Salt marsh dynamics<br />Professor Simon K. Haslett<br />Centre for Excellence in Learning and Teaching<br />Simon.email@example.com<br />1st July 2010<br />
Introduction<br />For most of the tidal cycle, the upper part of the intertidal zone is exposed to the air.<br />It is here in mainly tide-dominated situations that salt-tolerant plants may grow to create widespread vegetated intertidal surfaces, known as either salt marshes or mangroves.<br />Salt marshes are characterised by short plants, such as grasses, and are mostly restricted to temperate coastlines (Allen, 2000).<br />Mangroves on the other hand are found in tropical and subtropical environments, and comprise trees of various heights that can develop into extensive forests (Perry, 2007).<br />In this presentation, the features and morphology of a salt marsh and mangrove are described. <br />
Salt marsh features 1<br />‘The Marsh Shoreline’ (Allen, 1993)<br /><ul><li>There are three broad categories of marsh shore morphology:</li></ul>Ramped marsh shore. A smoothly sloping surface <br /> from un-vegetated mudflat gradually changing <br /> upwards and landwards into vegetated salt marsh. <br /> This shore-type is typical of an accreting salt marsh, <br /> advancing seaward and building up vertically. <br /> POSITIVE REGIME.<br />A ramped marsh shore. Stert Flats, Somerset (UK).<br />
Salt marsh features 2<br />Cliffed marsh shore. The opposite shoreline type are those that are cliffed. Their height can vary from 10 cm to several metres and they represent saltmarsh retreat which may proceed at an average rate of 1 ma-1. Cliff retreat can be caused by channel migration, changes in sediment supply, and changes in wind climate which effects wave energy. NEGATIVE REGIME. <br />Extensive cliffing of a salt marsh at Northwick Oaze in the Severn Estuary (UK). The sediment corer is c. 1.25 m long.<br />
Salt marsh features 3<br />Spur and furrow marsh shore. The third type is a ramp-like feature carved into finger-like spurs and furrows. It probably represents a regime of net erosion, but not as severe as the erosion on a cliffed shoreline. It may be neutral in that erosion mainly occurs in the furrows with deposition occurring on the spurs. Sand and gravel can accumulate in the furrows which appears to aid erosion. NEUTRAL REGIME.<br />Salt marshes, mudflats and a tidal creek at Northwick Warth, Severn Estuary (UK). The salt marsh shoreline is of spur and furrow type.<br />
Salt marsh features: definitions<br />Under changing wave conditions a salt marsh shoreline may advance or retreat, and such a dynamic salt marsh can be recognised by the presence of a stair-like or terraced set of OfflappingMorphostratigraphic units.<br />OVERSTEP – the overlying series lies on progressively older members of the underlying series. TRANSGRESSION<br /> OVERLAP – progressively younger members of the overlying series rest upon the underlying (older) series. TRANSGRESSION<br /> Overstep & Overlap = ONLAP<br />OFFLAP – the lower beds of the overlying series extend further over the underlying series than the younger members of the overlying series. REGRESSION<br />
OfflappingMorphostratigraphic units 1 <br />Allen (1993) describes a model for this process:<br />A salt marsh is eroded to produce a wave-cut platform and cliff during a period of high wave energy (unit A).<br />As wave energy decreases, a new marsh grows upward and outward to create unit B. The cliff separating units A & B represents the inland limit of retreat of unit A.<br />Further changes in wave condition would repeat the process, so producing subsequent units (e.g. unit C) and associated cliffs.<br />Unit A<br />Clifflet<br />Clifflet<br />Unit B<br />Clifflet<br />Unit C<br />Unit D<br />Advancing salt marsh<br />OfflappingMorphostratigraphic units in the hypertidal Severn Estuary (UK).<br />
OfflappingMorphostratigraphic units 2<br />Each of the newer units is deposited on an extensive wave-cut platform which landward turns upward into a buried cliff, which may only be represented by a small clifflet on the surface.<br />These erosion-accretion cycles appear to have a characteristic timescale of decades to a century or so but may be longer.<br />Where marsh-cliff retreat has not preceded uniformity, older shorelines can be truncated by younger ones creating offlappingonstep. This may be recognised by outcrop patterns of morphostratigraphic units.<br />
Salt marsh sedimentology: a model of salt marsh sedimentation (Allen, 1994).<br />This model considers a salt marsh as an idealised horizontal platform bounded on one side by a deep body of water which is the source of the water and sediment coming on to the marsh during a tidal cycle.<br />The other side of the marsh is limited by a physical barrier (cliff or sea wall).<br />Tidal flow velocity will decrease abruptly as it enters the marsh and will continue to decrease towards the marsh interior.<br />Due to this velocity decrease, suspended <br /> sediment in the water should at once begin to <br /> settle out as it is carried out over the marsh <br /> edge and towards the interior.<br />A typical salt marsh as considered by the model. Avon Estuary, Bristol (UK).<br />
