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Channel Coastal Observatory

Marine Environmental and Data Information Network (MEDIN)

Shoreline Evolution, Erosion and Flooding

The ancient Solent River flowed eastwards across the present site of Christchurch Bay and along the main axes of the West and East Solent into a major “English Channel”, south and east of the present day Isle of Wight. All of the modern rivers discharging into the Solent, Christchurch and Poole Bays fed into the Solent River, and therefore drained a large catchment throughout the Pleistocene stage of the Quaternary Period. During the early Holocene period, sea-levels rose rapidly, flooding the East Solent and widening through coastal erosion. Between 8600 and 6800BP the continuous chalk ridge that, extended between The Needles, Isle of Wight and Handfast Point, in the Purbecks was breached by rising sea levels, creating a connection with the shallow but rapidly expanding Christchurch Bay. Erosion of the shoreline and soft cliffs produced sediment that was transported eastwards to form Hurst Spit. Rising sea levels led to the formation of a permanent tidal channel at Hurst Narrows with powerful currents, linking the West Solent to Christchurch Bay.  Increased wave action within the Solent caused erosion of the shore and low soft cliffs, and led to the formation of barrier beaches and spits. Due to the sheltering effect of the Isle of Wight and Hurst Spit, the mainland coast was relatively sheltered against direct wave and tidal action and became characterised by the sedimentation of fine sediments and the growth of tidal flats and fringing saltmarshes.

The coastline is a complex and dynamic environment, continually changing and evolving due to the prevailing conditions, extreme events, natural processes and anthropogenic influences that operate there. Variations in the rate, scale or frequency of these coastal processes can have serious implications on the communities, land use and natural environments affected, often resulting in erosion of beaches and cliffs, or extensive tidal flooding.

Within the Solent, beaches have generally declined in front of defences over the past 100 years, a process that has been attributed to defences that have slowed or halted barrier transgression preventing reworking of gravel deposits of the coastal plain and reduced landward inputs from ebb tidal deltas potentially related to major dredging activities. Shoreline stability has been achieved only by continued management of coastal defences.

The scale and rate of historic and projected loss of saltmarshes and inter-tidal mudflats throughout the Solent has been identified and quantified through the Solent Dynamic Coastal Project, and Solent Coastal Habitat Management Plan (CHaMP) research projects. Rates of narrowing are likely to increase due to sea-level rise and increased wave energy associated with climate change; trends of saltmarsh and mudflat erosion are likely to continue into the future and total losses should be anticipated in the medium to long term on the exposed frontages. As these inter-tidal areas are lost, the upper shores will become exposed to increasing wave energy and erosion and low lying land will become increasingly vulnerable to flooding.

Monitoring of the shoreline and nearshore zone aids managers, planners and politicians in identifying and understanding the importance of existing and future requirements for coastal erosion and flood risk measures, and to determine, control or prevent development at or near the coast.