Andrew R. Black

Seasonality of flooding: a case study of North Britain

Abstract The seasonality of river flooding in North Britain displays considerable spatial variation. This paper identifies the geographical patterns of flood seasonality, using a database of events exceeding modest flood-flow thresholds at each of 156 gauging stations, and seeks to explain them in terms of climatological and catchment characteristics. Floods are found to occur at all times of year, but most rivers register at least 7846 of events in the October-March half-year, and these generally occur later in the year with distance from west to east. However, notable exceptions are superimposed upon this general pattern and, in particular, two areas of less pronounced seasonality occur on north-facing parts of the east coast. Seasonality is characterized using three complementary methods, including a four-fold seasonal classification which summarises the patterns found. In order to explain these patterns, reference is made to the seasonality of storm rainfall, soil moisture deficits, catchment size and lake storage. Seasonality class is correctly explained by reference to these catchment characteristics in 7446 of cases using discriminant analysis. The work is presented as an advance in the understanding of flood generation and, ultimately, in the assessment of flood risk.

Re-assessing the flood risk in Scotland

This paper presents a review of changes in flood risk estimation on Scottish rivers resulting from re-analysis of flood records or from the application of new methods. The review arises at a time when flood damages have received recent prominence through the occurrence of a number of extreme floods in Scotland, and when the possible impacts of climate change on flood risk are receiving considerable attention. An analysis of the nine longest available peaks-over-threshold (POT) flood series for Scottish rivers reveals that, for thresholds yielding two events per year on average, annual POT frequencies on western rivers have increased in the 1980s/1990s to maximum recorded values, while in the east, values were highest in the 1950s/1960s. These results support the results of flood modelling work based on rainfall and temperature records from the 1870s, which indicate that, in western catchments, annual POT frequencies in the 1980s/1990s are unprecedented. No general trends in flood magnitude series were found, but an unexpected cluster of extreme floods is identified as having occurred since 1988, resulting in eight of Scotlands 16 largest gauged rivers producing their maximum recorded flows since then. These shifts are related to recent increases in the dominance of westerly airflows, share similarities with the results of climate change modelling, and collectively point to increases in flood risk in many parts of Scotland. The paper also reviews advances in flood risk estimation arising from the publication of the UK Flood Estimation Handbook, developments in the collection and use of historic flood estimation and the production of maps of 100-year flood areal extent. Finally the challenges in flood risk estimation posed by climate change are examined, particularly in relation to the assumption of stationarity.

Major flooding and increased flood frequency in Scotland since 1988

Abstract Since 1988, major floods have occurred on several of the largest rivers in Scotland, causing damage estimated to be in excess of US

Approaching the physical-biological interface in rivers: a review of methods for ecological evaluation of flow regimes

170m. These events are clearly concentrated in western catchments, which have also shown increased frequencies of small events and increases in annual runoff. A climatic cause appears most likely and may also relate to similar changes observed in western Scandinavia; further work will examine these links more closely.

Development and utilization of a national web-based chronology of hydrological events/Développement et utilisation sur internet d’une chronologie nationale d’événements hydrologiques

New European legislation known as the Water Framework Directive (WFD) challenges catchment hydrologists and freshwater biologists to quantify the risk of damage to the organic communities of rivers that arises from anthropogenic distortion of the natural flow regime. Here, we take the first step towards this goal by collecting together relevant information from the two disciplines. An extensive biological literature is examined for insights into the ways in which the species and communities associated with rivers might change when the flow regime is altered. From the hydrological literature, the indicators of flow regime and flow regime change that are pertinent to ecology are described, and consideration is given to means of deriving flow regime data for ungauged river reaches. Attempts to combine hydrology and ecology in classifying rivers and in setting flow objectives to favour biota are then reviewed, together with integrated approaches to river management that aim to promote ecological quality. A significant scale disparity is noted between the disciplines, hydrology being studied at catchment, subcatchment and reach scales, and biology generally at local level. Nonetheless, both yield methods with potential applications in aspects of WFD implementation. The approach with most appeal for general risk assessment is based on the concept of hydrological alteration. This technique employs flow regime variables selected for their importance to aquatic and riparian ecology, and quantifies deviations from the natural values of these variables at reach scale. For WFD purposes, calibration of the scale of hydrological alteration in terms of risk to ecological status is desirable. In this, priority should be given to identification of the level of hydrological alteration that corresponds to the division between good and moderate ecological status.

