Flood Defence Systems Research Articles

Assessing the Suitability of the Flood Defense Policy of Republic of Korea for Risk Reduction in Local Rivers

This study examines whether the fluvial flood defense system of Korea is appropriate for risk reduction. Using spatial socioeconomic data and remote sensing, we estimated the potential economic damage that can be caused by the flooding of local streams and rivers along the Nakdong River (the longest river in Korea). For the analysis, a river risk map including return periods (50, 80, 100, and 200 years) and spatial inventories (residential, agricultural, industrial assets, and human lives) was employed to determine flood-prone areas and assess the damage within the inundation areas. A quantitative flood analysis was conducted using an object-based method to estimate the expected annual damage. We then compared the estimated damage for each tributary within the designed return periods and found no correlation. Numerous tributaries with low-defense targets were considered high-risk, while those with high-defense targets were assessed as low-risk. The dataset used in this study covered four damage categories. Among them, flood damage to residential assets appeared to have the highest value, whereas flood damage to industrial assets had the lowest value. The results demonstrate that the Korean government needs to tailor its flood defense policy based on quantitative risk assessments to effectively manage flood risks, especially given the increasing risk of climate change.

Risk-based maintenance and inspection of riverine flood defence systems

The condition of flood defence revetments is influenced by many different degradation processes such as animal burrowing, rutting and growth of weeds. Many of these processes are shock-based rather than progressive continuous. As shocks can cause a drop in performance, this means that the condition of a revetment can suddenly decrease, meaning that revetments can have significant initial damage at the beginning of a storm. Combined with the limited detection probability of common visual inspections of flood defences, this can have a significant influence on the reliability of flood defence systems, something typically not considered in reliability analysis. In this paper we study the reliability of a flood defence system subject to shock-based degradation. Various maintenance concepts are compared for a case study of a riverine flood defence of 20 kilometres length. This demonstrates that the current maintenance concept is insufficient to satisfy the reliability requirements for failure of the revetment. Overall, the joint influence of degradation and the existing maintenance concept leads to a 20 times higher failure probability estimate compared to a typical assessment without these aspects. Next, we demonstrate that both additional inspections, and targeted interventions to reduce the impact of for instance animal burrowing, can significantly reduce total cost and improve robustness of the considered flood defence system.

How natural foreshores offer flood protection during dike breaches: An explorative flume study

In this paper we describe one aspect of nature-based flood protection by foreshores in hybrid flood defences and discuss how foreshore ecosystem restoration can contribute to flood protection. Flood protection consists of flood prevention, by grey, green or hybrid defences, and flood impact reduction, by spatial planning to limit damage and crisis management to limit exposure. Reduction of flood impact is increasingly important because no flood defence system can provide 100% safety, especially with climate change and sea level rise. In this study we aim to understand and visualize the effect of foreshore characteristics (i.e., width, elevation and erodibility) on flooding impact in the hinterland of hybrid flood defences. As it is difficult to research real dike breaches, we do an explorative flume study to analyse the impact of a mimicked dike breach in the hinterland. Our physical scale model showed the presence of a non-erodible foreshore reduces flood damage in the hinterland. With regards to foreshore characteristics, mainly foreshore elevation and erodibility are important, while differing foreshore width has little additional influence. Already a narrow foreshore reduced flood impact in the flume hinterland. Our findings strengthen the appeal to integrate Nature-based flood protection by foreshores in hybrid flood defences. Grey flood defences can be turned into hybrid flood defences even if there is limited space for foreshore ecosystem restoration, for instance by managed realignment.

Discharge estimation in a compound channel with converging and diverging floodplains using ANN–PSO and MARS

Abstract The discharge estimation in rivers is crucial in implementing flood management techniques and essential flood defence and drainage systems. During the normal flood season, water flows solely in the main channel. During a flood, rivers comprise a main channel and floodplains, collectively called a compound channel. Computing the discharge is challenging in non-prismatic compound channels where the floodplains converge or diverge in a longitudinal direction. Various soft computing techniques have nowadays become popular in the field of water resource engineering to solve these complex problems. This paper uses a hybrid soft computing technique – artificial neural network and particle swarm optimization (ANN–PSO) and multivariate adaptive regression splines (MARS) to model the discharge in non-prismatic compound open channels. The analysis considers nine non-dimensional parameters – bed slope, relative flow depth, relative longitudinal distance, hydraulic radius ratio, angle of convergence or divergence, flow aspect ratio, relative friction factor, and area ratio – as influencing factors. A gamma test is carried out to determine the optimal combination of input variables. The developed MARS model has produced satisfactory results, with a mean absolute percentage error (MAPE) of less than 7% and an R2 value of more than 0.90.

