8.1 Wave transportation and deposition

demi b • 9 February 2019

Wave transportation and deposition

Sediment sources are varied and include: l onshore transport by waves
l longshore transport by waves
l rivers

l glacial and periglacial deposits l wind-blown deposits
l artificial beach replenishment

Sediment transport is generally categorised into two modes:
l Bedload – grains transported by bedload are moved with continuous contact

(traction or dragging) or by discontinuous contact (saltation) with the

seafloor. Traction, in which grains slide or roll along, is a slow form of transport. l Suspended load – grains are carried by turbulent flow and generally are

held up by the water. Suspension occurs when moderate currents are transporting silts or strong currents are transporting sands.

Deposition is governed by sediment size (mass) and shape. In some cases sediments will flocculate (stick together), become heavier and fall out in deposition.

Sediment cells

The coastal sediment system, or littoral cell system, is a simplified model that examines coastal processes and patterns in a given area (Figure 8.5). It operates at a variety of scales from a single bay, e.g. Turtle Bay, North Queensland, Australia, to a regional scale, e.g. the south California coast. Each littoral cell is self-contained, with inputs and outputs balanced.

If refraction is not complete, longshore drift occurs (Figure 8.6). This leads to
a gradual movement of sediment along the shore, as the swash moves in the direction of the prevailing wind, whereas the backwash moves straight down the beach, following the steepest gradient.