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CONTENTS
Foreword
Author's Note
Executive Summary
Introduction
• Climate Change
• Atmospheric
Hazards
• Geological Hazards
• Hydrological
Hazards
Sources & Further Reading
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Hazard & Risk Science Review 2007 6.Hydrological Hazards |
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| While
the July 2007 floods in the UK happened too late to form the basis
of papers included in this review, as one of the three (along with
wind and quake) natural hazards responsible for most deaths, damage
and insured and economic losses, flood hazard and risk continues
to form an intensively researched field. Crossing the channel to the continent, Rudolf Brázdil6 of Masaryk University in Brno, Czech Republic, and his co-authors, address the importance of historical hydrology in evaluating contemporary and future flood risk in Europe. In Hydrological Sciences they note, especially, that studies of flood events that predate the development of national hydrological networks can provide vital information on temporal and spatial patterns of river flow, particularly in relation to extreme events. The paper summarises the types of data that form the basis of historical hydrology and provides examples of its utility, and concludes by suggesting ways in which its role in flood risk evaluation may be augmented both in Europe and elsewhere. Contrasting influences on flood risk in Europe and the US are examined in a paper in the Journal of Hydrogeology by Nicholas Pinter66 of Southern Illinois University and colleagues. Pinter and his colleagues examine trends in flood stages and find significant differences between the Mississippi and German Rhine river systems. Analysis of trends in peak flood stages and peak flood stage frequencies reveals an increasing trend at most recording stations on the Mississippi and its tributaries, but no significant change in these parameters at stations along the German stretch of the Rhine. The authors note that while the effects of climate and land use changes can be detected, a stronger influence on flood trends comes from hard engineering and other modifications of the river channels. Most importantly – from a flood risk perspective - they conclude that the increasing flood trends on the Mississippi are driven by a more aggressive history of channel engineering, which has compromised the river’s capacity to efficiently convey flood flows. Coastal flood risk in the UK and Europe While the current focus, in the UK at least, is all on river flooding, there remains concern that the coastal flood risk will, in the future, become more of a problem, particularly in the light of climate change predictions for more intense European windstorms and rising sea levels. In relation to contemporary conditions, Philip Woodworth98, and others at the UK’s Proudman Oceanographic Laboratory in Liverpool, examine – in the journal Continental Shelf Research - links between the extreme sea levels and storm surges responsible for coastal flooding in the UK, and the North Atlantic Oscillation (NAO). The NAO is the major mode of North Atlantic atmospheric variability, with the NAO index defined by the difference between normalised sealevel pressures representative of the Azores High and Icelandic Low. Periods with large positive index correspond to strong westerly winds over the UK and northern Europe, and as the magnitude of storm surges depends primarily upon the wind stress over the continental shelf some kind of relationship between extreme sea levels and storm surges and the NAO can be expected. It is not surprising, therefore, that Woodworth and co-workers determine that extreme sea levels and storm surges around the UK currently exhibit dependence on the NAO, with a significant correlation with the positive NAO mode. The authors conclude, however, that, so long as the existing relationship between extreme high waters and the NAO persist, then the impact on the UK coastline in terms of increased flood risk should be low. In a paper published in Ocean Dynamics, Paolo Pirazzoli68 of the Laboratoire de Géographie Physique in Meudon, France, and co-researchers, look for trends in sea level, storm surges and related meteorological phenomena and relate these to possible changes in flood risk along the south coast of the UK and north coast of France. The authors undertake a statistical analysis of tide-gauge data, extending back to 170 years, measured at Weymouth, Portsmouth, Dover, Cherbourg, Dieppe, Calais and elsewhere, to identify major surges, most of which are related to north-westerly or south-westerly storms that push Atlantic Ocean waters into the Channel. For most of the French coast, the authors determine no medium-term trend in surge activity that would be likely to increase the flood risk, although this is still likely to increase in response to sealevel rise driven either by local circumstances or climate change. In contrast, medium-term coastal flood risk seems to be increasing along the south coast of England, which is exposed to southerly winds and that has undergone significant subsidence in the last two decades. Pirazzoli and his colleagues