13
CHAPTER 3
ESTUARINE PROCESSES
The extent and stability of inter-tidal areas are determined primarily by geomorphology
and the physical forces of wind, tides and wave-action; Biological processes also
influence the stability of sediments and the processes of accretion and erosion (Brown
et al, 1998).
3.1
Hydrodynamics
The hydrodynamics of estuaries are dependent on a combination of estuary geometry,
tidal dynamics, waves, bed roughness, longshore transport, tidal exchanges and
freshwater inputs. These in turn influence the morphological responses of tidalflat and
saltmarsh systems (Johnson, 1998). Tides are the most significant energy input for the
introduction of sediments into the system, dispersal of sediment over the marsh
surface, and maintenance of the marsh drainage network within estuaries (Frey and
Basan, 1985).
For a marsh to develop and mature it is important that the grains are not re-entrained by
high energy wave or current action. Low energy conditions, required for deposition of
fine-grained sediments, may be located within the shelter of coastal embayments,
behind barrier islands, or in estuaries. Tidal dynamics, sediment transport pathways
and locally generated waves will influence the establishment, development and
maintenance of marshes within each of these environments.
Colenutt, A. J. (1999). Beneficial Use of Dredged Material for Inter-tidal Recharge:
Management Options for the Lymington Saltmarshes
14
Strong wave and current processes at the seaward edge of the saltmarsh, may result in
eroded saltmarsh blocks and stacks, erosional hollows at the foot of the clifflet, (bluff
or micro-scarp) abrasion platforms, stressed vegetation on the seaward edge of
saltmarsh and offshore transport of sediments. Erosion of the front edge of saltmarshes
adjacent to navigation channels may be due to tidal current velocities resulting from
shipping-induced draw-down currents (Toft and Maddrell, 1995), or an increase in
tidal prism due to dredging.
3.2
Saltmarsh & Mudflat Sediment Processes
Sediment inputs to cohesive foreshores are principally as suspended loads (MAFF,
1993). Saltmarsh sedimentation processes (Figure 4) are directly related to tidal
hydraulics, sediment supply, plant colonisation, faunal activities and marsh growth
(Frey and Basan, 1985). Coastal saltmarshes are frequently viewed as long-term
sediment stores, which may be partially released by high magnitude, low-frequency
events. Dynamic coastal wetland systems, which may be eroding, accreting or stable,
prograde episodically by both vertical and lateral accretion (Pethick, 1981). The
processes controlling sediment deposition on vegetated tidal saltmarshes are not well
understood, yet rates of sediment deposition are of fundamental importance to marsh
ecology and hydrology (Christiansen and Wiberg, 1994).
Deposition is not uniform across the marsh, but is concentrated at the marsh edges and
on the sides of the creek system (Johnson, 1998). Tides provide energy for introduction
of sediments into the system, dispersal of sediment over the marsh surface, and
Colenutt, A. J. (1999). Beneficial Use of Dredged Material for Inter-tidal Recharge:
Management Options for the Lymington Saltmarshes
15
maintenance of the marsh drainage network (Frey and Basan, 1985). As the flood rises,
water spills from the tidal creeks onto broad wetland surfaces above the creeks. Sheet
flow replaces channel flow and entrained sediments begin to settle. The relatively short
duration of slack water, as the flood turns towards the ebb, inhibits deposition of all but
the finest of suspended sediments (Johnson, 1998).
The response of cohesive sediment to hydraulic forces is principally determined by the
shape, size, and density of the individual particles; typified by the settling velocity.
