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Riverine Flooding in a Warmer Britain
Abstract
A warmer Britain may also be wetter, at least seasonally, which might suggest that the frequency and magnitude of riverine flooding may increase. It may be difficult to distinguish the effects of climate warming from the effects of continuing land use change, given the naturally stochastic nature of the inputs into hydrological systems. Prediction of the effects of climate change on riverine flooding is clearly very uncertain. These uncertainties need to be assessed, particularly for more extreme events. Further research on data collection techniques and model improvements to reduce levels of predictive uncertainty is required. -Author
... Wheater (2002) reviewed the present capability of flood modelling and identified needed improvements: appropriate representation of spatial distribution of precipitation, simulation of ungauged catchments, land-use and land-management effects, urban flooding, parameterisations of in-channel and flood plain flows, and decision support modelling framework. Beven (1993) considered the impact of a warmer, wetter Britain, generally suggesting an increase in the frequency and magnitude of flooding. While it may be difficult to separate the effects of changing climate from those of land-use change, Beven (1993) provide guidance for reducing uncertainty through research that improved data collection and model improvements to reduce levels of predictive uncertainty. ...
... Beven (1993) considered the impact of a warmer, wetter Britain, generally suggesting an increase in the frequency and magnitude of flooding. While it may be difficult to separate the effects of changing climate from those of land-use change, Beven (1993) provide guidance for reducing uncertainty through research that improved data collection and model improvements to reduce levels of predictive uncertainty. Allamano et al. (2006) modelled a four similar rainfalldriven watersheds using the conceptual model IHACRES and downscaled climate from Coupled Global Climate Model 2. Using models well calibrated to present conditions, projections for floods in future climates produce contrary results ( Figure 3); models for two watersheds produce increasing floods in future climates and two decreasing floods, the combine effect of equifinality and extrapolation outside the observed range, and the assumption that the model parameters will remain constant. ...
... Conceptual models and physical models both present challenges when applied to future conditions. Beven (1993) considered the impact of a warmer wetter on flooding in Britain, generally suggesting an increase in the frequency and magnitude of flooding. While it may be difficult to separate the effects of changing climate from those of land-use change, Beven (1993) provide guidance for reducing uncertainty through research that improved data collection and model improvements to reduce levels of predictive uncertainty. ...
At the global scale, the warming of the atmosphere will increase the capacity of the atmosphere to hold and accelerate the redistribution of water in the atmosphere. This suggests that flood-generating processes linked to the atmosphere are likely to increase. However, the Intergovernmental Panel on Climate Change projections of future floods involve extremely complex issues that defy simple generalisations. Warming will alter other aspects of the water cycle increasing evaporation, changing precipitation patterns and intensity, and also affecting the processes involved in surface storage of water, including snowpack generation, snowmelt, river ice break-up, and glacial melt. Many of these are active in flood generation, and changes may cause floods to decrease as well as increase. However, these processes take place not at the global scale but at relatively local scale, making generalisations about flooding in future climates difficult and uncertain. At the global scale, the role of land use is generally unresolved, but at a watershed scale, land-use effects can be as important as changes in the meteorological processes. This review shows that while meteorologically driven flooding is expected to increase in a changed climate, making a precise pronouncement regarding all floods is unwise, as many types of floods will respond differently to changing climate and that because floods are watershed scale events, these local effects will remain important.
... global warming are uncertain, and likely to vary from region to region Jones, 1993 , they will probably include an increase in the seasonality of precipitation and runoff Ž . Beven, 1993;Marsh and Sanderson, 1997 , a marked increase in the frequency, Ž . magnitude and, possibly, seasonality of flooding Beven, 1993 , and an increase in both Ž rates of soil erosion and sediment yields Boardman and Favis-Mortlock, 1993;Wilby et ( ) . ...
... Beven, 1993;Marsh and Sanderson, 1997 , a marked increase in the frequency, Ž . magnitude and, possibly, seasonality of flooding Beven, 1993 , and an increase in both Ž rates of soil erosion and sediment yields Boardman and Favis-Mortlock, 1993;Wilby et ( ) . ...
Floodplain sediment cores collected from seven sites in the catchment of the River Ouse, in Yorkshire, UK, have been used to provide information on recent historical changes in both rates of overbank sedimentation and sediment sources. The environmental radionuclides 137Cs and unsupported 210Pb have been used to establish chronologies for each core and to estimate average sediment accumulation rates for the last ca. 30 and 100 years, respectively. Average sedimentation rates for the individual cores ranged from 0.11 to 1.04 g cm−2 yr−1. In all but one case, the estimates of average sedimentation rate during the last ca. 30 years for the individual cores are broadly similar to those for the last ca. 100 years, suggesting that overbank sedimentation rates have been essentially uniform over the longer time period. Composite fingerprints, based on a combination of geochemical and mineral magnetic properties, and a numerical mixing model have been used to investigate downcore changes in sediment source. In the case of source type, most of the cores reflect a primarily topsoil source, although there have been periods with increased contributions from subsoil/channel bank sources. Within the Ouse basin in general, the period commencing in the late 19th and early 20th century and extending through to the 1960s, was characterised by increased contributions from topsoil sources. However, contributions from subsoil/channel bank sources have increased over the last few decades. The source tracing results relating to sediment contributions from the three main geological/topographic zones are in broad agreement with the proportion of the area of the catchment underlain by each rock type. Temporal variations in the contributions from the three geological/topographic zones vary from site to site, but for the lower reaches of the River Ouse contributions from areas underlain by Permian and Triassic rocks, which mainly outcrop in the Vale of York, have increased since the turn of the century. The changes in sediment source identified are probably a reflection of changes in land use and management (and possibly changes in climate). These results enable estimates of contemporary suspended sediment fluxes and sources to be placed into a historical context and provide a means of considering the likely impact of potential future changes in land use and climate in the study basin.
... Increases in precipitation events with high precipitation quantiles have been observed in regional studies (Dai et al., 1997;Groisman et al., 2004;Hulme et al., 1998) which may increase the frequency of flooding. However, as stated by Beven (1993), flood generation and runoff are a highly nonlinear system, which is exposed to the natural and spatial/temporal variability of meteorology, topography, soil, vegetation, climate, groundwater conditions, and the channel drainage system. Therefore, other factors in addition to rainfall also need to be considered (Beurton and Thieken, 2009;Blöschl et al., 2017;Bronstert, 2003). ...
This study compares the flood regime of rivers in Ukraine and Austria over the last decades. We used data from mountain and lowland watersheds, where floods are caused by different processes. In order to identify possible shifts in the day of occurrence of annual flood maxima, we apply the kernel density method to the time series of two subperiods (1960-1987 and 1988-2015). We use the Mann Kendall test at a 5% significance level to identify significant positive or negative trends in the series of annual maximum discharges. In Austria, we observe an increasing trend in summer floods associated with increasing precipitation. In the lowland areas of Ukraine, a clear reduction in spring floods is observed, linked to shallower snow packs in a warming climate. In the Ukrainian Carpathians, on the other hand, where floods occur throughout the year, an increase in the portion of liquid precipitation during the cold period of the year leads to earlier floods and an increase in the probability of flooding in winter.
... Among recent streamflow trend studies, Lettenmaier et al. [12] stressed that the trend in streamflow are not fully parallel to the changes in precipitation and temperature due to a combination of climate and water management effects. Similarly, Beven [41] indicated that climate effects, such as changing rainfall patterns, are not the only possible causes of a change to flood regime, even when considering a change at a national scale. However, Sankarasubramanian et al. [2] and Fujihara et al. [39] found that there is a high regional sensitivity of streamflow to changes in precipitation. ...
