Rock Solid: How Armor Stones Provide A Natural Defense Against Coastal Erosion
Introduction
When Lake Superior’s fierce waves and ice sheets battered Duluth’s Lakewalk, engineers turned to a time-tested solution: armor stones. These massive, interlocking boulders absorb and redirect wave energy, forming a durable, natural-looking revetment that protects infrastructure while preserving shoreline beauty. Carefully selected and strategically placed, armor stones are nature’s defense—reshaping erosion control with strength, resilience, and rugged elegance.
Erosion, especially on coastlines, is a powerful force, steadily wearing away and reshaping the landscape over time. On the shores of Lake Superior, a relentless barrage of powerful waves and ice sheets hammer the coastline. This natural phenomenon can reshape the shoreline, negatively impacting coastal communities. To combat this relentless force, a solid object needs to be placed directly in its path. Meet, the humble armor stone.
What is an Armor Stone?
Armor stones — massive, durable rocks, strategically placed to absorb and dissipate wave energy — undergo a stringent examination process to ensure their quality. Not only do they provide exceptional resistance to erosion, but they also offer a rugged, natural aesthetic that blends into the landscape, subtly concealing their role as engineered barriers against water and ice.
One great example of armor stones in action can be found in Duluth, Minnesota, along the scenic Duluth Lakewalk. In October 2017, April 2018 and October 2018, severe storms caused significant damage and flooding, particularly compromising the shoreline infrastructure near Canal Park. In response, armor stones became the key element of a long-lasting, resilient design.
Unlike rigid concrete barriers, which are man-made and typically vertical, armor stones work by dispersing wave energy, rather than reflecting that energy back into the lake which can cause a secondary form of erosion at the base of the wall called scour. After being blasted out of a quarry, each armor stone is carefully examined for cracks or fissures. Only structurally sound stones are selected as armor stones. Others, still strong but smaller stones, may be repurposed as filter or chinking stones, which fill gaps between larger stones and help distribute loads more evenly.
Why Armor Stones?
Common materials for armor stones are granite and basalt, but in the case of the Duluth Lakewalk, an igneous rock similar to granite, called gabbro, was chosen for its rugged strength, durability, and local sourcing. Due to the spirit of their extraction of being blasted from quarries, armor stones are irregular in shape, which allow them to naturally interlock with each other to form a stable, and highly effective defense against erosion and wave action. This makes them perfect for use in protective structures, built of impact-resistant materials, called a revetment.
Armor Stones in Construction.
Even with their natural strength, armor stones require careful planning and placement to withstand the dynamic forces acting on a coastal structure. Constructing a revetment involves a practiced, precise, and layered approach:
Diagram of an armor stone revetment.
Core Layer: Smaller granular materials raise the elevation of the revetment. These helps cost-effectiveness by reducing the need of the large armor stones.
Geotextile Fabric: A high strength water-permeable fabric, that prevents sediment beneath the revetment from washing away as well as helping lock the stones in place.
Bedding Stone: A layer of smaller granular material is placed beneath the toe stones to prevent shifting and settling. The bedding stone also helps prevent larger stones from breaking through the geotextile fabric, which could lead to the revetment washing away over time.
Filter Stones: Larger than bedding stones but only about one-tenth the weight of armor stones, filter stones are placed over the geotextile fabric, where they allow water to flow through the revetment, dispersing wave energy and preventing larger stones from shifting, which helps maintain the structure's stability.
Armor Stone Layer: On Lake Superior, armor stones range from two to ten tons and absorb the majority of wave energy. These large, visible stones form the revetment’s main defensive bulwark. To ensure their stability, each stone is carefully positioned to interlock at three points, providing strength and preventing shifting under dynamic wave forces. This interlocking system is key to maintaining the integrity of the entire structure.
Toe Stones: Lake Superior toe stones weigh between four and sixteen tons. These are the largest and heaviest stones, critical for anchoring the base of the revetment. They are either embedded below the shoreline, trenched into bedrock, or mechanically anchored to shallow bedrock. Toe stones are carefully interlocked using heavy machinery to form the revetment’s foundation.
Chinking Stones: Smaller stones added to an armor stone revetment after the larger stones are placed. They fill gaps, enhance stability, prevent erosion, and help lock the armor stones in place. However, chinking stones are optional, as they can increase wave run-up, which may lead to erosion or damage to landward structures.
Throughout the process, AMI Engineers oversaw the work and contractors carefully positioned each stone. The use of specialized lifting equipment such as cranes and excavators ensure even the heaviest of stones are moved into position. Engineer divers and Remote Operated Vehicles (ROVs) help with underwater positioning using hand signals and real-time video feedback.
A Natural Solution for a Natural Problem.
Often overlooked, armor stones play a crucial role in coastal protection. Their ability to absorb and redirect wave energy makes them a natural defense against shoreline erosion. The Duluth Lakewalk is a prime example of how these stones safeguard infrastructure while preserving the natural beauty of the shoreline.
