SPIT
Wenyan Zhang
MARUM - Center for Marine Environmental Sciences,
University of Bremen, Bremen, Germany
Synonyms
Barrier spits; Headland spits; Sandspit
Definition
A spit is a coastal landform, a depositional ridge, or an
embankment of sediment (Evans, 1942) with one end
attached to a headland of the coast that serves as the source
of sediment (proximal end) and the other end extending
into open water (distal end). It is younger than the headland to which it is attached.
Description
Although spits can form and are maintained in a variety of
environmental settings, they develop most readily in large
lakes and wave-dominated coasts with a small tidal range,
which provides optimum conditions for undisturbed spit
development. Offshore waves normally approach the surf
zone of a coast at an oblique angle. A combination of
shore-oblique swash caused by the incoming waves and
shore-normal backwash caused by gravity creates
a longshore drift of sediment which is further strengthened
by longshore currents generated by wave breaking.
Sediment is entrained by strong turbulence induced by
wave breaking, and it is transported down-drift along the
coastline by longshore currents. Longshore sediment transport rate remains constant if a uniformity of waves and nearshore isobaths exist along the coastline (USACE, 1984).
Net deposition of sediment occurs where the longshore
uniformity is broken by a decrease of wave energy. This is
normally caused by a deepening of the bathymetry or
a change of the coastline orientation. In the latter case,
the boundary constraint of the longshore currents by the
coastline no longer exists, and the currents are veered by
a barotropic pressure induced by wave radiation stress.
On the side to which the currents are veered, turbulence
is dissipated by calm waters which cannot entrain the full
load. Much of the sediment is deposited as a result,
forming a submerged bar. This submerged bar subsequently acts to maintain the original direction of the
longshore currents and also serves as a reservoir for sediment accumulation. Deposition of sediment on the submerged bar will not cease until a uniformity of waves
and nearshore isobaths is again achieved. Eventually
a spit develops above water by this process.
When the submerged bar in front of a spit expands, it
may cause significant wave refraction which deflects the
longshore currents around the distal end of the spit to form
a hook or a secondary recurved spit. A change in
prevailing wind direction may also cause similar effects.
Wave refraction in multiple directions often induces the
formation of a complex spit system.
Bibliography
Evans, O. F., 1942. The origin of spits, bars and related structures.
Journal of Geology, 50, 846–863.
U.S. Army Corps of Engineers, 1984. Shore Protection Manual,
4th edn. Washington, DC: Department of the Army, U.S. Corps
of Engineers.
Cross-references
Bar
Barrier Spits
Beach Processes
Coastal Barriers
Coastal Cliffs
Coastal Landforms
Headland Breakwaters
Sediment Resuspension
Wave-Driven Sediment Resuspension
STORM SURGES
Harry C. Friebel
U.S. Army Corps of Engineers, Philadelphia District,
CENAP-EC-EH, Philadelphia, PA, USA
Synonyms
Inundation; Surge
Definition
Storm surge refers to the increased water level above the
predicted astronomical tide due to storm winds and atmospheric pressure changes.
Storm surge is principally produced by a storm’s wind
stress that pushes water in the same direction as the wind
(CHL, 2013). Low atmospheric pressure associated with
the storm makes a minor contribution to the overall storm
surge. Storm surge is calculated by subtracting the astronomical tide from the observed water level during the storm. It is
considered the most significant hazard to life and property, as
the increased water level permits large waves to break inland
of the “normal” surf zone. Storm surge is complex and dependent upon many contributing factors (NHC, 2013) including:
Intensity: The greater the wind speed, the greater the wind
stress and surge. Strengthening or weakening of the
storm before landfall will play a significant role in the
overall surge.