Mud, fine-grained sediment in the silt and clay fractions (Viles and Spencer, 1995), has
a low settling velocity. The time taken for a particle to reach the bed after the tidal
velocity has diminished to the point at which the particle can no longer be held in
suspension (settling lag) (Humby and Dunn, 1975), is typically greater than the actual
time of inundation during a tidal cycle. Clay grains possess short-range attractive and
repulsive forces. When the clay is suspended in sea water the attractive forces
Colenutt, A. J. (1999). Beneficial Use of Dredged Material for Inter-tidal Recharge:
Management Options for the Lymington Saltmarshes
16
predominate, which bind neighbouring clays particles together, to form large
aggregations known as ‘flocs’. The flocculation process is particularly likely to occur
in estuarine and nearshore environments, at the interface between fresh and salt water
(Figure 5), or water bodies of varying salinity; the presence of organic matter also
helps to bind and stabilise floc structures. These larger, denser aggregations settle
much more rapidly than their constituent grains. Mudflats are usually morphologically
dynamic, with sediment transported over all stages of tidal cycles (Viles and Spencer,
1995).
Colenutt, A. J. (1999). Beneficial Use of Dredged Material for Inter-tidal Recharge:
Management Options for the Lymington Saltmarshes
17
Saltmarshes experience episodic wave erosion that forms clifflets, or bluffs, typically
0.2-0.5m high (Figure 6). Sheet erosion (loss of material from the marsh surface), is
more common on mudflats than in saltmarshes and leads to the arrest or regression of
the vegetational development. Scarp erosion (intermittent bed failure at the edge of the
saltmarsh), releases ‘lumps’ of mud back into the water (Beeftink, 1977) (Plates 1 and
2). The fine-grained suspended sediments are transported into the inter- and sub-tidal
zones and offshore, out of the coastal system.
Accompanying erosion of the saltmarsh, coarse-grained sands, gravels and shells are
transported onshore (as bedload) by strong currents and waves. Such material is
deposited eventually onto the upper part of the saltmarsh edge, as low relief cheniers
(Plate 3). They are accretional features along net-erosional coasts, and indicate a
change in environmental conditions, from an environment of tidally dominated
deposition, to one of wave dominated erosion (Bradbury, 1995).
Colenutt, A. J. (1999). Beneficial Use of Dredged Material for Inter-tidal Recharge:
Management Options for the Lymington Saltmarshes
18
Plates 1 and 2.
Eroded Saltmarsh Clifflet, Lymington.
Note:
Background: Pennington seawall
Cliff 0.5-1.0m high
Photograph taken March 1999.
Colenutt, A. J. (1999). Beneficial Use of Dredged Material for Inter-tidal Recharge:
Management Options for the Lymington Saltmarshes
19
Plate 3.
Chenier, Lymington.
Note:
Saltmarsh cliff and eroded and slumped unvegetated mud blocks.
Chenier (shingle and shell bank/ridge) 0.3-0.8m high.
Gulls and waders roosting and nesting on the chenier.
Photograph taken March 1999.
Colenutt, A. J. (1999). Beneficial Use of Dredged Material for Inter-tidal Recharge:
Management Options for the Lymington Saltmarshes
20
Colenutt, A. J. (1999). Beneficial Use of Dredged Material for Inter-tidal Recharge:
Management Options for the Lymington Saltmarshes
21
3.3
Saltmarsh Ecology
Factors involved in the physiological and geomorphological development of vegetated
tidal wetlands include climate, elevation, hydrodynamics and overall coastal stability.
Saltmarshes can be defined as well-vegetated saline inter-tidal flats. The development
of a saltmarsh is dependent on the existence of a mudflat which is capable of reducing
wave and tidal energy sufficiently to allow sedimentation and salt-tolerant plants to
colonise bare inter-tidal mud or sediment (Toft and Maddrell, 1995). Halophytic
saltmarsh plants, with low species diversity, are adapted to a harsh, semi-aquatic
environment and saline soils, e.g. stout stems, small leaves, and physiological
adaptations for salt excretion and gas exchange (Anon, 1996).
If the mudflat fails to dissipate all the wave energy during extreme storm events then
waves pass into the saltmarsh vegetation where the increased frictional drag reduces
wave energy further. More than 50% of wave energy is dissipated within the first 2.5m
of the marsh; this rises to 80% at 10m and is fully dissipated at 30m (In Williams,
1990). Saltmarshes, therefore, reduce the danger of wave overtopping during storm
events (Hofstede, 1995) (Figure 7).
