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R.G. Hughes
has been replaced by pioneer zone species. The older slumps have also been
eroded, as they are relatively low lying and smoothly contoured.
At Tollesbury the saltmarsh creeks are inhabited predominantly by Nereis
and Hydrobia (Paramor and Hughes, in preparation). Corophium is not found
here but does occur in creeks elsewhere (Hughes and Gerdol1997). Hughes
(1999) suggested that the creeks, formed initially by water draining from the
marsh, were colonised by invertebrates that contributed to further erosion.
The channels increase in width progressively until they merge to leave large
areas of mud with residual mud mounds, within the marsh, as at Tollesbury
(Fig. 8.3), or ultimately with no original marsh surface remaining, as at
Clementsgreen Creek (Hughes 1999). This hypothesis is extended here. In
extensive creek systems such as those at Tollesbury, where large volumes of
tidal water flood into, and ebb from, several hundred meters of creek, erosion
of the creeks initiates a positive feedback in which erosion leads to more
erosion by increasing tidal flows, as described below. SLR may increase
saltmarsh loss in this manner indirectly as the invertebrates may colonise the
sediment higher up the sides of the creeks, contributing even more to their
erosion.
Consider a natural creek system, as in Fig. 8.6. The creeks are generally
2-4 m wide and 1-1.5 m deep, but are deeper and wider nearer to the front of
the marsh. Generally, there are no barriers, or ridges, at the mouths of the
creeks that usually drain completely leaving little standing water. Within the
creek systems waves are rare, but in some creeks the tidal currents are rapid.
Assume the flood tide first reaches the outer part of this creek system at midtide and rises for a further 3 h and reaches the surface of the marsh but does
not flood over it - a "bank-full" tide. During the flood and ebb tides the
volumes of water that flow past point X will have to fill the creeks at Y and Z.
The mean and maximum current speeds at X will be much higher than at Y
and Z. The slumping and subsequent erosion of soil at Y will, on a future
similar tide, increase the volume of water that flows past X, and the mean and
maximum current speed. Thus the erodability of the sediment at X will be
increased by an event that has occurred elsewhere on the marsh. The
slumping of a section of the saltmarsh into the creek at X will reduce the
cross-sectional area of the creek and increase the velocities of the tidal
currents, further eroding the sediment in the vicinity, usually on the bank
opposite to the slump.
One potential benefit of creeks is that they facilitate the transport of
suspended sediment, brought into the marsh by the tide and eroded from the
creeks, deep into the marsh where it would be available for deposition on the
surface, a process enhanced by the vegetation. Reed (1988) found that some of
the sediment that accreted on the marsh surface was transported into the
marsh via the creeks, but some was derived directly from the eroding creeks.
With rising sea level saltmarshes must accrete vertically at a rate at least equal
R.G. Hughes
has been replaced by pioneer zone species. The older slumps have also been
eroded, as they are relatively low lying and smoothly contoured.
At Tollesbury the saltmarsh creeks are inhabited predominantly by Nereis
and Hydrobia (Paramor and Hughes, in preparation). Corophium is not found
here but does occur in creeks elsewhere (Hughes and Gerdol1997). Hughes
(1999) suggested that the creeks, formed initially by water draining from the
marsh, were colonised by invertebrates that contributed to further erosion.
The channels increase in width progressively until they merge to leave large
areas of mud with residual mud mounds, within the marsh, as at Tollesbury
(Fig. 8.3), or ultimately with no original marsh surface remaining, as at
Clementsgreen Creek (Hughes 1999). This hypothesis is extended here. In
extensive creek systems such as those at Tollesbury, where large volumes of
tidal water flood into, and ebb from, several hundred meters of creek, erosion
of the creeks initiates a positive feedback in which erosion leads to more
erosion by increasing tidal flows, as described below. SLR may increase
saltmarsh loss in this manner indirectly as the invertebrates may colonise the
sediment higher up the sides of the creeks, contributing even more to their
erosion.
Consider a natural creek system, as in Fig. 8.6. The creeks are generally
2-4 m wide and 1-1.5 m deep, but are deeper and wider nearer to the front of
the marsh. Generally, there are no barriers, or ridges, at the mouths of the
creeks that usually drain completely leaving little standing water. Within the
creek systems waves are rare, but in some creeks the tidal currents are rapid.
Assume the flood tide first reaches the outer part of this creek system at midtide and rises for a further 3 h and reaches the surface of the marsh but does
not flood over it - a "bank-full" tide. During the flood and ebb tides the
volumes of water that flow past point X will have to fill the creeks at Y and Z.
The mean and maximum current speeds at X will be much higher than at Y
and Z. The slumping and subsequent erosion of soil at Y will, on a future
similar tide, increase the volume of water that flows past X, and the mean and
maximum current speed. Thus the erodability of the sediment at X will be
increased by an event that has occurred elsewhere on the marsh. The
slumping of a section of the saltmarsh into the creek at X will reduce the
cross-sectional area of the creek and increase the velocities of the tidal
currents, further eroding the sediment in the vicinity, usually on the bank
opposite to the slump.
One potential benefit of creeks is that they facilitate the transport of
suspended sediment, brought into the marsh by the tide and eroded from the
creeks, deep into the marsh where it would be available for deposition on the
surface, a process enhanced by the vegetation. Reed (1988) found that some of
the sediment that accreted on the marsh surface was transported into the
marsh via the creeks, but some was derived directly from the eroding creeks.
With rising sea level saltmarshes must accrete vertically at a rate at least equal