Processes That Affect Saltmarsh Erosion and Saltmarsh Restoration
179
4
§:
35
E
:>
3
;;;
"0
.. > 25
.8
'"
l!
'" 2
0;
~
15
0
200
400
Minutes after low wake,
600
800
Fig. 8.4. Diagram of the predicted tidal curve for West
Mersea on 1 January, 2000.
See text for explanation of
the shaded areas a, b, and c
water surface, and under the erosive influence of waves, for longer periods.
This is illustrated by reference to Fig. 8.4, which shows a tidal curve for a neap
tide at West Mersea (close to the Tollesbury realignment site) on 1 January
2000, when high water was at 4.0 m above CD. [Mean high water neap tide
level (MHWNTL) is at 3.8m]. Resuspension of sediment occurs when wave
and/or tidal currents impart a shear stress (To) to the sediment that exceeds
the critical bed shear stress (TO crit). As a first-order approximation, assume the
wind generates waves under which To> To crit to a water depth of 20 cm. If the
height of the sediment surface is 3.5 m, resuspension could occur for 48 min,
24 min each on the flood ("a" in Fig. 8.4) and ebb tide ("b" in Fig. 8.4).
Moreover, between these periods, when the depth of water is more than 20 cm,
deposition could occur during slack water around high tide. If the sediment
surface is raised to 3.8 m, under similar tide and wave conditions, sediment
resuspension could occur for 129 min over the period of high tide ("c" in
Fig. 8.4) and no deposition would occur around high tide either. Therefore, it
is expected that in low-lying realignment sites sediment will generally
accumulate until a dynamic equilibrium is reached where the height of the
sediment will fluctuate above and below a mean; sedimentation may occur,
for example, on days with low wind speeds and a high sediment load in the
incoming tide following a storm, whereas sediment may be exported from the
site on windy days.
8.2.2 Biological Factors
8.2.2.1 Effects of the Flora
If the sediment surface is sufficiently high it may become colonised by
microphytobenthos in high densities (see Underwood 1997), filamentous
179
4
§:
35
E
:>
3
;;;
"0
.. > 25
.8
'"
l!
'" 2
0;
~
15
0
200
400
Minutes after low wake,
600
800
Fig. 8.4. Diagram of the predicted tidal curve for West
Mersea on 1 January, 2000.
See text for explanation of
the shaded areas a, b, and c
water surface, and under the erosive influence of waves, for longer periods.
This is illustrated by reference to Fig. 8.4, which shows a tidal curve for a neap
tide at West Mersea (close to the Tollesbury realignment site) on 1 January
2000, when high water was at 4.0 m above CD. [Mean high water neap tide
level (MHWNTL) is at 3.8m]. Resuspension of sediment occurs when wave
and/or tidal currents impart a shear stress (To) to the sediment that exceeds
the critical bed shear stress (TO crit). As a first-order approximation, assume the
wind generates waves under which To> To crit to a water depth of 20 cm. If the
height of the sediment surface is 3.5 m, resuspension could occur for 48 min,
24 min each on the flood ("a" in Fig. 8.4) and ebb tide ("b" in Fig. 8.4).
Moreover, between these periods, when the depth of water is more than 20 cm,
deposition could occur during slack water around high tide. If the sediment
surface is raised to 3.8 m, under similar tide and wave conditions, sediment
resuspension could occur for 129 min over the period of high tide ("c" in
Fig. 8.4) and no deposition would occur around high tide either. Therefore, it
is expected that in low-lying realignment sites sediment will generally
accumulate until a dynamic equilibrium is reached where the height of the
sediment will fluctuate above and below a mean; sedimentation may occur,
for example, on days with low wind speeds and a high sediment load in the
incoming tide following a storm, whereas sediment may be exported from the
site on windy days.
8.2.2 Biological Factors
8.2.2.1 Effects of the Flora
If the sediment surface is sufficiently high it may become colonised by
microphytobenthos in high densities (see Underwood 1997), filamentous