178
R.G. Hughes
viewed. The hypotheses of Hughes (1999) and Hughes et al. (2000) are
developed and refined, and new hypotheses proposed, with particular reference to managed realignment schemes, and to the internal erosion of established saltmarshes. The biological factors, particularly the effects of the invertebrates, are emphasised as these have tended to be ignored in this context.
8.2 Managed Realignment
Much of the land behind the sea walls in SE England was previously saltmarsh
that has been enclosed over the past few centuries (e.g. Burd 1995). Since the
land was claimed its level would have fallen with respect to local sea level,
through compaction, shrinkage and because of the subsequent SLR. Consequently, in this area, as in others, managed realignment land usually will be
lower than the existing saltmarsh (see Reed et al.1999). Therefore, factors that
affect sediment accretion, which has to be sufficient to reach an elevation high
enough to allow plants to colonise, are crucial to the successful development
of new saltmarsh.
The following hypothesis is proposed. In low-lying managed realignment
sites sediment will accumulate but as the surface of the sediment becomes
higher the rate of accretion will decline and the rate of erosion will increase
until an equilibrium elevation is reached determined by the interaction of
physical and biological factors, as described below. If the sediment is suitable
invertebrates will colonise the accreted sediment first and prevent establishment of pioneer zone plants, directly by herbivory and bioturbation, and indirectly by making the sediment more erodable and decreasing the equilibrium height of the sediment.
8.2.1 Physical Factors
After the sea wall is breached deposition of sediment will occur from a layer
of water immediately above the sediment, the height of which will depend on
the settling velocity of the particles and the length of time when water movements are sufficiently slow to allow sedimentation to occur. In general, water
movements will be slowest at slack water on either side of high tide and when
wave-generated water movement does not reach down to this boundary layer.
As the height of the sediment rises the mean rate of sedimentation will decline
and the mean rate of erosion will increase because of the increasing effect of
internal wind-generated waves, the main source of water movement in such
semi-enclosed sites (French et al. 1999). As the height of the sediment surface
gets closer to the peak of the tidal curve the sediment remains close to the
R.G. Hughes
viewed. The hypotheses of Hughes (1999) and Hughes et al. (2000) are
developed and refined, and new hypotheses proposed, with particular reference to managed realignment schemes, and to the internal erosion of established saltmarshes. The biological factors, particularly the effects of the invertebrates, are emphasised as these have tended to be ignored in this context.
8.2 Managed Realignment
Much of the land behind the sea walls in SE England was previously saltmarsh
that has been enclosed over the past few centuries (e.g. Burd 1995). Since the
land was claimed its level would have fallen with respect to local sea level,
through compaction, shrinkage and because of the subsequent SLR. Consequently, in this area, as in others, managed realignment land usually will be
lower than the existing saltmarsh (see Reed et al.1999). Therefore, factors that
affect sediment accretion, which has to be sufficient to reach an elevation high
enough to allow plants to colonise, are crucial to the successful development
of new saltmarsh.
The following hypothesis is proposed. In low-lying managed realignment
sites sediment will accumulate but as the surface of the sediment becomes
higher the rate of accretion will decline and the rate of erosion will increase
until an equilibrium elevation is reached determined by the interaction of
physical and biological factors, as described below. If the sediment is suitable
invertebrates will colonise the accreted sediment first and prevent establishment of pioneer zone plants, directly by herbivory and bioturbation, and indirectly by making the sediment more erodable and decreasing the equilibrium height of the sediment.
8.2.1 Physical Factors
After the sea wall is breached deposition of sediment will occur from a layer
of water immediately above the sediment, the height of which will depend on
the settling velocity of the particles and the length of time when water movements are sufficiently slow to allow sedimentation to occur. In general, water
movements will be slowest at slack water on either side of high tide and when
wave-generated water movement does not reach down to this boundary layer.
As the height of the sediment rises the mean rate of sedimentation will decline
and the mean rate of erosion will increase because of the increasing effect of
internal wind-generated waves, the main source of water movement in such
semi-enclosed sites (French et al. 1999). As the height of the sediment surface
gets closer to the peak of the tidal curve the sediment remains close to the