of so many memories, the provider of calm moments, and a keystone of the shore economy. When you spend time at the beach, it’s easy to notice the dramatic changes that take place constantly. Beaches have a seasonal rhythm that add to their mystique.
Our beaches are also on a trajectory of change that is being driven by both the human engineering of the beach and dune system and Mother Nature’s will. As we welcome the promise of spring and the coming of another beach season, it’s fitting to talk about the patterns and rhythm of the beaches.
The beach is a marker of sea level; think of it as zero elevation. It makes sense when you are measuring the height of a mountain relative to it but gets a lot trickier when you factor in the tides. A beach is also highly dynamic: Variability in the level of the water (tides) and the energy of the waves (seasonal and storm) means that the actual beach face is constantly changing. Wave energy moves beach sand – both onshore and offshore, depending on its strength. The higher the wave, the stronger the wave energy. During the winter, larger waves take sand from the beach. The beach is steeper, but the sand isn’t gone – it’s stored just offshore, often as a series of sand bars separated by deeper troughs. You can see them when waves break on them, even though they are submerged.
During the spring and summer, when waves are gentle, the beach rebuilds itself. The sand stored in bars and troughs moves back and the gentle wave swash pushes the sand up onto the beach. Breezes blow the finer sand into the dunes, where vegetation traps it. Summer beaches are wide and relatively flat, and the bars and shallow pools make great places for kids to play – though the bars are constantly moving, so it is unlikely that they will be just where you remembered them. As sand moves onshore and offshore, it also moves to the south. Thus, rather than a zigzag pattern of sand moving on and off the beach, it is more of a sawtooth pattern, with sand moving offshore and south, and then back onshore even farther south. This is the repeatable rhythm of our beaches, creating narrow and steep beaches in the winter, and wide, gently sloping beaches in the summer. If you only visit the beaches in the summer, you may not notice this. But for those who walk them year-round, this change is apparent.
Superimposed on this regular rhythm of the seasons is the change caused by major storms – nor’easters, tropical storms, or hurricanes. During storms, the larger waves and storm surge push water higher onto the beaches and erode sand from both the beach and dunes. The beach works to protect itself by creating a broad, sloping ramp to spread wave energy, which again moves sand from the beach and dunes offshore into bars. However, these bars are usually farther offshore than the normal winter bars and can take longer to move back on to the beaches. During particularly large storms, beach sand can also be pushed inland as overwash sand banks. Again, the sand also moves farther south with each cycle of offshore and onshore movement.
You can feel this southward movement in the water that is carrying the sand when you swim or stand in the surf. There is a constant tug down beach; our beaches are actually like a river of sand. This is caused by how the waves break on the beach. They almost never break straight onshore, but rather at an angle, with the north end of the wave coming onshore and breaking first and the rest of the wave gradually breaking down beach. Thus, the sand (and water) on our beaches is constantly moving from north to south. This is one of the reasons the north end of Avalon is eroding and the south end of Stone Harbor at the Point is growing.
The other big variable of our ever-changing beaches is sea level. Remember, the beach elevation is a marker of sea level, and the tides rise and fall around this central point. Sea level has been rising since the end of the last ice age about 18,000 years ago. Along with this rise, the barrier islands – and their beaches – have been marching westward toward higher ground.
When we established our communities on the barrier islands, our property ownership, homes, and infrastructure have effectively tried to set a fixed line with the beach. Our practices are trying to hold the beach in place. The problem is that beaches don’t behave this way. Sea level has risen nearly 1½ feet since the early 1900s and almost 7 inches since the 1980s – and it continues to rise. This is moving wave energy and the beach-shaping forces higher onto the beach – and because we are trying to hold the beach in place, it is getting narrower.
Finally, with each beach replenishment project, we are intentionally shaping today’s beaches. These engineered beaches are shaped very differently than what Mother Nature would create. The flat beach is designed to be higher and the dunes much wider in an effort to provide resilience. This design does provide resilience, but it does so on a relatively short-term basis. When storm energy has time to reorganize the beach to a configuration that matches current sea level and energy, it does. Thus, we find ourselves in a cycle where we have significant storm-driven beach erosion followed by repetitive beach replenishment projects. Every spring, the beaches reflect the natural cycle of sand depletion from winter storms, and the number and intensity of winter storms determines how much change the beaches will experience. Thus, the spring beaches are somewhat narrower, and await the calmer waves to help rebuild them with some of the sand held in offshore bars.
Our beaches are wild and dynamic. For me, this is part of their beauty. I spend more time at the beaches when the energy is high and find my connection to that energy reaffirming. Beaches are shaped by the awesome energy of the sea. They have a rhythm that matches the gentle lapping of the waves on the shore. They are also equipped to handle the ferocious pounding of the surf during the worst storms.
Our desire to have the beach and dune fixed where they suit us most is what causes the challenges. A beach that is free to move landward, to higher elevation, to match sea-level rise, is a natural state that is incredibly rare. Moving beaches don’t fit into the community we have built, so we have set ourselves on a cyclic course of never-ending beach construction followed by erosion. I intentionally didn’t use the word restoration, because we aren’t actually restoring the beach. We are constructing one to meet our needs. It is expensive and becoming more expensive every year, but is now a necessity of island life.
