Engineering Shoreline Barriers

After figuring out how an ecosystem food web phenomena can be harmed, students become engineers, designing solutions that can help protect fragile systems. They focus on one kind of solution—an artificial reef—that can offer ecosystem services for coastal ecosystems.

• Oyster reefs are structures formed by generations of oysters settling on top of one another, and they provide many ecosystem services.
• Engineers have designed solutions to try to reduce shoreline erosion. In an effort to reduce erosion, many people have built seawalls—engineered hard structures designed to protect coastal areas from erosion.
• In recent years engineers have been looking for alternatives to the hard structure of a seawall. One of these solutions has been living shorelines, which are engineering solutions that use elements of natural habitats to maintain biodiversity and ecosystem services by reducing erosion and providing critical habitats for wildlife.

• How do oysters contribute to the marine food web?
• How has the disappearance of oyster reefs affected the food web?
• How do oyster reefs help to protect the coastline?
• How does a seawall prevent erosion of an area of coastline?
• What happens to the waves once they collide with the seawall?
• Why do seawalls increase erosion around the wall?
• How do technologies such as artificial reefs and reef balls improve upon the traditional seawall approach to reducing erosion?
• How does the design of reef balls help to slow the momentum of waves?
• What kinds of scientific knowledge would an engineer need to apply to design a solution that reduces coastal erosion?

Misconception: Species in an ecosystem “get along.”

Fact: Species compete with and feed on one another for survival. 

Misconception: Ecosystems are static.

Fact: Ecosystems are dynamic and constantly changing.  

Ecosystem service : any positive benefit that an ecosystem provides to people; includes decomposition, water purification, and erosion control

Living shoreline : an engineering solution that uses elements of natural habitats to maintain biodiversity and ecosystem services by reducing erosion and providing critical habitats for wildlife

Oyster reef : a structure formed by generations of oysters settling on top of one another; provides many ecosystem services, including a food source, habitats, and protection from erosion

Seawall : an engineered hard structure designed to protect coastal areas from erosion

A Disappearing Coastline

For the last two years, Jennifer Mattei has visited a section of Connecticut’s coastline. She checks on hundreds of giant concrete structures called reef balls that line the coast.

Jennifer is taking notes on how well the reef balls are reducing erosion (the transport of sediment by wind or water to a new location). She also wants to see whether any organisms are making the reef balls their new home.

For years before Jennifer got involved, the coastline was losing large amounts of sediment through erosion. Scientists believe that the coast had lost sediment up to 1.2 meters deep and 30 meters high.

One reason that erosion had become such a problem was the disappearance of oyster reefs. Oyster reefs are structures formed by generations of oysters settling on top of one another. The reefs have disappeared for many reasons, including overharvesting by people.

Oysters have been called “ecosystem engineers” because their reefs provide many ecosystem services. An ecosystem service is any positive benefit that an ecosystem provides to people.

Oyster reefs maintain a tidal exchange by allowing the movement of water between the shoreline and the ocean. Oysters are also an important part of the marine food web. They are primary consumers, filtering plankton and algae from the water. They are a food source for hundreds of species of fish, birds, shrimp, and crabs. Their reefs are also habitat for many of these organisms as well. Their disappearance has had a ripple effect across the food web, causing fewer organisms to survive.

Living Shorelines

In recent years engineers have been looking for alternatives to help reduce erosion of the shoreline. This is where Jennifer Mattei and scientists like her come in. They have been exploring living shorelines, which are engineering solutions that use elements of natural habitats to maintain biodiversity and ecosystem services by reducing erosion and providing critical habitats for wildlife.

Some engineers look to natural oyster reefs for inspiration. Engineers have designed artificial structures offshore that provide some of the ecosystem services provided by oysters.

Artificial Reefs

One common solution involves artificial reefs. Engineers take a hard material, such as fossilized shells, limestone, or concrete, to use as a platform that oyster larvae can attach to. Some places have experimented with used tires. Over time, oysters and other organisms that attach to surfaces cover the hard material. This draws in small fish and other animals that are looking for food and shelter.

Reef Balls Are a Solution

The reef ball technology also creates a living shoreline because each ball becomes a habitat for a wide range of marine organisms. The presence of algae, barnacles, and oysters is one of the things that Jennifer looks for as she walks up and down the coastline.

The reef balls also help to reduce erosion because they slow the momentum of the waves hitting the land. Momentum is the measurement of an object’s mass multiplied by its speed. Before the water collides with the coastline, it has momentum that depends on its mass and the speed at which it’s moving.

An object’s momentum decreases when energy transfers out of it. This goes back to the conservation of energy. Remember that energy is never created or destroyed. This means that if energy is transferred out of an object, it will have less energy to transfer during a collision. The wave’s momentum slows when some of its energy transfers to the reef (viewed from above).

The Design of Reef Balls

Reef balls have holes that water can pass through and come ashore. As the water passes through each hole, some of its energy is transferred to the reef ball. This reduces the wave energy and slows the wave’s momentum. Because there is less energy in the water that reaches the shore, sediments can settle, reducing the amount of erosion that takes place. The reef balls are working. In Connecticut, a layer of sediment about 0.3 meters (12 inches) thick has returned behind the reef ball barrier.

In this lesson, students use the engineering design process to find a solution that decreases the phenomena of shoreline erosion by mimicking natural oyster reefs. Students analyze their erosion data and observations collected during their prototype tests and use the data to evaluate the effectiveness of their shoreline protection barrier prototypes in keeping shoreline erosion to a minimum while maintaining the phenomena of tidal exchange.