Seawall Failure Series – 2. Seawall Failure Across Different Coastal Environments

The design, construction, and maintenance of seawalls that can withstand the ceaseless battering of marine forces are an ongoing challenge. These structures, while essential for safeguarding coastal assets and communities, are under perpetual siege by erosional processes, which can lead to catastrophic failures if not adequately addressed. The risks and complexities associated with seawall failure become magnified across diverse coastal terrains, where varying wave energy, sediment properties, and topographical features dictate the threats posed to the structural integrity of seawalls. The design, construction, and maintenance of seawalls that can withstand the relentless pounding of ocean waves present an ongoing challenge for engineers across a range of shoreline environments.

The Energy Equation: Wave Impact and Seawall Integrity

The energy imparted by waves onto seawalls stands as one of the primary drivers for both soil erosion and potential structural failure. The dynamics of this energy can vary considerably based on the coastal environment:

• Open Ocean Coasts: Immense wave energy, propelled by long-fetch waves that traverse vast ocean coastlines, put constant pressure on seawalls in these areas. The direct impact of stormy coastal waves can cause immediate structural damage to seawalls, especially if their design lacks adequate energy dissipation mechanisms. Over time, repeated battering can lead to structural fatigue and failure.
• Enclosed Bays or Lagoons: Here, the wave energy narrative is subtler. While direct wave impact might be lessened due to shorter fetches and natural barriers, other forces come into play. Tidal fluctuations and circulatory currents can induce water pressure differentials, potentially compromising the seawall’s structural integrity through the hydrostatic pressure it places on the soil from behind or below the structure.

Sediment Dynamics: From Support to Threat

The type of sediment underlying and surrounding a seawall directly influences its structural longevity. Each sediment type comes with its own set of challenges:

• Sandy Coasts: The mobile nature of sandy sediments makes them vulnerable to being displaced by wave actions. For seawalls, this often results in the undermining of foundational structures, creating voids or pockets. Over time, these voids can lead to partial or complete seawall collapses.
• Clay or Silty Coasts: While these sediments offer better cohesion, they can be prone to liquefaction, especially under specific wave or tidal conditions. Base failure or tilting is a risk if the supporting sediments suddenly lose their strength.
• Rocky Coasts: On the surface, rocky terrains might seem impervious to erosional forces. However, the natural fissures and cracks in rocky substrates can allow water intrusion, which over time can weaken the seawall’s foundational grip, leading to potential structural compromises.

The Role of Coastal Topography in Seawall Stability

The physical contour of a coastline isn’t just an aesthetic feature—it has direct implications on how seawalls respond to marine forces:

• Steep Coastlines: Such terrains can increase wave reflection, creating zones of turbulence near the seawall base. This turbulence can enhance forces that cause erosion, threatening the structural stability of the wall, especially if the protections are eroded or compromised.
• Gently Sloping Coasts: The gradual gradient can disperse wave energy over a more extensive area, reducing direct wall impact. However, this also means a larger zone of wave-soil interaction, potentially leading to widespread undermining of the seawall’s foundational structures.
• Irregular Coasts: Features such as coves, headlands, or peninsulas can induce wave refraction, concentrating wave energy onto specific segments of the seawall. This can lead to uneven structural stresses, with certain wall sections bearing disproportionate loads, heightening their failure risk.

The integrity of a seawall isn’t a mere reflection of its construction caliber. It’s a dynamic entity, deeply intertwined with the coastal environment it inhabits. Recognizing and understanding the multifaceted challenges posed by wave energy, sediment characteristics, and topographical intricacies is paramount for designing and repairing seawalls that can stand resilient against nature’s relentless forces.

Seawall Failure Series – 1. Understanding the Mechanics of Seawall Failure

Seawalls are continuously subjected to the relentless forces of nature. The integrity of these structures is challenged by the complex erosional processes and hydrostatic pressures they must withstand. Delving deep into the science underpinning seawall vulnerabilities provides invaluable insights for designing more resilient coastal protection systems. The enduring stability of seawalls is challenged by intricate physical processes.

Wave Action

Wave action stands at the forefront of erosional forces impacting seawalls. The dynamics of wave action on seawalls encompass several mechanisms:

• Wave Impact and Reflection: As waves strike the seawall, they exert direct hydraulic forces, leading to potential structural stresses. Additionally, waves that reflect off the wall can interact with incoming waves, creating zones of turbulence and amplified hydrostatic pressures.
• Wave Overtopping: In situations where waves breach the seawall’s crest, the overflow can lead to backside erosion or increased water pressures, further destabilizing the wall.
• Wave Run-up and Drawdown: Waves that surge up the face of the seawall can induce a downward flow, termed “drawdown,” leading to scouring at the base of the wall – a primary cause for undermining and structural failure.

Tidal Movements

Tides, with their cyclical rise and fall, exert their own brand of stress on seawalls:

• Tidal Erosion: Repeated tidal inundation can gradually erode the seawall’s foundational structures, especially if there are sedimentary voids or weaknesses that water can exploit.
• Hydrostatic Pressures: As the tide rises, the hydrostatic pressure on the wall will decrease due to the pressure on the landward and waterside equalizing with the water level rising. When the tide recedes, the hydrostatic pressure will increase dramatically, particularly if the wall has limited drainage. This is due to the extended time it takes for the water on the landward to escape from the wall. Allowing the water to drain properly and at a faster rate will decrease the hydrostatic pressure.

Erosion

Erosion can manifest in various forms, each presenting unique challenges:

• Toe Erosion: The base or “toe” of the seawall, being in constant contact with both water and sediment, is highly susceptible to erosion. As sediments are displaced, voids form, compromising the wall’s stability.
• Rear Erosion: This is often a consequence of wave overtopping or poor drainage. Water accumulating behind the wall can lead to soil erosion, creating pockets or channels that undermine the wall’s structural integrity.
• Seepage Erosion: Groundwater or trapped water behind the wall can seep through porous materials or cracks, mobilizing sediments and leading to internal erosion pathways.

Hydrostatic Pressure

Hydrostatic pressure, the force exerted by fluids in response to gravitational forces, can be particularly detrimental to seawalls:

• Pressure Build-up: Poor drainage or impermeable backfill materials can cause water to accumulate behind the seawall, increasing hydrostatic pressures. This pressure can push against the wall, straining its structural integrity.
• Pressure Oscillations: Rapid changes in water levels, be it from waves or tides, can induce oscillatory hydrostatic pressures, which cyclically load and unload the seawall, leading to material fatigue over time.

A symphony of forces challenges the durability of seawalls. Understanding the multifaceted mechanics of wave action, tidal influences, erosion, and hydrostatic pressures is pivotal for evolving engineering strategies, practices, and solutions. Only through a comprehensive grasp of these dynamics can we aspire to design and repair seawalls that not only stand firm against nature’s onslaught but also thrive amidst its challenges.