water can be referred to as the phreatic zone, where it
resides in the pore spaces of the sediments; and (2) water
films circumferential to sand particles in the wetted but
undersaturated sediment above the water table during
low tide; this wet zone can be referred to as the temporary
vadose zone and is tidal-flat pellicular water. The surface
of the phreatic zone is the groundwater table during low
tides. This groundwater table rises and falls with the tides.
The phreatic zone determines many of the biological and
chemical processes operating under the beach, and the
vadose zone during a low tide determines many of the
other biological and chemical processes operating under
the beach. Depending on the depth to the water table during the period of low tide, the beach may be wet (where the
water table is near the surface) or moistened by water rising by capillary action, or may be relatively dry (where the
water table is decimeters below the surface).
Location of estuarine beaches
As mentioned earlier, there are nine different environments within an estuary where beaches can form. Therefore, location will determine the suite of processes that
combine to form a beach in the first place, the type of sediments that comprise the beaches, and the types of processes that operate postdepositionally on beach sediments.
The most common locations for beaches are the shores
of the margins of the interior of an estuary and generally in
the central parts of estuaries (i.e., not deltas, spits,
cheniers, shoals, and the estuarine mouth). Here, the
shores are usually sandy, with sand derived from alongshore, washed up from subtidal zones, reworked from
the uplands, or delivered from the marine environment.
In microtidal and mesotidal settings, the beach is fronted
by low-tidal sand flats. Gravel sources and any eroding
rock in estuarine shore environments result in gravelly
beaches. Where beaches are bordered by sandy tidal flats
and skeletons of shelly benthos are transported onto the
beach by waves and storms to form shelly sand, or shell
gravel lags, or shell gravel lenses.
The next most common site for beach development is
the leeward margin of barrier dunes. Here again, the
shores are usually sandy with sand eroded from the dunes,
or derived from alongshore, or washed up from subtidal
zones. In microtidal and mesotidal settings, such beaches
are fronted by low-tidal sand flats which supply shell
gravel and shell grit to the beach to form shelly sand, or
shell gravel lags, or shell gravel lenses.
The shores of mid-estuarine-emergent shoals and
islands also are common sites for the development of
beaches. Because shoals and islands present differing
aspects to prevailing wind-wave fields, and to wind, there
is asymmetry in the suite of processes and in the products
developed. Beaches directly facing prevailing waves will
have different profiles to those on leeward sides of shoals
and islands, and similarly, the sediment response within
a beach in terms of lithology, granulometry, and
stratigraphic organization will differ from windward side
to leeward side of the shoal or island. Beaches peripheral
to shoals and sand islands are commonly sandy, while
those peripheral to an island of rock can have sandy, gravelly sand, and gravelly beaches.
Beaches can be developed at the mouth of an estuary
and along the margins of tidal exchange channels.
Beaches at the mouth of an estuary are subject to processes
similar to that of open coastal beaches, though the former
are more protected; these processes include oceanic wave
action, wind-wave action, tides, and wind. These beaches
may be backed by low beach ridges built by the prevailing
onshore winds. The sediment responses within such
beaches in terms of lithology, granulometry, and stratigraphic organization are similar to open coastal beaches
and include a larger proportion of floating debris derived
from marine sources. Beaches along the margins (banks)
of tidal exchange channels are also subject to processes
of oceanic wave action, wind-wave action, tides, and
wind. Orientation of the channel to the ocean wave field
determines how much wave action is involved in shaping
the beach morphology and lithology, and, in this context,
ebb and flood-tidal currents are more important in that
their effects are magnified in (relatively) narrow channels.
These shorelines also may be backed by low beach ridges
built by the prevailing winds. Beaches at the mouth of
estuaries and along the margins of tidal exchange channels
are most commonly sandy.
Beaches developed along the shores of spits and
cheniers are similar, though these coastal landforms
develop in different locations within an estuary, and for
spits, there often is a leeward basin. Spits, as linear emergent sandy bars and recurved emergent sandy bars, with
one end anchored to a shore, a shoal, or a promontory,
are developed along the margins of estuaries in
mid-estuarine locations, at the mouths of estuaries, peripheral to shoals and islands, and at the tips of promontories
of riverine deltas. Cheniers, as linear emergent sandy bars
and recurved emergent sandy bars that are isolated as
a sand body, are developed on tidal flats and at the tips
of promontories of riverine deltas. The shores of spits
and cheniers are developed by prevailing wave action or
by storms. Their beach slope is further shaped by tidal currents. For spits, as they are often recurved sand bodies
with a leeward lagoon or sheltered area and are subject
to hydrodynamic processes on both sides of the sand body,
there is a windward beach and leeward beach. Beaches
developed along the shores of spits and cheniers are most
commonly sandy. For both spits and cheniers, in tropical
regions, their sandy leeward (protected) slope often is
inhabited by mangroves.
The prograding front of a sandy delta is another location for the development of beaches. In this situation, the
beach-constructing agencies are mainly estuarine wind
waves, with lesser effect from tidal currents. These
beaches are mainly sandy and are peripheral to the delta
plain.
