Seawall Dynamics - Kailua Bay EarthCache

Welcome to Kailua Bay

Located along the coastline in Kailua-Kona, Hawai‘i, this concrete seawall stands as a frontline defense against the Pacific Ocean. Though it might look like a simple barrier, it's actually part of a larger geoscientific story about erosion, wave energy, sea level rise, and human adaptation. This EarthCache invites you to explore how seawalls are built, what they protect, and how they interact with the forces of nature around them. As you examine the wall and the surrounding shoreline, you’ll gain insight into coastal geology, engineering, and the challenges of managing dynamic marine environments.

The Geoscience Behind Seawalls

Seawalls are built to protect the land from coastal erosion caused by waves, tides, and rising sea levels. In coastal areas like Kailua-Kona, where the shoreline is shaped by volcanic rock and powerful ocean swells, seawalls play an essential role in preserving roads, sidewalks, and buildings located close to the water.

Seawalls come in two main designs, each with different impacts on wave energy and beach dynamics.

• Absorptive seawalls are built with a sloped, rough, and porous surface. Their shape allows them to absorb some of the wave’s energy, reducing the force of the impact and minimizing the amount of wave reflection. These designs attempt to mimic more natural shoreline defenses and reduce some of the damage caused by constant wave action.

• Reflective seawalls are typically vertical or curved. These structures deflect wave energy back out to sea, which can be effective in protecting infrastructure behind the wall. However, the reflected waves often collide with incoming waves, forming interference patterns that increase turbulence and can accelerate erosion at the base of the wall.

The energy hitting a seawall can be immense. A standard 3-foot wave delivers around 10 kilowatts of energy per meter of shoreline, similar to the force of a small car traveling at full speed. A 9-foot wave carries ten times that energy, and a 45-foot wave can release 250 times more. Because of this, seawalls must be built with durable materials, solid foundations, and sufficient depth to withstand this constant pressure.

However, seawalls also disrupt natural coastal processes. In a natural setting, waves gradually erode cliffs, dunes, or soil behind the beach, and this material is carried forward to replenish sand on the shore. When a seawall is built, it blocks that source of sediment from reaching the beach. Without this steady supply, the beach begins to narrow or disappear over time. In some cases, all that remains in front of the seawall is exposed rock or reef, especially during high wave seasons.

Additionally, the powerful backwash from reflected waves often pulls sand away from the base of the wall and transports it offshore. The increased turbulence caused by wave interference can create scouring, which is the removal of sand and sediment right at the foundation. Over time, this process may leave the base of the seawall unsupported, eventually weakening or damaging the structure.

Seawalls can also be overtopped during storms. When large waves rise above the top of the wall, water flows into the landward side and floods surrounding areas. As this water slowly seeps back toward the ocean, it often travels beneath the wall, gradually eroding the base and compromising its stability. To limit this damage, engineers sometimes place additional protective material, such as boulders or riprap, at the foot of the seawall to help absorb energy and reduce erosion.

Hawai‘i faces additional coastal hazards that make seawall design and maintenance even more critical. Sea level is steadily rising due to global climate change, and the massive weight of the volcanic islands causes the crust beneath them to subside. This combination increases the likelihood of wave overtopping. Storm surges and tsunamis, both of which bring extreme wave heights, also threaten coastal areas.

A dramatic example took place in 2011 after the Tohoku earthquake off the coast of Japan. Tsunami waves measuring 10 to 15 feet struck the west coast of Hawai‘i Island. In Kailua-Kona, waves overtopped the seawall, stripping away large sections of lava rock and reducing parts of the sidewalk to rubble. Businesses along the waterfront were damaged, despite the wall itself remaining mostly intact. Photos and records from the Pacific Tsunami Museum help document the force of the waves and the extent of the damage.

While seawalls are powerful tools for protecting coastal infrastructure, they are not permanent solutions. Their presence often accelerates beach loss, and without ongoing maintenance and adaptation, they too can fail under the persistent and growing power of the sea.

Why Was This Spot Chosen?

This location was chosen because it offers a clear, real-world example of how seawalls function in a dynamic and often challenging coastal environment. The seawall here runs along Ali‘i Drive, a busy road that parallels the shoreline and is highly exposed to daily wave action from the Pacific Ocean. This means the wall is regularly tested by both average surf conditions and larger storm-driven waves, making it an excellent site to observe how engineered structures interact with natural forces.

In addition to protecting the road and adjacent properties from erosion and flooding, the seawall plays a crucial role in preserving public access to the coastline. Without it, much of the infrastructure in this area could be undermined or damaged by the ocean over time. Observing this seawall up close gives visitors a chance to see how different design features are tailored to meet the demands of this specific shoreline.

Tasks for This EarthCache

To log this EarthCache, visit the location and complete the following tasks. Send your answers via Geocaching or email.

1. Include “Seawall Dynamics - Kailua Bay - GCB9WYX” as the first line of your message.

2. Describe the seawall’s shape. Is it sloped, vertical, or curved? Based on your observations, what type of seawall is this: absorptive or reflective? How do you think its design helps manage the force of the waves in this specific location?

3. What is the current wave height? Based on the information above, estimate the amount of energy that might be hitting each meter of the seawall. How might that energy impact the wall over time?

4. Describe the erosion you observe in front of the seawall. Do you see a wide sandy beach, a narrowed shoreline, exposed rock, or signs of sand loss? Based on your observations, how has the seawall affected the natural movement and buildup of sand in this area?

5. In your log, attach a photo of yourself or a personal belonging with the seawall in the background. (Note: photos predating the publication of this EarthCache are not accepted.)

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Works Cited

https://en.wikipedia.org/wiki/Seawall

https://www.geocaching.com/geocache/GC5C4VV

https://www.sciencedirect.com/topics/engineering/wave-energy

https://www.coastalwiki.org/wiki/Seawalls_and_revetments

https://www.surfrider.org/news/seawalls-are-stealing-our-sandy-beaches