Kevin Horsburgh

Tide‐surge interaction and its role in the distribution of surge residuals in the North Sea

[1] Storm surges are the sea level response to meteorological conditions. Scientists and engineers need to understand the interaction of surges with the tide in order to provide better estimates of extreme sea level for use in coastal defense. Using data from five tide gauges, spaced equally along the North Sea coastline around the UK, we show that the mode of peak residual occurrence is everywhere 3 to 5 hours before the nearest high water. We reveal a previously unobserved mode that falls 1 to 2 hours prior to high water, although this cluster is not associated with the highest residuals. A simple mathematical explanation for surge clustering on the rising tide is presented. The phase shift of the tidal signal is combined with the modulation of surge production due to water depth in a model that provides a good description of the residual data set. The results contain several features of interest for flood risk management. We show that large, locally generated surges are precluded close to high water. For physically realistic arrival times of any travelling surge component, the residual peak will avoid high water for any finite tidal phase shift. Furthermore, increasing the tidal range reduces the risk of residual peaks near high water. We draw attention to the existence of critical time and space scales for surge development and decay. For reliable operational forecasts of sea level, coastal numerical models need to reproduce both tides and surges with improved accuracy.

Past and Future Changes in Extreme Sea Levels and Waves

(1) Proudman Oceanographic Laboratory, Liverpool, UK (plw@pol.ac.uk) (2) The Hadley Centre, Met Office, UK (jason.lowe@metoffice.gov.uk) (3) Geophysical Fluid Dynamics Laboratory, Princeton, USA (tom.knutson@noaa.gov) (4) The Hadley Centre, Met Office, UK (ruth.mcdonald@metoffice.gov.uk) (5) CSIRO, Aspendale, Australia (kathleen.mcinnes@csiro.au) (6) GKSS, Geesthacht, Germany (woth@gkss.de) (7) GKSS, Geesthacht, Germany (storch@gkss.de) (8) Proudman Oceanographic Laboratory, Liverpool, UK (jaw@pol.ac.uk) (9) Environment Canada, Downsview, Canada (val.swail@ec.gc.ca) (10) Dalhousie University, Halifax, Canada (natacha.bernier@phys.ocean.dal.ca) (11) P.P. Shirshov Institute of Oceanology, Moscow, Russia (gul@sail.msk.ru) (12) Proudman Oceanographic Laboratory, Liverpool, UK (kevinh@pol.ac.uk) (13) National Institute of Oceanography, Goa, India (unni@darya.nio.org) (14) University of Hobart, Tasmania, Australia (john.hunter@utas.edu.au)

Thermohaline circulation of shallow tidal seas

The mechanisms controlling the temperature and salinity structure of shallow continental shelf seas have been understood for over thirty years, yet knowledge of what drives their large-scale circulation has remained relatively unknown. Here we describe a decade long programme of measurements, using satellite-tracked drifting buoys on the northwest European shelf, to draw attention to a striking picture of highly organised thermohaline circulation consisting of narrow, near surface, fast flowing jets. These are ubiquitous above sharp horizontal gradients in bottom temperatures and/or salinities. The circulation phenomena we describe are likely to be prevalent on all similar, wide, tidally energetic continental shelves including those off north-eastern China, Argentina and parts of the Arctic. The robust, repeatable observation of the key role of jets above bottom fronts results in a fundamental reassessment of how we view the dynamics of shelf seas.

Quantifying the Uncertainty in Future Coastal Flood Risk Estimates for the U.K.

Abstract Future sea-level rise will increase coastal flood risk in the U.K., yet the hazard uncertainties associated with such future risk estimates have not been fully explored. The sensitivity of coastal flood-risk mapping to future uncertainties was investigated by propagating ranges of plausible parameters through a LISFLOOD inundation model of a significant historic flood event to the North Somerset (U.K.) coast. Mean sea-level rise (including land movement) was found to have the greatest effect on the extent of flood inundation. Analysis of the latest research into the future storm-surge climate of the U.K. indicates no change above natural variability, thus, future, extreme water-level estimates (for the U.K.) should be based on observations and not Regional Circulation Models until research indicates otherwise. Evidence suggests that the current approach of forcing the inundation model with an extreme water level of a constant return period is incorrect. This uncertainty of the peak storm tide height along the coastline had a significant effect on our results. We present a new boundary-forcing technique to force the inundation model with (method C), based on the spatial characteristics of real events, which can account for the natural storm-surge variability. Indeed, if sea-level rise is included with method C, a great deal of the uncertainty surrounding such a future flood-hazard estimate can be quantified and communicated clearly and effectively.

A Three-Dimensional Model of Density-Driven Circulation in the Irish Sea

Abstract A semi-implicit version of the Princeton Ocean Model, ECOMsi, was used to simulate the cyclonic gyre that is found in the western Irish Sea during the spring and the summer. Mesoscale, seasonal, density-driven circulations such as this are an important component of the long-term flow in shelf seas, and they pose a challenge to coastal ocean models. Extensive comparisons are made here with observational data to assess model performance. The model successfully reproduced the development of the temperature field, and the associated density-driven currents, throughout seasonal simulations. The results demonstrate conclusively that the gyre is density-driven and reinforce the dynamical importance of strong nearbed horizontal density gradients. Maximum baroclinic currents of 0.14 m s−1 were obtained, and results showed that the regional kinetic energy due to the residual flow was 20%–25% of that due to tidal flow during periods in which density gradients were strongest. The model revealed important int...