This paper applies a procedure that identifies trends in streamflow variables in Turkey. The procedure utilizes the Mann-Kendall non-parametric test to detect trends. The research investigates three annual streamflow variables including annual minimum, mean and maximum streamflows and monthly mean streamflows for a network of 84 Turkish streamflow gauging stations in 26 basins of Turkey. The application of trend detection technique to fifteen streamflow variables has resulted in the identification of significant decreasing trends at the 0.05 level appearing mostly in the basins in western Turkey whereas almost no evidence of significant change was experienced with a general downward direction in the rest of the country. Of the 26 basins, however, only basins with the number of 14-16 and 22-25 exhibited significant increasing trend for small number of stations located mostly in the Black Sea and partly central and eastern Anatolia regions. The number of stations showing a decreasing trend is more than three-folds the number of stations with an upward trend whereas the significant downtrend exceeded the uptrends by twenty-folds. Besides, almost half of the decreasing trends were found statistically significant while approximately 8% of the increasing trends exhibited significant trend.
... Lettenmaier et al. [32] stressed that the trend in streamflow are not fully parallel to the changes in precipitation and temperature due to a combination of climate and water management effects. Similarly, Beven [33] indicated that climate effects, such as changing rainfall patterns, are not the only possible causes of change to flood regime, even when considering change at a national scale. Bates et al. [34] also indicated that trends in runoff are not always consistent with changes in precipitation. ...
This paper applies a procedure that identifies trends in monthly precipitation totals across Turkey. The procedure utilizes the regional average Mann-Kendall non-parametric test with both serial and cross-correlation to detect trends for a network of 52 Turkish precipitation gauging stations for a 30-year period (1968-1997) in seven geographical regions of Turkey. The application of regional trend analysis confirmed spatial and temporal changes in the behaviour of precipitation over Turkey. The regional trend analysis revealed that almost no evidence of significant change was experienced with a general upward regional direction appearing primarily in the Black Sea, Mediterranean Sea, Southeastern Anatolia, Eastern Anatolia regions and partly in the Marmara region compared to the Central Anatolia and Aegean regions. The regional insignificant increasing trends appeared to be dominant starting from June ending with December all over Turkey. In contrast, the most number of regional insignificant decreasing trends were obtained in January and spring months over Turkey except for the Southeastern Anatolia and Black Sea regions with some significant trends in the Aegean region.
... Among recent streamflow trend studies, Lettenmaier et al. [15] stressed that the trends in stream-flow are not fully parallel to the changes in precipitation and temperature due to a combination of climate and water management effects. Similarly, Beven [55] indicated that climate effects, such as changing rainfall patterns, are not the only possible cause of change to flood regimes, even when considering change at a national scale. However, Sankarasubramanian et al. [5] and Fujihara et al. [50] found that there is a high regional sensitivity of stream-flow to changes in precipitation. ...
The Mann-Kendall rank correlation test was performed to detect trends in this study. The research investigated 3 annual stream-flow variables including annual instantaneous minimum, mean and instantaneous maximum streamflows for a network of 57 Turkish streamflow gauging stations in 25 basins of Turkey, during 1968-2007. The application of trend detection technique to 3 stream-flow variables has resulted in the identification of significant decreasing trends at the 0.05 level, appearing mostly in the basins in western and partly in southeastern Turkey, whereas almost no.evidence of significant change was experienced with a general downward direction in the rest of the country. Of the 25 basins, however, only basins with the numbers of 12 and 22 exhibited significant increasing trend for one station each. The number of stations showing a decreasing trend is more than 9-fold that of stations with an upward trend whereas the significant downtrend exceeded the uptrends 49-fold. Besides, almost 2/3 of the decreasing trends (144 times over 159) were found to be statistically significant while approximately 13% of the increasing trends (15 times) exhibited significant trend.
... This has focused attention on the effects of floods and flood control measures, and the possible implications of future climate change (e.g. Beven, 1993). In many agricultural areas, flood embankments have been constructed to reduce the frequency of inundation. ...
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 more westerly regions exhibit the greatest shift in pattern. The significance of these shifts and future possible changes in flood magnitude and frequency for flood embankment failure risk are quantified; 5% increases in flood magnitude result in increases in the number of flood embankment failures of between 13 and 26%. Similarly a 5% decrease in flood peak discharges leads to decreases of between 11 and 25%. More generally it is shown that land next to rivers with flood embankments is sensitive to modest changes in flood generating conditions.
... Changes in river flow regimes in the UK over the past 20 years have already been noted (Gilvear et al. 2002), with increases in winter runoff (Smith and Bennett 1991) and the frequency and magnitude of flooding (Beven 1993, Black 1996. Prediction of the wider effects of climate influenced changes in river flow are difficult to make, because such changes will interact with socioeconomic considerations, such as land use and management Black 1999, Smith andBennett 1991). ...
Regulatory agencies use many chemical standards to help protect human health and the environment. Some of these standards could potentially be adversely influenced by climate change, so it is important to scope the likelihood of such changes, and develop a plan for addressing them. A review of the literature showed that many complex interactions could occur between climate change factors, chemicals, and receptors. However, the way in which chemical standards are currently set is already likely to take into account predicted climate changes in temperate regions, at least implicitly. In contrast, compliance monitoring to ensure that chemical standards are not breached may be compromised if climate change leads to more variability in measured parameters, with a consequent reduction in the statistical power of monitoring programs.
... Changes in the frequency of hydroclimatic extremes may be one of the most significant consequences of climate change (Beven, 1993;Jones, 1999). Kite (1993) showed an increase in maximum river flows that is consistent with the projected increase in extreme rainfall events in the Rocky Mountains of British Columbia. ...
The global climate change may have serious impacts on the frequency, magnitude, location and duration of hydrological extremes. Changed hydrological extremes will have important implications on the design of future hydraulic structures, flood-plain development, and water resource management. This study assesses the potential impact of a changed climate on the timing and magnitude of hydrological extremes in a densely populated and urbanized river basin in southwestern Ontario, Canada. An ensemble of future climate scenarios is developed using a weather generating algorithm, linked with GCM outputs. These climate scenarios are then transformed into basin runoff by a semi-distributed hydrological model of the study area. The results show that future maximum river flows in the study area will be less extreme and more variable in terms of magnitude, and more irregular in terms of seasonal occurrence, than they are at present. Low flows may become less extreme and variable in terms of magnitude, and more irregular in terms of seasonal occurrence. According to the evaluated scenarios, climate change may have favourable impacts on the distribution of hydrological extremes in the study area.
... if atmospheric CO 2 doubles. There is general agreement that as temperatures rise, evapotranspiration and precipitation will also change, but predicted effects on regional precipitation and stream flow patterns are variable (Beven 1993;Nikolaidis et al. 1994;Duell 1994;Rowe et al. 1994;Kwadijk and Middelkoop 1994;Easterling 1994a, 1994b). Whether the annual discharge of the Ohio River is increased or decreased by predicted regional warming, interactions of the river with its watershed will probably be affected. ...
To predict possible effects of global climatic change (via changes in ambient water temperatures and suspended sediments) on two exotic bivalves (zebra mussel, Dreissena polymorpha, and quagga mussel, Dreissena bugensis), we evaluated survival and growth at three temperatures (ambient, ambient + 2°C, and ambient + 4°C) and two turbidities (ambient and twice ambient) in outdoor tanks for approximately 3 months during both warm and cool seasons. We compared responses of zebra and quagga mussels from southwestern Lake Erie and zebra mussel from the Ohio River at Louisville, Kentucky. Experimental increases in temperature significantly enhanced growth rates in fall - early winter but not during summer - early fall. Elevated temperatures increased mortality in the warm season but not in the cool season. Zebra mussel survived better (especially the Ohio River population) than did quagga mussel at high temperatures. Inorganic turbidity had few detectable effects; relationships, where significant, varied with temperature and species. Based on these experiments and related laboratory studies, we predict that populations of Dreissena in the Ohio River and farther south will suffer overall if water temperatures increase. In contrast, more northern populations of Dreissena will probably benefit from predicted climatic change and may extend their range to higher latitudes and altitudes.