Reassessment of flood frequency using historical information for the River Ouse at York, UK (1200–2000)

Abstract Abstract The Chronology of British Hydrological Events (CBHE) has been created as an on-line information resource (http://www.dundee.ac.uk/geography/cbhe/) in order to enhance access to and use of historical facts pertaining to British hydrological phenomena. Its scope covers floods, droughts and all other notable historical phenomena of hydrological interest. This paper sets out the technical means by which the CBHE has been created, explains the benefits to hydrologists in creating such a facility, and illustrates its utility with a number of examples. Benefits include hydrological risk assessments, historical studies of individual events and objectives in environmental education. The scope for establishing comparable national chronologies for other countries is identified, along with prospects for enhancing the utility of such systems with additional functionality such as links to on-line gazetteers and maps.

Flood-induced embankment failures on the River Tay: implications of climatically induced hydrological change in Scotland

Abstract The reassessment of flood risk at York, UK, is pertinent in light of major flooding in November 2000, and heightened concerns of a perceived increase in flooding nationally. Systematic flood level readings from 1877 and a wealth of documentary records dating back as far as 1263 AD give the City of York a long and rich history of flood records. This extended flood record provides an opportunity to reassess estimates of flood frequency over a time scale not normally possible within flood frequency analysis. This paper re-evaluates flood frequency at York, considering the strengths and weaknesses in estimates resulting from four contrasting methods of analysis and their corresponding data: (a) single-site analysis of gauged annual maxima; (b) pooled analysis of multi-site gauged annual maxima; (c) combined analysis of systematic annual maxima augmented with historical peaks, and (d) analysis of only the very largest peaks using a Generalized Pareto Distribution. Use of the historical information was found to yield risk estimates which were lower and considered to be more credible than those achieved using gauged records alone. Citation Macdonald, N. & Black, A. R. (2010) Reassessment of flood frequency using historical information for the River Ouse at York, UK (1200–2000). Hydrol. Sci. J. 55(7), 1152–1162.

Evaluating uncertain flood inundation predictions with uncertain remotely sensed water stages

Abstract Agricultural flood embankment failure frequency within the Tay drainage basin in Scotland is explored by examination of breach data (228 breaches in total) collected during an eight-year period in which a large number of high discharge flood events (with return periods of up to 120 years) occurred. The data illustrates that overtopping is the main mechanism of failure, that certain reaches and specific locations are particularly vulnerable to failure, and that a near-linear increase in number and total length of flood embankment failures occurs with percentage increase in flood peak discharge. Non-hydrological factors contributing to flood embankment instability include construction over former river channels, location on the outside of meander bends and “honeycombing” of embankments as a result of rabbit burrowing. Hydrological data suggests an increase in the frequency of high magnitude flood events since 1988 in the drainage basin and an identifiable spatial variability; catchments draining mo...

Flood warning and the use of weather radar in Scotland: a study of flood events in the Ruchill Water catchment

Abstract On January 2 2003 the Advanced Synthetic Aperture Radar (ASAR) instrument onboard ENVISAT captured a high magnitude flood event on a reach of the Alzette River (G.D. of Luxembourg) at the time of flood peak. This opportunity enables hydraulic analyses with spatially distributed information. This study investigates the utility of uncertain (i.e. non error‐free) remotely sensed water stages to evaluate uncertain flood inundation predictions. A procedure to obtain distributed water stage data consists of an overlay operation of satellite radar‐extracted flood boundaries with a LiDAR DEM followed by integration of flood detection uncertainties using minimum and maximum water stage values at each modelled river cross section. Applying the concept of the extended GLUE methodology, behavioural models are required to fall within the uncertainty range of remotely sensed water stages. It is shown that in order to constrain model parameter uncertainty and at the same time increase parameter identifiability as much as possible, models need to satisfy the behavioural criterion at all locations. However, a clear difference between the parameter identifiability and the final model uncertainty estimation exists due to ‘secondary’ effects such as channel conveyance. From this, it can be argued that it is necessary not only to evaluate models at a high number of locations using observational error ranges but also to examine where the model would require additional degrees of freedom to generate low model uncertainty at every location. Remote sensing offers this possibility, as it provides highly distributed evaluation data, which are however not error‐free, and therefore an approach like the extended GLUE should be adopted in model evaluation.

Hydrology in Scotland: towards a scientific basis for the sustainable management of freshwater resources—foreword to thematic issue

This paper reports on work that demonstrates that weather radar can yield remarkably accurate rainfall measurements in Scotland. Analysis of data during 1999 and 2000 shows Nimrod radar data to have no consistent error bias and a 24% mean error in storm rainfall totals. For rivers such as the Ruchill Water in the Perthshire Highlands this highlights the fact that the observations and quality control corrections utilised by the Met Office Nimrod system may offer particular benefits in certain flood warning applications. Assessment of wind data indicates that smaller errors occur in Nimrod radar observations during lower wind speeds (less than 15 knots) and with an easterly airflow. However, errors may also be attributed to occasions when the Numerical Weather Prediction model fails to represent the wind conditions correctly at ground level. Copyright