BEACH PROCESSES
61
resides in the pore spaces of the sediments; and (2) water
films circumferential to sand particles in the wetted but
undersaturated sediment above the water table during
low tide; this wet zone can be referred to as the temporary
vadose zone and is tidal-flat pellicular water. The surface
of the phreatic zone is the groundwater table during low
tides. This groundwater table rises and falls with the tides.
The phreatic zone determines many of the biological and
chemical processes operating under the beach, and the
vadose zone during a low tide determines many of the
other biological and chemical processes operating under
the beach. Depending on the depth to the water table during the period of low tide, the beach may be wet (where the
water table is near the surface) or moistened by water rising by capillary action, or may be relatively dry (where the
water table is decimeters below the surface).
Location of estuarine beaches
As mentioned earlier, there are nine different environments within an estuary where beaches can form. Therefore, location will determine the suite of processes that
combine to form a beach in the first place, the type of sediments that comprise the beaches, and the types of processes that operate postdepositionally on beach sediments.
The most common locations for beaches are the shores
of the margins of the interior of an estuary and generally in
the central parts of estuaries (i.e., not deltas, spits,
cheniers, shoals, and the estuarine mouth). Here, the
shores are usually sandy, with sand derived from alongshore, washed up from subtidal zones, reworked from
the uplands, or delivered from the marine environment.
In microtidal and mesotidal settings, the beach is fronted
by low-tidal sand flats. Gravel sources and any eroding
rock in estuarine shore environments result in gravelly
beaches. Where beaches are bordered by sandy tidal flats
and skeletons of shelly benthos are transported onto the
beach by waves and storms to form shelly sand, or shell
gravel lags, or shell gravel lenses.
The next most common site for beach development is
the leeward margin of barrier dunes. Here again, the
shores are usually sandy with sand eroded from the dunes,
or derived from alongshore, or washed up from subtidal
zones. In microtidal and mesotidal settings, such beaches
are fronted by low-tidal sand flats which supply shell
gravel and shell grit to the beach to form shelly sand, or
shell gravel lags, or shell gravel lenses.
The shores of mid-estuarine-emergent shoals and
islands also are common sites for the development of
beaches. Because shoals and islands present differing
aspects to prevailing wind-wave fields, and to wind, there
is asymmetry in the suite of processes and in the products
developed. Beaches directly facing prevailing waves will
have different profiles to those on leeward sides of shoals
and islands, and similarly, the sediment response within
a beach in terms of lithology, granulometry, and
stratigraphic organization will differ from windward side
to leeward side of the shoal or island. Beaches peripheral
to shoals and sand islands are commonly sandy, while
those peripheral to an island of rock can have sandy, gravelly sand, and gravelly beaches.
Beaches can be developed at the mouth of an estuary
and along the margins of tidal exchange channels.
Beaches at the mouth of an estuary are subject to processes
similar to that of open coastal beaches, though the former
are more protected; these processes include oceanic wave
action, wind-wave action, tides, and wind. These beaches
may be backed by low beach ridges built by the prevailing
onshore winds. The sediment responses within such
beaches in terms of lithology, granulometry, and stratigraphic organization are similar to open coastal beaches
and include a larger proportion of floating debris derived
from marine sources. Beaches along the margins (banks)
of tidal exchange channels are also subject to processes
of oceanic wave action, wind-wave action, tides, and
wind. Orientation of the channel to the ocean wave field
determines how much wave action is involved in shaping
the beach morphology and lithology, and, in this context,
ebb and flood-tidal currents are more important in that
their effects are magnified in (relatively) narrow channels.
These shorelines also may be backed by low beach ridges
built by the prevailing winds. Beaches at the mouth of
estuaries and along the margins of tidal exchange channels
are most commonly sandy.
Beaches developed along the shores of spits and
cheniers are similar, though these coastal landforms
develop in different locations within an estuary, and for
spits, there often is a leeward basin. Spits, as linear emergent sandy bars and recurved emergent sandy bars, with
one end anchored to a shore, a shoal, or a promontory,
are developed along the margins of estuaries in
mid-estuarine locations, at the mouths of estuaries, peripheral to shoals and islands, and at the tips of promontories
of riverine deltas. Cheniers, as linear emergent sandy bars
and recurved emergent sandy bars that are isolated as
a sand body, are developed on tidal flats and at the tips
of promontories of riverine deltas. The shores of spits
and cheniers are developed by prevailing wave action or
by storms. Their beach slope is further shaped by tidal currents. For spits, as they are often recurved sand bodies
with a leeward lagoon or sheltered area and are subject
to hydrodynamic processes on both sides of the sand body,
there is a windward beach and leeward beach. Beaches
developed along the shores of spits and cheniers are most
commonly sandy. For both spits and cheniers, in tropical
regions, their sandy leeward (protected) slope often is
inhabited by mangroves.
The prograding front of a sandy delta is another location for the development of beaches. In this situation, the
beach-constructing agencies are mainly estuarine wind
waves, with lesser effect from tidal currents. These
beaches are mainly sandy and are peripheral to the delta
plain.
BEACH PROCESSES
61