Risk management and components’ coordination assessment in the design of a composite riverine flood defence system

ABSTRACT Flood control is one of the most important and difficult tasks in river basin management in terms of present several uncertainties. This study aims to follow the risk management and coordination analysis in designing a riverine flood defence system, actually, a composite multi-section flood control levee system considering the aleatory and epistemic uncertainties using contagious distributions. A stochastic optimization model was developed based on Monte Carlo analysis for uncertainty propagation, and then the system's optimum dimensions were computed in the single-section and multi-section cases for overall comparisons. Using the concept of ‘scenario probability’, the coordination of the system and its individual components was analysed. The results indicated that by increasing the complexity (from a single-section to a multi-section model) and by the separation of uncertainties (into epistemic and aleatory), the coordination of the system decreases 130% and 30%, respectively. Moreover, it was found that when economic uncertainty was considered, unexpected results may be obtained so that by increasing the system dimensions the total cost decreases. Finally, after analysing the sample histograms of the system's components, several distributions have been suggested. When there are vast datasets that might be confusing for decision-makers, the systematic method proposed here makes risk management easier.

Economic optimization of coastal flood defence systems including storm surge barrier closure reliability

AbstractStorm surge barriers are large movable hydraulic structures which close during a storm surge to prevent coastal floods. In the regions they protect, a failure to close the barrier is often the most likely cause for a catastrophic flood. Nevertheless, flood risk assessments usually only focus on raising flood defences behind the barrier. Despite its importance, there is no general method to assess the costs and benefits of improving the closure reliability. This paper presents a model that optimises investments considering both closure reliability improvements and raising flood defences behind the barrier, using the region protected by the Maeslant barrier as a case. We substantiate that constructing the Maeslant barrier was an optimal economic decision. Moreover, we demonstrate large investments such as a redundant barrier already being economically sound with a few decimetres of sea level rise. Based on our experience with this case study, we expect the model is useful in finding strategies to adapt to rising sea levels and other developments that cause coastal flood risk to rise worldwide.

Failure of levees induced by toe uplift: Investigation of post-failure behavior using material point method

Levees are essential structures in flood defense systems, and their failures can lead to devastating consequences on the surrounding territories. One of the failure mechanisms mostly controlled by the foundation soil stratigraphy is the instability of the land side slope, triggered by the development of high uplift pressures in the foundation. This complex phenomenon has been investigated experimentally with centrifuge tests or large-scale tests and numerically with the limit equilibrium method (LEM) and the finite element method (FEM). In this work, we applied a multiphase formulation of the material point method (MPM) to analyze the development of toe uplift instability mechanism, from the onset of failure to large displacements. The numerical model is inspired by an experiment carried out in a geotechnical centrifuge test by Allersma and Rohe (2003). The comparison with the experiment allows for understanding critical pore pressure triggering large displacements in the foundation soils. Moreover, we numerically evaluated the impact of different values of foundation soils' hydraulic conductivity on the failure mechanism. The results show that hydraulic conductivity mainly influences the time of failure onset and the extension of shear localization at depth. Finally, the advantages of using large displacement approaches in the safety assessment of earth structures are discussed. Unlike FEM, there are no issues with element distortions generating difficulties with numerical convergence, allowing for full post-failure reproduction. This capability permits precise quantification of earth structure damages and post-failure displacements. The ensuing reinforcement systems' design is no longer over-conservative, with a significant reduction in associated costs. ©2022 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Dynamic adaptive engineering pathways for mitigating flood risks in Shanghai with regret theory

