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E.J.C. Dupuits
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In risk analysis of riverine flood defence systems, sections of flood defences are often considered separately, herewith ignoring their interdependence, e.g. due to the hydraulic response following dike breaches in the system. In previous studies it has been found that such interdependence can have a significant influence on flood risk estimates and the spatial distribution. In this paper a method is proposed for the economic optimisation of riverine flood defence safety levels from a river system perspective. In order to deal with the computational challenge of integrating the hydraulic interactions in an economic optimisation, a surrogate model was developed. Despite the many simplifications, this model yields reasonably accurate results within acceptable time. The application of the model to a case study in the Netherlands has shown that taking into account interactions between flood defences has significant influence on optimal long term strategies for flood defences. The results suggest that accounting for interdependence in setting safety standards and reinforcement prioritisation yields a significant return on investment both in terms of lower investment cost and in terms of reduced risks.
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In risk analysis of riverine flood defence systems, sections of flood defences are often considered separately, herewith ignoring their interdependence, e.g. due to the hydraulic response following dike breaches in the system. In previous studies it has been found that such interdependence can have a significant influence on flood risk estimates and the spatial distribution. In this paper a method is proposed for the economic optimisation of riverine flood defence safety levels from a river system perspective. In order to deal with the computational challenge of integrating the hydraulic interactions in an economic optimisation, a surrogate model was developed. Despite the many simplifications, this model yields reasonably accurate results within acceptable time. The application of the model to a case study in the Netherlands has shown that taking into account interactions between flood defences has significant influence on optimal long term strategies for flood defences. The results suggest that accounting for interdependence in setting safety standards and reinforcement prioritisation yields a significant return on investment both in terms of lower investment cost and in terms of reduced risks.
Optimal system safety targets
Incorporating hydrodynamic interactions in an economic cost-benefit analysis for flood defence systems
Historically, people in flood-prone areas world-wide have (to a certain degree) accepted the risk of being flooded because of the benefits that flood-prone areas can provide; examples of such benefits are rich agricultural lands or trade advantages. Acceptance and benefits notwithstanding, people living in flood-prone areas have tried, and will continue to try, to reduce and manage their vulnerability and degree of exposure to floods. A typical risk reduction measure is building flood defences...
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Historically, people in flood-prone areas world-wide have (to a certain degree) accepted the risk of being flooded because of the benefits that flood-prone areas can provide; examples of such benefits are rich agricultural lands or trade advantages. Acceptance and benefits notwithstanding, people living in flood-prone areas have tried, and will continue to try, to reduce and manage their vulnerability and degree of exposure to floods. A typical risk reduction measure is building flood defences...
In recent years significant emphasis has been placed on quantifying coastal flood hazards in the U.S. using high resolution 2-D hydrodynamic and nearshore wave models. However, these studies are computationally expensive and often neglect to consider the flooding that arises from the combined hazards of precipitation and storm surge in coastal watersheds. This paper describes a method to stochastically simulate a large number of combinations of peak storm surge and cumulative precipitation to determine the hydraulic boundary conditions for a low-lying coastal watershed draining into a semi-enclosed tidal bay. The method is computationally efficient and takes into consideration five tropical cyclone characteristics at landfall: windspeed, angle of approach, landfall location, radius of maximum winds, and forward speed. A precipitation gage network and tidal gage data were used, along with observations from over 300 tropical cyclones in the Gulf of Mexico. A Non-parametric Bayesian Network was built to generate 100,000 synthetic storm events and used as input to an empirical wind set-up model to simulate storm surge within a tidal bay and at the downstream boundary of the watershed. Based on the results, probable combinations of cumulative precipitation and peak storm surge for the watershed during hurricane conditions are determined. These boundary conditions can be easily incorporated into a coastal riverine model to determine flood risk in the watershed.
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In recent years significant emphasis has been placed on quantifying coastal flood hazards in the U.S. using high resolution 2-D hydrodynamic and nearshore wave models. However, these studies are computationally expensive and often neglect to consider the flooding that arises from the combined hazards of precipitation and storm surge in coastal watersheds. This paper describes a method to stochastically simulate a large number of combinations of peak storm surge and cumulative precipitation to determine the hydraulic boundary conditions for a low-lying coastal watershed draining into a semi-enclosed tidal bay. The method is computationally efficient and takes into consideration five tropical cyclone characteristics at landfall: windspeed, angle of approach, landfall location, radius of maximum winds, and forward speed. A precipitation gage network and tidal gage data were used, along with observations from over 300 tropical cyclones in the Gulf of Mexico. A Non-parametric Bayesian Network was built to generate 100,000 synthetic storm events and used as input to an empirical wind set-up model to simulate storm surge within a tidal bay and at the downstream boundary of the watershed. Based on the results, probable combinations of cumulative precipitation and peak storm surge for the watershed during hurricane conditions are determined. These boundary conditions can be easily incorporated into a coastal riverine model to determine flood risk in the watershed.
Coastal flood defense systems can consist of multiple lines of defense. In case of a system with a front and a rear defense (e.g. a storm surge barrier and levees), the front defense can improve the reliability of the rear defense by reducing the load on this rear defense. This paper develops a framework in order to assess whether including the influence of such a load reduction influences the economically optimal safety targets of both defenses. The economic optimization is carried out using two approaches: a simplified method developed to explore the behavior of the economic optimization with a front and rear defense, and a numerical framework geared towards practical applications. The numerical framework provides more flexibility in defining risk, cost and damage functions, and emphasizes on the applicability and tractability of the necessary steps from an engineering perspective. Both approaches are used in a hypothetical case study in order to quantify the effect of including a load reduction on the economically optimal safety targets. The results indicate that if a front defense can create a significant risk reduction in a cost efficient manner, more efficient economically optimal safety targets can be found by including the load reduction.
