nearshore or low-tidal biota (and what is delivered to the
beach as shell and plant matter), the type of macrobiota
resident under and on the beach, the type of microbiota
resident under and on the beach, the amount of sediment
sheetwash delivered onto the beach from the adjoining
upland, the amount of freshwater seepage onto the beach
face, salinity of the estuarine waters, the chemistry of the
groundwater and pore waters under the beach
(dependent on estuarine setting), and the hydrological
(groundwater) through-flow.
These physical processes, biological processes, and
chemical processes and their products are described
below. Which process(es) on or under the beach is/are
dominant is determined by where the beach is located in
the estuary, how active are the hydrodynamic processes,
the extent of mobility of the sediments (and therefore to
what extent the organic matter is turned over), the amount
of influx of organic matter, and the extent of oxidation of
the sediments.
Sandy beaches commonly exhibit gradients normal to
their shore, e.g., a gradient in inundation and evaporation,
with attendant gradients in wave energy and tidal energy,
and hence a graded expression of the processes of sedimentation, erosion, and hydrochemical effects (Brocx
and Semeniuk, 2009). This results in variable, complex,
and diverse physical, biological, and geochemical products across the shore and variation in fine- to small-scale
stratigraphic sequences.
The physical processes on beaches are wave action by
prevailing wind waves and by oceanic waves propagating
through the estuary mouth, wind-generated currents, tidal
currents with maximum currents during spring tides and
lower-velocity currents during neap tides, formation of
cliffs and cusps, wind activity acting on dry beach surfaces
and on wet beach surface, storms resulting in chaotic wave
trains and waves often with elevated water levels, evaporation, freshwater seepage into the beach and freshwater
upwelling from under the beach, gas upwelling from
under the beach, rainfall effects (such as rain infiltration),
and wave-swash infiltration.
Wave action, tidal currents, and storms are involved in
sedimentation processes to develop beach sand lamination. Wave action, tidal currents, and storms transport sediment, and, depending on the wave energy, tidal-current
velocity, and degree of storm activity, they sort and separate sediments into grain-sized suites of sand with grains
of similar specific gravity (a monomineralic sand,
siliciclastic sand of quartz and feldspar grains, or
siliciclastic sand and carbonate sand) or hydraulically
equivalent suites (e.g., fine sand-sized grains of magnetite
as spheres with specific gravity of 5.2 are hydraulically
equivalent to medium sand-sized quartz and feldspar
grains as spheres with specific gravity of 2.6, and
2.6–2.7, respectively; Tourtelout, 1968; Selley, 2000).
Wave action, tidal currents, and storms also transport and
sort shells into size-graded and oriented accumulations
(Behrens and Watson, 1969; Reineck and Singh, 1980).
With run-up and backflow during wave action on
a beach slope, sediments, once sorted, are deposited as
granulometrically distinct and/or compositionally distinct
laminae that, with accretion, result in laminated beach
sand with laminae alternating in grain sizes, grain-sized
suites, or in composition (e.g., quartz fine sand laminae
alternating with quartz medium sand laminae, or with
mixed quartz fine to medium sand laminae, or with
grain-thick micro-laminae of rutile very fine sand or silt).
Wave action and tidal currents, during the high tide when
the beach slope is inundated, winnow the sand of the
beach slope leaving a lamination-scale lag of quartz
medium sand and coarse sand and laminae of opaque
(heavy) minerals such as rutile, tourmaline, and magnetite.
Where there is shell, or shell fragments, the action of
waves, tides, and storm waves can concentrate these particles leaving laminae of shell, shell fragments, and shell
grit within the sand laminae. As such, with accretion, the
beach is underlain by laminated sand, with lamination
defined by grain-sized variation, shell layers, shell grit
and fragments, and laminae of opaque minerals (heavy
minerals). Lamination that is formed by waves, tides,
and storms under the beach slope is parallel to the sloping
surface so that, with beach-slope accretion, the laminations of the sand in the beach environment are inclined
towards the estuary.
With a change of season and change in wave dynamics,
or in the change from spring tide to neap tide, or with
storms where water levels are higher than normal and
wave action is intense, chaotic, and short-term repetitive,
the beach slope (with erosion or accretion) can change
its inclination. Where such erosion is followed by accretion, the erosional surface is marked as a horizon of truncation of the underlying inclined lamination and
accretion of the additional laminated sediment takes place
parallel to the horizon of truncation. These horizons of
truncation are preserved as bedding discontinuities in the
small-scale stratigraphic record. Where there are small
channels or basins eroded into the sloping beach (scour
is effected by wave run-off, formation of beach cusps,
tidal drainage run-off, low-tidal seepage from the beach
slope, and freshwater seepage channels), followed by filling and accretion of these channels and scours by later
laminated sand, there is the development of small-scale
(10–50 cm wide and 5–20 cm deep) cut-and-fill
structures.
During storms, or periods of intense and sustained
wave action that may be atypical of prevailing conditions,
or during the change in water level from spring to neap
tide, or a change in the wave climate inter-seasonally, the
beach may erode to form a steeper beach slope or to form
a cliff (Figure 6). For such beaches, the steeper beach
slope is reflected in a change in dip of layering and lamination with a pre-erosion set of lamination less inclined
than the post-erosion set of lamination. The interface
between the sets of lamination can be marked by a lag
deposit of shells or pebbles (Figure 6). Where a cliff has
BEACH PROCESSES
63
beach as shell and plant matter), the type of macrobiota
resident under and on the beach, the type of microbiota
resident under and on the beach, the amount of sediment
sheetwash delivered onto the beach from the adjoining
upland, the amount of freshwater seepage onto the beach
face, salinity of the estuarine waters, the chemistry of the
groundwater and pore waters under the beach
(dependent on estuarine setting), and the hydrological
(groundwater) through-flow.
