Coastal Structures: Understanding the Basics of Shoreline Protection

 Lecture 17

Coastal structure

Coastal structures are human-made constructions built along coastlines or in nearshore areas to manage, protect, or enhance coastal environments.

Coastal structures are built for various purposes, driven by the need to develop and protect coastal and nearshore areas from waves and storm surges. They play a key role in coastal protection, either by directly controlling wave and surge action or by stabilizing beaches, which in turn safeguard the coast. Sandy beaches, besides their protective function, hold significant recreational value. However, sand is often limited and mobile, requiring structures to manage its movement and prevent erosion.

Types of Coastal Structures

1. Long Thin Cylindrical Structures

• Individual Piles: Used to support piers, docks, or navigation aids (e.g., lighthouses).
• Pipelines: Submerged pipelines for transporting oil, gas, or wastewater.
• Cables: Underwater cables for power transmission or communication.
• Jetty: A jetty is a long, narrow structure built perpendicular to the shore, typically at the entrance of rivers, harbors, or inlets. It is often constructed using piles, rocks, or concrete. Purpose is to stabilize navigation channels by preventing sediment buildup and directing currents and to protect harbor entrances from wave action and sediment deposition.

• Groin: A groin is a shore-perpendicular structure, typically shorter than a jetty, built to trap sand and reduce longshore sediment transport. It is often constructed using rocks, concrete, or wood. Purpose is to trap sand moving along the shore (longshore drift) and widen beaches and to reduce coastal erosion by stabilizing the beach.

• Design Concerns:
• Structural: Resistance to wave forces, currents, and corrosion.
• Functional: Ensuring pipelines and cables remain stable and intact under hydrodynamic loads.

2. Large Single-Unit Structures

• Submerged Breakwaters: Large concrete or rock structures placed offshore to reduce wave energy.
• Partially Submerged Caissons: Used in harbor construction or as wave barriers.

• Design Concerns:
• Structural: Stability against wave impact and scour at the base.
• Functional: Effective reduction of wave energy to protect shorelines or harbors.

3. Moored Floating Structures

• Examples:
• Floating Breakwaters: Used in calm waters to reduce wave action for small harbors or marinas.
• Offshore Mooring Platforms: For anchoring ships or floating oil storage facilities.
• Design Concerns:
• Structural: Anchoring system stability and resistance to wave and wind forces.
• Functional: Maintaining position and effectiveness in reducing wave energy or providing safe mooring.

4. Rubble Mound Structures

• Examples:
• Rubble Mound Breakwaters: Large piles of rock or concrete armor units to protect harbors or shorelines.

• Revetments: Sloped rubble mounds to protect embankments from erosion.

 

Image Source: Coastal Wiki

• Design Concerns:
• Structural: Determining the size of rock or armor units to withstand design waves.
• Functional: Ensuring the structure effectively dissipates wave energy and prevents erosion.

5. Vertical-Faced Rigid Structures

• Examples:
• Seawalls: Vertical concrete walls to protect coastal developments from waves and storm surges.

Image Source: Coastal Wiki

• Sheet Pile Walls: Used for shoreline stabilization or as retaining structures.
• Sea Dykes: A sea dyke is a vertical or sloping structure built parallel to the shoreline to protect inland areas from wave action, storm surges, and flooding. It is typically constructed from durable materials like concrete, stone, or steel.

 

Image Source: Coastal wiki

• Design Concerns:
• Structural: Resistance to wave impact, overturning, and sliding.
• Functional: Preventing wave overtopping and ensuring long-term stability.

6.   Combined Structures

• Example:
• Caisson on Rubble Mound: A vertical-faced concrete caisson placed on a submerged rubble mound base (e.g., used in breakwater construction).
• Geotextile Rubber Tube: Geotextile tubes are large, elongated bags made from high-strength, permeable geotextile fabric. They are filled with sand, slurry, or other locally available materials to create flexible, durable structures. Primary Function is to provide erosion control, shoreline stabilization, and coastal protection and to act as breakwaters, groins, or revetments in a more environmentally friendly way compared to rigid structures.
• Design Concerns:
• Structural: Stability of the caisson against wave forces and scour protection from the rubble mound.
• Functional: Effective wave energy dissipation and shoreline protection.