9.3 Coastal currents and sediment transport
4 min read•july 30, 2024
Coastal currents and sediment transport are crucial players in shaping our shorelines. From longshore drift to rip currents, these forces move and rocks, sculpting and barrier islands. Understanding these processes helps us grasp how our coasts change over time.
Waves, , and currents work together to create complex patterns of erosion and deposition. By studying how sediment moves along and across the shore, we can better predict coastal changes and manage our beaches. It's all about the dance between water and land.
Coastal currents and their drivers
Types and characteristics of coastal currents
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- Coastal currents categorized into alongshore currents (flow parallel to coastline) and cross-shore currents (flow perpendicular to coastline)
- Alongshore currents primarily driven by oblique approach of waves to shore
- Cross-shore currents include undertow, rip currents, and wave-induced currents
- Tidal currents caused by gravitational forces of moon and sun, influencing both alongshore and cross-shore currents
- Wind-driven currents generated by frictional force of wind on water surface, affecting surface and subsurface water movement
Driving mechanisms of coastal currents
- Density-driven currents caused by differences in water temperature, salinity, or sediment concentration
- Examples include coastal upwelling and downwelling
- Coriolis effect influences direction of large-scale coastal currents
- Deflects currents to the right in Northern Hemisphere and left in Southern Hemisphere
- Wave-induced currents result from breaking waves and wave setup
- Pressure gradients created by variations in water level or density drive some coastal currents
- Topographic steering affects current direction and strength due to bathymetry and coastline shape
Coastal currents in sediment transport
Sediment transport processes
- Coastal currents play crucial role in transport, erosion, and deposition of sediments along shorelines
- Longshore currents act as primary agents of longshore sediment transport
- Move sediment parallel to coast
- Contribute to formation of coastal features (spits, barrier islands)
- Cross-shore currents, particularly rip currents, transport sediment offshore
- Affect beach profiles
- Contribute to development of offshore bars
- Interaction between coastal currents and waves creates complex sediment transport patterns
- Influences shape and stability of beaches, dunes, and other coastal landforms
Impacts on coastal morphology
- Tidal currents in estuaries and tidal inlets redistribute sediments
- Affect morphology of tidal flats, salt marshes, and tidal channels
- Coastal upwelling currents transport nutrient-rich sediments from deeper waters to surface
- Influence coastal ecosystems
- Potentially affect sediment distribution patterns
- Strength and direction of coastal currents lead to areas of erosion or accretion along coastline
- Shape overall coastal profile
- Influence long-term coastal evolution
- of coastal systems affected by current-driven transport processes
- Determine whether a coastline experiences net erosion, accretion, or stability
Longshore drift vs rip currents
Longshore drift mechanisms and impacts
- Longshore drift transports sediment along coast by waves approaching shore at oblique angle
- Creates zigzag pattern of sediment movement
- Rate and direction of longshore drift influenced by:
- Wave energy
- Wave approach angle
- Sediment characteristics
- Contributes to formation and maintenance of various coastal features
- Examples include spits, barrier islands, and tombolos
- Affects sediment distribution along coastline
- Can lead to downdrift erosion or updrift accretion
- Longshore drift rates can be quantified using sediment transport equations
- Example: CERC formula (Coastal Engineering Research Center)
Rip currents and beach dynamics
- Rip currents form strong, narrow channels of water flowing seaward from shore
- Typically develop where breaks occur in sandbars or near structures (groins, jetties)
- Significantly impact beach morphology by:
- Eroding channels through sandbars
- Transporting sediment offshore
- Create cellular circulation pattern on beaches
- Onshore flow between rip currents
- Offshore flow within rip currents
- Affect sediment distribution and beach equilibrium
- Influence erosion and accretion patterns
- Impact overall stability of shoreline
- strength and frequency vary with wave conditions and beach morphology
- Stronger during storm events or high wave conditions
Factors influencing sediment transport
Wave and current characteristics