... Although periods of more frequent floods have occurred in the past (e.g. Beven, 1993), the recent major floods in Europe, such as the catastrophic central European flooding in 2002 and the UK flash flooding in 2004 and 2007, triggered the widespread perception that flood risk at the European level is increasing (e.g. EEA, 2005;Wilby et al., 2008). ...
The effects of human activities on flood propagation, during the period 1878–2005, in a 190-km reach of the middle—lower portion of the River Po (Northern Italy) are investigated. A series of topographical, hydrological and inundation data were collected for the 1878 River Po geometry and the June 1879 flood event, characterised by an inundated area of 432 km. The aim of the study is two-fold: (1) to show the applicability of flood inundation models in reconstructing historical inundation events, and (2) to assess the effects of human activities during the last century on flood propagation in the middle—lower portion of the River Po. Numerical simulations were performed by coupling a two-dimensional finite element code, TELEMAC-2D, with a one-dimensional finite difference code, HEC-RAS.
... The LOIS monitoring period has therefore been characterized by very high ¯ood frequency and magnitude and, in an historical context, signi®cantly enhanced sediment and contaminant metal ¯ux. Future trends in ¯ood frequency, magnitude and sediment ¯uxes Recent research in the USA ( Knox, 1993), Australia ( Smith, 1993) and Britain ( Beven, 1993) suggests that future climate change associated with global warming will cause marked changes in ¯ood frequency and magnitude. Climate change scenarios for the British Isles give a general consensus that with rising temperatures precipitation will increase in winter ( Rowntree, 1990;UKCCIPG, 1991;Arnell, 1992;Rowntree et al., 1993;Raper et al., 1997) and that this will be greatest in the north and west ( Santer et al., 1990;Arnell, 1992). ...
This study attempts to establish causes of historical variations in flood frequency and magnitude in a 119-year flood stage record at York, and assess its likely effects on fluxes of fine sediment and associated base metal contaminants. Climatic controls of flooding are evaluated through analysis of synoptic flood generation, and large-scale land use changes are also considered in terms of their influence on food frequency and magnitude and sediment availability. The late nineteenth century was characterized by low flood frequency and magnitude, but, as a consequence of upland metal mining, contaminant concentrations and downstream fine sediment delivery would have been high. A further decline in flood frequency and magnitude between 1904 and 1943, combined with the cessation of base metal mining, resulted in a decline of sediment and contaminant fluxes. Between 1944 and 1968 increases in upland drainage, changes in agricultural practices and high flood frequency and magnitude resulted in significantly enhanced sediment fluxes. Rates of flux declined between 1969 and 1977 owing to extremely low flood frequencies and magnitudes. In the last two decades there have been a series of extreme magnitude floods, which through remobilization of mining-contaminated alluvium have resulted in high pollutant metal mining loads, probably approaching those of the late nineteenth century. In an historical context it appears that the LOIS monitoring programme has been undertaken in a period of extremely high flood frequency and magnitude in the UK, and significantly enhanced sediment and contaminant metal flux. Analysis of synoptic flood generation suggests that any future increase in the frequency and vigour of cyclonic atmospheric circulations may result in a higher frequency of extreme floods and consequently increased sediment fluxes. Copyright © 1999 John Wiley & Sons, Ltd.
... If this is not the case, then implications for future ¯ood analyses will need to be assessed and allowed for. Climatic effects, such as changing rainfall patterns, are not the only possible causes of change to ¯ood regime, even when considering change at a national scale ( Beven, 1993). Amongst other things, the UK landscape has undergone widespread and progressive urbanization throughout the twentieth century (e.g. the area of urban land increased by 50 per cent between 1930 and 1990; Fuller et al., 1994). ...
Trends in UK flood behaviour are examined using extensive peaks‒over‒threshold and annual maxima data from 890 gauging stations. The analysis focuses on changes that have occurred at the national scale; data were therefore pooled across all available sites. Two annual series are examined, representing flood size and frequency of flood occurrence. Results are presented for the 40‒year period 1941–1980, for the 50‒year period 1941–1990 (annual maxima only), and for some limited data for 1870–1995. Three main tests for trend (linear regression, normal scores regression and Spearman's correlation) were applied using (i) conventional theoretical significance levels, and (ii) a permutation approach. Test results were generally significant under the conventional approach but were not significant under the permutation approach. This difference occurs because some of the independence assumptions required for the conventional approach are not satisfied. There is a high year‒to‒year variation inherent in the data and, relative to this variation, any underlying trends are not significant. Regional and seasonal effects are considered but no significant trends emerge.
... Flooding is the most common natural hazard in Europe and is increasingly perceived as a consequence of climate change, despite evidence of periods of more frequent occurrence of floods in the past (e.g. Beven, 1993). This perception is, no doubt, fuelled by the fact that Europe is currently experiencing a relatively flood-rich period with a spate of major floods across the continent over the last decade ( Figure 1). ...
The potential impact of climate change on fluvial flooding is receiving considerable scientific and political interest thanks to evidence from climate model projections and a widely held belief that flood risk may be increasing at European levels. This review compares published work on historical trends in UK rainfall and river flow records with high-resolution regional climate change projections, and attempts to reconcile apparent differences between the two. Attention is focused on the techniques used for climate change detection and attribution, as well as the potential confounding effects of land-use change. International and domestic efforts to build adaptive capacity rest on improved quantification of uncertainty in flood risk at very local, catchment and regional scales. This will involve further research to better integrate climate and land-management interactions, to understand changes in the dependence between different flood generating mechanisms, and to improve the characterization and communication of uncertainty at all stages of analysis. Resources are also needed to ensure that latest, but still uncertain, science is presented in an appropriate form to underpin policy development and is translated into sensible guidance for practitioners. Copyright © 2007 John Wiley & Sons, Ltd.
... They remain largely statistical estimates rather than physically-based predictions (cf. Wigley, 1989; Gregory et al, 1990; Beven, 1993; Holt & Jones, 1996a), although Gellens (1991) found an increase in winter floods and summer droughts in Belgium using a conceptual hydrological model. Drought and low flow frequencies are of major concern to water managers and there are widespread fears that these may increase. ...
Predicted climate change over the coming decades is likely to add measurable stress to water resources in many regions of the world, including some areas that are currently well endowed. The stresses are likely to involve changes in the frequency of extreme events as well as gradual changes in mean annual net resources. The paper analyses these predictions. It also attempts to place them in context, first, comparing their impact with other major barriers to sustainability, such as increasing demand, wastage, poor water resources assessment and international conflict, and, secondly, considering the limitations of current predictive techniques. Modification climatique et durabilité des ressources en eau: dans la perspective d'une menace de réchauffement planétaire Résumé Les modifications climatiques devant se produire au cours des prochaines décennies devraient aggraver le stress hydrique dans de nombreuses régions du monde, y compris dans certaines d'entre elles actuellement bien pourvues en eau. On doit s'attendre à des modifications concernant aussi bien la fréquence des événements extrêmes que les modules annuels. Cet article s'intéresse à ces prédictions et s'efforce de les remettre en perspective, en comparant leurs impacts à ceux d'autres obstacles à la durabilité comme la croissance de la demande, les rejets, les incertitudes pesant sur l'évaluation des ressources et les conflits internationaux, mais aussi en prenant en considération les limites actuelles capacités de prédiction.