Uncertainty in sea level rise and future extreme climate events presents a great planning challenge for flood defence in coastal mega cities like Shanghai. While academic literature has largely focused on uncertainty analysis, engineering solution design requires effective uncertainty management. Here we incorporate the regret theory of economics and decision science into the dynamic-adaptation-pathways framework and assess the impacts of high rates of changes on the flood defence systems in Shanghai. Specific options are developed to manage flooding on the Huangpu River from tidal water levels, river flows, rainfall, drainage inflows and combinations of these flood sources including sea level rises of up to 3 m. Dynamic adaptation pathways are developed where the timing of tipping points from one intervention to the next depends on the actual changes in sea level, rainfall and other variables that affect the future design. This framework is potentially applicable for planning ‘no regrets’ flood-defence systems in other low-lying coastal cities.

Expert judgment-based reliability analysis of the Dutch flood defense system

The Wettelijk Beoordelingsinstrumentarium (WBI) is the legal set of instruments for flood risk analysis in the Netherlands. Often, engineers have the impression that some failure probabilities of flood defenses resulting from these instruments are overestimated. In an effort to better estimate the failure probabilities of dikes along the Dutch river Rhine, this study sets out to assess them with experts and compare them to model results. We used Cooke’s method for combining experts’ estimates in a structured way and follow two approaches to estimate a system failure probability. In the first approach, experts estimate discharges that lead to at least one dike failure. This gives plausible results; failure probabilities between 1/30 and 1/17.000 in a year. The second approach is based on adjusting existing model-based assessment results, by estimating the model-bias and incorporating additional dependencies. This mostly leads to large, implausible, failure probabilities: Experts tend to give more conservative answers as they are asked for detailed estimates without clear reference values. This results in large uncertainty and consequently (too) high failure probabilities. Our research shows that when applied in a clear frame of reference, structured expert judgments can be successfully used for estimating the reliability of Dutch flood defenses.

Water surface profile in converging compound channel using gene expression programming

Abstract Assessment of water surface profile in compound channels is essential for flood defence systems. Agriculture and development activities in floodplains affect the floodplain shape over the length, leading in a converging compound channel. Few laboratory investigations proved overbank flow in converging floodplains. Therefore, innovative and precise approaches are still in great demand. In this paper, new approach has been proposed to forecast the water surface profile of various compound channels with converging floodplains using gene expression programming (GEP). The models are constructed utilising pertinent experimental data from past studies. A new equation is devised to compute water surface profile in such channels using non-dimensional geometric and flow parameters such as converging angle, width ratio, relative distance, relative depth, aspect ratio and bed slope. The findings demonstrate that the GEP-derived water surface profile is in good correlation with the experimental data and data from other studies (R2 = 0.99 and RMSE = 0.028 for the training data and R2 = 0.99 and RMSE = 0.027 for the testing data). According to the results of statistically based investigations, the GEP model created for the study of compound channel flow is reliable and can be used in this domain.

Characteristics of realigned dikes in coastal Europe: Overview and opportunities for nature-based flood protection

Managed realignment is the landward relocation of flood infrastructure to re-establish tidal exchange on formerly reclaimed land. Managed realignment can be seen as a nature-based flood defence system that combines flood protection by the realigned dike (artificial) and restored saltmarshes (nature-based). So far, research on coastal managed realignment is primarily directed to saltmarsh restoration on formerly reclaimed land. This study focuses on the realigned dikes. The aim of this research is to characterize realigned dikes and to indicate the characteristics that offer opportunities for nature-based flood protection. We categorized 90 European coastal managed realignment projects into two realigned dike groups: (1) Newly built landward dikes and (2) Existing landward dikes of former multiple dike systems. The second group has two subcategories: (2a) Former hinterland dikes and (2b) Realignments within summer polders. For each group we present the realigned dike characteristics of a representative case study. We consider that the use of existing landward dikes or local construction material make realignment more sustainable. From a nature-based flood protection perspective, the presence of an artificial dike is ambiguous. Our results show that targeted and expected saltmarsh restoration at managed realignment does not necessarily result in a greener realigned dike design that suits for combined flood protection with restored saltmarshes. We recommend coastal managers to explicitly take combined flood protection into account in the realigned dike design and steer the topography of the realignment site to facilitate nature-based flood protection and promote surface elevation increase seaward of the realigned dike in response to sea level rise. This makes managed realignment a nature-based flood defence zone for now and for the future.

