Abstract The assessment of flood risk is now widely recognised to need research and data on both the probability and the consequences of flooding; the research reported here concentrates on the latter data input. Building on the UK F oresight F uture F looding project, this paper describes the development of future scenarios through which to assess possible future flood risk in the T aihu B asin area of C hina. In addition, we describe the flood damage assessment model that was developed there to build on these scenarios so as to calculate anticipated risk. Acknowledged methodological limitations remain, but some important developments have resulted. First, the pre‐existing flood loss data that were available from S hanghai meant that this aspect of the risk model's input was more regionally appropriate than would otherwise have been the case. Second, the damage assessment has been related both to constructed scenarios and to an agreed N ational P lan, so that the two can be compared. Third, the scenario construction was linked in T aihu to the statistical base contained in the 2006 Y earbook and the F ifth N ational S ocio‐economic S urvey data, giving a sounder ‘base case’ of current flood vulnerability than used in the UK F oresight F uture F looding project. Finally, much more attention was given here to agricultural production and flood risk, given the importance of agriculture in the C hinese economy and its focus on food production for a growing population.
The authors analysis is an interesting and timely attempt to account for the effects of bank stability on the geometry of stable gravel rivers, as the neglect of the width dimension is a major limitation of channel morphology models. In the development of their model the authors encounter the problem that there are apparently more dependent variables than equations available for solution. To make the solution statistically determinate, they recognize that an additional equation is required and, therefore, use an extremal hypothesis. The authors acknowledge that the use of extremal hypotheses has been widely criticized on the grounds that the method lacks a physical basis. The main cause for concern is that the method only provides a method to calculate the channel width, although it does not suggest a mechanism by which width adjustment to a stable value is achieved (Bettess et al. 1988). In spite of this criticism, the authors argue that the use of an extremal hypothesis is justified on two grounds. First, they suggest that the method has enjoyed predictive success. Second, they argue that the inclusion of additional relations describing boundary shear stress and bank stability are insufficient to close the problem and that one extra relationship, in this case an extremal hypothesis, is still required. We would first like to discuss the apparent empirical success of the various extremal hypothesis approaches. There are a large number of extremal hypotheses to choose from, including the maximization of sediment transport rate, minimization of energy, and maximization of friction factor. Most studies have attempted to validate their results using comparisons between predicted and observed stable channel geometries, usually with a reasonable degree of success. However, no concerted effort was made until recently to directly verify the validity of various extremal hypotheses, by observing trends in the relevant parameters in unstable channels as they evolve toward equilibrium. Direct observations of the morphology and flow discharges of evolving channels were obtained in two diverse disturbed fluvial environments: the steep, high-energy coarse-grained Toutle River system in Washington, disturbed by the 1980 eruption of Mount Saint Helens; and the low-gradient, low-energy, fine-grained Obion-Forked Deer catchments in West Tennessee (Simon 1992), disturbed by channelization in the 1960s and 1970s. These data can be used in conjunction with step-backwater models to estimate temporal trends of flow energy and roughness variables in evolving channels. Simon (1992) showed that in both environments the stream power, total mechanical energy, and energy dissipation rate decreased with time toward minimum values, providing strong direct evidence in favor of the minimization of energy hypotheses. To illustrate this, Fig. 7 shows examples of temporal trends of the energy dissipation rate (energy slope) from the Toutle River system, in both aggrading and degrading reaches. As the authors recognized, the hypotheses of minimization of energy and maximization of sediment transport rate have been shown to be equivalent (Davies and Sutherland 1983); thus, this result also supports the maximization of sediment transport rate hypothesis, which was the one used by the authors in their analysis. However, support for other hypotheses is less clear. In particular, the estimated temporal trends of the Darcy-Weisbach friction factor in the Toutle River system following the 1980 eruption of Mount Saint Helens show a tendency to decrease or remain constant with time (Fig. 7), in contradiction to the maximization of friction factor hypothesis (Simon and Thorne, in press), which, according to Davies and Sutherland (1983), is also equivalent to the minimization of energy hypotheses. Shiqiang et al. (1986) conducted a comparison of the predictive abilities of the various extremal hypothesis methods. Of the tested hypotheses, the principles of minimum stream power, or maximum sediment concentration, gave the best agreement with the field data, which is consistent with the hypotheses. Although these results support the authors' choice of the maximization of sediment transport rate hypothesis, it appears that the basis of some of the extremal hypotheses may be open to question. Further, extremal hypotheses may give rise to very broad maximums or minimums, so that predicted channel equilibriums can exist over a large range, as is also suggested by the data shown in Fig. 7. This may make it difficult to obtain precise predictions of channel morphology when using the various extremal hypothesis approaches in practice. Independent of the validity and predictive power of the various extremal hypotheses, we
