4. Coastal Erosion and Protection Measures Including Case Studies
145
known as tidal prism. The inter-tidal prism volume can be expressed by the
relationship:
Q = HA
(4.2)
where, H: average tidal range and A: surface area of the basin.
If the tidal prism results in the huge volume of water, meaning that
when the tide ebbs, majority of the water is likely to be replaced. Hence,
the pollutants or any suspended toxic materials will be flushed away. This
occurs in very shallow estuaries. The tidal prism and flushing time of a
shallow water inlet is good, whereas, the replacement of water over the
entire column of a deeper inlet may not take place.
4.8.3 Stability of an inlet
Evaluation of the stability of tidal entrances on a littoral drift shore must
be based on thorough knowledge of tidal hydraulics, wave mechanics, and
sedimentary aspects. No absolutely “stable” tidal inlet exists on a littoral
drift shore. It is always subjected to changes in its plan form as well as in its
cross-sectional area and geometry. Regardless, it maintains its location and
cross-section with relatively small changes, due to seasonal changes in wave
conditions and to variations in tidal ranges. At tidal inlets on littoral drift
shores, the balancing forces are mainly the littoral drift which is carried to
the entrance by flood currents for deposits in inner and outer bars, shoals
and flats attempting to close the inlet, and ebb tidal and other currents
which try to flush these deposits away and maintain the cross-sectional
area of the inlet channel.
If an entrance cannot maintain a stable navigation channel by its own
flushing capability, then this must be supplemented by artificial means. It is
necessary to consider the various methods of improvements that mean control of the inlet including sédimentation. This may be obtained by diversion
of flows, dredging of traps etc. or by structural means including measures
like training walls, jetties, groins etc.
As per Brunn and Gerritsen [1959], based on a large number of analysis
on tidal inlets, classified them as tidal flow bypass and sand bar bypass
based on the ratio of annual littoral drift to the discharge from the inlet as
below.
• Bar bypassing [M/Q > 200].
• If annual littoral drift, M is very high and tidal discharge, Q is low, the
bypassing of sédiments would be directly across the inlet. This case leads
to the formation of sand bar. Tidal flow bypassing [M/Q < 7].
145
known as tidal prism. The inter-tidal prism volume can be expressed by the
relationship:
Q = HA
(4.2)
where, H: average tidal range and A: surface area of the basin.
If the tidal prism results in the huge volume of water, meaning that
when the tide ebbs, majority of the water is likely to be replaced. Hence,
the pollutants or any suspended toxic materials will be flushed away. This
occurs in very shallow estuaries. The tidal prism and flushing time of a
shallow water inlet is good, whereas, the replacement of water over the
entire column of a deeper inlet may not take place.
4.8.3 Stability of an inlet
Evaluation of the stability of tidal entrances on a littoral drift shore must
be based on thorough knowledge of tidal hydraulics, wave mechanics, and
sedimentary aspects. No absolutely “stable” tidal inlet exists on a littoral
drift shore. It is always subjected to changes in its plan form as well as in its
cross-sectional area and geometry. Regardless, it maintains its location and
cross-section with relatively small changes, due to seasonal changes in wave
conditions and to variations in tidal ranges. At tidal inlets on littoral drift
shores, the balancing forces are mainly the littoral drift which is carried to
the entrance by flood currents for deposits in inner and outer bars, shoals
and flats attempting to close the inlet, and ebb tidal and other currents
which try to flush these deposits away and maintain the cross-sectional
area of the inlet channel.
If an entrance cannot maintain a stable navigation channel by its own
flushing capability, then this must be supplemented by artificial means. It is
necessary to consider the various methods of improvements that mean control of the inlet including sédimentation. This may be obtained by diversion
of flows, dredging of traps etc. or by structural means including measures
like training walls, jetties, groins etc.
As per Brunn and Gerritsen [1959], based on a large number of analysis
on tidal inlets, classified them as tidal flow bypass and sand bar bypass
based on the ratio of annual littoral drift to the discharge from the inlet as
below.
• Bar bypassing [M/Q > 200].
• If annual littoral drift, M is very high and tidal discharge, Q is low, the
bypassing of sédiments would be directly across the inlet. This case leads
to the formation of sand bar. Tidal flow bypassing [M/Q < 7].