Research from the University of Southampton has developed and applied a method for understanding the effects and impacts of coastal flooding, which could contribute to more effective flood forecasting, defence design and land use planning.
By using observations from real coastal floods and numerical models, researchers simulated coastal floods within the Solent region of the South of England to approximate the consequences of synthetic flood events, using land and property as example measures of potential flood impacts from these hypothetical events. This is differentiated from existing information available in the Solent, by considering realistic defence responses and failures (overflow, outflanking, wave overtopping, and full breaching) and processes of water spreading across the floodplain.
PhD Researcher Matt Wadey, who worked on the study, explains: "The model allows a regional simulation to be made in 15 minutes across the entire Solent on a standard desktop PC. It also makes rare use of data sets on real events, such as the 10 March 2008 floods, to demonstrate the validity of the results."
Results were generated across a range of wave and still water level conditions over the timescale of one tidal cycle, to enable a view of present-day and 21st century impacts, and indicates the accompanying uncertainties from variations in what may be considered to be 'extreme' water level or wave events at any given time.
The Solent was selected as it provides a varied estuarine and wave exposed case study region. Importantly, it is experiencing a changing level of threat from coastal flooding, due to rising sea levels and new development in low-lying areas – these pressures are expected to continue through this century. Defended and undefended floodplains coexist. This study differed from most coastal flood research as it focused upon analysis of inundation over a region where coastal floodplains are relatively small, and water flowing over defences (overflow and wave overtopping) may be more significant compared to breach (a reduction in the effective crest height).
Simple 'bath-tub' methods (which assume that for a given sea level all areas below the elevation on the adjacent land are submerged),based only on contours, suggest that approximately 24,000 properties in the Solent are exposed to a 1 in 200 year coastal flood, with the largest concentration of risk in Portsmouth. Yet, when using the more realistic methods in this study, 5,000 to 17,000 properties might be flooded depending on how the defences function in these events. Comparison of the results with analysis of historic floods suggested less than 1,000 properties would be likely to suffer serious damage.
However, when viewing these results in the light of rising sea levels, which have been observed at Southampton over the last 75 years; it is apparent that the need to adapt or upgrade defences (or in some instances build new systems altogether) will become imperative as the nature of coastal floods changes with rising sea levels. Overtopping was found to be relatively important compared to breaching in the Solent region because coastal floodplains are small.
Robert Nicholls, Professor of Coastal Engineering who worked on the study, says: "Floods are an important long term risk to society. While society generally adapts to them, this is usually accomplished in response to real events, which can include significant losses and even deaths.
"Sea-level rise is increasing the probability of extreme events in most coastal regions; hence it is essential that methods are available to identify the likelihood and characteristics of flooding. High resolution datasets and numerical tools are currently available for many coastal areas, although are not routinely integrated; particularly where risks are not obvious or recently experienced. Our research provides a method for such integration that can support coastal management measures such as forecasting and warning, defence planning, and land-use zonation. This is intended to provide outputs that are easily communicated to the public and flood managers."
The concepts drawn from this study are transferable to similar coastal regions, whilst this research is ongoing with focus upon further validation and more detailed case studies within the Solent.
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