A tidal sandbar connecting the islands of Waya and Wayasewa of the
Yasawa Islands, Fiji
A
shoal,
sandbar (or just
bar in context), or
gravebar is a somewhat
linear landform within or extending into a body of
water, typically composed of
sand,
silt or small
pebbles. A
spit or
sandspit is a type of shoal. Shoals are characteristically long and narrow (linear) and develop where a
stream or
ocean current promotes
deposition of
granular material, resulting in localized shallowing (
shoaling) of the water. Shoals can appear in the
sea, in a
lake, or in a
river. Alternatively a bar may separate a lake from the sea, as in the case of an
ayre. They are typically composed of
sand, although could be of any granular matter that the moving water has access to and is capable of shifting around (for example,
soil,
silt,
gravel,
cobble,
shingle, or even
boulders). The
grain size of the material comprising a bar is related to the size of the waves or the strength of the currents moving the material, but the availability of material to be worked by waves and currents is also important.
The term bar can apply to
landform features spanning a considerable range in size, from a length of a few meters in a small stream to marine depositions stretching for hundreds of
kilometres along a
coastline, often called
barrier islands.
In a
nautical sense, a
bar is a shoal, similar to a
reef: a shallow formation of (usually) sand that is a
navigation or
grounding hazard, with a depth of water of six
fathoms or less. It therefore applies to a
silt accumulation that shallows the entrance to the course of a
river or
creek.
Sandbars and longshore bars
This bar forms (sometimes seaward of a
trough) where the waves are breaking, because the breaking waves set up a shoreward current with a compensating counter-current along the bottom. Also known as a trough bar. Sand carried by the offshore moving bottom current is deposited where the current reaches the wave break. Other longshore bars may lie further offshore, representing the break point of even larger waves, or the break point at low tide.
Harbour and river bars
A harbour or river bar is a sedimentary deposit formed at a
harbour entrance or
river mouth by the deposition of
sediment or the action of waves on the sea floor or adjacent beaches. A bar can form a dangerous obstacle to shipping, preventing access to the river or harbour in unfavourable weather conditions or at some states of the
tide.
Where beaches are suitably mobile, or the river’s
suspended and/or
bed loads are large enough, wave action can build up a bar to completely block a river mouth, damming the river, preventing access for boats or shipping, and causing flooding in the lower reaches of the river. This situation will persist until the bar is
eroded by the sea, or the dammed river develops sufficient
head to break through the bar.
Shoals as geological units
In addition to longshore bars discussed above that are relatively small features of a
beach, the term
shoal can be applied to larger geological units that form off a coastline as part of the process of coastal erosion. These include
spits and
baymouth bars that form across the front of embayments and
rias. A
tombolo is a bar that forms an
isthmus between an
island or offshore rock and a
mainland shore.
The largest of the geological units of this kind is a
barrier island, such as occur along the
East Coast of the United States, along the
Gulf coast, along the southern coast of
Belize and many other locations worldwide.
In places of re-entrance along a
coastline (such as
inlets,
coves, rias, and bays), sediments carried by a
longshore current will fall out where the current dissipates, forming a
spit. An area of water isolated behind a large bar is called a
lagoon. Over time, lagoons may silt up, becoming
salt marshes.
In some cases shoals may be precursors to beach expansion and dunes formation, providing a source of windblown sediment to augment such beach or dunes landforms.
Specific geology
The
barrier island can be separated into sections for easy study.
Lower shoreface
The
shoreface is the part of the barrier where the ocean meets the shore of the island. The barrier island body itself separates the shoreface from the backshore and
lagoon/
tidal flat area. Characteristics common to the lower shoreface are fine sands with muds and possibly silt. Further out into the ocean the sediment becomes finer. The effect from the waves at this point is weak because of the depth.
Bioturbation is common and many
fossils can be found here.
Middle shoreface
The middle shore face is located in the upper shoreface. The middle shoreface is strongly influenced by wave action because of its depth. Closer to shore the grain size will be medium size sands with shell pieces common. Since wave action is heavier, bioturbation is not likley.
Upper shoreface
The
upper shoreface is constantly effected by wave action. This results in development of herringbone
sedimentary structures because of the constant differing flow of waves. Grain size is larger sands.
Foreshore
The
foreshore is the area on land between high and low tide. Like the upper shoreface, it is constantly affected by wave action.
Cross bedding and lamination are present and coarser sands are present because of the high energy present by the crashing of the waves. The sand is also very well sorted.
Backshore
The
backshore is always above the highest water level point. The
berm is also found here which marks the boundary between the foreshore and backshore. Wind is the important factor here, not water. During strong storms high waves and wind can deliver and erode sediment from the backshore.
Dunes
The
dunes are located at the top of the backshore. The dunes are typical of a barrier island. The high sand dunes are only affected by wind because of their height. Similarly, strong storms are the only thing that really affect the size of the dunes. The dunes will display characteristics of typical
eolian wind blown dunes. The difference here is that dunes on a barrier island typically contain vegetation roots and marine bioturbation.
Lagoon and tidal flats
The
lagoon and
tidal flat area is located behind the dune and backshore area. Here the water is still and this allows for fine silts, sands, and muds to settle out. Lagoons can become host to an
anaerobic environment. This will allow high amounts organic rich mud to form. Vegetation is also common.
Human habitation
Since
prehistoric times humans have chosen some shoals as a site of habitation. In some early cases the locations provided easy access to exploit marine resources. In modern times these sites are sometimes chosen for the water amenity or view, but many such locations are prone to storm damage.
See also
