Coastal Geomorphology and Sediment Transport

Coastlines are restless landscapes, continually built up and worn away as waves and currents move sand and gravel along the shore, sculpting beaches, spits, barrier islands, and cliffs.

Definition

Coastal geomorphology is the study of the landforms of the coast and the processes that shape them; coastal sediment transport is the movement of sand, gravel, and finer material by waves, currents, and wind in the nearshore zone.

Scope

This topic covers the wave and current processes that mobilize coastal sediment, longshore and cross-shore transport, the sediment budget concept, the morphodynamics of beaches and bars, the formation of depositional landforms such as spits and barrier islands, and the erosion of cliffs and shorelines.

Core questions

• How do waves and currents mobilize and transport sediment in the nearshore?
• What is the difference between longshore and cross-shore sediment transport?
• How does the sediment budget determine whether a coast accretes or erodes?
• How do depositional landforms such as beaches, spits, and barrier islands form and evolve?

Key theories

Longshore transport and the sediment budget

Waves approaching at an angle drive a longshore current and transport of sediment, and the balance of sediment inputs and outputs (the budget) determines whether a stretch of coast grows or erodes.

Beach morphodynamics

Beaches adjust their profile and form through two-way feedback between waves, currents, and the evolving sediment surface, ranging from reflective to dissipative states depending on wave energy and sediment size.

Mechanisms

Breaking waves suspend and stir sediment, while the longshore current generated by oblique waves carries it along the coast and cross-shore flows move it between the beach and offshore bars. Where the sediment supply exceeds losses, beaches, spits, and barriers build out; where losses dominate, shorelines and cliffs erode, with the balance captured by the coastal sediment budget.

Clinical relevance

Understanding coastal sediment transport is essential for managing beach erosion, designing coastal defenses and nourishment schemes, maintaining navigable channels, and anticipating how shorelines will respond to storms and sea-level rise.

History

Quantitative coastal sediment science developed from the mid-twentieth century through field and laboratory studies by Inman, Komar, and others on longshore transport and beach processes, and through concepts such as the Bruun rule linking shoreline retreat to sea-level rise.

Key figures

• Douglas Inman
• Paul Komar
• Per Bruun

Seminal works

• masselink2011
• komar1998

Frequently asked questions

What is longshore drift?

It is the movement of sediment along a coast caused by waves arriving at an angle, which push sand up the beach obliquely and let it return straight down, producing a net transport parallel to the shore.

Why do beaches change so much between seasons?

Stormy seasons with high-energy waves tend to erode sand offshore into bars, leaving narrow beaches, while calmer seasons return sand to build wider beaches, so a beach's shape reflects recent wave conditions.