- Wave characteristics determine sediment transport rates and patterns
- Height, period, and approach angle are key factors
- Coastal currents influence sediment movement
- Longshore currents drive parallel sediment transport
- Cross-shore currents affect onshore-offshore sediment exchange
- Tidal range and tidal currents impact sediment transport rates
- Particularly important in estuaries and tidal inlets
- Stronger currents during spring tides lead to increased sediment movement
Sediment properties and coastal morphology
- Sediment properties significantly influence transport rates and modes
- Grain size, shape, and density affect mobility
- Determine mode of sediment movement (, saltation, bedload)
- Coastal morphology affects sediment transport patterns
- Beach slope and nearshore bathymetry alter wave and current dynamics
- Presence of coastal structures modifies local transport processes
- Seasonal variations in wave climate and storm frequency result in cyclical changes
- Affect beach profiles and coastal morphology
- Lead to alternating periods of erosion and accretion
Environmental and anthropogenic influences
- Wind patterns and storm events dramatically alter sediment transport
- Often lead to episodic, high-magnitude transport events
- Can cause significant shoreline changes in short periods
- Climate change impacts sediment transport through:
- Sea-level rise, altering coastal hydrodynamics
- Changes in storm frequency and intensity
- Human activities significantly alter natural sediment transport patterns
- Construction of coastal structures (groins, jetties, seawalls)
- Dredging operations modify local bathymetry and sediment availability
- projects introduce new sediment into coastal systems
Key Terms to Review (19)
Beach nourishment: Beach nourishment is the process of adding sand or sediment to an eroding beach to restore it to a wider and more stable condition. This technique is often used as a coastal management strategy to combat erosion, protect coastal infrastructure, and enhance recreational areas. By replenishing the beach with new sediment, it can also help support the ecosystem and maintain habitat for various species.
Beaches: Beaches are dynamic landforms located along the shoreline, typically composed of loose sediment like sand, gravel, or pebbles that have been shaped and deposited by the action of waves, tides, and currents. They serve as essential interfaces between land and sea, providing critical habitats for wildlife, recreational spaces for humans, and playing a significant role in coastal processes including erosion and sediment transport.
Bedload transport: Bedload transport refers to the movement of sediment along the bottom of a river or coastal area, primarily through rolling, sliding, or bouncing actions. This process plays a critical role in shaping riverbeds and coastlines as it contributes to the overall sediment transport dynamics influenced by water flow and wave action.
Clay: Clay is a fine-grained natural rock or soil material that is composed of a complex mixture of minerals, particularly hydrous aluminum silicates. It plays a crucial role in coastal environments, as its small particle size allows it to be easily transported by water, including coastal currents, which can carry sediments over long distances and influence sediment deposition along shorelines.
Deltas: Deltas are landforms created at the mouth of a river, where it meets a standing body of water, like an ocean or lake. They form from the accumulation of sediment carried by the river as the flow slows down upon entering the larger body of water, allowing the particles to settle and build up over time. Deltas are crucial for understanding sediment transport and coastal processes, as they illustrate how rivers interact with coastal currents and how sediments are redistributed in these environments.
Groynes: Groynes are structures built out from a coastline into the sea, designed to interrupt water flow and limit the movement of sediment. They help manage coastal erosion by trapping sand and sediment on one side while reducing the impact of waves on the beach. This creates a more stable shoreline, making groynes important in understanding coastal dynamics and effective management strategies.
Longshore current: A longshore current is a water current that moves parallel to the shore, generated by waves breaking at an angle to the coastline. This movement of water is crucial for the transportation of sediment along the coast, influencing beach erosion and deposition patterns. Longshore currents can vary in strength and direction based on wave conditions, local geography, and tidal influences, playing a vital role in shaping coastal landscapes.
