IV. Retreating Shorelines
203
Assuming that a longshore current moves from east to west, groins are to be built
first at the far western end of the shore to be protected. The field can then be
gradually extended eastward. Additionnally, an appropriate quantity of sand must
be placed on the eastern side of each groin - a technique called backfilling that
insures a continued westward flow of sediment.
In the former Democratic Republic of Germany 40 km (24.8 mi) of sea walls
were built and 150 km (93 mi) of coast lined with groins.
In Japan 50-ton concrete tetrapods have been used to protect the east coast of
Honshu. Tetrapod barriers have been built at Silt (Fed. Rep. Germany).
A well designed groin system can arrest or slow down the alongshore drift of
material and, by building up material in the groin bays, stabilise the foreshore
and protect the coastline (cf. fig. 19) and deflect strong tidal currents away from
the shoreline.
Groins may also help, in some situations, to hold material on a beach that has no
natural supply and has been artificially nourished ; control seasonal shifts of
material alongshore and hence swings in shoreline within a bay ; reduce the long
term erosive effect of wave activity in an area of coastal defence by accumulating
beach material in front of prominent hard beach heads such as sea walls,
revetments, bulkheads and cliffs. This requires an adequate supply of material
moving alongshore ; improve the extent and quality of an anemity beach and
increase the depth of beach material cover to an otherwise erodible sea bed soil.
(Flemming, C.A., 1990, Principles and effectiveness of groins. In : Pilarczyk, A.
(ed.), Coastalprotection ; Rotterdam, Balkema).
2.1.2
Jetties
Jetties are more massive than groins. Their principal function is to redirect tidal
flow to either protect a beach or a navigation channel. However, construction of
jetties results in groin-like effects by redirecting littoral drift. They are
occasionally used to cause by-passing. By-passing consists in dredging sand from
behind the upstream jetty and bringing it via pipeline to behind the downstream
jetty 16.
Port structures obstruct sediment drift and sediment accumulates updrift, causing
sometimes siltation of the port inlet. In the United States, Mexico, India and
elsewhere sediment by-passing, by means of pumping is frequently practiced.
203
Assuming that a longshore current moves from east to west, groins are to be built
first at the far western end of the shore to be protected. The field can then be
gradually extended eastward. Additionnally, an appropriate quantity of sand must
be placed on the eastern side of each groin - a technique called backfilling that
insures a continued westward flow of sediment.
In the former Democratic Republic of Germany 40 km (24.8 mi) of sea walls
were built and 150 km (93 mi) of coast lined with groins.
In Japan 50-ton concrete tetrapods have been used to protect the east coast of
Honshu. Tetrapod barriers have been built at Silt (Fed. Rep. Germany).
A well designed groin system can arrest or slow down the alongshore drift of
material and, by building up material in the groin bays, stabilise the foreshore
and protect the coastline (cf. fig. 19) and deflect strong tidal currents away from
the shoreline.
Groins may also help, in some situations, to hold material on a beach that has no
natural supply and has been artificially nourished ; control seasonal shifts of
material alongshore and hence swings in shoreline within a bay ; reduce the long
term erosive effect of wave activity in an area of coastal defence by accumulating
beach material in front of prominent hard beach heads such as sea walls,
revetments, bulkheads and cliffs. This requires an adequate supply of material
moving alongshore ; improve the extent and quality of an anemity beach and
increase the depth of beach material cover to an otherwise erodible sea bed soil.
(Flemming, C.A., 1990, Principles and effectiveness of groins. In : Pilarczyk, A.
(ed.), Coastalprotection ; Rotterdam, Balkema).
2.1.2
Jetties
Jetties are more massive than groins. Their principal function is to redirect tidal
flow to either protect a beach or a navigation channel. However, construction of
jetties results in groin-like effects by redirecting littoral drift. They are
occasionally used to cause by-passing. By-passing consists in dredging sand from
behind the upstream jetty and bringing it via pipeline to behind the downstream
jetty 16.
Port structures obstruct sediment drift and sediment accumulates updrift, causing
sometimes siltation of the port inlet. In the United States, Mexico, India and
elsewhere sediment by-passing, by means of pumping is frequently practiced.