The marsh acts as a reservoir of sediment, capable of feeding the mudflat during
extreme storm events producing overall widening and flattening of the inter-tidal zone.
The marsh provides an area of the inter-tidal zone into which the mudflat can extend
during storm waves (Toft and Maddrell, 1995).
Colenutt, A. J. (1999). Beneficial Use of Dredged Material for Inter-tidal Recharge:
Management Options for the Lymington Saltmarshes
22
Colenutt, A. J. (1999). Beneficial Use of Dredged Material for Inter-tidal Recharge:
Management Options for the Lymington Saltmarshes
23
Saltmarsh vegetation zonation is generally displayed as bands of characteristic species
assemblages, parallel to the shoreline, in response to increasing elevation (Figure 8).
Species composition and zonation in the saltmarsh are governed by salinity gradients in
combination with the extent of intertidal exposure, and saltmarsh elevation. Saltmarsh
vegetation shows clear successional trends over time, as accretion leads to changing
soil conditions which permit a different range of plants to grow. Characteristically,
saltmarshes are of two distinctive types;
(1)
Low, or pioneer, marshes are topographically lower (corresponding to the
upper-middle inter-tidal zone) and subject to the more active sedimentary environment
below mean high water mark. Low marshes are flooded more or less daily and are
usually dominated by one genus, such as Spartina.
(2)
High, or mature, marshes occupy a higher topographic position (the inter-tidal-
supra-tidal boundary above mean high water mark) and are floristically more varied.
The primary factor that influences distribution of low and high marsh is the amount of
area subject to regular tidal inundation; a product of topography, tidal range and length
or duration of the sedimentation process, as well as the stability of the coastal area. In a
mature marsh, the areas of high and low marsh are approximately equal. Viles and
Spencer (1995), state that the presence of absence of vegetation is not a good basis for
zonation: many wetlands are partly or wholly typified by tidal flats, barren except for
algae, which may never acquire a vegetation cover.
Spartina anglica, a sturdy, rhizomatous grass, can tolerate various levels of tidal
immersion (McCorry, 1999), and can grow a metre in height and send down roots a
metre below the mud surface (Tubbs, 1977). S. anglica has prograded into many areas
Colenutt, A. J. (1999). Beneficial Use of Dredged Material for Inter-tidal Recharge:
Management Options for the Lymington Saltmarshes
24
of nature conservation value, replacing mudflat habitat with a less diverse,
monospecific sward, lowering invertebrate infauna diversity and densities, thereby
reducing feeding grounds for wader and wildlife species (McCorry, 1999).
Spartina marsh changes the velocity profile of tidal water and thus increases the rate of
silt and mud deposition so that the marsh grows rapidly upwards (Viles and Spencer,
Colenutt, A. J. (1999). Beneficial Use of Dredged Material for Inter-tidal Recharge:
Management Options for the Lymington Saltmarshes
25
1995). However, Brown et al (1998) state that the pioneer cord grass S. anglica is not
active in enhancing sediment deposition, but is more important as a sediment stabiliser
during periods of erosion. If marsh accretion is less than the rise in relative sea-level,
the sediment deficit causes the marsh to deteriorate. As the sward ages, the rapid
accretion of poorly drained sediment results in waterlogging of the marsh soil, causing
the development of extreme anaerobic conditions in the mud surrounding the Spartina
root system (Viles and Spencer, 1995). Concentrations of plant toxins, increase as a
result of organic matter building up and reduced flushing (Johnson, 1998). This
reduces the ability of the marsh to retain its relative elevation. The overall effect is
plant (e.g. Spartina) dieback, slumping and eroding (Tubbs, 1977) and the conversion
of vegetated saltmarsh to low-elevational tidal flats or open water (Reed, 1990;
Johnson in prep). This regression of Spartina marsh, ‘die-back’, was first noticed in
1931 in Poole Harbour, spreading along the south coast estuaries during the following
decades (Carter, pers. com. 1999).