... Flooding events may cause enormous economical, social and environmental damage and even loss of lives. Flooding is the most common natural hazard in Europe and is increasingly perceived as a consequence of climate change, despite evidence of periods of more frequent occurrence of floods in the past (Beven, 1993). This perception is, no doubt, fuelled by the fact that Europe is currently experiencing a relatively flood-rich period with a spate of major floods across the continent over the last decade (see figure 1); however, in a number of evaluations, the role of changing land use over time, that may have a fundamental influence on the flood generation mechanisms, is not discussed. ...
This report is the final document related to the third year, second activity whose title is: General methodologies and hydrological-hydraulic parameters supporting the definition of a climate index for changing flood risk assessment. The purpose of the collaboration between LAMPIT (department of Soil Defence, University of Calabria) and CMCC is to develop an hydrometeorological chain in order to obtain a reliable tool in the context of flood evolution prediction able to provide quantitative information of practical importance within the civil protection activities. The LAMPIT contribution to the project concerns the mathematical description of both generation and propagation of flood events at basin scale. The work here presented has been carried out in close cooperation with dr. Pasquale Schiano and dr. Paola Mercogliano. In order to embrace the problem as a whole, this report starts from a general overview of the characteristics of climate change according to IPCC evaluations, highlighting the main causes that may induce a variation of flood features (chapter 1). The term flood is often generically associated to a number of natural events that significantly differ in terms of phenomenological generation. So it seems necessary to recall in mind a possible main classification of flood phenomena in order to underline the flood type discussed herein and his peculiarities in relation to climate change (chapter 2). In a catchment-wide perspective there is the need to take into account non-climatic drivers, such as land-use change, in the evaluation of changing flood risk: land cover change within a watershed is recognized as an important factor affecting runoff and it is possible that the transformation of land across the globe could have a greater influence on runoff than climate change; that question is explained in the chapter 3. Chapter 4 is devoted to the general methodologies developed to evaluate the effects of climate change on runoff. They are mainly based on the interconnection between climate and hydrologic models; some features of each model are also analysed to highlight those aspects useful for the evaluation of climate impact on river flooding that link climate models to hydrological model. In order to better characterize the general methodologies, a number of typical results, presented in the literature, are illustrated in the chapter 5. The analysis related to the impact of climate change on flood risk is affected by a significant degree of uncertainty; some consideration on the uncertainty source is presented in the chapter 6. Finally in the chapter 7, the concept of climate elasticity of streamflow, considered to be an important indicator identifying the sensitivity of streamflow to climate change, is presented.
... Rumsby and Macklin, 1994; Tipping, 1994; Wilby et al., 1997; Foster et al., 2000). Furthermore, it is likely that future changes in climate will increase soil erosion rates and sediment yields (Boardman and Favis-Mortlock, 1993; Wilby et al., 1997), and increase the frequency and magnitude of flooding (Beven, 1993). Separating the effects of changes in land use and climate on geomorphic activity is, however, a difficult task, particularly given the general absence of long-term records of geomorphic activity. ...
Evidence from floodplain cores collected from three sites in the middle reaches of the Tweed basin in Scotland is used to reconstruct changes in sediment sources and overbank floodplain deposition rates over the last c. 100 years. Core chronologies and sedimentation rates are established using 137Cs and unsupported 210Pb measurements. The average sedimentation rates since 1963 range from 1·9 ± 0·2 to 2·2 ± 0·2 kg m−2 a−1 and are lower than the average rates for the period 1894/95 to 1963, which range from 2·7 ± 0·6 to 5·9 ± 0·9 kg m−2 a−1. There is also evidence of significant downcore variations in sediment source, defined in terms of both type (i.e. topsoil or channel bank/subsoil material) and spatial location (i.e. main geological/topographic zones). There is no clear link between the changes in overbank sedimentation rates and sediment sources and the trends shown by precipitation, weather pattern and river flow records over the past 100 years, suggesting that changes in climate alone cannot explain the downcore trends. Instead, the temporal changes in overbank sedimentation rates and sediment source appear to be linked more closely to changes in land use and land management over the past 100 years and, more particularly, the introduction of land drainage at the end of the 19th century, the rapid increase in afforestation since the 1940s and the post-war conversion of grassland to arable land. Copyright © 2002 John Wiley & Sons, Ltd.
... It can be surmised that the interest in the non-stationarity of the Polish annual maximum series (AMS) has revived with the continued discussion of climate Table 1 Origin and characteristics of¯ood data considered Basin/river (Aliev and Vishnevskyi, 1998;Beven, 1993;Burn, 1998;Caspary, 1998;Krasovskaia and Gottschalk, 1993;Kuusisto, 1992;Lindstrom, 1999;Schaake, 1990;van Montfort and Witter, 1986 and many others). All these studies, however, deal with a trend in the mean value only. ...
This paper, the last of a three-part paper, investigates into trends in annual maximum flows of Polish rivers using 39 flow series for a period of 70 years from 1921 to 1990. The gradients of trends in the mean and the standard deviation (SD) are estimated by the weighted least squares method and the best fitting linear model of trend is with the aid of the Akaike Information Criterion (AIC). It is shown that for every time series, a trend in the variance has a considerable effect on the trend estimators of the mean value. The analysis also includes seasonal peak flow series in order to obtain further insight into the detected non-stationarity of the peak flow series.Using the maximum likelihood method for parameter estimation and AIC for identification of an optimum model, the best fitting probability distribution and trend model are identified, following the methods discussed in part 1. Then, the 1% quantiles for the first and the last years of observation, denoted, respectively, as Q1%(1921) and Q1%(1990), are estimated and analysed. The influence of the distribution assumption on the values of estimators of time-dependent moments is demonstrated. In general, a decreasing tendency in both the mean and the SD of annual peak flow series is detected. This tendency is more pronounced on rivers with a high contribution of winter floods to the annual peak flow discharge series. Summer season peak flow series is found to be stationary.
... Kuusisto et al. (1994) and Dooge et al. (1998) provided a comprehensive list of references. To conclude, Beven's (1993) statement is quoted: ªThe possibility of estimating the change in¯ood frequency characteristics under different GCMgenerated scenarios would appear to be remote as a result of both a lack of disaggregation theory in moving from GCM predictions to local storm rainfalls and limitations in the accuracy of hydrological models.º ...
For dealing with hydrological non-stationarity in flood frequency modelling (FFM) and hydrological design, it is necessary to account for trends. Taking the case of at-site FFM, statistical parametric techniques are discussed for investigation of the time-trend. The investigation entails (1) an identification of a probability distribution, and (2) development of a trend software. The Akaike Information Criterion (AIC) was used to identify the optimum distribution, i.e. the distribution and trend function, which enabled an identification of the optimum non-stationary FFM in a class of 56 competing models. The maximum likelihood (ML) method was used to estimate the parameters of the identified model using annual peak discharge series. A trend can be assumed in the first two moments of a probability distribution function and it can be of either linear or parabolic form. Both the annual maximum series (AMS) and partial duration series (PDS) approach were considered in the at-site frequency modeling.
... Nonetheless, circulation pattern data have been used extensively to forecast a range of hydrological phenomena and to account for spatial-temporal trends in extreme events (e.g., Roy and Fox, 1995;Grew and Werrity, 1995). Furthermore, reliable estimates of high-magnitude precipitation events are also essential for a variety of climate change impact analyses, such as assessments of future flood hazards (Beven, 1993), engineering design (Cowpertwait, 1994), rates of soil erosion (Favis-Mortlock and Boardman, 1995) and water quality changes (Wilby, 1993). ...
