Geoscience Reference
In-Depth Information
Drift
Longshore
Estuary
River
Cliff
Delta
Offshore
loss/gain
Transgressive
dune
In situ
coastal
production
Dune
Inlet
Nourishment
Extraction
Beachridge plain
Longshore
River input
Cliff erosion
Onshore/
Offshore
Inlet exchange
Lagoon
Nourishment
Urban
Groynes
Fig. 8.3
Diagrammatic representation of coastal sediment
budget showing main sources and exchanges both natural and
anthropogenic.
Dam
Drift
during which sediment is reworked by coastal
processes (Cooper 1993).
The continental shelf is a vital sediment source
for many temperate coastlines. Here relict ter-
restrial, marine or coastal sediments from lower
sea-levels or past glaciations may be reworked
landward by contemporary wave action. Off-
shore sediment abundance was accompanied by
millennial-scale coastal progradation in south-
east Australia (Thom 1984) and decadal-scale
beach ridge accumulation in south-east Spain
(Goy et al. 2003). In conditions of sediment
scarcity, wave action may, however, erode the
sea-floor to transport older deposits to the shore-
line. In North Carolina, grains eroded from
underlying Tertiary lithologies are a component
of contemporary beach sediment (Pilkey et al.
1998). Sediment transport on the shelf varies
according to wave conditions and during storm-
wave action extends to deeper water. Thus the
shelf sediment supply may also be temporally
variable (see Chapter 10).
The coastline itself is a source of sediment
to adjacent beaches. Often, longshore drift of
sand from adjacent beaches and cliffs is a major
element in the sediment supply, although this is
diminished in strongly embayed coasts. Erosion
of relict glacial deposits yields abundant gravel
and boulder beaches in the higher latitudes
(Davies 1980). Elsewhere, the lithology and
texture of coastal outcrops strongly influence
the nature of eroded clasts that supply beaches.
In southern England, for example, the prefer-
ential preservation of chert (flint) clasts eroded
from less resistant limestone (chalk) is marked
(Carr 1969). The supply of sediment is not con-
stant and is controlled to a large extent by patterns
of slope failure on coastal cliffs, and the capacity
of waves to transport sediment. Slope failure
depends on a range of factors including the
porosity and permeability of the rock, stratifica-
tion, rainfall intensity and the extent of wave
undercutting (Emery & Kuhn 1982). Often sedi-
ment input is in the form of large-scale landslides