3.4
Sea-Level Rise and Coastal Estuarine ‘Squeeze’
In the UK sea-level rise resulting from vertical land movements, such as positive
isostatic forces and subsidence, already affects the southern and eastern coasts; this
will be exacerbated by the additional sea level rise from human-induced climate
change (Figure 9). Relative sea-level rise has or may create(d) conditions of increased
wave energy with the consequential threat of flooding low-lying land and erosion of
inter-tidal areas (Environment Agency, 1998). The frequency, intensity and trajectories
of storms, wind direction and wave climate are potentially serious because they can
Colenutt, A. J. (1999). Beneficial Use of Dredged Material for Inter-tidal Recharge:
Management Options for the Lymington Saltmarshes
26
affect the stability of sediment transport cells, and are likely to affect patterns of marsh
edge recession and zones of sedimentation (Bray et al, 1992). An increase in the
incident wave energy is the most significant and widespread factor causing erosion of
the seaward edge of saltmarshes, hence narrowing of the saltmarsh strip may indicate
that the incident local wave climate is becoming more severe.
Coastal wetlands are particularly vulnerable environments because they encompass a
limited vertical range. Inter-tidal erosion is part of a broader adjustment of estuary
morphology to relative sea-level rise that is, in turn, driven primarily by regional
subsidence (French and Watson, 1999). Relative sea-level rise will continue to disrupt
mudflat and saltmarsh zonation by causing a steepening of the shore profile and
erosion of seaward habitats (Goudie, 1996). Marshes in areas receiving an adequate
fine-grained sediment supply will keep pace with sea-level rise (Pye and French, 1993).
Whether accumulation on marshes can continue as the rate of sea-level rise increases
depends upon the nature of the processes causing sedimentation and overall marsh
accretion, some of which may be episodic. The factors which govern the position that
the halophyte marsh communities occupy within the tidal frame (French, 1997) are:
elevation; hydroperiod (inundation frequency and duration); rate of sediment supply;
and local relative sea-level rise.
Colenutt, A. J. (1999). Beneficial Use of Dredged Material for Inter-tidal Recharge:
Management Options for the Lymington Saltmarshes
27
Colenutt, A. J. (1999). Beneficial Use of Dredged Material for Inter-tidal Recharge:
Management Options for the Lymington Saltmarshes
28
The response of estuarine habitats and ecosystems to rising sea-level would be to
retreat inland under natural conditions. However, some of the most valuable coastal
habitats, for instance saltmarshes and mudflats, may be progressively lost or drowned,
‘caught’ between coastal defences and rising sea levels (Figure 10). Many current
forms of flood defence and coast protection use static ‘hard engineering’ structures,
such as sea walls, which prevent the landward migration of eroding and retreating
coastal habitats. Where further erosion occurs, and the inter-tidal mudflats retreat
landwards, then the saltmarsh area in front of the defences narrows, and/or disappears
completely. Not only does this allow greater wave action on the flood banks, but the
loss of a priority habitat is also a major environmental concern. In these circumstances
such defence works become an important contributory factor to coastal estuarine
‘squeeze’ (Reed, 1990).
Selective use of ‘soft’ engineering management options, such as inter-tidal foreshore
recharge, may restore width to the inter-tidal zone and provide space for the
regeneration of coastal habitats. The impact of accelerated sea-level rise emphasises
the potential conflict between conservationists, seeking to maintain the areal extent of
Colenutt, A. J. (1999). Beneficial Use of Dredged Material for Inter-tidal Recharge:
Management Options for the Lymington Saltmarshes
29
inter-tidal habitats, and engineering requirements to stabilise the present shoreline
and/or maintain levels of flood protection. An understanding both of the mechanisms
by which coastal wetland habitats adjust to present tidal levels, and of the morphosedimentary adjustments consequent upon future sea-level rise scenarios is crucial to
the effective resolution of these management issues.
Colenutt, A. J. (1999). Beneficial Use of Dredged Material for Inter-tidal Recharge:
Management Options for the Lymington Saltmarshes