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Saltmarsh Erosion

NFDC Coastal Group have no reports regarding the specific causes of saltmarsh loss. It is thought that the Lymington saltmarshes and mudflats are eroding due to a combination of factors, some of which are little understood. However, the primary factors which are thought to influence the distribution and erosion of saltmarsh are:-

·Wave action

·Lack of sediment supply (mainly from offshore, alongshore and to a much lesser extent from downstream, resulting in minor or no sediment accretion, which would raise marsh surface levels)

·Waterlogging of estuarine soils (increased inundation frequency and duration)

·Dieback of saltmarsh vegetation (possibly a natural cycle, or as a result of successful establishment of vegetation)

·Tidal currents (changes in velocity, direction and duration)

·Sea level rise

Prior to the 1890s there were extensive gently sloping mudflats in the Lymington estuary. With the arrival of Spartina in ballast water, in Southampton Water around 1920 this invasive species quickly spread, hybridised and established itself on mudflats throughout the Solent. Spartina anglica, a sturdy, rhizomatous grass, could tolerate various levels of tidal immersion, and could grow a metre in height and send down roots a metre below the mud surface. At Lymington, the saltmarshes rapidly expanded, extending out to the Jack-in-the-Basket navigation pile. Since the 1920s these saltmarshes have been declining.

In general for a marsh to develop and mature it is important that the fine-grained sediments (silts and clays) are not re-entrained by high energy wave or current action. Low energy conditions, required for deposition of fine-grained sediments, may be located within the shelter of coastal embayments, behind barriers (such as Hurst Spit), or in estuaries. Compared to the Keyhaven saltmarshes, the Lymington saltmarshes are located in a relatively energetic environment; although they are protected from the prevailing south westerly and southerly storms by Hurst Spit and the Isle of Wight, they are exposed to easterly storms.

The extent and stability of inter-tidal areas are determined primarily by geomorphology and the physical forces of wind, tides and wave-action, although biological processes also influence the stability of sediments and the processes of accretion and erosion. Tidal dynamics, sediment transport pathways and locally generated waves will influence the establishment, development and maintenance of marshes within each of these environments.

Spartina vegetation has an extensive root mass, approx 0.3m deep, and the surface elements of the plants help to stabilise the mudflat surface. Strong wave and current processes at the seaward edge of the saltmarsh may result in erosion of the mud underneath the root mass, causing the overhanging mass to collapse, and thereby form saltmarsh blocks, stacks and cliffs (typically 0.2 to 0.5m high). Loss of sediment from the marsh surface results in abrasion platforms, and regression of vegetation on the seaward edge of saltmarsh. Due to the ebb-dominance of the Solent, fine-grained suspended sediments are eroded offshore. This exacerbates the lack of sediment supply feeding the saltmarshes, and results in some quite rapid rates of erosion.

Accompanying the erosion of the saltmarsh, coarse-grained sands, gravels and shells are transported onshore by strong currents and waves, particularly from easterly or south-easterly storm events in the West Solent. Such material is deposited onto the upper part of the saltmarsh edge, as low relief cheniers. They are accretional features along net-erosional coasts, and indicate a change in environmental conditions, from an environment of tidally dominated deposition, to one of wave dominated erosion.

If saltmarsh accretion is less than the rise in relative sea-level, the sediment deficit causes the marsh to deteriorate. Water-logging of the inner areas of saltmarsh results in poorly drained sediments and causes the development of anaerobic conditions in the mud surrounding the Spartina root system. The overall effect is Spartina dieback, slumping, eroding and conversion of vegetated saltmarsh to low-elevational tidal flats or open water.

NFDC Coastal Group, through the Strategic Regional Coastal Monitoring Programme for the Southeast of England, have had an extensive monitoring programme in place for approx 15 years, in order to advance the understanding of the physical, ecological and socio-economic environments, encountered in the coastal zone. The survey programme includes collection and analysis of aerial photography, topographic data, nearshore bathymetry, vegetation type mapping, sediment budgets, and hydrodynamics (wave climate, tide levels, wind speed and direction and climate). Such datasets have been invaluable in improving the understanding of the conditions and processes acting on our coast.

Analysis of historic and archived aerial photography, covering the period from the 1940s to the present enabled an average annual rate of saltmarsh loss to be determined. In the West Solent, for Lymington and Keyhaven, the rate of saltmarsh loss from the 1920s to today has been at an almost linear trend, and has not accelerated. Saltmarsh erosion is not confined to the Lymington River, but is evident across the whole Solent region.

The extent of the vegetation and drainage channels has also been mapped. Extrapolation of these data has enabled predictions on the indicative effective life expectancy for the saltmarshes at the mouth of the Lymington River to be produced, which assumes the rate of loss remains constant, Results indicate that by 2030 the effectiveness of these natural flood defences would be limited, and the saltmarshes would have disappeared by about 2050.

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