Flood defence

Sets out the duties and powers of the NRA, part of the Environment Agency, m relation to water resources management, abstraction and impounding, control of pollution of water resources, flood defence. 

Uniscott Surveys Ltd (1990) [3]. The monitoring of d ormations due to coal mining subsidence of flood defences using the Global Positioning System. Report to WRC pic (Water Research Centre)... 

Risk based design of coastal flood defences - A Vietnam case... 

In this paper methods are critically reviewed, developed and explored on how to deal with this problem and how to include them in a risk analysis of coastal-and fluvial floods. An investigation of accepted risks in the coastal and fluvial flood-prone areas is made to answer the question if safe is safe enough and to define the acceptable risk levels. A risk-based approach in defining optimal safety levels of water defence system is developed. Application is made to define the optimal safety standmd for a case of coastal flood defences in Nam Dinh province, Vietnam. 

In Vietnam the existing coastal flood defences are required to meet 1/20 year design safety standard. Therefore, the inundation probability can be taken as 0.05 per year. 

In application of the second approach we consider four different scenarios of flooding events by means of different return periods. Each scenario associates with its certain flooding water level corresponding to the return period of the event. This leads to different in expected flood depths and, as consequence, the potential loss of life also differs by scenarios. Base on scenario analysis the FN-curve for Nam Dinh is found (see Fig. 3). With present situation of flood defences, the immdation probability is in range of 0.1 to 0.15 (6-10 years return periods) and the potential loss of life due to sea flood in Nam Dinh is expected at 41 fatalities. 

Based on the observed water level data at Phu Le station, water level along the Nam Dinh Coast is best fitted modeling by a mixed Gamma and Exponential distributions. In the frequency domain of interest for the design purpose of the sea flood defences (0.1 to 10 ) the tail of exceedance frequency curve is best modeled by exponential distribution ... 

Economic optimal safety for Nam Dinh sea dikes Two situations are considered in economic risk analysis of Nam Dinh coastal flood defences. 

Economic risk analysis while considering the Nam Dinh coastal flood defences as a whole dike ring shows that the safety standard of 1/50 years or more should be applied for the present economie situation (Situation 1). While faking into account the fast economic growth and acciunulated wealth in the future the safety standard of 1/100 or more should be chosen. 

Potential loss of life due to flood is preliminary estimated in this study based on the actual information. The coastal flood defence in Nam Dinh should be upgraded to ensure that the inundation probability is less than 1/145 per year, in order to satisfy the individual and societal acceptable risk. 

Economic risk analysis showed that the actual safety standards in design of coastal flood defences of the Vietnamese case study (1/20 years) are not safe enough in views of the current Vietnamese development. An optimal choice of the acceptable risk level was found at a return period of 100 years. This is in good agreement with the upper bound of the societal acceptable risk level of Vietnam. 

CUR/TAW 1990. Probabilistic design of flood defences. Report 141, Gouda, Netherlands. 

Mai Van, C., van Gelder, P.H.A.J.M., Vrijling J.K. Mai, TC (2008). Risk analysis of coastal flood defences - A Vietnam case. Proceedings of the 4th international symposium on flood defence managing flood risk, reliability vulnerability (Pp. 93-1-93-8). Toronto Institute for catastrophic loss reduction. 

In the Netherlands, the government is currently inves-tiging how potential loss of life due to flooding can be taken into account in a revised, risk-based flood defence policy, see also The potential use of individual and societal risk criteria within the Dutch flood safety policy (part 1) basic principles (Jongejan et al., 2009) for further background. 

Over 500 million is spent on coastal and maritime construction work in the UK every year. Approximately half of this expenditure relates to commercial projects including ports, harbours, docks and jetties while the other half is spent on coastal and flood defence work. 

A flood protection scheme is to be provided in a river valley where the flood level reaches up to 4.5 m above ground level (agl). Prepare an outline scheme that ensures protection of facilities based to the right of the line X shown in Figure A1 if the river reaches 4-4.5 m for a period of up to 7 days. The length of the flood defence system is 500 m. 

A-1. A suitable combination of flood causing events depends on the specific characteristics of the site and involves considerable engineering judgement. The following is an example of a set of combinations of events that cause floods for use in determining the design conditions for flood defence in an estuary where the following items are of importance ... 

R. A. Holman, Wave set-up. Handbook of Coastal and Ocean Engineering, Vol. 1, ed. J. Herbich (Gulf Publishing, 1990), Chapter 11, pp. 635-646. van der Meer, TAW Technical Report Wave Run-up and Wave Overtopping at Dikes, Technical Advisory Committee for Flood Defence, Delft (2003). 

TAW Technical Report on wave run-up and wave overtopping at dikes, Techn. Advisory Committee on Flood Defence, Delft, The Netherlands. 

The benefit of a risk-based approach, compared to the other approaches to design or decision making, is that it deals with outcomes. A risk-based approach enables informed choices to be made based on comparison of the expected outcomes and costs of alternative courses of action. This is distinct from, for example, a standards-based approach that focuses on the severity of the load that a particular flood defence is expected to withstand. Using a risk-based approach, risk assessment often involves more complete representation of the flood/erosion system. This more holistic approach will be able to account for (a) complex physical mechanisms, spatial, and temporal variations in natural hazards (e.g., natural variability in wind, wave, rainfall, and water level conditions) (b) descriptions based on sparse/incomplete data and (c) multiple stakeholders with differing, often conflicting, values and objectives. ... 

In Fig. 38.2, is the predicted overall probability of failure for flood defences and E D) is the expected economic and noneconomic damages in the flood prone areas. The predicted flood risk probability is defined as Rj = Pf E D). 

Ferreira and Soares describe the joint probability distribution function of longterm hydraulic conditions. Especially when the main interest is in the design of flood defence structures, the extreme conditions are important, which implies that the dependence between hydraulic conditions needs to be accounted for. The joint probability analysis of extreme waves and water levels thus is significant in order to estimate more accurately the extreme environmental loading on a coastal structure. Because wind setup (storm surge) and wave conditions depend on the same driving force, a strong dependence between them is observed under extreme conditions. 

Risk pathways describe the way how the risk travels from the source to the receptors. This part of a risk analysis includes loading and failure modes of flood defences, morphologic changes, breaching initiation and breach growth, reliability analysis, and flood inundation (Fig. 38.10). All these processes help to understand the performance of the entire flood defence system and its components and therefore contribute significantly to obtain an estimate of the overall probability of flooding. 

H. G. Voortman, Risk-based design of large scale flood defence systems, PhD. thesis, Delft University of Technology, the Netherlands (2003). 

S. N. Jonkman, P. van Gelder and J. K. Vrijling, Loss of life models for sea and river floods. Flood Defence 2002, eds. B. Wu et cd. (Science Press, New York Ltd., 2001), http //www.waterbouw.tudelft.nl/public/gelder/paperl20b-vl0210.pdf (2002). 

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