Low-frequency, high-magnitude daily rainfall amounts recorded at several sites in central and southern England were related to the prevailing Lamb Weather Types (LWTs), daily weather front and airflow data. Three statistically distinct weather-type clusters were indentified and used to construct a simplified frontal model of daily precipitation occurrence/amount. The model was calibrated against station data for the period 1970–1990 and used to reconstruct observed daily precipitation between 1875 and 1969 given the historic sequence of LWTs. Although the model reproduced the incidence of low-frequency, high-magnitude events, it failed to capture variations in mean wet day probabilities and wet/dry spell persistence. This inability was attributed to the general limitations of the weather classification methodology, which did not capture all aspects of the precipitation regime with equal levels of proficiency. Therefore, the prospects for downscaling high-resolution precipitation series directly from indices of mean sea-level pressure rather than via weather patterns was discussed. Preliminary results indicate that relationships can be established between mean daily precipitation occurrence and airflow indices such as vorticity and strength of air flow. However, further research is required to establish the value of such indices for modelling low-frequency, high-magnitude precipitation events.
... Some evidence was found in the Elan basin to suggest that this might result from increases in extreme flood events towards the end of the century under the sulphate scenario. If so, then this could support suggestions that there will be significant increases in the frequency of extreme events at both ends of the spectrum (Beven, 1993; Cameron et al., 2000). However, this must remain speculative on the present evidence. ...
The research reported here uses a methodology that has been developed to generate riverflow scenarios for future climate change from GCMs. The method involves simulation of daily flows to assess the degree of change in flow frequencies, lengths of low flow periods and seasonal distributions in catchments in mid-Wales under two climate change scenarios derived from the Hadley Centre HadCM2 GCM. The method consists of: (1) establishing statistical relationships between interval-scale airflow indices (vorticity, the strength of the geostrophic wind, and its zonal and meridional directional components) and recorded precipitation and potential evapotranspiration (PET) for the period of instrumental record within each catchment, (2) using the derived transitional probabilities and correlations as input to a daily Stochastic Weather Generator, and (3) inputting this synthesised daily weather sequence into a physicallybased hydrological simulation model (HYSIM). Split-sample tests indicate good agreement between predicted and recorded daily flow frequencies, with a Nash-Sutcliffe efficiency criterion of 0.7 for daily flows and 0.9 for monthly totals. The results suggest that there will be increased frequency of low flow days during the summer and a slight increase in the length of individual low flow spells.
... Increases in winter runoff (Smith and Bennett, 1991) and the frequency and magnitude of flooding (Black, 1996) have been observed. Climate change scenarios indicate that wetter winters with greater climatic extremes may be the trend in the 21st century (Beven, 1993). One possible scenario will be that winter temperatures in the UK may rise by up to 1.5 8C, with the largest increases in the north and a possible winter precipitation increase of 5%. ...
Hydrology is a primary control on the ecological quality of river systems, through its influence on flow, channel geomorphology, water quality and habitat availability. Scottish rivers are widely perceived to be of high ecological quality, with abundant flow volumes and high water quality. However, historical and current river flow regulations, and land use change have altered the physical and chemical characteristics of Scottish rivers, with adverse consequences for aquatic biota. Baseline hydrological, geomorphological and water quality conditions in Scottish rivers are thus summarised. The impacts of river regulation and land use change on the hydrology, geomorphology and water quality of Scottish rivers are then discussed. Consequences of these changes for aquatic habitat are examined, with particular reference to the economically significant salmonid species (Salmo salar and Salmo trutta). Policy and management issues relating to the future ecological quality of Scottish rivers are reviewed. These include the impacts of climate change on ecological quality, the calculation and implementation of ecologically acceptable flows, and river restoration and best management practices within integrated catchment planning.
The research on restoration of the past environment is called for by hydrology as well as other learning fields. Then, the restoration methods of the past hydrological environment were considered for flood in River Kamo at Kyoto as an example. First, it arranged in three types (Restoration from earth surface, Restoration from geological structure and Restoration from historical records) about the restoration methods of the flood. The outline was described about each restoration methods. About the flood of River Kamo, two examples in the Heian period and the Edo period were taken up, and the above-mentioned restoration methods were applied. When using the restoration methods of three types together, it was made to become clear with any example that the past hydrological environment could be restored certainly.
The paper attempts to determine the extent to which rainfall trends and land use changes influence the river flow regime and flood response of a predominantly agricultural catchment (210 km²) in southwest England. Temporal changes in daily rainfall totals were analysed over annual, seasonal and monthly timescales but although annual average October rainfalls exhibited a weak rising trend (r=0.3, p>0.05), no long-term patterns were observed. Analysis of daily mean discharge data for the River Camel over the period 1965–2000, using the Gumbel distribution, revealed an increase in the magnitude and frequency of peak flows. Land use changes were examined using Agricultural Census Data for 1969, 1979, 1988, 1997 and 2000. Both the number of stock and the area under cereals increased from 1969 to 1997, but cereal production declined substantially between 1997 and 2000. Spatial response patterns were also examined through a comparison of the flow response of the entire Camel catchment with its more complex land use, with the predominantly pastoral De Lank headwater sub-catchment (22 km²), to ascertain the amount of change due to increased livestock numbers alone. No single factor was found to be responsible for the increases in flood frequency and magnitude in the River Camel. Rather, long-term changes in the response of the Camel system appear to emanate from the cumulative impact of subtle changes in climate, combined with increased farming activity plus urban expansion. The paper highlights the inherent difficulties of attempting to disentangle the precise effects of individual land uses on flood responses at the catchment scale and underlines the need for a holistic approach to the management of floods. By the same token, sustainable catchment management can only be achieved through greater awareness of the potential for complex, small-scale land use decisions to result in large-scale hydrological changes.
Definitions of extreme climatological and meteorological events may depend on the objective of the study, the type of data, or the researcher’s subjective opinion. This chapter discusses definitions of extreme climatic events and presents indices of precipitation extremes found in the literature. The spatial and seasonal variability in a range of these indices were analysed, and the results were used to select criteria for identifying extreme precipitation in Europe adopted in this study. These criteria were then applied in subsequent chapters. During the study period, the maximum daily precipitation totals in Europe ranged from 37.6 to 520 mm. An overwhelming majority of stations (89 %) recorded daily precipitation greater than 50 mm less than once per year. Because there is great variability between precipitation regimens in Europe, this study adopted a statistical definition of extreme precipitation events. The events were identified separately for each of the weather station and in each of the months using an empirical distribution of the daily precipitation totals. Precipitation totals exceeding or equal to the 95th percentiles of daily precipitation were selected as extreme. The 95th percentile (95P) was calculated from days with daily totals ≥1 mm during the period 1961–1990. The resulting spatial and seasonal variability of the threshold values corresponding to the 95P daily precipitation across Europe is similar to the spatial and seasons variability of the monthly totals.
This study presents the simulation of hydrological processes and nutrient transport and turnover processes using two integrated numerical models: Soil and Water Assessment Tool (SWAT) (Arnold et al., 1998), an empirical and semi-distributed numerical model; and Modelo Hidrodinâmico (MOHID) (Neves, 1985), a physics-based and fully distributed numerical model. This work shows that both models reproduce satisfactorily water and nitrate exportation at the watershed scale at annual and daily basis, MOHID providing slightly better results. At the watershed scale, both SWAT and MOHID simulated similarly and satisfactorily the denitrification amount. However, as MOHID numerical model was the only one able to reproduce adequately the spatial variation of the soil hydrological conditions and water table level fluctuation, it proved to be the only model able of reproducing the spatial variation of the nutrient cycling processes that are dependent to the soil hydrological conditions such as the denitrification process. This evidences the strength of the fully distributed and physics-based models to simulate the spatial variability of nutrient cycling processes that are dependent to the hydrological conditions of the soils.
