Processes That Affect Saltmarsh Erosion and Saltmarsh Restoration
183
water movement, in more open areas tidal or wave disturbance may wash the
seeds away, bury them, or prevent seedlings establishing (see Houwing et al.
1999).
Reducing the fetch over which the wind can generate internal waves would
reduce erosion and disturbance to seeds and seedlings. These measures could
take a variety of forms, including the use of brushwood groynes, such as those
used on the coast of the Wadden Sea (e.g. Houwing et al. 1999), or by excavating earth banks prior to the initial flooding. Obviously, the position of
any wave breaks needs to be perpendicular to the longest fetch and/or the
direction of the prevailing wind, but not in positions where they would create
bottlenecks that generate rapid, and erosive, tidal water movement. Possible
designs include single rows of detached short banks, or double rows of the
"open chevron" discussed below.
In some unmanaged realignment sites the mud remains flat, unconsolidated and saturated throughout periods of low tide with no apparent
natural creek formation (French 1996; R.G. Hughes, personal observation).
The tide floods as a "sheet" and ebbs in the same manner. The accreted
sediment in managed realignment sites may develop the same characteristics
and become too wet for plant colonisation. It has been suggested that the
digging of creeks, a practice in some man-made marshes in the Wadden Sea
(e.g. Dijkema 1997), would aid development of vegetation by draining the
sediment and by aiding transport of sediment into the developing marsh
(French 1996). However, channelling tidal water into creeks will increase the
frictional erosive forces of the tides, which may lead to the lateral creek
erosion prevalent on some established marshes (see below). Whether artificial
creeks are effective or not may be site-related and specific research would be
required to estimate their benefits.
8.3.2 Biological Factors
If invertebrates can reduce sediment accretion and plant colonisation then the
factors that affect their abundance may be managed to reduce their effects.
Figure 8.5 shows an interaction web that summarises some of the biological
interactions that may affect the abundance of animals and plants on these
mudflats. Some interactions are not different to a food web; for example, birds
eating invertebrates that would otherwise eat microphytobenthos. A similar
trophic cascade was described by Daborn et al. (1993) where sandpipers contributed to sediment accretion by eating Corophium. An example of a nontrophic interaction is the possible negative effect of birds on sediment stability through physical disturbance of the sediment with their feet. Reducing
disturbance by humans could increase bird predation on invertebrates. At
Tollesbury the old and the new sea walls have footpaths on the top and wading
183
water movement, in more open areas tidal or wave disturbance may wash the
seeds away, bury them, or prevent seedlings establishing (see Houwing et al.
1999).
Reducing the fetch over which the wind can generate internal waves would
reduce erosion and disturbance to seeds and seedlings. These measures could
take a variety of forms, including the use of brushwood groynes, such as those
used on the coast of the Wadden Sea (e.g. Houwing et al. 1999), or by excavating earth banks prior to the initial flooding. Obviously, the position of
any wave breaks needs to be perpendicular to the longest fetch and/or the
direction of the prevailing wind, but not in positions where they would create
bottlenecks that generate rapid, and erosive, tidal water movement. Possible
designs include single rows of detached short banks, or double rows of the
"open chevron" discussed below.
In some unmanaged realignment sites the mud remains flat, unconsolidated and saturated throughout periods of low tide with no apparent
natural creek formation (French 1996; R.G. Hughes, personal observation).
The tide floods as a "sheet" and ebbs in the same manner. The accreted
sediment in managed realignment sites may develop the same characteristics
and become too wet for plant colonisation. It has been suggested that the
digging of creeks, a practice in some man-made marshes in the Wadden Sea
(e.g. Dijkema 1997), would aid development of vegetation by draining the
sediment and by aiding transport of sediment into the developing marsh
(French 1996). However, channelling tidal water into creeks will increase the
frictional erosive forces of the tides, which may lead to the lateral creek
erosion prevalent on some established marshes (see below). Whether artificial
creeks are effective or not may be site-related and specific research would be
required to estimate their benefits.
8.3.2 Biological Factors
If invertebrates can reduce sediment accretion and plant colonisation then the
factors that affect their abundance may be managed to reduce their effects.
Figure 8.5 shows an interaction web that summarises some of the biological
interactions that may affect the abundance of animals and plants on these
mudflats. Some interactions are not different to a food web; for example, birds
eating invertebrates that would otherwise eat microphytobenthos. A similar
trophic cascade was described by Daborn et al. (1993) where sandpipers contributed to sediment accretion by eating Corophium. An example of a nontrophic interaction is the possible negative effect of birds on sediment stability through physical disturbance of the sediment with their feet. Reducing
disturbance by humans could increase bird predation on invertebrates. At
Tollesbury the old and the new sea walls have footpaths on the top and wading