A model of salt marsh sedimentation (Allen, 1994).<br />This model suggests:<br />Flow velocity varies linearly with distance across the marsh, decreasing away from the water body.<br />The amount of sediment deposited on the marsh over a tidal cycle declines with increasing distance from the water body.<br />Mean settling velocity of the sediment deposited over a tidal cycle decreases with increasing distance across the marsh from the water body.<br />Particles with higher density fall out over shorter horizontal distances than finer particles; thus, grain size will decrease with distance from the water body.<br />Particles will also settle out over shorter horizontal distances as the height of the marsh surface increases in relation to stationary tide levels.<br />
Implications of the model for vertical sediment texture patterns 1 (Allen, 1996). <br />These model results have important implications for the evolution of sediment accumulating at a fixed point on a marsh that is building up while the shoreline varies in its horizontal position. There are 3 main cases which illustrate the implication (see Fig. 3.15 in Haslett, 2008):<br /><ul><li>CASE 1 – This case represents neutrality, whereby the shoreline maintains a constant horizontal position. The model predicts that as the marsh surface grows upward towards HAT (Highest Astronomical Tide) level, the sediment becomes progressively finer. NEUTRAL CURVE.
CASE 2 – This case represents a seaward advancing salt marsh and progressive timelinesdepict an increasing distance between the positioning the sampled profile and the marsh-edge. Consequently, the model predicts a more rapid upward decrease in grain size than the neutral curve.</li></li></ul><li>Implications of the model for vertical sediment texture patterns 2 (Allen, 1996).<br /><ul><li>CASE 3 – This case represents a landward retreating saltmarsh and progressive timelines depict a decreasing distance between the sampled profile and the marsh edge. The model predicts that grain size should increase as the marsh edge retreats towards the sampled profile.
Allen (1996) compares his model results with actual vertical sediment profiles from the Severn Estuary, where he measures grain size with reference to the rubidium content of sediment (rubidium content decreases as grain size increases). He was also able to compare his results with historical data (maps and aerial photographs).
The two sites show grain size variation which is consistent with shoreline movement, and thus agrees with the model prediction.
Allen (1996) concludes that because salt marsh surfaces suffer little erosion (the marsh edge does), this method of vertical sediment analysis should be able to provide continuous records of salt marsh dynamics.</li></li></ul><li>Salt marshes and sea-level rise<br />The response of salt marshes to sea-level rise depends very much on the sediment supply into the salt marsh system.<br />If the rate of accretion and surface elevation is less than the rate of sea-level rise then the salt marsh will submerge and drown.<br />If the rate of elevation is higher than that of sea-level rise the salt marsh will emerge and perhaps terrestrialise.<br />If the rate of surface elevation may be equal to that of sea-level rise, in which case the salt marsh will neither emerge nor submerge and will exist in equilibrium.<br />Sea defences and salt marsh on the Severn Estuary (UK).<br />
Mangroves 1<br />The principal difference between salt marshes and mangroves is greater above-ground biomass in mangroves.<br />Amongst the commonest mangrove species are Rhizophora, which possess dense networks of prop roots that extend into the sediment surface from the above ground trunk, and Avicennia that have a horizontal below-ground root system that sends up shoots called pneumatophores.<br />Mangroves of the eastern Australian coastline: (a) Avicennia mangroves with abundant pneumatophores (Minnimurra Estuary, New South Wales); (b) Rhizophora with prop root networks (Thomatis Creek, Queensland).<br />
Mangroves 2<br />Both strategies assist in anchoring the trees to the substrate, and the partial above-ground roots help to facilitate oxygen intake, as the substrate is invariably anaerobic.<br />Such root networks are also considered to promote the trapping and deposition of sediment that is introduced into the mangal system, up to approximately 2 mm/year.<br />Mangrove swamp. Queensland, Australia.<br />