Global warming is likely to increase annual precipitation over a large part of Europe, but changes in seasonality, variability and extreme events could also be significant and cause problems, especially during the transition period. Increased drought risk is expected in the south, but this is likely to be accompanied by a more widespread increase in flood risk. Analyses based on instrumental records, palaeohydrological analogues and models are compared. Techniques generally require considerable development before definitive predictions can be claimed, but progress is rapid in both the development of appropriate scenarios and the adaptation of methods for hydrological prediction.
The Aggregates Levy Sustainability Fund (ALSF) tax on aggregate (sand and gravel) producers in England has provided the funding for a large number of aggregate-related archaeological projects by English Heritage since 2002. One of these projects was a review, presented herein, of aggregate-related archaeology both prior and subsequent to the introduction by the UK government of new Planning Guidance Note 16 (PPG16) in 1990. This guidance advised local authorities to impose archaeological conditions upon aggregate producers seeking new permissions and extensions for aggregate working. The review commissioned by English Heritage also examined the temporal and spatial variation in interventions using data collected by the Archaeological Investigations Project (AIP) based at Bournemouth University. This paper reveals trends over time largely related to the state of the economy and aggregate demand, and more signifi cantly, large regional variations in both the number of archaeological interventions and the number of interventions encountering archaeology. While some of this variation is geologically related (hard aggregate- vs. soft aggregate-dominated regions), some variation is not, and may relate to variations in the archaeological planning process. Observations are also made as to the problems faced by archaeological interventions in aggregate quarries in accordance with planning procedures. This paper also draws attention to the probable direct and indirect effects of the changing hydrological and legislative climate on the aggregate industry and the implications this could have for aggregate-related archaeology in England, and by implication other developed countries.
The Mekong River is an attractive river basin because of many natural environmental and development projects. Also, the river basin is located in six different countries and the basin countries are composed of different situations of economic, culture, history, policies of planning of the river basin, etc. There is considerable world interest in the large Mekong river. The authors propose a new approach to planning river basin management of this river basin to be sustainable for the future. The Hydro GIS is the unique method of flood simulation and appropriate methods to clarify flood status on low relief floodplains such as the Mekong delta.
The changes to catchment scale flood risk following river restoration works, including the addition of large wood logjams to the channel, are poorly quantified in the literature. Key concerns following river restoration for river managers and other stakeholders are changes to flood hydrology at the reach and catchment scale and changes in the mobility of large wood pieces. The effects of accumulations of large wood (logjams) on local flood hydrology have been documented in the literature, showing logjams slow flood wave travel time and increase the duration and extent of local overbank inundation. Modelling studies conducted at a reach scale have shown that these local effects can slow flood wave travel time through a reach and delay the timing of flood peak discharge at the reach outflow. How these local and reach scale effects translate to the catchment scale remains to be illustrated in the literature.
In this thesis a combination of field and modelling studies are used to; elucidate the link between logjam form and function, to quantify the mobility of pieces of large wood relative to their physical characteristics, to predict the changes in floodplain forest restoration over time and to provide predictions of changes to catchment scale flood hydrology following river restoration at a range of scales and locations.
It is shown that logjams inducing a step in the water profile are most effective at creating diverse geomorphology and habitats. Logjams were found to account for 65% of flow resistance in forested river channels, rising to 75-98% of flow resistance where the logjam was inducing a step in the water profile.
Large wood in small forested river channels was found to be highly mobile with 75% of pieces moving, with the longest transport length of 5.6km. Large wood mobility is governed primarily by the length of a piece of wood with wood in excess of 1.5x channel width a threshold for a lower probability of movement.
Hydrological modelling using OVERFLOW shows that reach scale river restoration can lead to modest changes in catchment scale flood hydrology. It is concluded that flood risk management can incorporate river restoration, but that results are likely to be unpredictable if engineered logjams are used alone. Substantial benefits in reducing catchment outflow peak discharge (up to 5% reduction) are modelled for floodplain forest restoration at the sub-catchment scale (10-15% of catchment area), rising to up to 10% reductions as the forest matures and becomes more complex.
The flow series for the River Thames near its tidal limit is one of the most studied in the world. Its length and completeness, and the richness of the historical information which augments the formal flow record, ensures that the series is of immense value. However, the variability in flood magnitude and frequency that it captures needs to be interpreted with caution. The homogeneity of the time series is influenced by a wide range of factors, including changes in the hydrometric capability of the gauging station and the impact of differing water, river and land management practices on the flow regime. Nevertheless, both the daily flow series and the record of lock levels provide some reassuring signals regarding the resilience of the Thames to fluvial flood risk in a warming world. Since routine flow measurement began in 1883, the Thames basin has seen a substantial rise in air temperature and a tendency for both winter rainfall and annual runoff to increase. There is no trend in fluvial flood magnitude however, partly reflecting a decline in snowmelt contributions to major floods and annual maximum lock levels show a significant decline, reflecting a highly sustained programme of river management.
Hydrological processes are an integral component of both global climate change arising from increasing concentrations of greenhouse gases and the assessment of subsequent terrestrial impacts. This article examines the potential sensivity of water resources in the UK to climatic change as exemplified by the 1988-92 drought. The representation of hydrological processes at three distinct model scales is then discussed with reference to global hydrology, regional downscaling and catchment-scale responses. A final section speculates on future directions of research for an emerging greenhouse hydrology.
Because floodplains are 'attractors' of human activities and settlement, Late Pleistocene and Holocene palaeohydrological changes are an important part of archaeology. Indeed there has been a tradition of palaeohydrological writing within archaeology centring around topics such as river transport, drought/floods and floodplain farming, rivers as physical and cultural barriers, riverine resources, and the flood hazard. An agenda for continuing research in this inter-disciplinary area in a more quantitative and rigorous manner than in the past has been developed. This recognizes that the potential of palaeohydrological research to be applied to the study of past and future environmental change revolves around three areas: (i) the estimation of past frequency distributions and their relationships to climate; (ii) the past and future representations of risks; such as flooding; (iii) the uses and limitations of analogies (e.g. by using the Medieval climatic optimum as an analogy for future global warming).
Stemflow leaching from the above-ground vegetative surfaces of broadleaved deciduous canopy trees has been well documented during the growing season. Winter stemflow leaching from the leafless crowns of deciduous trees is less well understood, especially in the context of global climate change. Boreal and northern temperate forests are forecast to have a lower incidence of snow events and an increased frequency of rain and mixed precipitation events. A change in the seasonal precipitation regime may be significant due to linkages among global change, stemflow leaching, and biogeochemical processes. The aim of this paper is to (1) demonstrate the extent of winter stemflow nutrient leaching from deciduous trees; (2) explore how winter stemflow leachate quantity may vary as a function of the type of precipitation event; and (3) quantify the extent to which an increase in the incidence of snow-to-rain events would enhance stemflow leaching. Measuring meteorological conditions, stemflow volumes, and stemflow chemistry over two successive winter seasons in New England demonstrated that winter stemflow drainage was significantly enriched compared to the incident bulk precipitation: 162 times greater for K+, 44 times greater for Ca2+, and 29 times greater for Mg2+. Snow-to-rain events leached considerably greater quantities of base cations from the deciduous trees than all other types of precipitation events. An increased frequency of snow-to-rain events from 13.8% to 25% of winter precipitation events would substantially increase mean stemflow nutrient inputs to the bases of forest trees during winter. Implications for significantly increased winter stemflow leachate inputs, corresponding to an increased incidence of snow-to-rain events, include a shift in the biogeographic range of species, delayed leaf emergence, and increased soil respiration.
