Salt marsh and mudflat sedimentation dynamics

Saltmarshes and mudflats play a vital role in the accretion and storage of sediment in estuaries (Boorman et al., 1998; Pejrup & Mikkelsen, 2010). These environments are typically found within sheltered bays or estuaries where the wave action and tidal currents are relatively weak, allowing for salt grass to settle and survive in the marsh (Friedrichs & Perry, 2001, Davidson-Arnott, 2002). This leads to sedimentation, which is the most important factor of a salt marsh, as without it, colonization by salt tolerant plants cannot occur. Sedimentation occurs when water movement is slowed and the suspended particles can settle. Particle size (controlled by flocculation i.e.

effective particle size) and flow velocity determine how well the particle of sediment is suspended in the water. Smaller particles are more easily carried by water. Silt particles, for example, react more slowly to the velocity change and are therefore carried further in the direction of the flood (Bartholdy, 2000). These conditions allow fine sediment particles to fall out of suspension and to be deposited on the bed, creating an environment of mudflats and salt marshes (Schostak et al; 2002, Hung et al., 2006). The overall conclusion that can be drawn from previous studies is that

increased tidal velocities increase the concentration of sediment suspended in the water and this can be further augmented by the flow conditions of the river.

Sedimentation on the marsh surface depends on various factors. Firstly, sediments carried by tidal current get deposited on the marsh surface by tidal inundation (Christiansen, 1998). Sedimentation from tidal inundation is at its ma ximum in the marsh located along the tidal creeks, in the direct vicinity of sediment delivery. A study by Khalequzzaman (1989) showed that the marsh within a distance of 15 – 20 m from tidal creeks received 2.8 mm/year of sediments from tidal inundation, which

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was 93% of the annual required sediment to keep pace with the rising sea level of 3mm/year in Delaware Bay. Marshes away from the tidal creeks, however, received only 0.6 mm/year of sediment, 20% of annual required sedimentation, from tidal inundation.

Secondly, episodic events that inundate the marsh surface are one of the major processes by which sediments are deposited on the marsh surface. Storm inundations play a vital role in increasing sediment accumulation on the marsh surface (LeMay, 2007) especially with coincidence of high flow and suspended load in the river. Church et al., (1987) found that storm events may provide sufficient sediment for marsh maintenance for a large area of the marsh, with one extreme storm depositing 30% to 165% of annual demand in a salt marsh in Kelly Island, Delaware.

Sedimentation rates are also influenced by the elevation of the marsh and its relationship to the duration of tidal flooding (Brown, 1998; Reed et al., 1999). These two factors, elevation and time of inundation, are linked. As elevation is increased, the time that the area will spend inundated by the tide decreases. Therefore, the amount of sediment deposition will decrease with an increase in elevation (LeMay, 2007). A balance is required between the time in which an area is inundated and the time in which it is exposed to dry conditions. Inundation must not be too long or frequent because the chemical stress associated with water logging will prevent grass survival (Friedrichs & Perry, 2001). If inundation is too long, the result could be massive plant death due to salt intrusion or water logging. This plant death could lead to the rapid loss of elevation due to erosion forces and the reduced cohesion of the marsh sediment. The elevation loss could be in the order of 10-15 cm (Friedrichs

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& Perry, 2001). Regarding the stratigraphy of a marsh in the Severn Estuary UK, Allen (1990) indicates that the rate of vertical accretion of a marsh surface is related to flood frequency of the marsh surface: the lower the marsh, the more often it is flooded and the more rapidly it increases in elevation. Cahoon & Reed (1995) show that the amount of sediment deposited on a Louisiana marsh is proportional to increased inundation time of the marsh surface; and French & Stoddart (1992) observed large increases in suspended load on tides of higher elevation. Allen (1990) hypothesizes that a relationship exists among marsh elevation, relative tidal elevation and rate of organic matter accumulation. In the extreme case of the marsh having accreted to an elevation where it is no longer flooded, 100 % of the accretion is due to organic matter accumulation. Allen (1990) does not account for the possibility of organic matter deposition from exterior sources, suggested by Cahoon

& Reed (1995) to being an important contribution to organic matter accumulation on the marsh. Vertical accretion in the Hut Marsh, England, was found to vary between 8 mm/year in low areas of the marsh to 1 mm/year in higher inland areas.

The third factor that influences sedimentation on the marsh surface is vegetation type and density, which affects the sediment movement and settling on the marsh by reducing flow velocities. The total of sediment deposition has been frequently increased as more of the incoming sediment is intercepted and trapped, as the increased surface area of the vegetation causes an increase in friction.

Consequently, increased friction will result in a reduction in flow velocities as well as the ability of the water to suspend and transport sediment (Boorman et al., 1998).

Another important aspect of the vegetation is that the presence of plants tends to reduce the re-suspension of the sediment in a marsh while adding organic matter to the surface of the marsh (Boorman et al., 1998). At low flow velocities, retardation of

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water flow by vegetation is proportional to vegetation height; but, with increasing water flow velocities, retardation of tall vegetation can be less than that of short vegetation (Boorman et al., 1998). Mudflat areas are more complex in the behaviour of these environments (Deloffre et al., 2007; Tomchou Singh & Nayak, 2009).