The sloping sandy bank of a river is also a location for
the development of a beach. Here, the beach-constructing
agencies are mainly river current, estuarine wind waves,
with lesser effect from tidal currents. These beaches are
mainly sandy and locally gravelly. Since these sandy
banks are usually protected, in tropical regions, they often
support stands of mangroves.
An important aspect of some estuaries is the occurrence
of a peat-filled lagoon leeward of a shore-parallel barrier
(such as a prograded ribbon of sand or a prograded narrow
beach ridge system). With high freshwater levels in the
lagoon after rain or floods, freshwater from the peat-filled
lagoon may discharge from under the beach slope of the
most seaward beach ridge.
Sediment sources
There are four sources of sediment particles that comprise
estuarine beaches; these are (1) rivers, (2) estuarine biota,
(3) margins of the estuary, (4) preexisting estuarine (relict)
estuarine sediment, and (5) marine sources. These sedimentary particles form the “raw material” (viz., the sediment) upon which the physical, biological, and chemical
processes in the estuary in the beach environment will
act and imprint various structures and products.
River sediment, sediment from the margins of an estuary, and marine sediment are exogenic, deriving from outside the estuarine basin and being delivered to the estuary.
Estuarine biota generates endogenic sediment.
Rivers deliver sand, some gravel, and mud, with sand
and mud being the dominant particle types. The margins
of the estuary through erosion by waves, tidal currents,
wind, and (rain) sheetwash provide sand, mud, and gravel.
Depending on the type of upland, i.e., whether it is rocky,
sandy, or muddy, the sediment eroded into the margin of
the estuary by waves, tides, rain, and storms varies from
coarse, medium, and fine sand to gravel, mud, or mixtures
of these. Preexisting estuarine (relict) sediment is sediment within the estuary relict from an earlier depositional
phase that is remobilized by waves, tides, and storms and
transported onto the beach. Though it may comprise
a range of grain sizes such as sand, mud, and (rock and/or
shell) gravel, only sand, rock, gravel, and shell are of relevance to estuarine beaches. Delivery of marine sediment
to the estuary (and specifically to estuarine beaches) can
be complicated and occurs in three pathways: (1) through
the entrance or mouth of the estuary by tidal currents,
wind-induced currents, and waves; this sediment is coarse,
medium-to-fine sand, rock gravel, and shell; and this
source also includes the floating mollusks such as Spirula
and cuttlefish; (2) initially accumulating from the marine
environment as a dune barrier which, on its leeward margin, is then eroded by estuarine processes of waves, tides,
and sheetwash into the estuarine shore zone; this sediment
usually is medium and fine sand; and (3) aeolian transport
from the dune barrier into the estuarine environment; this
sediment is usually fine sand.
Endogenic biogenic sediment in an estuary is variable
depending on biogeography, climate, and salinity of the
environment. At macrofaunal scale, it ranges from molluskan shell and their fragments (deriving from benthic infaunal bivalves, epifaunal gastropods, and oysters that
colonize rocks or form biostromes, mussels that also colonize rocks or form biostromes), crustacean fragments,
ostracods, and minor components such as bryozoans,
echinoderms, hydrozoans, and sponges. Shelly fauna that
contributes shell to the sediment in an estuary varies in
composition according to their location in terms of salinity
and the variability of salinity (Day, 1981): (1) near the
estuary mouth where the environment is marine, organisms are stenohaline marine in character; (2) in
mid-estuarine environments, biota is truly estuarine or
euryhaline marine in composition (the latter are species
represented in the marine environment but which tolerate
salinities of 5–50 %); (3) in the river mouth and river
channel where it is freshwater, biota is euryhaline freshwater (i.e., species primarily are freshwater types, but some
tolerate salinities greater than 5 %) or stenohaline freshwater. At microbiota scale, biota that contributes to estuarine sediment includes foraminifera and diatoms. Some of
these biotas inhabit beach environments and so directly
contribute to skeletal accumulation in situ, but a majority
live externally to the beach and are transported to the
beach site.
Seagrass; algal beds; algal mats; saltmarsh; mangroves
logs, stumps, and branches; mangrove leaves; and terrestrial wood and logs washed into the estuary from rivers
also contribute to beach sediment. Seagrass and algal beds
such as Heterozostera, Halophila, Ruppia, Zostera,
Chaetomorpha, Fucus, Gracilaria, and Ulva occur in the
intertidal or shallow subtidal zone and are delivered to
beaches by waves, tidal currents, and storms. Leaves of
saltmarsh species and mangroves and wood are washed
onto the beach from alongshore (and, for saltmarsh, also
from behind the beach), particularly during storms.
Although not growing on the beach, estuarine plants from
these various intertidal and subtidal environments are
transported from their habitat to accumulate on the beach
as “flotsam and jetsam” (also termed “wrack”), as
scattered plant fragments or as organic beds (incipient
peat) of plant matter some 10–50 cm thick. Most typically,
plant material from their respective habitats occurs as
scattered fragments and detritus on the beach slope to be
buried later in the beach sediment.
The physical, biological, and chemical processes
operating on an estuarine beach
Processes operating on a beach can be categorized as
physical, biological, or chemical. Many of the processes
on beaches are ubiquitous within beaches throughout an
estuary, but there is a distinct suite of processes in different
parts of the estuary, depending on the extent of wave
action, tidal currents and river currents, wind, the
62
BEACH PROCESSES
the development of a beach. Here, the beach-constructing
agencies are mainly river current, estuarine wind waves,
with lesser effect from tidal currents. These beaches are
mainly sandy and locally gravelly. Since these sandy
banks are usually protected, in tropical regions, they often
support stands of mangroves.