Lake sediment-based estimates of sediment yield have frequently been used to reconstruct changing patterns of sediment supply arising from environmental change. Such analyses have often emphasized the importance of anthropogenic factors and, in particular, changing land use of management practices over timescales beyond the scope of direct process monitoring. This paper examines several U.K. sediment yield chronologies within the context of mesoscale atmospheric circulation patterns. Changes in the frequency of the winter cyclonic Lamb weather type since 1861 were found to account for a significant proportion of the variation in sediment yields. The results also have implications for future sediment accumulation rates given the potential geomorphological consequences of global climate change. © 1997 by John Wiley & Sons, Ltd.
Critical to restoring the nature conservation value of many river corridors is an understanding of how alluvial landscapes will respond to cessation of river management and land use practices that have previously degraded the environment. This paper analyses changes in valley floor landforms and vegetation patch dynamics, in relation to fluvial disturbance, over a period of almost 100 years following flood embankment abandonment on a wandering gravel-bed river, namely the River Tummel, Scotland. Such rivers were once typical of many draining upland areas of northern maritime Europe. Prior to abandonment the valley floor landscape was agriculturally dominated and the river for the most part was single thread confined between flood embankments. The pattern of landform change and vegetation patch development over time following a decision in 1903 not to maintain embankments was tracked by geomorphic and land cover mapping utilizing successive sets of aerial photography for the period 1946 to 1994. A historical context for these changes was also feasible because the channel planform in 1900 and earlier channel planform changes dating back to 1753 were known due to the availability of old maps and earlier geomorphic studies. The land cover mapping was validated by comparison of results produced from the interpretation work on the 1994 aerial photographs with the field-based UK National Vegetation Classification protocol. The findings of the study illustrate that bordering the River Tummel fluvial landforms and vegetation patch mosaics, presumably resembling those that occurred before valley floor land use intensification, evolved in less than 50 years after flood embankment abandonment with a resultant increase in habitat diversity. The change relates primarily to flood-induced channel planform change and moderate levels of fluvial disturbance. The general significance of this change to plant species diversity on the valley floor of the River Tummel and elsewhere is discussed as is possible implications of the upstream impoundment and scenarios for climatically induced changes in flood frequency and magnitude. The overall outcome is the strong possibility that simple changes in river management and land use practices could result in re-establishment of the nature conservation value of similar river corridors in Europe over the medium term without active restoration efforts. Copyright © 2002 John Wiley & Sons, Ltd.
A 236-year time series (1757–1992) of monthly areal average precipitation for Scotland is produced using the ‘official’ record compiled by the Meteorological Office (UKMO) from 1869 onwards, corrected for known inhomogeneities, and extended back to 1757 using previously unpublished data from the UKMO. The complete series is evaluated and examined for fluctuations and extreme values with particular reference to observed precipitation increases over the last two decades. Recent increases in annual precipitation, and associated seasonal variations, are judged to be unusual despite the recognition of a longer term trend to higher annual and winter values. Correlations of precipitation since 1869 with Lamb weather types show differences within the record and from other work, which suggest that this index does not fully capture the synoptic conditions relevant to precipitation over Scotland as a whole. The series since 1869 was also used to test the predictive value of climatic normals for 3, 5, 10, 15, 20, and 25 years alongside the standard 30-year averages of precipitation. It is suggested that the best estimate of seasonal and annual precipitation totals for the following year can be achieved by the application of 3- and 5-year normals, rather than by using precipitation averages of longer duration.
This paper examines the timing, nature and magnitude of river response in upland, piedmont and lowland reaches of the Tyne basin, northern England, to high-frequency (20–30 year) changes in climate and flood regime since 1700 AD. Over this period fluvial activity has been characterized by alternating phases of river-bed incision and stability coinciding with non-random, decadal-scale fluctuations in flood frequency and hydroclimate that appear to be linked to changes in large-scale upper atmospheric circulation patterns. Episodes of widespread channel bed incision (1760–1799, 1875–1894, 1955–1969) result from a higher frequency of large floods (> 20 year return period) and cool, wet climate under meridional circulation regimes. Phases of more moderate floods (5–20 year return period), corresponding to zonal circulation types (1820–1874, 1920–1954), are characterized by enhanced lateral reworking and sediment transfer in upper reaches of the catchment, and channel narrowing and infilling downstream. Rates of fluvial activity are reduced in intermediate periods (1800–1819, 1895–1919) with no dominant circulation regime associated with lower flood frequency and magnitude. The results of this study provide a valuable guide for forecasting probable drainage basin and channel response to future climate change.
One of the most significant anticipated consequences of global climate change is the increased frequency of hydrologic extremes. Predictions of climate change impacts on the regime of hydrologic extremes have traditionally been conducted using a top-down approach. The top-down approach involves a high degree of uncertainty associated with global circulation model (GCM) outputs and the choice of downscaling technique. This study attempts to explore an inverse approach to the modelling of hydrologic risk and vulnerability to changing climatic conditions. With a focus targeted at end-users, the proposed approach first identifies critical hydrologic exposures that may lead to local failures of existing water resources systems. A hydrologic model is used to transform inversely the main hydrologic exposures, such as floods and droughts, into corresponding meteorological conditions. The frequency of critical meteorological situations is investigated under present and future climatic scenarios by means of a generic weather generator. The weather generator, linked with GCMs at the last step of the proposed methodology, allows the creation of an ensemble of different scenarios, as well as an easy updating, when new and improved GCM outputs become available. The technique has been applied in Ontario, Canada. The results show significant changes in the frequency of hydro-climatic extremes under future climate scenarios in the study area. Copyright © 2007 John Wiley & Sons, Ltd.
Analysing the impact of future climate change on hydrological regimes is hampered by the disparity of scales between general circulation model (GCM) output and the spatial resolution required by catchment-scale hydrological simulation models. In order to overcome this, statistical relationships were established between three indices of atmospheric circulation (vorticity and the strength and direction of geostrophic windflow) and daily catchment precipitation and potential evapotranspiration (PET) to downscale from the HadCM2 GCM to the Upper Wye experimental catchment in mid-Wales. The atmospheric circulation indices were calculated from daily grid point sea-level pressure data for: (a) the Climatic Research Unit observed data set (1975–90); (b) the HadCM2SUL simulation representing the present climate (1980–99); and (c) the HadCM2SUL simulation representing future climate conditions (2080–99). The performance of the downscaling approach was evaluated by comparing diagnostic statistics from the three downscaled precipitation and PET scenarios with those recorded from the Upper Wye catchment. The most significant changes between the downscaled HadCM2SUL 1980–99 and 2080–99 scenarios are decreases in precipitation occurrence and amount in summer and autumn combined with a shortening of mean wet spell length, which is most pronounced in autumn. A hydrological simulation model (HYSIM) was calibrated on recorded flow data for the Upper Wye catchment and ‘forced’ with the three downscaled precipitation and PET scenarios to model changes in river flow and hillslope hydrological processes. Results indicate increased seasonality of flows, with markedly drier summers. Analysis of extreme events suggests significant increases in the frequency of both high- and low-flow events. Copyright © 2002 John Wiley & Sons, Ltd.
