Dependence between sea surge, river flow and precipitation in south and west Britain

Abstract

Estuaries around Great Britain may be at heightened risk of flooding because of the simultaneous occurrence of extreme sea surge and river flow, both of which may be caused by mid-latitude cyclones. A measure especially suited for extremes was employed to estimate dependence between river flow and sea surge. To assist in the interpretation of why flow-surge dependence occurs in some areas and not in others, the dependence between precipitation and surge and between precipitation and river flow was also studied. Case studies of the meteorological situations leading to high surges and/or river flows were also carried out. The present study concerns catchments draining to the south and west coasts of Great Britain. Statistically significant dependence between river flow and daily maximum sea surge may be found at catchments spread along most of this coastline. However, higher dependence is generally found in catchments in hilly areas with a southerly to westerly aspect. Here, precipitation in south-westerly airflow, which is generally the quadrant of prevailing winds, will be enhanced orographically as the first higher ground is encountered. The sloping catchments may respond quickly to the abundant rainfall and the flow peak may arrive in the estuary on the same day as a large sea surge is produced by the winds and low atmospheric pressure associated with the cyclone. There are three regions where flow-surge dependence is strong: the western part of the English south coast, southern Wales and around the Solway Firth. To reduce the influence of tide-surge interaction on the dependence analysis, the dependence between river flow and daily maximum surge occurring at high tide was estimated. The general pattern of areas with higher dependence is similar to that using the daily maximum surge. The dependence between river flow and daily maximum sea surge is often strongest when surge and flow occur on the same day. The west coast from Wales and northwards has slightly stronger flow-surge dependence in summer than in winter, whereas dependence is stronger in winter than in summer for the southern part of the study area.

Estimations of statistical dependence as joint return period modulator of compound events. Part I: storm surge and wave height

“…Overall, the low-level flow characteristics (prevailing and dominant winds) appear to be first in harmony with the transient nature of (extra-tropical) storms and their footprints (storm tracks). This seems to be consistent with similar findings (even if they apply for different pair of variables) in Defra TR1 & TR3 Reports and in Svensson and Jones (2002, 2004a, 2004b documenting that (storm) surge and (river) flow dependence appears to be largely influenced by the storm track of the depressions. 15…”

“…It seems that there is a clear tendency for the northern extreme compound events to take place mostly with southwestern components of stronger wind intensity (compared to the southern ones) with emphasis during the cold months. This appears mainly to be in harmony with the transient nature of winter storms and their storm tracks as already indicated in Jones (2004a &2004b) in a similar analysis for surge and discharge compound events around Britain.…”

“…If one variable exceeds a certain extreme (high-impact) threshold, then the value of  represents the risk that the other variable will also exceed a high-impact threshold as explained in Hawkes (2004), Jones (2004a &2004b), Petroliagkis et al (2016). 10 Following Coles et al (2000), if all of the extreme observations of two variables exceed a given threshold at the same time, this indicates total dependence (  1).…”

“…In the coastal and inter-tidal zones, high waves and extreme tidal surge events can occur simultaneously with extreme precipitation events and high river flows, leading to increased flood severity, duration or frequency as highlighted in Svensson and Jones (2002, 2004a, 2004b, Hawkes and Tawn (2000), Hawkes et al (2005). These interactions are generally referred to as coincident or compound events (IPCC, 2012).…”

Epistemic uncertainties and natural hazard risk assessment. 1. A review of different natural hazard areas

“…Assessing the joint frequency of such events has been receiving increasing attention (e.g. Svensson and Jones 2004). In particular, the covariation of different causes of the hazard, and joint occurrences across multiple locations has been investigated using flexible functional relationships based on overlap likelihood relationships (Gill and Malamud, 2014) and copulas (e.g.…”