which is ultimately derived from the sun, is transmitted via
the climate system onto the coast. In contrast to normal
ocean waves, tsunami waves are characterized by a longer
wavelength (200–300 km) with periods from minutes to
hours. The speed of a tsunami wave within the deep sea
reaches up to 800 km/h with a very low amplitude of less
than 1 m. The amplitude increases in shallow water
(shoaling) as the wave gets compressed. A tsunami wave
moves the entire water column, which causes refraction
effects. Bryant (2005) notes 124 tsunami events as a result
of 15,000 earthquakes for the period 1861–1948. In general, storm surges occur much more frequently than tsunami waves.
Storm surges
Storm surges are long-wavelength, low-amplitude sea surface displacements resulting in superelevated water levels
that are over and above the regular astronomically forced
highest water levels (high tides). Storm surges are a climatically driven phenomenon, accompanied with reduced
atmospheric pressure. They are therefore surface gravity
waves. The low pressure results in the reduction of the vertical force on the sea surface and hence leads to a rise of
the water level. This phenomenon is also known as the
“inverted barometer effect” (Wunsch and Stammer,
1997). A linear relationship exists between atmospheric
pressure and sea-level. A sea-level drop of 1 cm is
observed with an increase of 1 mbar of atmospheric pressure (Ross, 1854). Storm surges are short-term changes in
water level. This is due to the presence of wind which
moves the low-pressure system. The variations in sealevel can be about Æ15 cm.
The coastal hazards associated with storm surge
impacts are coastal flooding, erosion, and salinization.
The flooding of a normally dry coastal zone occurs where
there are large low-lying areas; hence, flooding is especially severe in estuaries and other coastal areas characterized by geologically young and unconsolidated
sedimentary strata. The situation is enhanced in cases of
local land subsidence which may be induced by humans,
e.g., due to excessive groundwater extraction (Galloway
and Burbey, 2011). Artificial drainage of swampy coastal
areas, as practiced for centuries in the Netherlands, leads
to peat compaction and oxidization. As a consequence,
the surface is lowered and the flooding potential
amplified.
Wave impacts can result in significant erosion along the
shore, a process which is sometimes amplified by additional erosional agents such as floating debris (e.g., trees)
and sea ice. In addition to the effects along the coastline,
the water level in rivers may rise. This is because severe
weather conditions are often accompanied by extraordinary precipitation events, which can cause a noncoastal
flooding hazard. Coastal flooding in river mouths
(especially in backwater areas) may also occur without
additional water from precipitation. This is because the
base level changes during times of storm-induced higher
sea level. Coastal storms have an “event” character as they
result in comparatively sudden coastal changes and are the
main reason for coastal retreat. This retreat is not a gradual
and/or linear process but the sum of sporadic events and
the intermittent movement of sediments. At least 70 %
of sandy beaches around the world are recessional (Bird,
1985; Schlacher et al., 2007).
Coastal storms are generated by low-pressure weather
systems such as tropical cyclones (synonyms: hurricanes,
typhoons) as well as extratropical storms (low-pressure
systems including blizzards). Extratropical cyclones form
over both land and sea, in contrast to tropical cyclones that
only form over the sea. Cyclones rotate anticlockwise in
the Northern Hemisphere and clockwise in the Southern
Hemisphere due to the Coriolis effect. A sea surface temperature above 26
C is one of the most important requirements for a tropical cyclone to form. Energy in the form of
latent heat is transferred from the ocean into the climate
system by the condensation of water vapor. Frontal activity associated with the interaction of warm and cold air
masses is the main driving force in the formation of
extratropical cyclones.
The severity of a storm surge is influenced by the wind
conditions, the drag on the sea surface due to the wind
fetch, the timing of the tides, the wind speed, and also
the duration of the event. Of equal importance is the orientation of the coast relative to the storm path, as wind
directed at a right angle causes the highest rise in water
level. Furthermore, the storm surge extent depends on
the speed of movement of the pressure system, the topography, the shape of the coastline, and the bathymetry.
The passing of an atmospheric pressure system and the
associated storm surge activity may last from hours to
days. Extratropical cyclones are usually larger and longer
lasting than tropical cyclones, which are always
nonfrontal storms (Gray, 1979). However, tropical
cyclones are more severe as they contain more potential
energy. The severity of the storm surge hazard increases
if the situation persists over several tidal cycles, because
the risk of flooding increases significantly if high tide
and storm surge coincide. Furthermore, the maximum
height of the water level is controlled by instantaneous
water level changes caused by waves.
Tsunami
Tsunamis are the result of a large mass of water being
displaced by different processes. Earthquake-induced tsunamis have small wave heights offshore and very long
wavelengths which can reach several hundred kilometers
(line source tsunamis). This is in contrast to tsunamis
caused by mass movements (subaerial or underwater origin), volcanic eruptions and explosions, glacier calvings,
or meteorite impacts, which have very high waves and
very short wavelengths close to the origin (point source
tsunamis). More than 85 % of tsunamis have tectonic
causes such as fault movements; 5 % are caused by volcanic activity, 5 % by landslides, and 5 % by a combination
GEOHAZARDS: COASTAL DISASTERS
277
the climate system onto the coast. In contrast to normal
ocean waves, tsunami waves are characterized by a longer
wavelength (200–300 km) with periods from minutes to
hours. The speed of a tsunami wave within the deep sea
reaches up to 800 km/h with a very low amplitude of less
than 1 m. The amplitude increases in shallow water
(shoaling) as the wave gets compressed. A tsunami wave
moves the entire water column, which causes refraction
effects. Bryant (2005) notes 124 tsunami events as a result
of 15,000 earthquakes for the period 1861–1948. In general, storm surges occur much more frequently than tsunami waves.
