IV. Retreating Shorelines
.....
167
A longshore current, being continuous, it can carry (littoral drift) materials over
quite long distances. Littoral drift value can vary widely from storm conditions to
normal situations. Any structure placed in its way will cause accumulation on one
side and erosion downdrift.
A frequently used littoral drift formula is given by (9) in which Qs is the volume
in m~/day units, and Pe, improperly named the longshore component of wave
power, is in W/m units, evaluated with the square root-mean-square height
(I-Irms) :
Pe = (ECn)b sin c~ o cos u b
(8)
Qs = 6.8 Pe
(9)
It permits to estimate sand quantities topped by hard structures, the effects of
sediment dredging on the inner (viz. shallow) continental shelf on adjacent
beaches, and effects of longshore variations in the littoral drift.
2.7
Sea-Level Changes
Sea-level changes are caused by tides and storm surges, but we are accustomed to
use the term in connection with a world-wide rise in the level of the oceans, a
serious treat to coastal zones. Indeed, over the last 20,000 years that level has
risen by more than 100 m (328 f0.
Sea-level changes are not a XXth century scourge. They have occurred rather
frequently during geological history. Major level variations, perhaps during the
Cenozoic, certainly thereafter, have been the result of glaciations and warmingup stages, and the contemporary level is comparable to that attained during
earlier interglacial periods. A rise of 100 m (328 ft) since the Wisconsinian cf.ms
a conservative estimate. Annual variations are related to long-term weather
"tides". Longer trends are still discussed as to their causes. It has been suggested
that the eustatic level rise may reach 1 cm/year (0.4 in/y). In some places the rate
is double : in Louisiana subsidence (both natural and man enhanced) worsens the
situation. The erosion at a given site is gauged by the net-sea-level change, the
sum of the eustatic rise and local land-level changes. Indeed in some areas
subsidence has compounded the effects of the global change (Louisiana,
Galveston TX) and in others (Juneau, AK) land rise results in a lowering of the
mean sea-level.
.....
167
A longshore current, being continuous, it can carry (littoral drift) materials over
quite long distances. Littoral drift value can vary widely from storm conditions to
normal situations. Any structure placed in its way will cause accumulation on one
side and erosion downdrift.
A frequently used littoral drift formula is given by (9) in which Qs is the volume
in m~/day units, and Pe, improperly named the longshore component of wave
power, is in W/m units, evaluated with the square root-mean-square height
(I-Irms) :
Pe = (ECn)b sin c~ o cos u b
(8)
Qs = 6.8 Pe
(9)
It permits to estimate sand quantities topped by hard structures, the effects of
sediment dredging on the inner (viz. shallow) continental shelf on adjacent
beaches, and effects of longshore variations in the littoral drift.
2.7
Sea-Level Changes
Sea-level changes are caused by tides and storm surges, but we are accustomed to
use the term in connection with a world-wide rise in the level of the oceans, a
serious treat to coastal zones. Indeed, over the last 20,000 years that level has
risen by more than 100 m (328 f0.
Sea-level changes are not a XXth century scourge. They have occurred rather
frequently during geological history. Major level variations, perhaps during the
Cenozoic, certainly thereafter, have been the result of glaciations and warmingup stages, and the contemporary level is comparable to that attained during
earlier interglacial periods. A rise of 100 m (328 ft) since the Wisconsinian cf.ms
a conservative estimate. Annual variations are related to long-term weather
"tides". Longer trends are still discussed as to their causes. It has been suggested
that the eustatic level rise may reach 1 cm/year (0.4 in/y). In some places the rate
is double : in Louisiana subsidence (both natural and man enhanced) worsens the
situation. The erosion at a given site is gauged by the net-sea-level change, the
sum of the eustatic rise and local land-level changes. Indeed in some areas
subsidence has compounded the effects of the global change (Louisiana,
Galveston TX) and in others (Juneau, AK) land rise results in a lowering of the
mean sea-level.