So what is a Shoal?
A shoal, sandbar (or just bar in context), or gravelbar 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
Shoaling
When surface waves move towards shallow water, such as a beach,
they slow down, their wave height increases and the distance
between waves decreases. This behaviour is called shoaling, and the
waves are said to shoal. The waves may or may not build to the
point where they break, depending on how large they were to begin
with, and how steep the slope of the beach is. In particular, waves
shoal as they pass over submerged sandbanks or reefs. This can be
treacherous for boats and ships.Shoaling can also diffract waves,
so the waves change direction. For example, if waves pass over a
sloping sandbank which is shallower at one end than the other, then
the shoaling effect will result in the waves slowing more at the
shallow end. Thus the wave fronts will refract, changing direction
like light passing through a prism. Refraction also occurs as waves
move towards a beach if the waves come in at an angle to the beach,
or if the beach slopes more gradually at one end than the
other.
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.
To claim credit for the Earthcache please answer the
following
1.) Can you see the shoal today?Why or why not?
2.) Which way is the water shoaling?
3.) An according to the historic marker what was the shoal used
for?
As with all of my earthcaches please enjoy them and if you like
to include a picture in your log you may do so but it is not
required.