also note, however, that the flood potential everywhere will increase substantially if climate change brings about a significant rise in global sea levels. Flood modelling and mapping At least for medium and large events, most hazardous natural phenomena (earthquakes, volcanic eruptions, landslides, wildfires and asteroid impacts) satisfy a power-law frequency size distribution – in essence showing that there are far fewer very large events than smaller ones – but the relationship has not yet been established for floods. Bruce Malamud55 of Kings College London and Donald Turcotte of the University of California now claim, in a paper in the Journal of Hydrology, to have accomplished this. Using a combination of historical USGS hydrological station data and palaeo-flood estimates, the authors demonstrate convincingly that floods can also be described by power-law statistics. From a hazard perspective, Malamud and Turcotte note that a power-law estimate of flood hazard will always consistently provide higher flood frequency estimates than other probability distributions currently in use, and as such represents a conservative approach to the issue. They also note that the applicability of power-law statistics to flood hazard may be compromised by climate change, which they see as introducing a major uncertainty. While statistics as applied to flood size-frequency distributions, may be important in making broad assumptions about flood hazard and forecasting, a far more immediate and specific role is played by hydrological catchment modelling, one of the most important applications of which is flood forecasting. Inevitably, uncertainty and its treatment forms a critical element of such forecasting, and its importance, in this context, is discussed by E. Todini90, of the University of Bologna, in a paper published in Hydrology and Earth System Science. Todini usefully addresses the basic issues that underpin hydrological modelling and flood forecasting, before examining the role of uncertainty and its significance. In a similar vein, K. Guganesharajah29 of the UK’s University of Surrey, and colleagues, look at the various types of uncertainty that can influence the estimate procedure in relation to floodwater level. Writing in the Journal of Hydraulic Engineering, the authors note that in practice, flood levels in channels are generally estimated for a design return period by using the discharges of the same return period. The flood levels are also influenced by other factors such as bed roughness, flow area, wetted perimeter, and friction slope which are random in nature. The surveyed cross sections and calibrated values of roughness coefficients are generally used without any allowance for their variability to assess the water levels based on discharge of a selected return period. Guganesharajah and his co-workers point out that when various uncertainties are considered, this traditional approach results in an under-estimation of water level at high return periods and over-estimation of water level at low return periods. Sample studies using the variation in channel roughness and friction slope indicated that the return period of the water level, based on a 100 year return period discharge, varied from 32 to 82 years, depending on the statistical properties of the influencing random parameters. Uncertainty is also addressed in a paper published in the Journal of the American Water Resources Association by Christopher Smemoe82 and colleagues of Brigham Young University in Utah. Smemoe and his co-authors examine the important issue of floodplain uncertainty, which they demonstrate using flood probability maps. They explain that while flood plain boundaries in respect of various return periods are denoted on flood hazard maps as single lines, no flood plain extent can actually be defined in this way. The authors present a new approach to flood plain mapping that takes account of accepted methodologies in hydrologic and hydraulic analysis while also incorporating the effects of uncertainty. Using this approach, the extents of computed floodplain boundaries are defined as a continuous map of flood probabilities, rather than as a single line. Smemoe and his team suggest that engineers and planners can use these flood probability maps for viewing the uncertainty of a floodplain boundary at any recurrence interval. They also point out that they also have application in determining graduated flood insurance rates. The utility of flood-risk maps is also demonstrated by B. Büchele9 of Germany’s University of Karlsruhe, and colleagues, who show, with application to Germany, how such maps may be used as a means of improving assessment of extreme events and associated risks. Writing in Natural Hazards and Earth System Sciences, the authors present a hydrological simulation that provides hazard mapping across the full spectrum of relevant flood events, with particular reference to extreme historical floods, coupled with a GIS tool for flood-damage assessment, based upon established stage-damage functions. «back to top« |
Hydrological Hazards UK, European and US flood: lessons and prospects Coastal flood risk in the UK and Europe Flood modelling and mapping |
