Cross-shore transport is the movement of sediment perpendicular to the shoreline, essentially moving sediment either towards the land (onshore) or towards the sea (offshore).
This process plays a crucial role in shaping coastlines, influencing beach erosion and accretion, and affecting nearshore environments. It's a dynamic process influenced by waves, tides, and coastal morphology.
Understanding Cross-Shore Transport
Cross-shore transport can be broadly categorized as either:
- Onshore Transport: Sediment moves from the offshore zone towards the beach. This typically leads to beach accretion (building up).
- Offshore Transport: Sediment moves from the beach towards the offshore zone. This typically leads to beach erosion.
Factors Influencing Cross-Shore Transport
Several factors govern the direction and magnitude of cross-shore sediment transport:
- Wave Characteristics: Wave height, period, and angle of approach significantly impact sediment movement. High-energy waves (typically during storms) tend to cause offshore transport, while lower-energy waves can facilitate onshore transport.
- Tidal Fluctuations: Tides influence the water depth and wave energy reaching the shoreline, affecting the extent of wave breaking and sediment mobilization.
- Beach Profile: The slope and shape of the beach influence wave run-up and backwash, determining whether sediment is transported onshore or offshore.
- Sediment Characteristics: The size, density, and shape of sediment particles influence their susceptibility to transport. Finer sediments are more easily transported by weaker wave action.
- Storm Events: Storms are a major driver of offshore transport, eroding beaches and depositing sediment in deeper waters.
- Coastal Structures: Structures like groins and jetties can disrupt natural sediment transport patterns, leading to localized erosion or accretion.
Destructive vs. Constructive Cross-Shore Transport
As indicated, cross-shore transport can be both destructive and constructive:
- Destructive (Erosion): High-energy wave action removes sediment from the beach and transports it offshore, resulting in beach erosion and a flattening of the beach profile. This often occurs during storms.
- Constructive (Accretion): Low-energy wave action transports sediment onshore, building up the beach and steepening the beach profile. This typically occurs during calmer weather conditions.
Examples of Cross-Shore Transport
- Summer Beach Building: Gentle waves transport sand from offshore sandbars onto the beach, widening and building up the beach for the summer season.
- Winter Beach Erosion: Storm waves erode the beach, transporting sand offshore and creating offshore sandbars. This reduces the beach width and leaves it more vulnerable to future storms.
- Formation of Coastal Dunes: Onshore winds transport sand from the beach inland, forming coastal dunes that act as a natural barrier against storm surges.
In summary, cross-shore transport is a dynamic process that constantly reshapes coastlines by moving sediment onshore and offshore, influenced by a variety of factors including wave energy, tides, and sediment characteristics.
