forced out of the aerated zone but trapped by descending
swash water. As such, air is trapped in bubbles in the sand
in the upper tidal level. In areas with low wave action but
large tidal range (mesotidal and macrotidal), with a rapidly
rising water table, air is also trapped in the sand to form air
bubbles. Where air is entrapped in the beach sand, the
structure is termed “bubble sand” and is a distinctive structure of sand in tidal zone (Emery, 1945; De Boer, 1979;
Reineck and Singh, 1980). In estuaries, it occurs in all
beaches with a tidal fluctuation (viz., mouth of the estuary
and margins of tidal exchange channels, leeward shoreline
of a dune barrier, beaches along the margins of the interior
of the estuary, the shores of spits and cheniers, and of
mid-estuarine emergent shoals and islands); it is less
developed to absent on beaches that comprise the sandy
front of deltas.
As noted earlier, storms and wave action during times
of elevated water levels are also instrumental in developing lithologically distinct sedimentary deposits
(Semeniuk and Johnson, 1982; Semeniuk, 1997). These
may be marked by the concentrated occurrence of marine
and estuarine plant wrack, wood and log debris, shell
deposits, and accumulations of floating mollusks such as
Spirula and cuttlefish. The marine-derived accumulations
of floating mollusks are more common on beaches near
estuarine mouths or within estuaries that are widemouthed
and have a strong marine influence at their seaward parts.
A summary of the products of the physical processes
acting on beach sediments is shown in Figure 7a.
Freshwater through-flow from the uplands bordering an
estuarine beach, or from water ponded by a beach barrier,
can discharge over or through a beach. With a beach barrier
that bars/ponds a freshwater lagoon to leeward, or where
the uplands provide general sheet flow of freshwater, the
seepage across and through the beach can be a broad front
(a seepage front, or interface). Such seepage may not be
perennial but linked to the wettest time of the year. On the
other hand, due to drainage channels and buried drainage
channels from the uplands, or because of hydrological conduits, the freshwater through-flow may be channeled and
restricted in its passage in corridors across and in the subsurface through the beach. The through-flow of freshwater
across and through a beach will have biological,
hydrochemical, and geochemical effects (see later). In particular, it may affect the composition of macrobiota and
microbiota that are ecologically linked to a specific salinity.
Freshwater discharging under a beach, because of its
buoyancy in relation to denser marine or brackish water,
can escape to the beach surface in a discharge “pipe.” This
water escape, or freshwater upwelling from under the
beach, results in physical disruption of the lamination of
beach sediment. The upflow can entrain sand and bring
it to the surface. The lamination within and in an aureole
around the discharge “pipe” is contorted, and the surface
Beach Processes, Figure 7 (a) Structures produced in beach sediment where physical processes are dominant. (b) Structures
produced in beach sediment where physical processes and biological processes are co-dominant.
BEACH PROCESSES
65
swash water. As such, air is trapped in bubbles in the sand
in the upper tidal level. In areas with low wave action but
large tidal range (mesotidal and macrotidal), with a rapidly
rising water table, air is also trapped in the sand to form air
bubbles. Where air is entrapped in the beach sand, the
structure is termed “bubble sand” and is a distinctive structure of sand in tidal zone (Emery, 1945; De Boer, 1979;
Reineck and Singh, 1980). In estuaries, it occurs in all
beaches with a tidal fluctuation (viz., mouth of the estuary
and margins of tidal exchange channels, leeward shoreline
of a dune barrier, beaches along the margins of the interior
of the estuary, the shores of spits and cheniers, and of
mid-estuarine emergent shoals and islands); it is less
developed to absent on beaches that comprise the sandy
front of deltas.
As noted earlier, storms and wave action during times
of elevated water levels are also instrumental in developing lithologically distinct sedimentary deposits
(Semeniuk and Johnson, 1982; Semeniuk, 1997). These
may be marked by the concentrated occurrence of marine
and estuarine plant wrack, wood and log debris, shell
deposits, and accumulations of floating mollusks such as
Spirula and cuttlefish. The marine-derived accumulations
of floating mollusks are more common on beaches near
estuarine mouths or within estuaries that are widemouthed
and have a strong marine influence at their seaward parts.
A summary of the products of the physical processes
acting on beach sediments is shown in Figure 7a.
Freshwater through-flow from the uplands bordering an
estuarine beach, or from water ponded by a beach barrier,
can discharge over or through a beach. With a beach barrier
that bars/ponds a freshwater lagoon to leeward, or where
the uplands provide general sheet flow of freshwater, the
seepage across and through the beach can be a broad front
(a seepage front, or interface). Such seepage may not be
perennial but linked to the wettest time of the year. On the
other hand, due to drainage channels and buried drainage
channels from the uplands, or because of hydrological conduits, the freshwater through-flow may be channeled and
restricted in its passage in corridors across and in the subsurface through the beach. The through-flow of freshwater
across and through a beach will have biological,
hydrochemical, and geochemical effects (see later). In particular, it may affect the composition of macrobiota and
microbiota that are ecologically linked to a specific salinity.
Freshwater discharging under a beach, because of its
buoyancy in relation to denser marine or brackish water,
can escape to the beach surface in a discharge “pipe.” This
water escape, or freshwater upwelling from under the
beach, results in physical disruption of the lamination of
beach sediment. The upflow can entrain sand and bring
it to the surface. The lamination within and in an aureole
around the discharge “pipe” is contorted, and the surface
Beach Processes, Figure 7 (a) Structures produced in beach sediment where physical processes are dominant. (b) Structures
produced in beach sediment where physical processes and biological processes are co-dominant.
BEACH PROCESSES
65