Flood Defence Research Articles

The ethical dilemmas of risky decisions.

Even in a pandemic there seem to be inherent conflicts of interest between the individual and societal consequences of remedial actions and strategies. Actions taken in the sole interests of patients, as required by the Hippocratic oath, can have broadly inconvenient economic implications for the State. ("Average" benefits for a population can impose individual inconveniences for the vulnerable.). Understandably these decisions are not normally made explicitly and transparently by governments. This leads to seemingly illogical and inhumane strategies which are not understood and hence mistrusted and often ignored by the public. Vaccination sentiments on social media are often an unwanted symptom of this dilemma. This article outlines and discusses a number of examples of such situations with a focus on ethical aspects. It concludes that each case must be considered individually as to the issues that need to be weighed in these difficult decisions; and that there are no clear and universally acceptable ethical solutions. What can be learned from the COVID-19 crisis is that short term utilitarianism has consequences that in the eyes of the population are unacceptable. This lesson seems equally valid for cost benefit evaluations regarding other risks, such as from hazardous industries, flood defenses, and air transport. Decisionmakers and politicians can learn that persuasion only goes so far. In the end the people appear to prioritize in terms of deontology.

Cost–benefit analysis of coastal flood defence measures in the North Adriatic Sea

Abstract. The combined effect of global sea level rise and land subsidence phenomena poses a major threat to coastal settlements. Coastal flooding events are expected to grow in frequency and magnitude, increasing the potential economic losses and costs of adaptation. In Italy, a large share of the population and economic activities are located along the low-lying coastal plain of the North Adriatic coast, one of the most sensitive areas to relative sea level changes. Over the last half a century, this stretch of coast has experienced a significant rise in relative sea level, the main component of which was land subsidence; in the forthcoming decades, climate-induced sea level rise is expected to become the first driver of coastal inundation hazard. We propose an assessment of flood hazard and risk linked with extreme sea level scenarios, under both historical conditions and sea level rise projections in 2050 and 2100. We run a hydrodynamic inundation model on two pilot sites located along the North Adriatic coast of Emilia-Romagna: Rimini and Cesenatico. Here, we compare alternative extreme sea level scenarios accounting for the effect of planned and hypothetical seaside renovation projects against the historical baseline. We apply a flood damage model to estimate the potential economic damage linked to flood scenarios, and we calculate the change in expected annual damage according to changes in the relative sea level. Finally, damage reduction benefits are evaluated by means of cost–benefit analysis. Results suggest an overall profitability of the investigated projects over time, with increasing benefits due to increased probability of intense flooding in the near future.

The role of tides and winds in shaping seed dispersal in coastal wetlands

AbstractGlobal changes such as sea level rise and enhanced storminess motivate the use of saltmarshes as nature‐based flood defenses. Yet, it remains poorly understood about how shifted environmental conditions may shape processes governing long‐term stability of saltmarshes. Here, we integrated data from in situ measurements and field experiments in several Dutch salt marshes to probe the impacts of changes in tides and winds on seed arrival and seed retention on adjacent tidal flats, which is key to marsh regeneration following wave‐driven lateral erosion. The results show that both the quantity and viability of the seeds transported toward adjacent tidal flats relate positively with the peak water level of each tide. Spring tides are more powerful in seed dispersal than neap tides, and storm‐induced extreme water levels can serve as “Window of Opportunity” that deliver disproportionately higher amounts of viable seeds than average conditions. Seed retention decreased with growing onshore wind speed. Storm‐induced strong wave disturbance can function as “Windows of Risk” that wipe out seeds on tidal flats at wind‐exposed marshes. This study highlights the importance of variability in tides and winds for regulating the potential of seed‐based marsh recovery on adjacent tidal flats and thus their resilience to lateral degradation. These findings are relevant for assessing the long‐term marsh stability and sustainability of nature‐based coastal defenses with saltmarshes under global environmental changes.

How strong are salt marshes? Geotechnical properties of coastal wetland soils

AbstractSalt marshes are globally distributed, vegetated intertidal wetlands and marsh edge erosion is common on many shores. To understand how and why marsh edge erosion occurs, the response of salt marsh substrates to applied shear and vertical stress must first be quantified. This response is likely influenced by marsh substrate biological, geochemical and sedimentological composition. However, currently there is little systematic research into the between‐marsh variability in these properties and how they affect both marsh edge erosion processes and the ability of a marsh to maintain its position vertically within the tidal frame.This paper compares two marshes of contrasting sedimentology at Tillingham marsh, East England and Warton marsh, Northwest England. Soil shear strength and compressibility are determined by applying geotechnical methods to determine marsh resistance to shear and vertical effective stresses. This research was able to isolate the influence of roots on substrate shear strength in a three‐dimensional sample. In response to vertical effective stress, both the expected displacement magnitude and the vertical recovery potential of a marsh substrate are affected by past stress conditions on the marsh, particularly those resulting from desiccation. The substrate response to vertical effective stress also influences substrate shear strength through the effect of consolidation on the void ratio (or bulk density).We present evidence for the connection between marsh composition and substrate behaviour under applied stress. The results shed light on potential determinants of marsh resistance to edge erosion, which is ultimately essential for the informed implementation of both nature‐based coastal flood defences and coastal restoration schemes.

