188
R.G. Hughes
placed across some creeks, but these will be largely ineffective as the amounts
of water moved by the tides past the baffles will not be reduced by their
presence, nor will its erosive capacity. Baffles reduce the effects of wave action
not those of tidal currents that erode the sediment in the creeks. Reducing
creek erosion requires one or both of two strategies, reducing the volumes of
water flowing through the creeks, and/or reducing their erosive effects.
Reducing the erodability of the sediment could be achieved by reducing the
effects of the invertebrates, as discussed above. There may be one difference,
however, as birds and their footprints are seen rarely in the creeks at
Tollesbury indicating that the invertebrates in the creeks suffer less predation
than those on more open mudflats. Paramor and Hughes (in preparation)
have stimulated sediment accretion in some eroding creeks by using matting
to exclude invertebrates in experiments similar to those of Hughes (1999).
Reducing the water flow in creeks could be achieved simply by infilling,
perhaps with dredged sediment if it were available. If this sediment were
relatively coarse (sand, gravel, shingle) the tidal volumes would be reduced
but the drainage of the marsh by the creeks, which enhances plant development, could be maintained to some extent. A partial infilling with coarse sediment, as a lining to the creek bottoms, could reduce erosion in three ways; the
flows would be reduced, the To crit will be reduced because of the larger particle
sizes, and the invertebrates may be deterred. If this measure leads to a natural
accumulation of fine sediment, which invertebrates could then colonise and
destabilise, it could be repeated to create an increasing depth of alternate
layers of coarse and fine sediments.
The factors that determine creek erosion and stability have relevance to
current management schemes designed to increase sedimentation. One
example is on the Hindenburg Dam that joins Sylt (northern Wadden Sea,
Germany) to the mainland. Periodically the saltmarsh is excavated in varied
ways but often in parallel channels, sometimes separated from the sea by a
perpendicular wall with small openings (Figs 8.7, 8.8A). These designs
contain bottlenecks, at the gaps in the wave breaks and at the mouths of the
creeks, where tidal movement will produce relatively high current speeds and
deter sedimentation, if not cause erosion. A better design to minimise current
velocities is shown in Fig. 8.8B. The excavated creeks are triangular in plan,
where the width increases progressively to seaward, so that the ratio of the
cross-sectional area of the creek to the volume above (inland) remains constant. Any wave breaks should be a single row of short walls set at an angle
appropriate for the particular location, or a double row of overlapping relatively short walls, to create an "open chevron" pattern (Fig. 8.8e) which will
prevent ingress of waves from any offshore direction. These arrangements of
short wave breaks are advantageous as there are many gaps that allow the
tidal currents to flow relatively unimpeded with slow velocities.
R.G. Hughes
placed across some creeks, but these will be largely ineffective as the amounts
of water moved by the tides past the baffles will not be reduced by their
presence, nor will its erosive capacity. Baffles reduce the effects of wave action
not those of tidal currents that erode the sediment in the creeks. Reducing
creek erosion requires one or both of two strategies, reducing the volumes of
water flowing through the creeks, and/or reducing their erosive effects.
Reducing the erodability of the sediment could be achieved by reducing the
effects of the invertebrates, as discussed above. There may be one difference,
however, as birds and their footprints are seen rarely in the creeks at
Tollesbury indicating that the invertebrates in the creeks suffer less predation
than those on more open mudflats. Paramor and Hughes (in preparation)
have stimulated sediment accretion in some eroding creeks by using matting
to exclude invertebrates in experiments similar to those of Hughes (1999).
Reducing the water flow in creeks could be achieved simply by infilling,
perhaps with dredged sediment if it were available. If this sediment were
relatively coarse (sand, gravel, shingle) the tidal volumes would be reduced
but the drainage of the marsh by the creeks, which enhances plant development, could be maintained to some extent. A partial infilling with coarse sediment, as a lining to the creek bottoms, could reduce erosion in three ways; the
flows would be reduced, the To crit will be reduced because of the larger particle
sizes, and the invertebrates may be deterred. If this measure leads to a natural
accumulation of fine sediment, which invertebrates could then colonise and
destabilise, it could be repeated to create an increasing depth of alternate
layers of coarse and fine sediments.
The factors that determine creek erosion and stability have relevance to
current management schemes designed to increase sedimentation. One
example is on the Hindenburg Dam that joins Sylt (northern Wadden Sea,
Germany) to the mainland. Periodically the saltmarsh is excavated in varied
ways but often in parallel channels, sometimes separated from the sea by a
perpendicular wall with small openings (Figs 8.7, 8.8A). These designs
contain bottlenecks, at the gaps in the wave breaks and at the mouths of the
creeks, where tidal movement will produce relatively high current speeds and
deter sedimentation, if not cause erosion. A better design to minimise current
velocities is shown in Fig. 8.8B. The excavated creeks are triangular in plan,
where the width increases progressively to seaward, so that the ratio of the
cross-sectional area of the creek to the volume above (inland) remains constant. Any wave breaks should be a single row of short walls set at an angle
appropriate for the particular location, or a double row of overlapping relatively short walls, to create an "open chevron" pattern (Fig. 8.8e) which will
prevent ingress of waves from any offshore direction. These arrangements of
short wave breaks are advantageous as there are many gaps that allow the
tidal currents to flow relatively unimpeded with slow velocities.