Mangroves 3<br />Three main physical mangrove settings occur (Woodroffe, 1993):<br />River-dominated setting – refers to substantial mangroves that exist on many low-latitude deltas, where the sediment is principally supplied by river water; for example, the Fly River Delta in Papua New Guinea.<br />Tide-dominated setting – mainly associated with estuarine situations in which sediment is supplied by tides and tidal currents; for example, the South Alligator River in the Northern Territory of Australia.<br />Carbonate setting – restricted to coral reef settings where mangroves may grow around cays and in protected areas behind storm shingle ramparts and coral islands. The sediment here is of local derivation being supplied by wave erosion of coral. An example is Grand Cayman.<br />
Mangroves 4<br />Mangroves are ecologically rich and are refuges for many species of birds and other animals.<br />Nurseries for a number of fish species which are an important food source for local communities, and potentially profitable.<br />Dense mangals offer significant protection to coastal communities from tropical storm waves and associated erosion.<br />However, this fact is often neglected and mangrove sites in developing countries are being converted to agriculture, such as rice cultivation.<br />For example, in some west African states original mangroves have declined in area by 46%, and in the Philippines by as much as 75% (French, 1997).<br />
Mangroves and sea-level rise 1<br />Woodroffe (1995) suggests that mangroves will respond in different ways to sea-level rise according to their overall morphology and physical setting.<br />In the tide-dominated Darwin Harbour, mangroves occur on the fringe of the estuary, with many tidal creeks supplying the system with sediment.<br />Therefore, under gradual sea-level rise, this type of fringing mangrove may migrate landward, supplied with sediment from erosion of the submerging lower intertidal zone.<br />Palaeoecological studies suggest that all the studied mangroves migrated onshore in this way during the post-glacial Holocene transgression (when the melting of the glaciers caused the sea-level to rise).<br />
Mangroves and sea-level rise 2<br />However, in river-dominated settings, extensive deltaic plains of mangroves have been built as the shore has prograded seaward, following the stabilisation of the Holocene transgression, using sediment delivered to the coast by rivers.<br />These near-horizontal plains are susceptible to any future sea-level rise, because they are less likely to migrate landward, and may simply become submerged.<br />A mangrove-lined tidally influenced channel. A distributary of the channel of the Barron River Delta near Cairns, Queensland (Australia).<br />
Summary<br />Salt marshes and mangroves are geographically distinct. <br />Common presence of halophytic plants which are often zoned.<br />They each vary in their morphology and setting. <br />Successive phases of salt marsh erosion and accretion can result in the occurrence of offlappingmonostratigraphic units.<br />Salt marshes and mangroves rely on river, tidal and wave currents for sedimentation.<br />Mangroves have the majority of their biomass above-ground.<br />Salt marshes and mangroves possess distinctive ecosystems that are often under pressure from human society.<br />Generally, both salt marshes and mangroves have evolved through interaction with sea-level in the past and will continue to do so in the future, though this is not to say that they are without risk.<br />
References<br />Allen , J.R.L. 1993. Muddy alluvial coasts of Britain: field criteria for shoreline position and movement in the recent past. Proceedings of the Geologists’ Association, 104: 241-262.<br />Allen, J.R.L. 1994. A continuity-based sedimentological model for temperate-zone tidal salt marshes. Journal of the Geological Society, London, 151: 41-49.<br />Allen, J.R.L. 1996. Shoreline movement and vertical textural patterns in salt marsh deposits: implications of a simple model for flow and sedimentation over tidal marshes. Proceedings of the Geologists’ Association, 107: 15-23.<br />Allen, J.R.L. 2000. Morphodynamics of Holocene salt marshes: a review sketch from the Atlantic and Southern North Sea coasts of Europe. Quaternary Science Review, 19: 1155-1231, 1839-1840 (erratum).<br />French , P.W. 1997. Coastal and Estuarine Management. Routledge, 251pp.<br />Haslett, S.K. 2008. Coastal Systems (2nd ed.). Routledge, 240pp.<br />Perry, C. 2007. Tropical coastal environments: coral reefs and mangroves. In: Perry, C. and Taylor, K. (eds). Environmental Sedimentology. Blackwell Publishing, 302-350.<br />Woodroffe, C.D. 1993. Geomorphological and climatic setting and the development of mangrove forests. In: Lieth, H. and Al Masoom, A. (eds). Towards the Rational Use of High Salinity Tolerant Plants. Kluwer Academic Publishers, the Netherlands, 13-20.<br />Woodroffe, C.D. 1995. Response of tide-dominated mangrove shorelines in Northern Australia to anticipated sea-level rise. Earth Surface Processes and Landforms, 20: 65-85.<br />
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