Exploring the Combined Risk of Sea Level Rise and Storm Surges Using a Bayesian Network Model: Application to Saemangeum Seawall

Jung, M.-K.; Kim, J.-Y.; Lee, B., and Kwon, H.-H., 2021. Exploring the combined risk of sea level rise and storm surges using a Bayesian network model: Application to Saemangeum seawall. In: Lee, J.L.; Suh, K.-S.; Lee, B.; Shin, S., and Lee, J. (eds.), Crisis and Integrated Management for Coastal and Marine Safety. Journal of Coastal Research, Special Issue No. 114, pp. 186–190. Coconut Creek (Florida), ISSN 0749-0208. In terms of natural hazards, typhoon-induced storm surge along with heavy rainfall has been recognized as the most frequently reported hazard among water-related disasters in the coastal areas in South Korea. Moreover, it has been widely acknowledged that the frequency and intensity of typhoons (or abnormal low-pressure systems) are likely to increase over time due to the potential impact of climate change. A risk analysis covering different loading conditions has been a tool for flood risk management in these contexts. We propose a Bayesian network-based generic risk analysis tool for flood defense systems such as a levee, dike, and seawall. On the other hand, this study explores various failure modes of flood defense structures for the use of risk analysis. In this study, the proposed modeling framework is applied to Saemangeum seawall in South Korea. Based on the literature review, various failure modes for Saemangeum seawall was identified, and the selected failure mode is then translated into nodes in the Bayesian network framework. The failure probability was estimated quantitatively by computing the limit state equation, which is composed of a set of random variables. Moreover, we investigated a Bayesian network model to assess the impact of compounding risk associated with revetment erosion of the seawall from sea-level rise informed by climate change scenarios. A further discussion on the role of the uncertainty for overall risk is provided.

Experimental Investigation of Flood Energy Dissipation through Embankment Followed by Emergent Vegetation

The combination of hard (artificial) and soft (natural) solutions i.e., composite defense systems against flooding and tsunami opens a new window for engineering innovation for researchers nowadays. In this study, the experimental investigation of flood energy dissipation phenomena through composite defense systems comprising of embankment and rigid vegetation models in an open channel flume, is conducted. The flow regime through the composite defense system is classified in two main types, which are further subdivided in two sub-categories. Various combinations of embankment and vegetation and spacing between embankment and vegetation are analyzed. Against the selected range of initial Froude numbers, three different sizes of embankment models, three spacings between the embankment and vegetation (Ldv) and vegetated corridors of two different porosities (PR), are tested to examine the effect of these three parameters on the characteristics of the generated hydraulic jumps and flood energy dissipation within the defense system. It is found that embankment size and vegetation porosity have a greater impact on flood energy dissipation while the selected range of Ldv is less effective. Amongst the assessed composite flood defense systems, the maximum energy dissipation (55%) is observed for the combination of maximum embankment height and vegetation porosity (93%). For fixed combinations of embankment size and Ldv, the maximum increase of energy dissipation (18%) is found for decreasing vegetation porosity from 97% to 93%.

