3.1 Beach and Shoreface Sediments
cases, by eolian dunes. The migrating sand creates
prominent coastal features such as hooks and spits
behind headlands or tombolas connecting former
islands with the main coast (Fig. 3.2b). This sand is
also a main factor in the formation of broad beachridge strand plains (ehenier plains, see below).
3.1.2 Beach and Shoreface Sands
and Their Budget
The Beach-Shoreface Zone
The complicated and permanently changing hydraulic
regime of the nearshore zones is reflected by the
beach-shoreface profile and its sediments. This profile results from a specific dynamic equilibrium between the wave type and the grain size of the sediments. Apart from special features like sand bars, the
slope of the beach-shoreface profile usually diminishes from foreshore to deeper water. The mean angle of this slope is low along shorelines subjected to
steep high-energy waves (high H/L ratio), because a
high energy input is dissipated most efficiently by a
wide, flat beach profile. In contrast, low energy input
under flat waves enables a rather steep beach gradient. Furthermore, the presence of coarse-grained
sand or gravel increases the slope angle of the beachshoreface profile. For these reasons, the slope angles
vary considerably between 0.2 0 (high-energy waves,
very fine sand) and approximately 10 0 (low-energy
waves, coarse sand).
A gentle slope profile allows the formation of a
broad coastal sand belt. A further consequence of the
above mentioned rules is the fact that many coastlines exhibit a seasonal change in the beach profile.
Due to a higher proportion of steep waves in winter
time, the winter beaeh tends to be lower and its slope
gentler than the summer beaeh. The lost beach sand
is usually stored in submerged sand bars parallel to
the coast at some meters depth below mean sea level
(Fig. 3.3a). Later, under a subdued wave regime, the
sand is slowly swept back onto the beaches. Loss of
beach sand to deeper water is also caused by single
storm events (see below).
Bed Forms and Sedimentary Structures
The bed forms and internal sedimentary structures
along the beach-shoreface profile are shown in Fig.
3.la for fairweather wave conditions. They reflect
the transformation of deep-water waves (oscillatory
flow) to shoaling waves, generating land-directed
flow and return flow into the foreshore zone under
upper flow regime conditions (cf. Sect. 2.2).
97
At depths near, or below the fairweather wave base (often
10 to 20 m), long-crested symmetrical wave ripples, produced earlier by rare storm waves, are bioturbated during
normal fairweather conditions. Landward, these inactive
ripples pass into active ripples which become increasingly
asymmetrie, irregular, and short-crested. Larger, lunateshaped megaripples are frequently observed in the breaker
zone. All these bedforms are associated with small- or
larger-scale cross-bedding which is predominantly oriented
toward the land. In the foreshore zone the most characteristic feature is parallel to low-angle cross-bedding dipping
seaward.
The upper face of beach and foreshore sand may display
distinctive swash marks inc1uding fine shell debris, as weil
as small bones and teeth, backwash rills and kolk marks
around gravel or shells, and sometimes rhomboid ripple
marks formed under a very thin cover of running water.
Foreshore and backshore sands frequently contain
dark laminae or layers consisting of heavy mineral
coneentrations. Locally, these plaeer deposits are of
economic interest, if they contain significant quantities of certain minerals (e.g., rutile, zircon, ilmenite,
and monazite).
High-Energy Shorelines
The general tendency in the lateral sequence of
bedforms and sedimentary textures can be modified
on high-energy co asts or on shorefaces which exhibit
one or several shore-parallel sand ridges or ojJshore
bars (barred shorelines).
On a nonbarred high-energy shoreface, two zones
with predominantly planar bedding occur, an inner
(upper) foreshore planar zone and an outer (lower)
planar zone within the breaking waves (Fig. 3.la). In
between the two planar zones the sea bed becomes
rough due to dotted and irregular erosional and
depositional features.
Offshore sand bars are complex structures, because they often mi grate in different directions, either landward, seaward, or parallel to the shoreline.
Prominent internal structures are landward-dipping
large-scale foresets and more or less horizontallamination, but small-scale cross-bedding is also corumon
(Fig. 3.3b).
Gravelly shoreface deposits are characterized by
large, asymmetric gravel wave ripples (ripple wave
length frequently between 100 and 150 cm, height 10
to 20 cm).
