Coastal and Aeolian Processes

Coastlines are dynamic environments shaped by the constant movement of sediment. The primary driver of this movement is Longshore Drift (LSD). When waves approach a beach at an oblique angle—determined by the prevailing wind—the swash (the water rushing up the beach) carries sediment at that same angle. However, the backwash (the water returning to the sea) pulls sediment straight down the beach slope due to gravity. This creates a zigzag motion that transports sand and pebbles along the coast. Over time, this process can move millions of tons of material, significantly altering beach morphology and creating new landforms where the coastline changes direction.

High-energy coastlines with resistant rock headlands undergo a predictable sequence of erosion. It begins with hydraulic action and abrasion attacking faults or joints in the rock. This creates a cave. When caves on opposite sides of a headland meet, or a single cave is bored through, an arch is formed. As the arch is widened, the roof becomes unstable and collapses under its own weight, leaving an isolated pillar of rock known as a stack. Eventually, marine erosion at the base causes the stack to collapse, leaving a small, low-lying stump that may only be visible at low tide.

In arid regions, wind (aeolian) processes replace water as the primary agent of change. Wind transports sediment through three main mechanisms based on particle size and wind velocity. Suspension occurs when very fine particles (less than $0.05\text{ mm}$) are lifted high into the atmosphere and carried long distances. Saltation is the most common form, where medium-sized grains ($0.1$ to $0.5\text{ mm}$) 'bounce' along the surface. When these bouncing grains hit others, they trigger surface creep, where larger particles are pushed or rolled along the ground. The efficiency of this transport is governed by the threshold velocity—the minimum wind speed required to start moving particles.