Rip Current: A rip current is a strong, narrow channel of water that flows away from the shore, often formed when water from breaking waves returns to the ocean. These currents can pose significant risks to swimmers, as they can pull individuals out to deeper waters with considerable force. Understanding rip currents is essential for recognizing their role in coastal dynamics and sediment transport processes.
Sand: Sand is a granular material composed of finely divided rock and mineral particles, typically ranging in size from 0.0625 to 2 millimeters. In the context of coastal currents and sediment transport, sand plays a vital role in shaping shorelines, supporting ecosystems, and influencing human activities such as beach nourishment and coastal engineering.
Sediment budget: A sediment budget is the accounting of sediment volumes within a particular area over a specific time frame, essentially tracking the sources, transport pathways, and sinks of sediment. This concept is crucial for understanding landscape evolution and coastal dynamics as it helps assess how sediment is redistributed due to natural processes and human activities. By analyzing sediment budgets, scientists can predict changes in landforms, sediment supply, and the impact of factors like erosion and deposition on ecosystems.
Sediment size distribution: Sediment size distribution refers to the range and proportion of different particle sizes present in a sediment sample. This distribution is crucial in understanding how sediments are transported and deposited in coastal environments, particularly as influenced by coastal currents. The specific sizes and proportions of sediments can determine the behavior of sediment transport processes, influencing shoreline stability, habitat availability, and the formation of various sedimentary features.
Sediment transport model: A sediment transport model is a mathematical or computational framework used to simulate and predict the movement of sediment in various environments, including rivers, coastal areas, and oceanic systems. These models are essential for understanding how sediment is mobilized, transported, and deposited, which is crucial for coastal management, habitat restoration, and assessing the impacts of human activities and natural events on sediment dynamics.
Shallow water wave theory: Shallow water wave theory describes the behavior of waves in water that is shallower than half their wavelength. This theory explains how waves interact with the seabed, leading to changes in their speed, height, and direction as they approach the coast. Understanding this theory is crucial for analyzing coastal currents and sediment transport, as these factors are influenced by wave dynamics and energy dissipation.
Silt: Silt is a fine-grained sediment, ranging in size from 0.002 to 0.05 millimeters, that plays a crucial role in various geological and environmental processes. It is smaller than sand but larger than clay, making it a key component in soil composition and sediment transport. Its ability to retain moisture and nutrients contributes to fertile soils, particularly in areas influenced by riverine and coastal dynamics.
Suspension: Suspension refers to a mixture where solid particles are dispersed throughout a fluid but are not dissolved, allowing them to remain suspended in the liquid or gas. In natural systems, this concept is crucial for understanding how sediments are transported by water, wind, and coastal currents, impacting geomorphological processes and ecosystem dynamics.
Swash and Backwash: Swash refers to the movement of water up the beach after a wave breaks, while backwash is the water that flows back down the beach into the ocean after the swash has occurred. These two processes are integral to coastal dynamics, affecting sediment transport and shoreline shape as they work in tandem to reshape the beach environment with each wave cycle.
Tides: Tides are the regular rise and fall of sea levels caused by the gravitational forces exerted by the Moon and the Sun, combined with the Earth's rotation. These oscillations lead to periodic changes in water levels along coastlines, which play a crucial role in shaping coastal environments and influencing sediment transport processes.
Wave action: Wave action refers to the energy and movement generated by waves as they break upon the shoreline, influencing coastal processes such as erosion, sediment transport, and the formation of landforms. This dynamic interaction between waves and the coastal environment plays a crucial role in shaping coastlines, redistributing sediments, and contributing to the development of various coastal features.
Wave refraction: Wave refraction is the bending of waves as they approach shallow water or obstacles, causing them to change direction. This phenomenon occurs because waves travel at different speeds in varying water depths, leading to alterations in their energy distribution and shoreline impact. Understanding wave refraction is crucial for analyzing coastal dynamics and sediment transport processes, as it influences how waves interact with the coastline and how sediment is moved along the shore.