These ripples have been observed in modem environments
at water depths down to more than 6 m (Hart and Plint
1989). Gravel forms decimeter-thick, massive or cross-bedded layers altemating with thinner sand beds. Pebble fabric
and cross-bed orientation indicate predominantly alongshore sediment transport.
cases, by eolian dunes. The migrating sand creates
prominent coastal features such as hooks and spits
behind headlands or tombolas connecting former
islands with the main coast (Fig. 3.2b). This sand is
also a main factor in the formation of broad beachridge strand plains (ehenier plains, see below).
3.1.2 Beach and Shoreface Sands
and Their Budget
The Beach-Shoreface Zone
The complicated and permanently changing hydraulic
regime of the nearshore zones is reflected by the
beach-shoreface profile and its sediments. This profile results from a specific dynamic equilibrium between the wave type and the grain size of the sediments. Apart from special features like sand bars, the
slope of the beach-shoreface profile usually diminishes from foreshore to deeper water. The mean angle of this slope is low along shorelines subjected to
steep high-energy waves (high H/L ratio), because a
high energy input is dissipated most efficiently by a
wide, flat beach profile. In contrast, low energy input
under flat waves enables a rather steep beach gradient. Furthermore, the presence of coarse-grained
sand or gravel increases the slope angle of the beachshoreface profile. For these reasons, the slope angles
vary considerably between 0.2 0 (high-energy waves,
very fine sand) and approximately 10 0 (low-energy
waves, coarse sand).
A gentle slope profile allows the formation of a
broad coastal sand belt. A further consequence of the
above mentioned rules is the fact that many coastlines exhibit a seasonal change in the beach profile.
Due to a higher proportion of steep waves in winter
time, the winter beaeh tends to be lower and its slope
gentler than the summer beaeh. The lost beach sand
is usually stored in submerged sand bars parallel to
the coast at some meters depth below mean sea level
(Fig. 3.3a). Later, under a subdued wave regime, the
sand is slowly swept back onto the beaches. Loss of
beach sand to deeper water is also caused by single
storm events (see below).
Bed Forms and Sedimentary Structures
The bed forms and internal sedimentary structures
along the beach-shoreface profile are shown in Fig.
3.la for fairweather wave conditions. They reflect
the transformation of deep-water waves (oscillatory
flow) to shoaling waves, generating land-directed
flow and return flow into the foreshore zone under
upper flow regime conditions (cf. Sect. 2.2).
97
At depths near, or below the fairweather wave base (often
10 to 20 m), long-crested symmetrical wave ripples, produced earlier by rare storm waves, are bioturbated during
normal fairweather conditions. Landward, these inactive
ripples pass into active ripples which become increasingly
asymmetrie, irregular, and short-crested. Larger, lunateshaped megaripples are frequently observed in the breaker
zone. All these bedforms are associated with small- or
larger-scale cross-bedding which is predominantly oriented
toward the land. In the foreshore zone the most characteristic feature is parallel to low-angle cross-bedding dipping
seaward.
The upper face of beach and foreshore sand may display
distinctive swash marks inc1uding fine shell debris, as weil
as small bones and teeth, backwash rills and kolk marks
around gravel or shells, and sometimes rhomboid ripple
marks formed under a very thin cover of running water.
Foreshore and backshore sands frequently contain
dark laminae or layers consisting of heavy mineral
coneentrations. Locally, these plaeer deposits are of
economic interest, if they contain significant quantities of certain minerals (e.g., rutile, zircon, ilmenite,
and monazite).
High-Energy Shorelines
The general tendency in the lateral sequence of
bedforms and sedimentary textures can be modified
on high-energy co asts or on shorefaces which exhibit
one or several shore-parallel sand ridges or ojJshore
bars (barred shorelines).
On a nonbarred high-energy shoreface, two zones
with predominantly planar bedding occur, an inner
(upper) foreshore planar zone and an outer (lower)
planar zone within the breaking waves (Fig. 3.la). In
between the two planar zones the sea bed becomes
rough due to dotted and irregular erosional and
depositional features.
Offshore sand bars are complex structures, because they often mi grate in different directions, either landward, seaward, or parallel to the shoreline.
Prominent internal structures are landward-dipping
large-scale foresets and more or less horizontallamination, but small-scale cross-bedding is also corumon
(Fig. 3.3b).
Gravelly shoreface deposits are characterized by
large, asymmetric gravel wave ripples (ripple wave
length frequently between 100 and 150 cm, height 10
to 20 cm).
These ripples have been observed in modem environments
at water depths down to more than 6 m (Hart and Plint
1989). Gravel forms decimeter-thick, massive or cross-bedded layers altemating with thinner sand beds. Pebble fabric
and cross-bed orientation indicate predominantly alongshore sediment transport.