Sediment cells function as largely self-contained systems, comprising zones of erosion, transport, and deposition of sediment along a coastline.
Here's a breakdown of how they work:
- Erosion Zone: This is where sediment is sourced. Common examples include:
- Coastal bluffs and cliffs: Wave action and weathering erode these areas, releasing sediment into the system.
- River mouths: Rivers carry sediment from inland, which is then deposited into the coastal environment.
- Transport Zone: This zone involves the movement of sediment. Sediment is carried along the coast by:
- Longshore currents: These currents run parallel to the shoreline and are responsible for transporting sand along beaches.
- Tidal currents: These currents can transport sediment into and out of inlets and estuaries.
- Wind: Wind can directly transport sand, particularly in dune systems.
- Deposition Zone: Here, sediment accumulates. Examples include:
- Beaches: Waves deposit sand, creating beaches.
- Spits: Long, narrow ridges of sand extending from the mainland, formed by longshore drift.
- Salt marshes and mudflats: Fine-grained sediment settles in these sheltered areas.
Sediment Budget: Within a sediment cell, there's a sediment budget, which is the balance between sediment inputs (erosion and river input) and outputs (deposition and loss offshore or to inlets). A positive budget leads to accretion (growth), while a negative budget leads to erosion.
Containment: Sediment cells are considered relatively self-contained because much of the sediment remains within the cell. However, there are losses:
- Inlets and estuaries: Some sediment may be transported into inlets and estuaries, effectively removing it from the coastal system.
- Offshore: Fine sediment can be carried offshore by tides or storms.
- Human Intervention: Coastal defenses (like groynes and sea walls) can interrupt the natural flow of sediment, impacting the sediment budget in a cell and affecting down-drift areas.
In essence, a sediment cell is a dynamic system where sediment is constantly being eroded, transported, and deposited, shaping the coastline. The processes within a sediment cell are interdependent. Changes in one zone can affect the others.
