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are built, many of the techniques used do sequence of causes and consequences may be a way of prioritising expenditure and not necessarily take full advantage of the need to connect or coincide for a breach resources on flood risk reduction measures. experience developed in other countries. to occur and often this sequence starts Flood risk analysis can be an important tool with a deterioration process. An important in this process of prioritising new works, In addition, economic growth, future point here is that the risk of failure can be repair and adaptation or for the planning of climate chage preassures, urbanisation and significantly reduced by good maintenance. operations and maintenance programmes. the subsequent concentration of population and property, mean that increasing Dealing with levee failure A flood risk management framework numbers of people are moving to flood- provides the organisational arrangements, prone areas in many countries. Systems approach policies and procedures that will embed A typical flood defence system consists of flood risk management throughout a However, despite their critical importance a chain of different structures, sometimes responsible organisation at all levels. As in mitigating flood risk, interest and well thought-out and designed, but part of this framework, an organisation investment in levees has tended to be sometimes haphazard because of historical responsible for managing flood risks should lower than in other critical water retaining development and adaptation. However, in have a policy or strategy for deciding when infrastructure such as dams. In particular, every case, it is important to understand and how risks should be assessed and in many countries, levees have lacked the how the whole system works. If any one communicated. legal and technical framework necessary component of a flood defence system to promote an appropriate level of deteriorates, the risk of failure of that A levee risk assessment is the part of performance. element increases and so the performance risk management which provides a of the whole system can be compromised. structured process that identifies how levee How do levees fail? management objectives may be affected, The subject of levee performance and and analyses the flood risk in terms ofIn this situation, consideration should be failure is of great importance to the consequences and their probabilities beforegiven to the potential ‘cascade effect’, management of levee systems. The term deciding on whether further treatment isie the impact of the failure of one flood ‘failure’ can be used to describe many required.defence system on an adjacent system due to extra hydraulic loading. For example, different aspects of poor performance The risk that a levee may breach overflow flood defence systems are normally such as collapse during construction, or overtop at a given location is assessed by designed to resist wave or fluvial erosion shallow slippage, overtopping, seepage, a risk analysis based on: on the waterside but can be comparatively erosion, piping or complete breach. So, to 1 The nature of the area protected byfragile of the landward side. So they can remove ambiguity and achieve a consistent the levee.be vulnerable to overflowing, overtopping approach to the description of failure, the author team decided to use three separate 2 The consideration of the source of theor post-breach flows parallel to the levee. descriptive categories: initiating event (eg an extreme rainfallThe performance of a flood defence system during an event depends on the event, storm surge). deterioration – a long-term process condition of its components at the time 3 The performance or response of the damage – a short-term process given the of flood as well as on the influence of the levee when exposed to that event. deteriorated state of the levee underlying ground conditions. So for the breach. system to perform during a flood event, 4 The consequences incurred given each of these components needs to be that the flood occurs, reflecting the Deterioration encompasses those adequately maintained on an ongoing basis. vulnerability of the receptors (eg people, processes (settlement, desiccation etc) It is impossible to improve long lengths of property, environment). which change the state of the structure levee in a matter of days in the event of over a time-scale significantly greater than an adverse weather warning. However, At a screening level, qualitative or semi- that of a single loading event. As the state history suggests that when these extreme quantitative methods are used to make of the levee deteriorates, the probability flood events occur only occasionally, levee informed decisions for classification of damage (and consequential breaching) maintenance tends to get postponed and and prioritisation. At a base condition during a given flood event increases. delayed. As a result, these systems may not risk assessment level, relatively simple be adequate during times of emergency. quantitative methods are used to confirm levee safety issues and adjust portfolio Ultimate failure can be further sub- divided into two groupings, breach/ It should be noted that the transitions priorities. At more demanding levels, collapse (structural failure) and functional between different components are often increasingly sophisticated methods can be (performance) failure such as overtopping the weak points in the system and so an used to identify specific risk management occurring during a high water event area for careful design, construction and options, recommend specific actions, and greater than the design flood water level. management. Without a secondary line adjust portfolio priorities. The term ‘overtopping failure’ is used to of flood defence structures, any system is describe overtopping at less than the design only as strong as its weakest link. This is a Risk attribution can be defined as the standard of the levee. Deterioration or concern when there are multiple owners contribution of a specific component damage, when coupled with a high water and different levels of flood risk mitigation, (eg a levee segment), to the level of event can lead to overtopping and/or resulting in varying levels of engineering flood risk incumbent on the receptors ultimately trigger a structural failure. quality and/or management competence in the system. Attributing risk to each within a single flood risk management area. component of a system helps to target management interventions on those aspects A large range of factors can affect the performance of a levee during a flood It is important to be able to analyse and that pose the greatest risks, to make event and a failure will rarely be because evaluate flood risk to be effective in cost-effective and environmentally-sound of a single factor. A combination of factors mitigating against floods in an efficient and investment decisions. and events will usually be required to equitable manner. It is recognised that trigger a breach. So, a potentially complex finance is limited and that there should 25


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