Intertidal Mud and Sandflats

Subtidal Mobile Sandbanks

The hydrophysical regime of an area is regarded as the net result of all factors affecting water movement and any interference with this regime will affect the physical integrity of the sedimentary systems. In particular, the regime indicates an area’s energetic nature; in this report the low energy to high energy continuum refers to the dynamics of sedimentary habitats and thus it excludes the very high energy conditions responsible for exposed rock areas. An understanding of the hydrodynamic (current) regime is important as it has the primary role of delivering particles, food and dispersal stages of organisms to an area:

• The wind-driven currents are dependent on the direction and strength of the prevailing wind climate and on the wave fetch.
• The tidal currents will depend on the location and topography in relation to amphidromic points where the tidal range is always zero (Open University, 1989) and the wider nature in relation to tidal surges. The currents close to the shore are influenced by the shape of the coastline with prominent conical headlands increasing the speed of tidal currents and causing gyres within adjoining bays (Barne et al, 1995). Subtidal sandbanks then often occur within those gyres. Within semi-enclosed areas such as estuaries there will be greater erosion potential during spring tides and less so during neap tides. These spring-neap erosion-deposition cycles will influence the stability of the sediment and the dispersal of organisms.
• The freshwater-mediated currents are important within estuarine and coastal areas receiving run-off. The less-dense freshwater will produce vertical stratification which will influence the transport and settlement of particles and dispersive stages of organisms. The high flows during the winter and the low flow during the summer may produce winter-summer erosion-deposition cycles. These may allow sediment to be washed downstream in winter and then tidally-pumped back in the summer. Associated salinity patterns will in turn affect the dispersal of organisms.
• The residual currents comprise that component of the hydrophysical regime which remains after the above influences have been removed. In particular this includes large scale hydrographic features such as Coriolis force which pushes water counter-clockwise when moving into an estuary or bay. The interaction of these currents together with the topography may produce coastal counter-currents (Dyke, 1996). The residual currents may also be manifest as long shore drift which will act as a sediment transport mechanism especially for intertidal sand flats. In addition, summer conditions may induce gyres which may influence bed conditions.
• In addition to the above, hydrographic features such as fronts will influence the delivery of physical and biological materials to an area of seabed (Open University, 1989).

Intertidal areas are highly dynamic systems which are constantly influenced by local energy levels and, especially in the case of high energy sand flat areas, exhibit a micro-structure which is governed by repeated erosion and deposition during the reworking of the sediment (Swart, 1983). Although mud and sand flats have complex interactions between physical, chemical, geological and biological factors, the determining factors affecting beach systems is their exposure to wave, current and wind action (Eagle, 1973, Swart 1983).

Wave action, particle size and intertidal gradient are related to and influence each other in a cyclical manner (Pethick, 1984). Waves breaking on the shore cause sediment to be pushed up the shore by the swash and back by the backwash. The backwash is weaker because water percolates into the sediment. Coarse sediments with high percolation rates encourage the build up of steep beach profiles because the backwash is too weak to move the sediment down the beach, whereas fine sediments lead to flatter slopes. Gentle waves surge a long way up the beach where they lose energy by carrying sediment and lose water through percolation. In this situation backwash is weak hence beaches build up which makes them steeper. Steep storm waves break over a narrow area and do not move as far up the beach. Less energy is lost carrying sediment and less water through percolation. The backwash is strong and so sediment is carried seawards thus eroding the beach and giving it a shallower profile (Pethick, 1984).

Onshore winds from winter storms increase wave action and erode material from beaches and transport it to sea where it is deposited as a longshore bar. In the summer, less dynamic swell waves return the sediment to the beach. The strong seaward movement of sediment during storms is normally counter balanced by its slower rate of return during the rest of the year (Swart, 1983).

The above hydrographic and sedimentary processes on intertidal sand flats also control the mixing and dispersal of sediment bound contaminants (Dolphin et al, 1995). Sediment entrainment by strong spring tidal currents may be restricted to the middle and lower regions of the sand flat which are inundated during the peak tidal flows. The upper 2-3 cm of sediment is then re-worked across the middle and upper sand flat by mild storm events.

The hydrological regime affects the water characteristics in terms of salinity, temperature and dissolved oxygen. It also influences the rate of deposition and remobilisation of the sand and hence the nature of the substratum and the depth of the sand bank. The speed of the water movement and the rate of erosion and deposition of the sand are important in maintaining the integrity of these habitats. Some subtidal sandbanks experience very strong currents and are primarily physically controlled especially in high energy situations away from coastal silt input or where currents are sufficiently strong to prevent accumulation of fine sediment (Pethick, 1984). At certain times, particularly during storms, the top of a sand bank can be removed and then replaced during calmer conditions.

Tidal streams and wave action cause sediment transport and erosion which will affect the grain size of sandbanks; sediment will range from fine to very coarse depending on current strength and may be well to poorly sorted. Very strong currents may either produce channels around banks, where the sediment may be extremely coarse, or they may remove all of the surficial sediment. Large scale sand ripples (mega-ripples) may also develop and accumulate silt in their troughs.

The presence of headlands on cliffed coasts are important in determining the hydrodynamic regime, sediment dispersal and deposition and shoreline evolution. Headlands capture wave energy and their influence on tidal streams may lead to the development of residual current gyres.

Figure – The influence of headlands on the hydrodynamic regime

Sediment moving along shore will tend to enter the tidal stream at the headland and then accumulate within the gyre to form banks and shoals (Carter, 1988). The cyclic nature of tidal currents may result in the ebb and flow currents following different paths. This often leads to an ellipsoidal flow pattern which produce conditions suited for sandbank formation. The residual currents produced by asymmetric flood and ebb patterns are also extremely important in the transport of water and waterborne material such as sediment, pollutants and planktonic larvae (Pethick, 1984).

The water currents affect the distribution patterns of both permanent (holoplanktonic) and temporary (meroplanktonic) plankton species including the larvae of benthic species as well as waterborne pollutants. The fauna may be determined by chance settlement of species brought from external areas by these currents although the presence of a gyre over a subtidal bank can limit dispersion and to some extent limit the influx of larvae. This may help to maintain the integrity of the population as was shown for an Abra community in Oxwich Bay (Tyler & Banner, 1979). In addition, water movements may also affect the organisms through physical stress (Wood, 1987).

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