An important aspect of some estuaries is the occurrence
of a peat-filled lagoon leeward of a shore-parallel barrier
(such as a prograded ribbon of sand or a prograded narrow
beach ridge system). With high freshwater levels in the
lagoon after rain or floods, freshwater from the peat-filled
lagoon may discharge from under the beach slope of the
most seaward beach ridge.
Sediment sources
There are four sources of sediment particles that comprise
estuarine beaches; these are (1) rivers, (2) estuarine biota,
(3) margins of the estuary, (4) preexisting estuarine (relict)
estuarine sediment, and (5) marine sources. These sedimentary particles form the “raw material” (viz., the sediment) upon which the physical, biological, and chemical
processes in the estuary in the beach environment will
act and imprint various structures and products.
River sediment, sediment from the margins of an estuary, and marine sediment are exogenic, deriving from outside the estuarine basin and being delivered to the estuary.
Estuarine biota generates endogenic sediment.
Rivers deliver sand, some gravel, and mud, with sand
and mud being the dominant particle types. The margins
of the estuary through erosion by waves, tidal currents,
wind, and (rain) sheetwash provide sand, mud, and gravel.
Depending on the type of upland, i.e., whether it is rocky,
sandy, or muddy, the sediment eroded into the margin of
the estuary by waves, tides, rain, and storms varies from
coarse, medium, and fine sand to gravel, mud, or mixtures
of these. Preexisting estuarine (relict) sediment is sediment within the estuary relict from an earlier depositional
phase that is remobilized by waves, tides, and storms and
transported onto the beach. Though it may comprise
a range of grain sizes such as sand, mud, and (rock and/or
shell) gravel, only sand, rock, gravel, and shell are of relevance to estuarine beaches. Delivery of marine sediment
to the estuary (and specifically to estuarine beaches) can
be complicated and occurs in three pathways: (1) through
the entrance or mouth of the estuary by tidal currents,
wind-induced currents, and waves; this sediment is coarse,
medium-to-fine sand, rock gravel, and shell; and this
source also includes the floating mollusks such as Spirula
and cuttlefish; (2) initially accumulating from the marine
environment as a dune barrier which, on its leeward margin, is then eroded by estuarine processes of waves, tides,
and sheetwash into the estuarine shore zone; this sediment
usually is medium and fine sand; and (3) aeolian transport
from the dune barrier into the estuarine environment; this
sediment is usually fine sand.
Endogenic biogenic sediment in an estuary is variable
depending on biogeography, climate, and salinity of the
environment. At macrofaunal scale, it ranges from molluskan shell and their fragments (deriving from benthic infaunal bivalves, epifaunal gastropods, and oysters that
colonize rocks or form biostromes, mussels that also colonize rocks or form biostromes), crustacean fragments,
ostracods, and minor components such as bryozoans,
echinoderms, hydrozoans, and sponges. Shelly fauna that
contributes shell to the sediment in an estuary varies in
composition according to their location in terms of salinity
and the variability of salinity (Day, 1981): (1) near the
estuary mouth where the environment is marine, organisms are stenohaline marine in character; (2) in
mid-estuarine environments, biota is truly estuarine or
euryhaline marine in composition (the latter are species
represented in the marine environment but which tolerate
salinities of 5–50 %); (3) in the river mouth and river
channel where it is freshwater, biota is euryhaline freshwater (i.e., species primarily are freshwater types, but some
tolerate salinities greater than 5 %) or stenohaline freshwater. At microbiota scale, biota that contributes to estuarine sediment includes foraminifera and diatoms. Some of
these biotas inhabit beach environments and so directly
contribute to skeletal accumulation in situ, but a majority
live externally to the beach and are transported to the
beach site.
Seagrass; algal beds; algal mats; saltmarsh; mangroves
logs, stumps, and branches; mangrove leaves; and terrestrial wood and logs washed into the estuary from rivers
also contribute to beach sediment. Seagrass and algal beds
such as Heterozostera, Halophila, Ruppia, Zostera,
Chaetomorpha, Fucus, Gracilaria, and Ulva occur in the
intertidal or shallow subtidal zone and are delivered to
beaches by waves, tidal currents, and storms. Leaves of
saltmarsh species and mangroves and wood are washed
onto the beach from alongshore (and, for saltmarsh, also
from behind the beach), particularly during storms.
Although not growing on the beach, estuarine plants from
these various intertidal and subtidal environments are
transported from their habitat to accumulate on the beach
as “flotsam and jetsam” (also termed “wrack”), as
scattered plant fragments or as organic beds (incipient
peat) of plant matter some 10–50 cm thick. Most typically,
plant material from their respective habitats occurs as
scattered fragments and detritus on the beach slope to be
buried later in the beach sediment.
The physical, biological, and chemical processes
operating on an estuarine beach
Processes operating on a beach can be categorized as
physical, biological, or chemical. Many of the processes
on beaches are ubiquitous within beaches throughout an
estuary, but there is a distinct suite of processes in different
parts of the estuary, depending on the extent of wave
action, tidal currents and river currents, wind, the
62
BEACH PROCESSES