Processes That Affect Saltmarsh Erosion and Saltmarsh Restoration
185
ing invertebrates are deterred by coarse or shallow sediment, raising the surface of the land appropriately prior to flooding may prevent them colonising
it. Burd (1995) discussed the direct advantage of using dredged material to
raise the level of the sediment prior to flooding and, if this sediment deterred
burrowing invertebrates, then a double advantage may accrue. Even if a shallow layer of finer sediment were to develop, any invertebrates that burrowed
into it could be more prone to predation by birds.
Artificial barriers to burrowing invertebrates may offer another management solution. In experiments on small spatial and temporal scales removing
Corophium, or preventing Nereis reaching the surface, has enabled plants to
colonise sediments lower than they occur naturally (see above). Expanding
the size and duration of these experiments, for example, by use of matting of
the sort used to stabilise the banks of new road cuttings, may favour longer
term sediment accretion and colonisation by vascular plants. This may affect
a switch from an invertebrate-dominated community to a plant-dominated
community proposed by the alternative state hypothesis of Hughes et al.
(2000).
8.4 Loss of Saltmarsh Vegetation by Lateral
Erosion of Creeks
Adam (1990) reviewed several possible reasons for the formation of creeks in
marshes but concluded that in specific cases the causes were unknown. Creek
density and shape may be related to sediment type and to tidal range. In SE
England, where fine sediments predominate, and the tidal range is comparatively moderate, the marshes have a high creek density. In some other
regions saltmarsh creeks are notably absent. Adam (1990) noted that although
creeks give the impression of instability, with cliffs and eroding banks, many
are remarkably stable over periods of many decades, a view supported by
Gabet (1998). However, this is not the case in many locations in SE England
(see above) (Burd 1992). At Tollesbury creek erosion is noticeable as severely
undercut creek banks followed by the slumping of sections of the saltmarsh
surface into the creeks. This erosion is apparent even in the head of the creeks,
where an explanation based on purely physical processes is untenable because
of the virtual absence of waves and very slow tidal water movement, even
during spring tides. The slumped blocks are characterised by the downward
displacement of the vegetation, which may change subsequently because of
the greater degree of inundation by seawater. A possible successional
sequence of vegetation is apparent; on recent slumps the vegetation is similar
to that of the saltmarsh surface, while on some older slumps the original flora
185
ing invertebrates are deterred by coarse or shallow sediment, raising the surface of the land appropriately prior to flooding may prevent them colonising
it. Burd (1995) discussed the direct advantage of using dredged material to
raise the level of the sediment prior to flooding and, if this sediment deterred
burrowing invertebrates, then a double advantage may accrue. Even if a shallow layer of finer sediment were to develop, any invertebrates that burrowed
into it could be more prone to predation by birds.
Artificial barriers to burrowing invertebrates may offer another management solution. In experiments on small spatial and temporal scales removing
Corophium, or preventing Nereis reaching the surface, has enabled plants to
colonise sediments lower than they occur naturally (see above). Expanding
the size and duration of these experiments, for example, by use of matting of
the sort used to stabilise the banks of new road cuttings, may favour longer
term sediment accretion and colonisation by vascular plants. This may affect
a switch from an invertebrate-dominated community to a plant-dominated
community proposed by the alternative state hypothesis of Hughes et al.
(2000).
8.4 Loss of Saltmarsh Vegetation by Lateral
Erosion of Creeks
Adam (1990) reviewed several possible reasons for the formation of creeks in
marshes but concluded that in specific cases the causes were unknown. Creek
density and shape may be related to sediment type and to tidal range. In SE
England, where fine sediments predominate, and the tidal range is comparatively moderate, the marshes have a high creek density. In some other
regions saltmarsh creeks are notably absent. Adam (1990) noted that although
creeks give the impression of instability, with cliffs and eroding banks, many
are remarkably stable over periods of many decades, a view supported by
Gabet (1998). However, this is not the case in many locations in SE England
(see above) (Burd 1992). At Tollesbury creek erosion is noticeable as severely
undercut creek banks followed by the slumping of sections of the saltmarsh
surface into the creeks. This erosion is apparent even in the head of the creeks,
where an explanation based on purely physical processes is untenable because
of the virtual absence of waves and very slow tidal water movement, even
during spring tides. The slumped blocks are characterised by the downward
displacement of the vegetation, which may change subsequently because of
the greater degree of inundation by seawater. A possible successional
sequence of vegetation is apparent; on recent slumps the vegetation is similar
to that of the saltmarsh surface, while on some older slumps the original flora