Interpreting Century-Scale Changes in Southern North Sea Storm Surge Climate Derived from Coupled Model Simulations

Abstract This paper describes numerical experiments using a climate–storm surge simulation system for the coast of the United Kingdom, with a particular focus on the southern North Sea and the Thames estuary in southeastern England. Time series of surges simulated in the southern North Sea by a surge model driven by atmospheric data from a regional climate model and surges simulated by the same surge model driven by atmospheric data from a global climate model are compared. A strong correspondence is demonstrated, and a linear scaling factor relating them is derived. This factor varies slowly with location. Around the Thames estuary, extreme surges are compared in the same way, and the linear scaling factor for the extremes is found to be similar to that for the full time series. The authors therefore assert that in seeking significant trends in surge at this location using this model arrangement, the regional model downscaling stage could be avoided, if observations were used to establish a suitable scal...

A user-friendly database of coastal flooding in the United Kingdom from 1915-2014

Coastal flooding caused by extreme sea levels can be devastating, with long-lasting and diverse consequences. Historically, the UK has suffered major flooding events, and at present 2.5 million properties and £150 billion of assets are potentially exposed to coastal flooding. However, no formal system is in place to catalogue which storms and high sea level events progress to coastal flooding. Furthermore, information on the extent of flooding and associated damages is not systematically documented nationwide. Here we present a database and online tool called ‘SurgeWatch’, which provides a systematic UK-wide record of high sea level and coastal flood events over the last 100 years (1915-2014). Using records from the National Tide Gauge Network, with a dataset of exceedance probabilities and meteorological fields, SurgeWatch captures information of 96 storms during this period, the highest sea levels they produced, and the occurrence and severity of coastal flooding. The data are presented to be easily assessable and understandable to a range of users including, scientists, coastal engineers, managers and planners and concerned citizens.

A comparison of the 31 January–1 February 1953 and 5–6 December 2013 coastal flood events around the UK

A North Sea storm surge during 31 January-1 February 1953 caused Northwest Europe’s most severe coastal flood in living memory. This event killed more than 2,000 people on the coasts of England, the Netherlands and Belgium. In the UK, where this study focuses, this event was a pivotal influence for flood risk management. Subsequent progress included a national tide gauge network, a storm surge forecasting and warning service, and major defence upgrade such as the Thames Barrier. Almost 60-years later, on 5-6 December 2013 Storm “Xaver” generated a surge event of similar magnitude. This paper describes a detailed comparison of these two events in the UK in terms of: (1) the meteorological conditions; (2) the observed high sea levels; and (3) the coastal flooding and impacts. The 1953 storm had a more southerly track and generated bigger waves due to the north-northwesterly onshore winds off East Anglia. The 2013 storm had a more west-to-east path from the north Atlantic to Scandinavia. Consequently, the 1953 high waters were more extreme in the southern North Sea. However, the 2013 event coincided with larger astronomical tides, resulting in a larger spatial ‘footprint’ (the length of coastline impacted by extreme high waters and floods). The extreme sea levels impacted communities on the west, east and south coasts, with 2,800 properties flooded during the 2013 event, compared to 24,000 properties mainly between the Humber and Thames in 1953. The 1953 floods remain a modern benchmark in the UK of potential flood consequences in terms of failed defences, damaged property and infrastructure and loss of life. Measures taken after 1953 greatly reduced the consequences of the 5-6 December 2013 storm. However, the latter event is a reminder of the potential for national-scale coastal storm events and impacts. Continued monitoring of extreme sea levels and their consequences is important to inform a realistic perspective on future planning and resilience.

Ensemble Forecasting of Storm Surges

The overtopping of flood defenses by coastal storm surges constitutes a significant threat to life and property. Like all forecasts, storm surge predictions have an associated uncertainty, but this is not directly predicted by current operational systems. The dominant source of this uncertainty is thought to be uncertainty in the driving atmospheric forecast of conditions at the sea surface, which can vary substantially depending on the meteorological situation. Ensemble prediction is a technique used to assess uncertainty in forecasts of complex nonlinear systems such as weather, where small errors can quickly grow to produce significantly different outcomes. It works by running not one but several forecasts, using slightly different initial conditions, boundary conditions, and/or model physics. These are chosen to sample the range of uncertainty in model inputs and formulation so that the corresponding forecasts will sample the range of possible results that are consistent with those uncertainties. The United Kingdom Met Office has recently developed the Met Office Global and Regional Ensemble Prediction System (MOGREPS), which provides 24 different predictions of meteorological evolution over a North Atlantic and European domain with a 24 km grid length. The aim of the present project is to run a barotropic storm surge prediction for each MOGREPS ensemble member, and thereby estimate the risk of damaging events given the forecast uncertainties which are sampled by the ensemble. The system forecasts 54 hours ahead and runs twice per day. In most situations, the ensemble develops rather little spread, suggesting a fairly predictable situation and a high degree of confidence in the forecast. On some occasions, however, the spread is much larger, suggesting a greater degree of uncertainty. Initial verification results are encouraging, although statistical evaluation suggests the ensemble spread is generally too small.

Spatial and temporal analysis of extreme sea level and storm surge events around the coastline of the UK

In this paper we analyse the spatial footprint and temporal clustering of extreme sea level and skew surge events around the UK coast over the last 100 years (1915–2014). The vast majority of the extreme sea level events are generated by moderate, rather than extreme skew surges, combined with spring astronomical high tides. We distinguish four broad categories of spatial footprints of events and the distinct storm tracks that generated them. There have been rare events when extreme levels have occurred along two unconnected coastal regions during the same storm. The events that occur in closest succession (<4 days) typically impact different stretches of coastline. The spring/neap tidal cycle prevents successive extreme sea level events from happening within 4–8 days. Finally, the 2013/14 season was highly unusual in the context of the last 100 years from an extreme sea level perspective.