148
Coastal Engineering: Theory and Practice
Structures for improvements include:
(a) Jetties or training walls possibly and/or with spurs if the banks of the
inlet need to be stabilised
(b) Breakwaters built parallel to shore
(c) Weirs in breakwaters with traps inside for periodic dredging
(d) Dredging of navigation channel
Jetties are the most common adopted structural measure. They intercept the free passage of the littoral drift and prevent or delay the formation
of sand bars in the channel. The efficiency dépends upon the length of the
jetties in relation to bottom slope, wave exposure and the drift of sédiments, mainly sand, caused by the waves. Jetties may be single or double.
Generally, it is true that the single jetty is not a good protective measure.
Its effect is bénéficiai updrift, but downstream it créâtes two problems:
(a) Erosion of the downdrift shore.
(b) Déposition in a leeside eddy, which encroaches upon the navigation
channel when expanding updrift.
It is therefore not advisable to build single jetty/groin unless a spécial natural entrance configuration invites one to do so, but there are only
a very few cases of nature controlling. The lengths of the pair of jetties,
(Fig. 4.31 (a)) are the most important parameter, the length of which on
the updrift side is usually longer and has to certainly pierce the surf width
for it to trap the sédiments effectively from silting the mouth of the inlet.
The length is often determined based on the normal undisturbed distribution of drift in the profile or offshore area, but this distribution will be
often considerably altered by the presence of the jetties. Sometimes updrift
side jetties are provided with small “spurs” that means jetties built perpendicular to the main jetty to trap sand. Double training walls on the
updrift side also constitute a littoral drift barrier. One of the methods
adopted for pumping sand across an approach channel is by means of floating pipelines mounted on pontoons. It would be necessary to detach the
pipeline frequently for the movement of boats in the channel and as such,
during periods of heavy traffic, bypassing will be seriously restricted. Under
these circumstances, pipelines buried below the bed of the channel could be
adopted for the uninterrupted bypassing of drift across the inlet. However,
it is obvious that this pipeline would be considerably more expensive and
heavier than the floating pipeline as this has to be withstand the pressure
of the water column above.
Coastal Engineering: Theory and Practice
Structures for improvements include:
(a) Jetties or training walls possibly and/or with spurs if the banks of the
inlet need to be stabilised
(b) Breakwaters built parallel to shore
(c) Weirs in breakwaters with traps inside for periodic dredging
(d) Dredging of navigation channel
Jetties are the most common adopted structural measure. They intercept the free passage of the littoral drift and prevent or delay the formation
of sand bars in the channel. The efficiency dépends upon the length of the
jetties in relation to bottom slope, wave exposure and the drift of sédiments, mainly sand, caused by the waves. Jetties may be single or double.
Generally, it is true that the single jetty is not a good protective measure.
Its effect is bénéficiai updrift, but downstream it créâtes two problems:
(a) Erosion of the downdrift shore.
(b) Déposition in a leeside eddy, which encroaches upon the navigation
channel when expanding updrift.
It is therefore not advisable to build single jetty/groin unless a spécial natural entrance configuration invites one to do so, but there are only
a very few cases of nature controlling. The lengths of the pair of jetties,
(Fig. 4.31 (a)) are the most important parameter, the length of which on
the updrift side is usually longer and has to certainly pierce the surf width
for it to trap the sédiments effectively from silting the mouth of the inlet.
The length is often determined based on the normal undisturbed distribution of drift in the profile or offshore area, but this distribution will be
often considerably altered by the presence of the jetties. Sometimes updrift
side jetties are provided with small “spurs” that means jetties built perpendicular to the main jetty to trap sand. Double training walls on the
updrift side also constitute a littoral drift barrier. One of the methods
adopted for pumping sand across an approach channel is by means of floating pipelines mounted on pontoons. It would be necessary to detach the
pipeline frequently for the movement of boats in the channel and as such,
during periods of heavy traffic, bypassing will be seriously restricted. Under
these circumstances, pipelines buried below the bed of the channel could be
adopted for the uninterrupted bypassing of drift across the inlet. However,
it is obvious that this pipeline would be considerably more expensive and
heavier than the floating pipeline as this has to be withstand the pressure
of the water column above.