Forward Speed: In a cyclonic storm, the relative wind
speed at any point is the vector sum of the local wind
speed and the forward speed of the storm. For example,
a cyclonic storm with a maximum wind speed of
622
SPIT
Wenyan Zhang
MARUM - Center for Marine Environmental Sciences,
University of Bremen, Bremen, Germany
Synonyms
Barrier spits; Headland spits; Sandspit
Definition
A spit is a coastal landform, a depositional ridge, or an
embankment of sediment (Evans, 1942) with one end
attached to a headland of the coast that serves as the source
of sediment (proximal end) and the other end extending
into open water (distal end). It is younger than the headland to which it is attached.
Description
Although spits can form and are maintained in a variety of
environmental settings, they develop most readily in large
lakes and wave-dominated coasts with a small tidal range,
which provides optimum conditions for undisturbed spit
development. Offshore waves normally approach the surf
zone of a coast at an oblique angle. A combination of
shore-oblique swash caused by the incoming waves and
shore-normal backwash caused by gravity creates
a longshore drift of sediment which is further strengthened
by longshore currents generated by wave breaking.
Sediment is entrained by strong turbulence induced by
wave breaking, and it is transported down-drift along the
coastline by longshore currents. Longshore sediment transport rate remains constant if a uniformity of waves and nearshore isobaths exist along the coastline (USACE, 1984).
Net deposition of sediment occurs where the longshore
uniformity is broken by a decrease of wave energy. This is
normally caused by a deepening of the bathymetry or
a change of the coastline orientation. In the latter case,
the boundary constraint of the longshore currents by the
coastline no longer exists, and the currents are veered by
a barotropic pressure induced by wave radiation stress.
On the side to which the currents are veered, turbulence
is dissipated by calm waters which cannot entrain the full
load. Much of the sediment is deposited as a result,
forming a submerged bar. This submerged bar subsequently acts to maintain the original direction of the
longshore currents and also serves as a reservoir for sediment accumulation. Deposition of sediment on the submerged bar will not cease until a uniformity of waves
and nearshore isobaths is again achieved. Eventually
a spit develops above water by this process.
When the submerged bar in front of a spit expands, it
may cause significant wave refraction which deflects the
longshore currents around the distal end of the spit to form
a hook or a secondary recurved spit. A change in
prevailing wind direction may also cause similar effects.
Wave refraction in multiple directions often induces the
formation of a complex spit system.
Bibliography
Evans, O. F., 1942. The origin of spits, bars and related structures.
Journal of Geology, 50, 846–863.
U.S. Army Corps of Engineers, 1984. Shore Protection Manual,
4th edn. Washington, DC: Department of the Army, U.S. Corps
of Engineers.
Cross-references
Bar
Barrier Spits
Beach Processes
Coastal Barriers
Coastal Cliffs
Coastal Landforms
Headland Breakwaters
Sediment Resuspension
Wave-Driven Sediment Resuspension
STORM SURGES
Harry C. Friebel
U.S. Army Corps of Engineers, Philadelphia District,
CENAP-EC-EH, Philadelphia, PA, USA
Synonyms
Inundation; Surge
Definition
Storm surge refers to the increased water level above the
predicted astronomical tide due to storm winds and atmospheric pressure changes.
Storm surge is principally produced by a storm’s wind
stress that pushes water in the same direction as the wind
(CHL, 2013). Low atmospheric pressure associated with
the storm makes a minor contribution to the overall storm
surge. Storm surge is calculated by subtracting the astronomical tide from the observed water level during the storm. It is
considered the most significant hazard to life and property, as
the increased water level permits large waves to break inland
of the “normal” surf zone. Storm surge is complex and dependent upon many contributing factors (NHC, 2013) including:
Intensity: The greater the wind speed, the greater the wind
stress and surge. Strengthening or weakening of the
storm before landfall will play a significant role in the
overall surge.
Forward Speed: In a cyclonic storm, the relative wind
speed at any point is the vector sum of the local wind
speed and the forward speed of the storm. For example,
a cyclonic storm with a maximum wind speed of
622
SPIT