Tidal-flat reclamation aggravates potential risk from storm impacts

A better understanding of how tidal-flat reclamation changes the flood hazard is critical for climate-proofing coastal flood defense design of heavily urbanized areas. Since the 1950s, large-scale reclamation has been performed along the Shanghai coast, China, to fulfill the land demands of city expansion. We now show that the loss of tidal flats may have resulted in harmful impacts of coastal storm flooding. Using the foreshore profiles measured before and after reclamation (i.e., wide vs. narrow tidal flat), we determined the long-term changes in flood risk using a numerical model that combines extreme tidal level and wave overtopping analysis. Results show that wide tidal flats in front of a seawall provide efficient wave damping even during extreme water levels. Reclamation of these tidal flats substantially increased wave heights and correspondingly reduced the return period of a specific storm. As a result, estimates of overtopping are aggravated by more than 80% for the varying return periods examined. It is concluded that the disasters of coastal flooding after the 1997 tidal-flat reclamation in Hangzhou Bay, China are a consequence of both anthropogenic and natural activities. Moreover, our model calculations provide an equation describing the equivalent dike height needed to compensate for the loss of every km tidal flat of a certain elevation, and vice versa. For example, for every km of tidal flat ranging from high marsh to bare tidal flat that is being regained, the dike can be lowered by 0.84 m–0.67 m, when designing for a 1 in 200 years storm event. Overall, we suggest that wide tidal flats are ideally restored in front of dikes, and that when tidal areas are reclaimed, the seawall height is raised as part of the intertidal reclamation procedure. Using such an equivalent protection standard is relevant to designing hybrid flood defense system worldwide.

Prototip sistema za rano upozoravanje na bujične poplave u brdsko-planinskim predelima zasnovan na internetu stvari

Bosnia and Herzegovina is predominantly mountainous country with many settlements located near rivers. The characteristic terrain and seasonal occurrences of large amounts of water, due to rainfalls or snowmelt, make Bosnia and Herzegovina a country at a constant risk of flooding. Although the weather and hydrological situation in the territory of BiH is monitored by two hydrometeorological services, one from the Republika Srpska and the other from the Federation of BiH, automatic measuring stations have been set on the main watercourses, while remote monitoring of distant mountain areas subject to sudden emergence of torrential waters has not been established. A prototype of an IoT based torrential floods early warning system presented in this paper could solve this problem, being capable of monitoring watercourses in remote and inaccessible terrain. This system may be set to function autonomously, or as an integral part of an existing flood defense system. The basic unit of the system is an automatic measuring station (AMS) based on an Arduino controller with appropriate sensors, modules and power supply. Being small in size and affordable, easy to manufacture and setup, autonomous in operation and capable of wireless data transmission, makes AMS suitable for applications where use of standard metering stations is not possible. Number of AMSs in the system may vary and depend on the size of territory and water basin being monitored. Web server with custom developed Web application collects, processes, analyses and distributes data, while access level is set in correspondence with real-life authorization levels within the organizational structure of the flood defense system. Implementation of such system may provide monitoring of the situation in inaccessible areas in the basin, and assessment of time remaining until the flood wave reaches populated places. This will provide enhanced situation awareness and an additional window of opportunity for timely notification, reaction and protection of the local population and material goods.

Optimal planning of flood defence system reinforcements using a greedy search algorithm

Climate change and deterioration require a continuous effort to reinforce flood defences and meet reliability requirements. To efficiently upgrade flood defence systems, insight in costs and benefits of measures at a system level is required throughout the process of planning and design. Due to the size of flood defence systems the number of possible decisions is large, which hampers system optimization. We describe a greedy search algorithm that can find (near-)optimal combinations of reinforcement measures for dike segments. The algorithm has been validated by comparing results for 2800 different dike segments to an integer programming implementation. The difference in objective value (Total Cost) is only 0.04% on average, which is small compared to other uncertainties in assessment and design of dike segments. The algorithm is applied to a reinforcement project for a dike segment of 41 independent sections, and compared to the common design practice which uses reliability-based requirements on a section level. It is found that the resulting reinforced dike segment is 42% cheaper to construct than the one obtained from the common approach, based on the same input information. This illustrates the practical and societal value of the design approach using a greedy search algorithm in this context.

A systems-level approach to managing flood defences in the River Thames estuary, UK

Communities in London and along the tidal River Thames in the UK already benefit from world-class tidal defences, but climate change means flood risk continues to increase. In 2012, the Environment Agency published a plan for how 1.3 million people and £275 billion worth of property will continue to be protected from tidal flood risk until the year 2100. The first 10 years of the plan, which includes innovative asset management solutions on tidal flood defence systems, is being delivered through the £308 million Thames Estuary Asset Management 2100 (Team2100) programme. As described in this paper, the integrated delivery team is focusing on delivering value for public money through innovation, collaboration with the supply chain and a systems-level approach to asset and programme management.