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Coastal flood defense systems can consist of multiple lines of defense. In case of a system with a front and a rear defense (e.g. a storm surge barrier and levees), the front defense can improve the reliability of the rear defense by reducing the load on this rear defense. This paper develops a framework in order to assess whether including the influence of such a load reduction influences the economically optimal safety targets of both defenses. The economic optimization is carried out using two approaches: a simplified method developed to explore the behavior of the economic optimization with a front and rear defense, and a numerical framework geared towards practical applications. The numerical framework provides more flexibility in defining risk, cost and damage functions, and emphasizes on the applicability and tractability of the necessary steps from an engineering perspective. Both approaches are used in a hypothetical case study in order to quantify the effect of including a load reduction on the economically optimal safety targets. The results indicate that if a front defense can create a significant risk reduction in a cost efficient manner, more efficient economically optimal safety targets can be found by including the load reduction.
Journal article
(2017)
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Egidius Johanna Cassianus Dupuits, Ferdinand Lennaert Machiel Diermanse, Matthijs Kok
Flood defence systems can be seen as multiple interdependent flood defences. This paper advances an approach for finding an optimal configuration for flood defence systems based on an economic cost-benefit analysis with an arbitrary number of interdependent flood defences. The proposed approach is based on a graph algorithm and is, thanks to some beneficial properties of the application, able to represent large graphs with strongly reduced memory requirements. Furthermore, computational efficiency is achieved by delaying cost calculations until they are actually needed by the graph algorithm. This significantly reduces the required number of computationally expensive flood risk calculations. In this paper, we conduct a number of case studies to compare the optimal paths found by the proposed approach with the results of competing methods that generate identical results. The proposed approach is set up in a generic way and implements the shortest-path approach for optimising cost-benefit analyses of interdependent flood defences with computationally expensive flood risk calculations.
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Flood defence systems can be seen as multiple interdependent flood defences. This paper advances an approach for finding an optimal configuration for flood defence systems based on an economic cost-benefit analysis with an arbitrary number of interdependent flood defences. The proposed approach is based on a graph algorithm and is, thanks to some beneficial properties of the application, able to represent large graphs with strongly reduced memory requirements. Furthermore, computational efficiency is achieved by delaying cost calculations until they are actually needed by the graph algorithm. This significantly reduces the required number of computationally expensive flood risk calculations. In this paper, we conduct a number of case studies to compare the optimal paths found by the proposed approach with the results of competing methods that generate identical results. The proposed approach is set up in a generic way and implements the shortest-path approach for optimising cost-benefit analyses of interdependent flood defences with computationally expensive flood risk calculations.
Economically efficient flood protection levels
Effects of system interdepencies
In the Netherlands, economic cost-benefit analysis plays an important role when deciding on safety levels for flood defenses. The cost of increasing the safety level is weighed against the reduction in flood risk (the benefit). The optimal level occurs where the sum of the cost and benefits is at its minimum; this is shown graphically in Figure 2. However, when conditions change over time, due to for example economic growth, the optimal safety levels change as well. This is illustrated in Figure 3. An in-depth description of the current use of cost-benefit analyses in the Netherlands can be found in Kind (2014).
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In the Netherlands, economic cost-benefit analysis plays an important role when deciding on safety levels for flood defenses. The cost of increasing the safety level is weighed against the reduction in flood risk (the benefit). The optimal level occurs where the sum of the cost and benefits is at its minimum; this is shown graphically in Figure 2. However, when conditions change over time, due to for example economic growth, the optimal safety levels change as well. This is illustrated in Figure 3. An in-depth description of the current use of cost-benefit analyses in the Netherlands can be found in Kind (2014).
Flood risk reduction systems optimization
Protecting Galveston Bay shores and the Barrier Islands
Many alternatives can reduce the flood risk around Galveston Bay (for an overview, see page 146). But which combination of alternatives suits the society most? This is, no doubt, a political decision, with various interests each playing a role. Nevertheless, we can always ask which combination is most attractive from an economic point of view.
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Many alternatives can reduce the flood risk around Galveston Bay (for an overview, see page 146). But which combination of alternatives suits the society most? This is, no doubt, a political decision, with various interests each playing a role. Nevertheless, we can always ask which combination is most attractive from an economic point of view.
A breach in a flood defence will affect the downstream water levels in a riverine system, and therefore the flood risk of the system. The effect of this changed flood risk is used in an economical optimization to assess if this significantly changes the economically optimal safety targets of the flood defences in a riverine system. The impact of breaches on the flood risk and the economically optimal safety targets is modelled using simplified hydrodynamic relations and a number of conceptual case studies for small systems. Significant differences were found, but are limited to cases with a relatively high chance of breaching and/or high impact breaches. These differences seem to mostly affect the optimal heights of the flood defences, which means that including the effect of breaches can result in a different-optimal investment path.
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A breach in a flood defence will affect the downstream water levels in a riverine system, and therefore the flood risk of the system. The effect of this changed flood risk is used in an economical optimization to assess if this significantly changes the economically optimal safety targets of the flood defences in a riverine system. The impact of breaches on the flood risk and the economically optimal safety targets is modelled using simplified hydrodynamic relations and a number of conceptual case studies for small systems. Significant differences were found, but are limited to cases with a relatively high chance of breaching and/or high impact breaches. These differences seem to mostly affect the optimal heights of the flood defences, which means that including the effect of breaches can result in a different-optimal investment path.