Mudflats • Saltmarshes • Sea level changes •
Process relationships

Saltmarshes

Saltmarshes are, in effect, vegetated mudflats; although the morphology and the processes which act upon them are distinct in several respects. Saltmarsh surfaces are higher than mudflat surfaces and so they are flooded less frequently. The tidal currents that normally flow over saltmarshes have much lower velocities than those flowing over mudflats (and this allows initial vegetation colonisation of the surface). This is because, as the tide floods the highest parts of the intertidal area, current velocities gradually decrease to zero towards the limit of high tide. The presence of vegetation on the saltmarsh also generally increases the attenuation of both tidal currents and waves as they pass over the vegetated area.

As the upper mudflats grow higher through the vertical accretion of sediment, the number and duration of tidal inundations decrease. At a critical point in this upward growth, the mudflat becomes exposed long enough for vegetation to become established (see figure below). The elevation at which vegetation can colonise a mudflat depends on several factors, one of the most important being the availability of plant species able to survive within this environment (see Saltmarsh Ecology).

Schematic diagram showing the profile of a saltmarsh and mudflat and their main physiographical features (www.abdn.ac.uk/geospatial)

Spartina and Salicornia are the two most common UK pioneering salt-tolerant plants. The presence of these plants helps to reduce flow, in many cases, encouraging further deposition of mud. This is the process by which a forming saltmarsh can accrete sediment more rapidly than mudflats and help facilitate change in elevation across the intertidal profile. Care is needed, however, in applying the general assumption. Small swards of Spartina, for example, may suppress accretion rates compared to surrounding mudflats. Clumps of Spartina less than 2m in diameter have been shown to (in effect) raise the flow up above the bed (Pethick et al., 1990). This leaves a zone of near zero velocity in the plant stems but high shearing velocity above them. As sedimentation is proportional to the depth of water this effectively “cuts off” the sediment laden water above the vegetation, thus reducing sedimentation compared to the open mudflat. Where this process occurs, Spartina can be found in water logged depressions within an accreting mudflat. Once the sward is wider than 2m, turbulence breaks down this separation. Care is also needed in simply considering vegetation as a sediment trap where energy levels (waves or tides) are increasing from historic levels. If energy levels increase, then vegetation can enhance erosion rates by being rotated and physically scouring the surface.

Numerous factors dictate the height at which a particular mudflat surface will start to be colonised by vegetation. The most important factor is that suitable plant species have to be available for colonisation; different plants are able to colonise at lower levels than others (see Saltmarsh Types). Other factors include tidal current velocity and its ability to remove plant seeds, the availability of light for growth of plants and the salinity of the water. Once a saltmarsh is established, biological activity and sedimentation at a particular level on a saltmarsh depend on the pattern and extent of tidal inundation at that level. A youthful saltmarsh may be submerged by around half the tides in an average year, whereas a mature saltmarsh may be drowned on just a few tens of occasions.