The Comings & Goings of Our Dynamic Beaches

Originally published in Seven Mile Times' July 2016 Issue

60

If you want to know how much I love you count the waves in the sea. ~ Unkno wn

The

COMINGS & GOINGS

Of Our Beaches

By Dr. Lenore Tedesco of The Wetlands Institute

Storm ramp

ne of the main attractions of the shore is its beaches.

They drive our love for this area, are the cornerstone of so many memories, and are a keystone of the shore economy. If you spend any time at the beach, it’s easy to notice the dramatic changes that take place there. Beaches have a seasonal rhythm. 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 kick off the 2016 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. This variability in the level of the water (tides) and the energy of the waves (seasonal and storms) means that the actual beach face is highly dynamic and constantly changing. Wave energy is responsible for moving sand on the beach and it moves sand both onshore and offshore depending on wave energy.

The higher the wave, the stronger the July 2016

Sandbars and troughs visible at low tide

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 in sandbars just offshore of the beach. It is common to have a series of sandbars separated by deeper troughs sitting just offshore. Even though they are submerged, you can see them when waves break on them.

During the spring and summer, when waves are gentle, the beach rebuilds itself. The sandbars and troughs move back onto the beach 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 sandbars and

SUMMER SummerPROFILE Profile High Tide Line

Wide Berm

Winter Profile High Tide Line

Eroded or No Berm Eroded Dune Scarp Sand Bar

Sand Bar

Steep Beach

shallow pools make great places for kids to play. The bars are constantly moving, so it is unlikely they will be just where you remember 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 further south. This is the repeatable rhythm of our beaches. Narrow and steep beaches in the winter, and wide, gently sloping beaches in the summer.

But, superimposed on this regular rhythm of the seasons, is the change caused by major storms – nor’easters, tropical storms and hurricanes. During storms, the intense waves push water higher onto the beaches and erode sand from the beach and dunes. The beach works to protect itself by creating a broad, sloping ramp to spread wave energy. Sand from the beach and dunes is again moved offshore into sandbars.

These bars, however, are usually further offshore than the normal winter bars and can take longer to move back on to the beaches. During particularly large continued on page 62 WWW.SEVENMILETIMES.COM

62

Man cannot discover new oceans unless he has the courage t o l o s e s i g h t o f t h e s h o re . ~ A n d re G i d e

UPCOMING AT

CRABULOUS CRAB DAY Friday July 22, 2016 9:30am–3pm Non-Member: $10 Adult, $8 child, $30 family pack of 4

Winter Storm Jonas dune scarp

Member: $8 Adult, $6 child, $25 family pack of 4 continued from page 60

storms, beach sand can also be pushed inland. Again, the sand also moves further south with each offshore and then onshore movement.

You can notice 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 so that our beaches are actually like a river of sand. This is caused by the angle waves break on the beach. They almost never break straight onshore, but rather at an angle to the beach with the north end of the wave coming onshore and breaking first, with 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 a central level that is effectively sea level. Sea level has been rising since the end of the last ice age about 18,000 years ago. As sea level has been rising, 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 to the east of the dunes and the town to the west. Our practices are trying to hold the beach in place. The problem is, beaches don’t behave this way. Sea level continues to rise and has July 2016

risen more than 1.3 feet since the early 1900s and more than 6 inches since the 1980s. This rise is moving wave energy, and the beach-shaping forces higher onto the beach. Because we are trying to hold the beach in place, the beach is getting narrower.

Finally, today’s beaches are shaped by design with each beach-replenishment project. These engineered beaches are shaped very differently than Mother Nature would design them. 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 shortterm basis. When storm energy has time to reorganize the beach to the configuration to match current sea level and energy, it does. Thus, we find ourselves in a cycle where we have significant beach erosion following most storms and have committed to repetitive beach-replenishment projects. This year, the beaches will reflect the natural cycle of sand depletion from winter storms, and Winter Storm Jonas was a particularly strong storm. Thus, the beaches are somewhat narrower and awaiting the calmer waves to help rebuild some of the sand being held in offshore sandbars.

Our beaches are wild and dynamic.

For me, that is part of their beauty.

They 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 well equipped to handle the ferocious pounding of the surf during the worst storms. It is our desire to have the beach and dune fixed where they suit us most that causes the challenges.

Join us at The Wetlands Institute for a day filled with Crabulous fun and see just how much you can learn about crabs!

• Crab Races and Games • Crabbing Tips and Tricks • Crab Teaching Tank • Crabbing Tournament • Crabby Crafts • And more!

SUMMER HAPPENINGS The Wetlands Institute will be open daily from 9:30am-4:30pm with extended evening hours and activities until 8pm Tuesday-Thursday from June 13-Sept. 5, 2016!

Summer at the Institute is packed with fun for the whole family: • Totally Turtle Tuesday and Horseshoe Crabmania Thursday • Crabbing at the Dock • Catch o’ the Day • Summer Nature Programs • Aquarium Teaching and Touch Tank • Salt Marsh Safari • Beach and Dune Walks • Hooked on Fishing • Back-Bay Birding and Wildlife Tours • Back-Bay Kayaking and Paddleboarding • And much more!

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