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Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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Discussion papers
https://doi.org/10.5194/hess-2018-632
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/hess-2018-632
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 31 Jan 2019

Research article | 31 Jan 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Hydrology and Earth System Sciences (HESS).

Assessing the characteristics and drivers of compound flooding events around the UK coast

Alistair Hendry1, Ivan D. Haigh1, Robert J. Nicholls2, Hugo Winter3, Robert Neal4, Thomas Wahl5, Amélie Joly-Laugel3, and Stephen E. Darby6 Alistair Hendry et al.
  • 1Ocean and Earth Science, University of Southampton, National Oceanography Centre Southampton, University of Southampton, Waterfront Campus, European Way, Southampton, SO14 3ZH, United Kingdom
  • 2School of Engineering, University of Southampton, Highfield, Southampton, SO17 1BJ, United Kingdom
  • 3Natural Hazards and Environmental Group, EDF Energy R&D UK Centre, London, UK
  • 4Department of Weather Science, Met Office, Exeter, UK
  • 5Civil, Environmental, and Construction Engineering, National Center for Integrated Coastal Research, and Sustainable Coastal Systems Cluster, University of Central Florida, 12800 Pegasus Drive, Suite 211, Orlando, FL 32816-2450, USA
  • 6Geography and Environmental Sciences, University of Southampton, Highfield, Southampton, SO17 1BJ, United Kingdom

Abstract. In low-lying coastal regions, flooding arises from oceanographic (storm surges plus tides and/or waves), fluvial (increased river discharge) and/or pluvial (direct surface runoff) sources. The adverse consequences of a flood can be disproportionately large when these different sources occur concurrently, or in close succession, a phenomenon that is known as ‘compound flooding’. In this paper, we assess the potential for compound flooding arising from the joint occurrence of high storm surge and high river discharge around the coast of UK, using observed sea level and river discharge data. First, we map the spatial dependence between high skew surges and high river discharge, considering 326 river stations linked to 33 tide gauge sites. We find that the joint occurrence of high skew surges and high river discharge occurs more frequently during the study period (15–50 years) at sites on the south-west and west coasts of the UK (between 3 and 6 joint events per decade), compared to sites along the east coast (between 0 and 1 joint events per decade). Second, we investigate the meteorological conditions that drive compound (i.e. joint occurrence of high skew surge and high river discharge) and non-compound events (i.e. high skew surge or high river discharge only) events across the UK. We show, for the first time, that spatial variability in the dependence and number of joint occurrences of high skew surges and high river discharge is driven by meteorological differences in storm characteristics. On the west coast of the UK, the storms that generate high skew surges and high river discharge are typically similar in characteristics and track across the UK on comparable pathways. In contrast, on the east coast, the storms that typically generate high skew surges are mostly distinct from the types of storms that tend to generate high river discharge. Third, we briefly examine how the phase and strength of dependence between high skew surge and high river discharge is influenced by the characteristics (i.e. flashiness, size, elevation gradient) of the corresponding river catchments. We find that high skew surges tend to occur more frequently with high river discharge at catchments with a lower base flow index, smaller catchment area and steeper elevation gradient. In catchments with a high base flow index, large catchment area and shallow elevation gradient the peak river flow tends to occur several days after the high skew surge. The previous lack of consideration of compound flooding means that flood risk has likely been underestimated around UK coasts, particularly along the southwest and west coasts. It is crucial that this is addressed in future assessments of flood risk and flood management approaches.

Alistair Hendry et al.
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Short summary
Flooding can arise from multiple sources including waves, extreme sea levels, rivers, and severe rainfall. When two or more of these sources combine, the consequences can be greatly multiplied. We find the potential for the joint occurrence of extreme sea levels and river discharge is greater on the west coast of the UK when compared to the east coast. This is due to the weather conditions generating each flood source around the UK. These results will help increase our flood forecasting ability.
Flooding can arise from multiple sources including waves, extreme sea levels, rivers, and severe...
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