Modeling Storm Surge Attenuation by an Integrated Nature-Based and Engineered Flood Defense System in the Scheldt Estuary (Belgium)

There is increasing interest in the use of nature-based approaches for mitigation of storm surges along coasts, deltas, and estuaries. However, very few studies have quantified the effectiveness of storm surge height reduction by a real-existing, estuarine-scale, nature-based, and engineered flood defense system, under specific storm surge conditions. Here, we present data and modelling results from a specific storm surge in the Scheldt estuary (Belgium), where a hybrid flood defense system is implemented, consisting of flood control areas, of which some are restored into tidal marsh ecosystems, by use of culvert constructions that allow daily reduced tidal in- and outflow. We present a hindcast simulation of the storm surge of 6 December 2013, using a TELEMAC-3D model of the Scheldt estuary, and model scenarios showing that the hybrid flood defense system resulted in a storm surge height reduction of up to half a meter in the estuary. An important aspect of the work was the implementation of model formulations for calculating flow through culverts of restored marshes. The latter was validated comparing simulated and measured discharges through a physical scale model of a culvert, and through a real-scale culvert of an existing restored marsh during the storm surge.

Implementation and evaluation of coupled discontinuous Galerkin methods for simulating overtopping of flood defenses by storm waves

In coastal regions, accurate prediction of the inundation risks posed by tropical cyclones and other severe weather is key to mitigating disaster impacts. Predictions of these risks are often provided by numerical models; however, these models also frequently do not consider inundation due to wind-sea wave run-up. Neglecting such effects may be misleading for evaluating the risks posed to areas protected by flood defense systems such as levees and dykes. To effectively evaluate the flooding risks posed by storm waves, accurate wind-sea modeling should be undertaken in parallel with storm surge modeling, and accurate wave-overtopping flow rates at flood barriers must be prescribed based on the modeled sea conditions and structure geometries. To this end, we present development of a loosely coupled finite element based shallow-water equations and parametric spectral wave model. Wave-overtopping flow rates are computed by formulas provided in the guidance of the EurOtop manual. We demonstrate that the parametric spectral wave modeling approach is sufficiently accurate for modeling of wave-overtopping events, despite its simplifying assumptions, such as those based on deep water (e.g., discounting foreshore effects such as wave shoaling and breaking), and disregarding internal forces due to wave breaking and turbulence, as opposed to more commonly used discrete (third generation) spectral wave models, which provide more detailed descriptions of wave fields but are much more computationally expensive. Comparisons of numerically modeled wave-overtopping flow rates with measurements from the CLASH database show good agreement by linear regression (slope: 0.9911, R2: 0.9584) for more than 90% of the test cases considered, where for the relatively few outlier cases the model performs poorly due to either uncertainties in the EurOtop guidance or the lack of foreshore effects in the wave model.

Inversion Algorithm for Civil Flood Defense Optimization: Application to Two-Dimensional Numerical Model of the Garonne River in France

The objective of this study is to investigate inversion-approach for optimization of flood defense in a inundated area. This is a new methodology in this engineering field which consists in definition of a criterion (as for instance the water level in a given location must be lower than a given value) and analysis of all uncertain controlled parameters (i.e. flood defense geometry, location, and so on) combinations allowing to not exceed safety objective for all possible uncontrolled parameters combination (i.e. flow hydrograph parameters) representing natural phenomenon. In order to estimate this safety set, we will use a transient meta-modelling approach, which reduces widely number of model evaluations required. This algorithm relies on a kriging surrogate built from few model evaluations, sequentially fulfilled with new numerical model evaluations as long as remaining uncertainty of whole safety set remains too high. Also known as Stepwise Uncertainty Reduction, this algorithm is embedded in Funz engine in charge of bridging numerical model and any design of experiments algorithm. We applied this algorithm to a real bi-dimensional numerical model of Garonne River. Especially, we focused our attention on water depth at a given area when considering influence of a simplified flood defense during a flooding event. In order to simplify problem, we only analyze two parameters describing slab and dyke elevations of flood defense system. The results will be analyzed in terms of safety control inside operating range of river. It appears that once properly evaluated, this constrained zone gives highly valuable data for a fully risk informed management of area to protect.