Attempts to increase European biodiversity by restoring rivers and floodplains are based on inadequate data on natural systems. This is particularly the case for NW European rivers because all catchments have been impacted by agriculture and river engineering. If river restoration is to have an ecological, as opposed to `cosmetic' design basis then baseline models are required. However, this poses three questions; (a) what is the natural river-floodplain state, (b) how can it be defined and modelled and (c) can this state be recreated today? The first two questions can only be addressed by using palaeohydrological and palaeoecological data. A second and equally vital consideration is the stability/instability of any restored system to change in external forcing factors (e.g. climate) and in this context it may not be realistic to expect baseline models to provide equilibrium solutions but instead to define process-form domains. Over the last two decades evidence has accumulated that the natural state of lowland rivers in much of NW Europe was multi rather than single thread-braided, anastomosing or anabranching. Until recently our knowledge of floodplain palaeoecology was generally derived from pollen diagrams, which have source-area of problems and lack of taxonomic specificity. The precision and breadth of palaeoecological reconstruction (including richness and structure) has been greatly increased by the use of multiple palaeo-indicators including macrofossils, diatoms and beetles. The dynamics of small to medium sized, low-energy, predeforestation floodplains were dominated by disturbance (windthrow, beavers, etc.) and large woody debris. In order to compare the hydrogeomorphological basis of floodplain ecology, both temporally and spatially, a simple index of fluvial complexity is presented. Palaeoecological and geomorphological investigations have the potential to provide in-depth models of the natural range of channel conditions and sensitivity to external change that can be used to provide a scientific basis for floodplain restoration. There is also the possibility that floodplain-channel restoration may be a valuable tool in the mitigation of future geomorphological change forced by climatic instability.
This chapter examines the dimensions and some implications of sustainable development — ecological, economic and social —
the principles and criteria for environmental sustainability. A method is set out which may be used to help the decisionmakers
in a climate of justification in terms of benefit/cost analysis related to a single function (flood defence). The roles of
land use planning and environmental economics are assessed. The greater the scale and complexity of development in terms of
land use, the more likely it has been that the hazards associated with the water environment are inadequately regarded. This
situation is set to change as understanding of the relationship between land use and the water environment grows alongside
experience of the consequences of unsustainable decision-making. The needs of flood defence should therefore grow from holistic
appraisal of the other aspects of a catchment’s water environment, not in isolation from them. In most situations, there is
a choice of actions to be taken in response to a perceived hazard. This choice extends in several dimensions: on what principles
and with what criteria to appraise the problem (doing nothing may be the best option); what to do, where, and how much (effectiveness
— function, location and extent), how to do it (efficiency), and at what cost (economy). Some implications for the professional
project manager, society and ecology are drawn, together with some conclusions from experience in the R. Thames catchment.
One of the objectives of floodplain management planning is to ensure that decisions over land use do not place property at
risk from flooding, nor cause increased risk of flooding elsewhere. To be effective, the plan must be related to, and influence,
land use in the rest of the catchment. In this sense, the plan should act as a pivot between the development needs of the
catchment (and probably the wider region of the country) and the floodplain. The environmental capacities of the floodplain
may act as a constraint on catchment and regional activities in many ways, including the provision of satisfactory standards
of flood defence. This may in itself involve control of other processes such as the catchment’s hydrological response and
fluvial geomorphology, using environmentally sensitive techniques which benefit ecology in terms of habitat conservation and
restauration and improved water quality. Some implications of this approach are indicated, such as the need to apply source
control techniques for quality and quantity of rainfall runoff, the need to provide buffer zones and protect areas of sensitivity
and critical natural capital, and the need for environmental audit. To ensure sustainability, the floodplain plan should not
only involve the public but be subject to a democratic process, for example through being reflected in development plans produced
by local authorities.
This paper offers an alternative to the use of geomorphological and sedimentological evidence for the reconstruction of flood and low flow frequencies. It is based on a technique developed to estimate the hydrological impact of future climate change and it uses either observed or calculated meteorological parameters. It is possible to use this method directly without modification to ‘hindcast’ events within the period of regional meteorological records, which in areas like the UK extend back over the last 150 years. It should also be possible to extend the approach to earlier periods using reconstructed meteorological parameters based on surrogate data, such as ships' logs.The resulting extreme flow sequences may be used to study past hydrological regimes or to improve estimates of present-day risks by extending the flow records. The technique uses an airflow index-based stochastic weather generator to create hydrometeorological parameters to input into a physically based hydrological simulation model. The method is illustrated here in a reconstruction of daily flow series for the River Wye catchment above Rhayader, mid-Wales, for the period 1889–1998. The method makes use of observed climatic variables for the entire period, with the aim of capturing actual climate variability occurring over the 110-year period. Changes in high flow characteristics are assessed using the mean annual flood (MAF), Q5 flow and peaks-over-threshold (POT) calculated from the 110-year simulated daily flow series. This particular application shows evidence of a possible trend towards increasing magnitude and frequency of high flow events, which, if continued, would have implications for flood and water resource management. Looking at evidence from the last 110 years helps to place possible future trends within the context of past variations in high flow extremes due to both natural and anthropogenically influenced fluctuations in climate.
The paper attempts to determine the extent to which rainfall trends and land use changes influence the river flow regime and flood response of a predominantly agricultural catchment (210 km2) in southwest England. Temporal changes in daily rainfall totals were analysed over annual, seasonal and monthly timescales but although annual average October rainfalls exhibited a weak rising trend (r=0.3, p>0.05), no long-term patterns were observed. Analysis of daily mean discharge data for the River Camel over the period 1965–2000, using the Gumbel distribution, revealed an increase in the magnitude and frequency of peak flows. Land use changes were examined using Agricultural Census Data for 1969, 1979, 1988, 1997 and 2000. Both the number of stock and the area under cereals increased from 1969 to 1997, but cereal production declined substantially between 1997 and 2000. Spatial response patterns were also examined through a comparison of the flow response of the entire Camel catchment with its more complex land use, with the predominantly pastoral De Lank headwater sub-catchment (22 km2), to ascertain the amount of change due to increased livestock numbers alone. No single factor was found to be responsible for the increases in flood frequency and magnitude in the River Camel. Rather, long-term changes in the response of the Camel system appear to emanate from the cumulative impact of subtle changes in climate, combined with increased farming activity plus urban expansion. The paper highlights the inherent difficulties of attempting to disentangle the precise effects of individual land uses on flood responses at the catchment scale and underlines the need for a holistic approach to the management of floods. By the same token, sustainable catchment management can only be achieved through greater awareness of the potential for complex, small-scale land use decisions to result in large-scale hydrological changes.
This paper explores the potential for assessing the impacts of climate change upon flood frequency for the gauged, upland Wye catchment at Plynlimon, Wales, UK, while taking account of uncertainty in modelling rainfall-runoff processes under current conditions. A continuous simulation methodology which uses a stochastic rainfall model to drive the rainfall-runoff model TOPMODEL is utilised. Behavioural parameter sets for both the rainfall model and TOPMODEL are identified prior to the climate change runs using the Generalised Likelihood Uncertainty Estimation (GLUE) methodology. The "medium-high" UKCIP98 climate change scenario, obtained from the HadCM2 GCM simulations, is used as a starting point for a variety of different scenarios at the catchment scale. It is demonstrated that while the scenarios have only a small impact upon the likelihood weighted flood frequency uncertainty bounds in comparison with the current condition scenario, the risk of a given discharge as an element in the distribution of T year floods is changed. This underlines the need to account explicitly for uncertainty within hydrological modelling, especially in estimating the impacts of climate change.
Keywords: Climate change; Floods; Frequency; TOPMODEL
Whether the floods experienced during the last decade in Germany and in other European countries are triggered or worsened by human activities has been the subject of a great deal of debate. Possible anthropogenic activities leading to increased flood risk include river regulation measures, intensified land use and forestry, and emissions of greenhouse gases causing a change in the global climate. This article discusses the latter by reviewing the existing knowledge on the subject. First, the relevance, capabilities, and limitations of climate models for the simulation and analysis of flood risk under aspects of the anthropogenic climate change are described. Special consideration is given here to differences between the "typical" spatial scale of climate models and hydrological flood models. Second, observations of trends in climate variables relevant for river flooding issues are summarized. Special emphasis is put on the Rhine and other German catchment areas. Third, the possibilities of modeling the different parts of the "cascade of flood risk" are summarized, introducing the special features of meteorological, hydrological, and river hydraulic models.
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