14.3 Tidal Flats
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exists over tidal flats, apart from the main oscillatory tidal water movement
(Posma, 1988).
Since water depth on the tidal flats is very small, waves propagating towards
shore are considered 'long' when the Vrsell number is greater than 75 (see
Sect. 4.2). Waves play an important role in the resuspension of sediments on
tidal flats. They can either amplify or reverse the flux of suspended sediment.
Thorne (1979) found that a near-bed oscillatory flow of only 5 mmls could
double the transport of fine sand by tidal currents. Also, the sse is of an order
of magnitude greater during storms than at other times. Amos (1995) observed,
in the upper Bay of Fundy, that wave erosion prevails with a consequent export
of suspended sediment when the significant wave heights reach 4-6 m.
Storm waves, due to their periodic character, may induce pore-pressure amplification and subsequent liquefaction of tidal flat sediment. However, little
is still known about these processes, in contrast to liquefaction in offshore soft
marine sediments (Mei and Foda, 1981).
14.3.2 Salinity and Sediment Transport on Tidal Flats
During the flooding stage of a tidal cycle, currents in the creek transport salt
water further inward. This creates a salinity gradient between the creek and
the tidal flat. When no freshwater is available and temperature is high, sea
water evaporates and salinity increases above the normal level. In subtropical
areas, when there is little or no rainfall, evaporation proceeds until the stage
of salt saturation and salt flats are formed (Amos, 1995). Similar variations
are also observed in temperature; high extremes can be found in tropical tidal
pools, but flood tide causes a sudden drop of temperature.
The movement of tidal water affects complex processes such as the transport,
erosion, deposition and consolidation of sediments. When intertidal sediments
are exposed, the surface layer is drained of water. The depth of the drainage
strongly depends on grain size and elevations. Beaches and sandflats drain
more quickly than mudflats.
Tidal flat sands are extremely well sorted with the median grain size varying
between 100 and 300 {tm. On some tidal flats these limits are even narrower.
However, the grain size distribution over a tidal flat is not totally homogeneous.
The median grain size decreases over the flat with distance from the offshore
boundary of the tidal flat, as should be expected. This is mainly caused by the
loss of wave energy when travelling over the flat.
According to Groen (1967) and Amos (1995), the shallow-water asymmetry
of the flood and ebb current controls vertical exchanges of sediment within the
benthic boundary layer. The maximum ebb current is preceded by a period
of low current velocities much longer than the flood current maximum. This
means that during the period preceding maximum ebb flow, particles have
a much longer time to settle out of suspension, and the suspended load decreases. Therefore, the suspended load at peak ebb is lower. This results in a
shoreward sediment residual motion which may be up to 38% greater than the
427
exists over tidal flats, apart from the main oscillatory tidal water movement
(Posma, 1988).
Since water depth on the tidal flats is very small, waves propagating towards
shore are considered 'long' when the Vrsell number is greater than 75 (see
Sect. 4.2). Waves play an important role in the resuspension of sediments on
tidal flats. They can either amplify or reverse the flux of suspended sediment.
Thorne (1979) found that a near-bed oscillatory flow of only 5 mmls could
double the transport of fine sand by tidal currents. Also, the sse is of an order
of magnitude greater during storms than at other times. Amos (1995) observed,
in the upper Bay of Fundy, that wave erosion prevails with a consequent export
of suspended sediment when the significant wave heights reach 4-6 m.
Storm waves, due to their periodic character, may induce pore-pressure amplification and subsequent liquefaction of tidal flat sediment. However, little
is still known about these processes, in contrast to liquefaction in offshore soft
marine sediments (Mei and Foda, 1981).
14.3.2 Salinity and Sediment Transport on Tidal Flats
During the flooding stage of a tidal cycle, currents in the creek transport salt
water further inward. This creates a salinity gradient between the creek and
the tidal flat. When no freshwater is available and temperature is high, sea
water evaporates and salinity increases above the normal level. In subtropical
areas, when there is little or no rainfall, evaporation proceeds until the stage
of salt saturation and salt flats are formed (Amos, 1995). Similar variations
are also observed in temperature; high extremes can be found in tropical tidal
pools, but flood tide causes a sudden drop of temperature.
The movement of tidal water affects complex processes such as the transport,
erosion, deposition and consolidation of sediments. When intertidal sediments
are exposed, the surface layer is drained of water. The depth of the drainage
strongly depends on grain size and elevations. Beaches and sandflats drain
more quickly than mudflats.
Tidal flat sands are extremely well sorted with the median grain size varying
between 100 and 300 {tm. On some tidal flats these limits are even narrower.
However, the grain size distribution over a tidal flat is not totally homogeneous.
The median grain size decreases over the flat with distance from the offshore
boundary of the tidal flat, as should be expected. This is mainly caused by the
loss of wave energy when travelling over the flat.
According to Groen (1967) and Amos (1995), the shallow-water asymmetry
of the flood and ebb current controls vertical exchanges of sediment within the
benthic boundary layer. The maximum ebb current is preceded by a period
of low current velocities much longer than the flood current maximum. This
means that during the period preceding maximum ebb flow, particles have
a much longer time to settle out of suspension, and the suspended load decreases. Therefore, the suspended load at peak ebb is lower. This results in a
shoreward sediment residual motion which may be up to 38% greater than the