Advancing stakeholder participation beyond consultation offers a range of benefits for local flood risk management, particularly as responsibilities are increasingly devolved to local levels. This paper details the design and implementation of a participatory approach to identify intervention options for managing local flood risk. Within this approach, Bayesian networks were used to generate a conceptual model of the local flood risk system, with a particular focus on how different interventions might achieve each of nine participant objectives. The model was co-constructed by flood risk experts and local stakeholders. The study employs a novel evaluative framework, examining both the process and its outcomes (short-term substantive and longer-term social benefits). It concludes that participatory modelling techniques can facilitate the identification of intervention options by a wide range of stakeholders, and prioritise a subset for further investigation. They can help support a broader move towards active stakeholder participation in local flood risk management.
Abstract The T aihu B asin, situated on the south side of the Y angtze delta, is a large flood‐prone area that has urbanised rapidly. The risk of flooding is set to increase in future because of continued economic development and the impacts of climate change. Scenario analysis has been adopted to help understand the potential impacts of long‐term change on flood risk, including the effects of climate change and socio‐economic development, thus providing the basis for the development of sustainable flood risk management strategies. This paper describes the flooding processes in the T aihu B asin and sets out strategic questions that the scenario analysis sought to address. It provides an overview of the assessment framework adopted in the C hina/ UK scientific cooperation project ‘ C hina– UK S cenario A nalysis T echnology for R iver B asin F lood R isk M anagement in the T aihu B asin’. Further details of the component studies carried out as part of the project are given in the associated papers in this special issue.
Abstract Fluvial maintenance is frequently undertaken to preserve the flood capacity, visual amenity, conservation value and geomorphic stability of managed river channels. Maintenance tasks include the management of both riparian and in‐channel vegetation and maintenance dredging. Riparian vegetation is traditionally managed by physical methods such as cutting of grasses or removal of trees. Less environmentally severe alternative practices include grazing or shading for grasses, and practices such as pollarding or coppicing for trees. While a range of alternative maintenance practices, with varying environmental impact, are usually available for river managers to select, the potential for improving maintenance practices varies according to the particular task considered and the constraints imposed by the need to reach and maintain the target standard of service in terms of flood defence and land drainage. This review shows that economic and environmental impacts associated with fluvial maintenance operations may be reduced at three scales. First, at the smallest scale, it is shown that there is often potential for improving the local operational efficiency of individual fluvial maintenance tasks. Second, it may be possible to reduce the intensity of maintenance in channel reaches which are presently over‐serviced. Third, at the largest scale, it is shown that efficient maintenance is best achieved within the framework of Integrated Basin Management, and by giving appropriate consideration to future maintenance requirements at the design stage of new projects to reduce the overall need for fluvial maintenance. Examples of the way in which these policies may be implemented to reduce environmental impact without compromising engineering objectives are illustrated through case studies from the UK and the USA.
The ability to predict the stability of eroding riverbanks is a prerequisite for modeling alluvial channel width adjustments and a requirement for predicting bank-erosion rates and sediment yield associated with bank erosion. However, there are a number of limitations of existing bank-stability analyses that limit their physical basis and predictive ability. Some of these limitations are addressed through the development of a new bank-stability analysis. The new approach is applicable to steep, cohesive, nonlayered riverbanks that fail along planar failure surfaces. Pore-water and hydrostatic confining pressure terms are included in the analysis. The failure plane is not constrained to pass through the toe of the bank. The predictive abilities of four bank-stability analyses (Lohnes and Handy 1968; Huang 1983; Osman and Thorne 1988; and the present analysis) were assessed using field data. The new analysis is the most successful of the tested analyses in terms of predicting the stability of riverbanks with respect to mass failure.
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