The work of the sea - Fundamentals of the Physical Environment - page 549

marsh during the tidal cycle. The upper reed marsh (right) is
inundated less often and grades inland into a freshwater
marsh.
Photo: Ken Addison.
Coast-parallel nearshore bars form on dissipative beaches and offshore at zones of either
lower water velocity or higher sediment concentration. They normally form as single or
multiple low ridges parallel to the coast, or in crescentic form as linked beach-cusp bars,
broken by backwash or rip channels. Bars may be stable in the energy
Figure 17.10 Beach-dune system morphology.
Source: Partly after Goudie (1995).
environment at which they form, absorbing 80-100 per cent of wave energy, but unstable
in any other when they wash out or migrate. Bars accentuate as well as respond to
longshore currents and are capable of extending across coastal embayments and estuaries
as spits in the direction of longshore drift. They also connect islands to mainland with
tombolos such as Chesil Beach, linking the Isle of Portland to the Dorset coast of
southern England.
Dune systems develop by deflation of dry sand from the backshore landwards and, as
with estuarine landforms, develop in association with vegetation succession (see Chapter
16). Biogenic processes assume even greater importance on some tropical coasts. In
addition to the role of biogenic debris and bioherms in lagoons, described earlier, reefs
form more permanent wave-resistant and biomorphological structures 10 1-2 m thick, 10 1
km wide and 10 1-3 km long. Living reef corals, etc., grow on the cemented debris of dead
organisms and in that way can contend with slow rates of sea-level change. Despite this,
they are sensitive to other conditions and have a predominantly tropical distribution today
away from the turbidity of terrigenous sediment fluxes. Two main forms exist. Fringing
reefs weld themselves to the shore, whilst barrier reefs parallel the shore beyond an
impounded lagoon. The Great Barrier Reef off Queensland (Australia) is the modern