Mudflats, Sandflats & Saltmarshes

Mudflats/Sandflats

• In estuarine environments sediment that becomes too heavy to be ‘transported’ will ‘settle’ and be deposited

• What is an estuary?• Characteristics provide a clue to the ‘depositional’ environment• What are these characteristics? How would define an estuary?• Consider the Ythan estuary• Mudflats and sandflats with small amounts of saltmarsh• Tidal regime: flood and ebb tides• Water velocity & flow• Sediment sequence (where?)• What are the processes active? Stokes Law?• Problem is that time period over which ‘settling’ takes place is ‘too short’• What other processes are active? • Fresh water and salt water mixes: clay particles flocculate (stick together)• Large areas of inter-tidal mud flats form• Natural coastal defence

There are four main types of estuary:

Coastal Plain Estuaries. These are typically wide and shallow estuariesformed by the flooding of pre-existing valleys at the end of the last ice-age.

Bar-Built Estuaries. These are quite widespread around the UK, they have a sediment bar across the mouth of a partially drowned river valley.

Complex Estuaries. Quite rare in the UK, these are formed by various influences such as sea level change, erosion and geological constraints from hard rock outcrops.

Ria Estuaries. Drowned river valleys with estuarine features restricted to the upper reaches.

Estuaries are partly enclosed bodies of water at the mouths of rivers which are subject to the tide. They are thus open to saline water from the sea and fresh water from the rivers. They are

complex environments with a range of habitats grading from riverine to marine.

The total area of estuarine habitat in the UK is in the order of 588,000ha representing approximately 15% of the northwest European total. Of this just over half is intertidal habitat (sand and mud flats with a lesser amount of saltmarsh). Intertidal flats in the UK cover an area of approximately 270,000ha.

Mudflats are defined as "sedimentary intertidal habitats created by deposition in low energy coastal environments, particularly estuaries and other sheltered areas". Sediments generally consist of silts and clays with a high organic content. Physical processes active e.g. erosion and deposition, link mudflats and other coastal habitats such as saltmarshes and maritime cliffs. Mudflats frequently occur as part of the natural sequence of habitats between the sub-littoral zone and vegetated saltmarshes. Like most other intertidal areas they dissipate wave energy and thus have an important role to play in reducing the risk of erosion damage to saltmarshes and coastal defences, and of tidal flooding in low-lying coastal areas.

Saltmarsh

• Accumulation of mud• Increase in elevation• What happens? Water covers the mudflat less and less• Gradual invasion of ‘salt loving’ plants: pioneer plant species (link to Plant Ecology)• For example: Spartina sp (vegetation type will vary in different parts of the world)• The result is the development of saltmarshes• Differentiation in plant species between upper and lower levels of saltmarsh. Why?• Vegetation succession starts to take place (competition and domination)• Once vegetation starts to grow this affects the sedimentation. Why?• Vegetation acts as a ‘baffle’ allowing sediment to build up• Roots act as sediment traps• But there are many other factors that affect the saltmarsh development and

longevity e.g. activities of man such as dredging…sea defences…. Pollution?

The most recent saltmarsh surveys of the UK estimate the total extent of saltmarsh (including transitional communities) to be approximately 45,500 ha (England 32,500 ha, Scotland 6747 ha, Wales 6089 ha, and Northern Ireland 215 ha).This resource is concentrated in the major estuaries of low-lying land in eastern and north-west England and in Wales, with smaller areas in the estuaries of southern England, the firths of eastern and south-west Scotland and the sea loughs of Northern Ireland; north-west Scotland is characterised by a large number of very small saltmarsh sites at the heads of sea lochs, embayments and beaches. It is estimated that, at the mean high water line, 24% of the English coastline, 11% of the Welsh coastline and 3% of the Scottish coastline consists of saltmarsh vegetation.

Saltmarshes are usually restricted to comparatively sheltered locations in five main physiographic situations: in estuaries, in saline lagoons, behind barrier islands, at the heads of sea lochs, and on beach plains. The development of saltmarsh vegetation is dependent on the presence of intertidal mudflats.

Saltmarsh continued…

• Saltmarshes also develop ‘characteristics’

• Cliffs• Creeks (network of channels: major

importance for supply and loss of sediment, water and nutrients)

• Diversity of vegetation• Salt pans• Creek bank levees• They can be classified as ‘low’ or

pioneer and ‘high’ or mature• The frontal edge can be characterised

by a small ‘mud cliff’ (0.5- 2m high)• May be a mud or sand (some parts of

the world they are more peaty)• Often saltmarshes are reclaimed,

drained to form agricultural land• Consequences are that defences need

to be constructed to prevent flooding• Loss of intertidal area results in

constricted estuarine channels etc…• Why? Effects on hydrodynamics such

as tidal flow, current velocities, ebb/flood symmetry etc…

Saltmarsh processes are influenced by a number of external factors, including:

exposure

salinity

temperature

tidal range

Tidal range is the most prominent influence as all saltmarshes are within the intertidal zone and are, therefore, exposed to tidal immersion. The once daily (diurnal), or more commonly, twice daily (semi-diurnal) flooding of the saltmarsh and the associated impact of tidal waters, are likely to be the most important influence on its development. Marshes can also withstand severe buffeting, and can act as a buffer through the stilling of waves. It should be noted, however, that not all saltmarshes are inundated at least once a day. High saltmarsh may only be inundated a few times a year at the very highest tides.

Erosion and 'coastal squeeze'

Erosion of the seaward edge of saltmarshes occurs widely in the high energy locations of the larger estuaries as a result of coastal processes. There is evidence that this process is exacerbated both by the isostatic tilting of Britain towards the south-east, and by climatic change leading to a relative rise in sea level and to increased storminess. Many saltmarshes are being 'squeezed' between an eroding seaward edge and fixed flood defence walls. The erosional process is exacerbated in some locations by a reduced supply of sediment. 'Coastal squeeze' is most pronounced in south-east England, where, for example, it is estimated that 20% of the saltmarsh resource in Kent and Essex was lost between 1973 and 1988. The best available information suggests that saltmarshes in the UK are being lost to erosion at a rate of 100 ha a year. In more western and northern regions, there is recent evidence of a trend towards net sea level rise which may be causing saltmarsh erosion, although the rates of loss are not known.

Accretion

Accretion and development of saltmarsh is occurring on parts of the British coastline, notably in north-west England where sediments are comparatively coarse and isostatic uplift largely negates sea level rise. However this accretion is not sufficient to offset the national net loss of saltmarsh, and in many cases the newly created habitats differ from those being lost due to regional differences.

Sediment dynamics

Local sediment budgets may be affected by coast protection works, or by changes in estuary morphology caused by land claim, dredging of shipping channels and the impacts of flood defence works over the years.

Saltmarsh - typical cross-section

Saltmarsh - restoration