These physical processes, biological processes, and
chemical processes and their products are described
below. Which process(es) on or under the beach is/are
dominant is determined by where the beach is located in
the estuary, how active are the hydrodynamic processes,
the extent of mobility of the sediments (and therefore to
what extent the organic matter is turned over), the amount
of influx of organic matter, and the extent of oxidation of
the sediments.
Sandy beaches commonly exhibit gradients normal to
their shore, e.g., a gradient in inundation and evaporation,
with attendant gradients in wave energy and tidal energy,
and hence a graded expression of the processes of sedimentation, erosion, and hydrochemical effects (Brocx
and Semeniuk, 2009). This results in variable, complex,
and diverse physical, biological, and geochemical products across the shore and variation in fine- to small-scale
stratigraphic sequences.
The physical processes on beaches are wave action by
prevailing wind waves and by oceanic waves propagating
through the estuary mouth, wind-generated currents, tidal
currents with maximum currents during spring tides and
lower-velocity currents during neap tides, formation of
cliffs and cusps, wind activity acting on dry beach surfaces
and on wet beach surface, storms resulting in chaotic wave
trains and waves often with elevated water levels, evaporation, freshwater seepage into the beach and freshwater
upwelling from under the beach, gas upwelling from
under the beach, rainfall effects (such as rain infiltration),
and wave-swash infiltration.
Wave action, tidal currents, and storms are involved in
sedimentation processes to develop beach sand lamination. Wave action, tidal currents, and storms transport sediment, and, depending on the wave energy, tidal-current
velocity, and degree of storm activity, they sort and separate sediments into grain-sized suites of sand with grains
of similar specific gravity (a monomineralic sand,
siliciclastic sand of quartz and feldspar grains, or
siliciclastic sand and carbonate sand) or hydraulically
equivalent suites (e.g., fine sand-sized grains of magnetite
as spheres with specific gravity of 5.2 are hydraulically
equivalent to medium sand-sized quartz and feldspar
grains as spheres with specific gravity of 2.6, and
2.6–2.7, respectively; Tourtelout, 1968; Selley, 2000).
Wave action, tidal currents, and storms also transport and
sort shells into size-graded and oriented accumulations
(Behrens and Watson, 1969; Reineck and Singh, 1980).
With run-up and backflow during wave action on
a beach slope, sediments, once sorted, are deposited as
granulometrically distinct and/or compositionally distinct
laminae that, with accretion, result in laminated beach
sand with laminae alternating in grain sizes, grain-sized
suites, or in composition (e.g., quartz fine sand laminae
alternating with quartz medium sand laminae, or with
mixed quartz fine to medium sand laminae, or with
grain-thick micro-laminae of rutile very fine sand or silt).
Wave action and tidal currents, during the high tide when
the beach slope is inundated, winnow the sand of the
beach slope leaving a lamination-scale lag of quartz
medium sand and coarse sand and laminae of opaque
(heavy) minerals such as rutile, tourmaline, and magnetite.
Where there is shell, or shell fragments, the action of
waves, tides, and storm waves can concentrate these particles leaving laminae of shell, shell fragments, and shell
grit within the sand laminae. As such, with accretion, the
beach is underlain by laminated sand, with lamination
defined by grain-sized variation, shell layers, shell grit
and fragments, and laminae of opaque minerals (heavy
minerals). Lamination that is formed by waves, tides,
and storms under the beach slope is parallel to the sloping
surface so that, with beach-slope accretion, the laminations of the sand in the beach environment are inclined
towards the estuary.
With a change of season and change in wave dynamics,
or in the change from spring tide to neap tide, or with
storms where water levels are higher than normal and
wave action is intense, chaotic, and short-term repetitive,
the beach slope (with erosion or accretion) can change
its inclination. Where such erosion is followed by accretion, the erosional surface is marked as a horizon of truncation of the underlying inclined lamination and
accretion of the additional laminated sediment takes place
parallel to the horizon of truncation. These horizons of
truncation are preserved as bedding discontinuities in the
small-scale stratigraphic record. Where there are small
channels or basins eroded into the sloping beach (scour
is effected by wave run-off, formation of beach cusps,
tidal drainage run-off, low-tidal seepage from the beach
slope, and freshwater seepage channels), followed by filling and accretion of these channels and scours by later
laminated sand, there is the development of small-scale
(10–50 cm wide and 5–20 cm deep) cut-and-fill
structures.
During storms, or periods of intense and sustained
wave action that may be atypical of prevailing conditions,
or during the change in water level from spring to neap
tide, or a change in the wave climate inter-seasonally, the
beach may erode to form a steeper beach slope or to form
a cliff (Figure 6). For such beaches, the steeper beach
slope is reflected in a change in dip of layering and lamination with a pre-erosion set of lamination less inclined
than the post-erosion set of lamination. The interface
between the sets of lamination can be marked by a lag
deposit of shells or pebbles (Figure 6). Where a cliff has
BEACH PROCESSES
63