Storm surges
Storm surges are long-wavelength, low-amplitude sea surface displacements resulting in superelevated water levels
that are over and above the regular astronomically forced
highest water levels (high tides). Storm surges are a climatically driven phenomenon, accompanied with reduced
atmospheric pressure. They are therefore surface gravity
waves. The low pressure results in the reduction of the vertical force on the sea surface and hence leads to a rise of
the water level. This phenomenon is also known as the
“inverted barometer effect” (Wunsch and Stammer,
1997). A linear relationship exists between atmospheric
pressure and sea-level. A sea-level drop of 1 cm is
observed with an increase of 1 mbar of atmospheric pressure (Ross, 1854). Storm surges are short-term changes in
water level. This is due to the presence of wind which
moves the low-pressure system. The variations in sealevel can be about Æ15 cm.
The coastal hazards associated with storm surge
impacts are coastal flooding, erosion, and salinization.
The flooding of a normally dry coastal zone occurs where
there are large low-lying areas; hence, flooding is especially severe in estuaries and other coastal areas characterized by geologically young and unconsolidated
sedimentary strata. The situation is enhanced in cases of
local land subsidence which may be induced by humans,
e.g., due to excessive groundwater extraction (Galloway
and Burbey, 2011). Artificial drainage of swampy coastal
areas, as practiced for centuries in the Netherlands, leads
to peat compaction and oxidization. As a consequence,
the surface is lowered and the flooding potential
amplified.
Wave impacts can result in significant erosion along the
shore, a process which is sometimes amplified by additional erosional agents such as floating debris (e.g., trees)
and sea ice. In addition to the effects along the coastline,
the water level in rivers may rise. This is because severe
weather conditions are often accompanied by extraordinary precipitation events, which can cause a noncoastal
flooding hazard. Coastal flooding in river mouths
(especially in backwater areas) may also occur without
additional water from precipitation. This is because the
base level changes during times of storm-induced higher
sea level. Coastal storms have an “event” character as they
result in comparatively sudden coastal changes and are the
main reason for coastal retreat. This retreat is not a gradual
and/or linear process but the sum of sporadic events and
the intermittent movement of sediments. At least 70 %
of sandy beaches around the world are recessional (Bird,
1985; Schlacher et al., 2007).
Coastal storms are generated by low-pressure weather
systems such as tropical cyclones (synonyms: hurricanes,
typhoons) as well as extratropical storms (low-pressure
systems including blizzards). Extratropical cyclones form
over both land and sea, in contrast to tropical cyclones that
only form over the sea. Cyclones rotate anticlockwise in
the Northern Hemisphere and clockwise in the Southern
Hemisphere due to the Coriolis effect. A sea surface temperature above 26
C is one of the most important requirements for a tropical cyclone to form. Energy in the form of
latent heat is transferred from the ocean into the climate
system by the condensation of water vapor. Frontal activity associated with the interaction of warm and cold air
masses is the main driving force in the formation of
extratropical cyclones.
The severity of a storm surge is influenced by the wind
conditions, the drag on the sea surface due to the wind
fetch, the timing of the tides, the wind speed, and also
the duration of the event. Of equal importance is the orientation of the coast relative to the storm path, as wind
directed at a right angle causes the highest rise in water
level. Furthermore, the storm surge extent depends on
the speed of movement of the pressure system, the topography, the shape of the coastline, and the bathymetry.
The passing of an atmospheric pressure system and the
associated storm surge activity may last from hours to
days. Extratropical cyclones are usually larger and longer
lasting than tropical cyclones, which are always
nonfrontal storms (Gray, 1979). However, tropical
cyclones are more severe as they contain more potential
energy. The severity of the storm surge hazard increases
if the situation persists over several tidal cycles, because
the risk of flooding increases significantly if high tide
and storm surge coincide. Furthermore, the maximum
height of the water level is controlled by instantaneous
water level changes caused by waves.
Tsunami
Tsunamis are the result of a large mass of water being
displaced by different processes. Earthquake-induced tsunamis have small wave heights offshore and very long
wavelengths which can reach several hundred kilometers
(line source tsunamis). This is in contrast to tsunamis
caused by mass movements (subaerial or underwater origin), volcanic eruptions and explosions, glacier calvings,
or meteorite impacts, which have very high waves and
very short wavelengths close to the origin (point source
tsunamis). More than 85 % of tsunamis have tectonic
causes such as fault movements; 5 % are caused by volcanic activity, 5 % by landslides, and 5 % by a combination
GEOHAZARDS: COASTAL DISASTERS
277