Soft cliff retreat adjacent to coastal defences, with particular reference to Holderness and Christchurch Bay, UK
University of Southampton, School of Civil Engineering and the Environment,
Coastal defences reduce sediment input and modify the sediment budget, usually resulting in a sediment deficit down-drift and an accumulation up-drift. This process results in set-back adjacent to defences. Three types of set-back were identified and these occur due to the:
• terminal groyne effect, where defences stop or dramatically reduce erosion, induce a sediment deficit down-drift and cause an increase in retreat rate;
• perceived terminal groyne effect, where defences stop or dramatically reduce erosion, and down-drift retreat rates remain the same or decrease;
• initial groyne effect, where defences stop or dramatically reduce erosion, induce sediment accumulation up-drift and cause a decrease in retreat rate. Set-backs are found on defended coasts world-wide, and are complex evolving features dependent on numerous natural and anthropogenic factors.
200 set-back sites were identified in England and Wales, half on cliffed coasts. The terminal groyne effect theory was investigated on 17 sites on the soft cliffs of Holderness, Christchurch Bay and north-east Norfolk, UK, all of which erode naturally at 0.5m/yr-2.0m/yr. Historical shoreline analysis and a history of human intervention was undertaken for each study region and site.
For 13 out of the 17 case studies, a terminal groyne effect appeared to have occurred. As time passed and the magnitude of set-back increased, the terminal groyne effect became increasingly apparent. Where increased retreat resulted, the coast was affected for tens to thousands of metres down-drift. For the remaining case studies, a perceived terminal groyne effect occurred. An initial terminal groyne effect occurred at all sites.
Longshore, the continued set-back caused outflanking of defences prompting emergency works, such as repeated defence extensions up and down-drift. Over several decades of set-back, the defences formed an artificial headland
and created a crenulate shaped embayment down-drift. The planform of an embayment expanded rapidly, then reduced to a steadier retreat rate.
As shoreline management evolves from a highly defended to a less heavily managed coast, defence abandonment will result in rapid retreat. Set-backs may be created due to the juxtaposition of maintained and abandoned defences,
as illustrated at Happisburgh, Norfolk. In the coming decades, set-backs, artificial headlands and the terminal groyne effect will remain important issues for shoreline management.
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