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

Assessing the characteristics and drivers of compound flooding events around the UK coast
Assessing the characteristics and drivers of compound flooding events around the UK coast
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 run-off) 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 the UK. We hypothesise that there will be spatial variation in compound flood frequency, with some coastal regions experiencing a greater dependency between the two flooding sources than others. We map the 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-western and western coasts of the UK (between three and six joint events per decade) compared to sites along the eastern coast (between zero and one joint events per decade). Second, we investigate the meteorological conditions that drive compound and non-compound 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 western 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 eastern 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, and 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 south-western and western coasts. It is crucial that this be addressed in future assessments of flood risk and flood management approaches.
1607-7938
3117-3139
Hendry, Alistair
42b38e3f-9651-4c7e-bce1-64816b0eb103
Haigh, Ivan
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Nicholls, Robert
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Winter, Hugo
8728682d-c3b1-42d2-a946-37c96ef8243e
Neal, Robert
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Wahl, Thomas
fbac2b37-5fd8-41d1-85b1-024e9f61ff34
Joly-Laugel, Amelie
49b8dbe7-0b42-4793-a0ef-e5cb10876bd0
Darby, Stephen
4c3e1c76-d404-4ff3-86f8-84e42fbb7970
Hendry, Alistair
42b38e3f-9651-4c7e-bce1-64816b0eb103
Haigh, Ivan
945ff20a-589c-47b7-b06f-61804367eb2d
Nicholls, Robert
4ce1e355-cc5d-4702-8124-820932c57076
Winter, Hugo
8728682d-c3b1-42d2-a946-37c96ef8243e
Neal, Robert
0d0d93b7-6739-4501-b83a-b4b65ed79ab5
Wahl, Thomas
fbac2b37-5fd8-41d1-85b1-024e9f61ff34
Joly-Laugel, Amelie
49b8dbe7-0b42-4793-a0ef-e5cb10876bd0
Darby, Stephen
4c3e1c76-d404-4ff3-86f8-84e42fbb7970

Hendry, Alistair, Haigh, Ivan, Nicholls, Robert, Winter, Hugo, Neal, Robert, Wahl, Thomas, Joly-Laugel, Amelie and Darby, Stephen (2019) Assessing the characteristics and drivers of compound flooding events around the UK coast. Hydrology and Earth System Sciences, 23, 3117-3139. (doi:10.5194/hess-23-3117-2019).

Record type: Article

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 run-off) 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 the UK. We hypothesise that there will be spatial variation in compound flood frequency, with some coastal regions experiencing a greater dependency between the two flooding sources than others. We map the 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-western and western coasts of the UK (between three and six joint events per decade) compared to sites along the eastern coast (between zero and one joint events per decade). Second, we investigate the meteorological conditions that drive compound and non-compound 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 western 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 eastern 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, and 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 south-western and western coasts. It is crucial that this be addressed in future assessments of flood risk and flood management approaches.

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Accepted/In Press date: 7 May 2019
Published date: 23 July 2019

Identifiers

Local EPrints ID: 432932
URI: https://eprints.soton.ac.uk/id/eprint/432932
ISSN: 1607-7938
PURE UUID: 4bd9e50b-e909-4223-bd73-8690dda34616
ORCID for Robert Nicholls: ORCID iD orcid.org/0000-0002-9715-1109
ORCID for Stephen Darby: ORCID iD orcid.org/0000-0001-8778-4394

Catalogue record

Date deposited: 01 Aug 2019 16:30
Last modified: 21 Aug 2019 00:37

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