Hydrodynamic Modelling of Wave Overtopping over a Block-Covered Flood Defence

Wave overtopping can cause erosion on the landward slope due to high flow velocities and turbulence that cause high stresses on the cover. Innovative block revetments such as Grassblocks protect the subsoil of the dike against erosion. The blocks are permeable, which reduces the flow velocity and the pressures along the landward slope. The performance of these blocks is assessed in physical tests, which provides insights into the stability of the blocks. However, such experiments are expensive and accurate measurements are difficult due to highly turbulent conditions. Therefore, the goal of this study is to determine the hydrodynamic conditions at the dike cover caused by the wave run-up on the seaward slope and by the overtopping flow over the crest and landward slope. The geometry and wave conditions from the physical test at the Deltares Delta flume are implemented in an OpenFOAM® numerical model. Using the porousWaveFoam solver, a porous layer on the crest and landward slope is implemented, where the flow resistance of this porous layer largely depends on the resistance coefficients α [-] and β [-]. The numerical model is calibrated based on resistance coefficients as introduced earlier in the literature, which showed that the resistance coefficients of α=500 and β=2.0 performed best for the peak flow velocities and the peak pressures. The numerical model is evaluated by using these resistance coefficients in other time series of the physical tests. The evaluated model is then used to determine the hydrodynamic conditions on the landward slope, which showed that the pressure was the most influential hydrodynamic condition at the time of failure. Finally, the model showed that a porosity of n=0.6 and the porous layer thickness η=36 mm reduced the peak pressure the most.

Estuarine Infauna Within Incidentally Retained Sediment in Artificial Rockpools

Artificial coastal structures (ACSs) are primarily designed to provide services for human use, such as flood defence or shipping, and are generally poor for marine biodiversity. Consequently, there has been significant research effort to enhance these hard structures to increase biodiversity and habitat availability via eco-engineering. On seawalls and breakwaters, this has included the creation of habitats for benthic species found on natural rocky shores, including the provision of cracks, crevices and water retaining features, such as artificial rockpools. When sediment retention in these features has occurred, it has often been deemed detrimental to the overarching aim of the intervention. Yet, it is soft sediment habitat that is impacted the most through coastal construction. As ecological enhancement of a flood defence scheme, nine concrete retrofit rockpools were installed at three different tidal elevations between mean high water neap tide and mean tide level on steel sheet piling on the Arun Estuary in Littlehampton Harbour, United Kingdom, which naturally filled with mud 1 year after installation. To explore how analogous the faunal assemblages and sediment profile of rockpool mud were to two local mudflats, core samples were taken and analysed for species richness, abundance, biomass, assemblage structure, median grain size, and organic matter content. More benthic species were observed in the artificial rockpool than in the local mudflats. Although the rockpools were placed at higher tidal levels than the lower shore mudflat, their assemblage structure and species richness were more similar to the lower shore mudflat at the base of the sheet piling than the upper shore mudflat. This study demonstrates that retained sediment within eco-engineered features on hard ACSs can create habitat for benthic assemblages. Providing sediment-retentive features on ACSs has the potential to provide a novel eco-engineering option that may be appropriate for some heavily modified waterbodies on sheltered, depositional coasts.

Assessment of maintenance efforts and probabilities of failure at German inland waterways to advance the design of bank revetments

Revetments protect waterways or flood defenses against erosion from waves and currents. In Germany a high percentage of about 7235 km of waterways is secured by revetments. Like many stakeholders of various infrastructures, the German Federal Waterways and Shipping Administration increasingly aims for a more economic and ecological design and maintenance strategy. Thus, new methodologies must be introduced that relate the structural condition of the revetment to resulting consequences such as required maintenance. In this paper we investigate the correlation of maintenance and revetment stability. Using the example of German inland waterways, maintenance measures conducted over at least six years are correlated with a deterministic and a probabilistic stability assessment. To account for realistic traffic loads, the stability assessment employs field measurements which provide data on ship-induced waves. It was found that at least a linear correlation between revetment stability and maintenance must be assumed. A comparison between the deterministic and the probabilistic stability assessment and thereby obtained correlations shows that less maintenance is predicted with the deterministic stability assessment. Particularly for small sample sizes and small probabilities of failure, the probabilistic approach should be favored over the deterministic approach to account for various uncertainties. In the case that only maintenance is of relevance for design considerations, the results of the probabilistic approach indicate that β = 1.3 (pf ≈ 10-1) may be a suitable annual target reliability.

НАВОДНЕНИЯ В ЗАПАДНОЙ СИБИРИ В КОНТЕКСТЕ ПРИРОДНО-ХОЗЯЙСТВЕННЫХ ВЗАИМОСВЯЗЕЙ (XVIII — НАЧАЛО XX в.)

In terms of scale and devastating consequences, floods are the most dangerous thing among natural disasters. The article is an attempt to assess their impact on the settlements and economic development in the Ob-Irtysh river system within the West Siberian region in the 18th — early 20th centuries. Floods which had high waters were associated with spring floods, but the water could not subside until the fall or even before the ice break. There were also catastrophic ones with a very high level. In addition, some complications such as long high-water cycles accrued at the time when the level and frequency of flooding increased. During severe and catastrophic floods settlements and agricultural land were flooded, livestock died, houses and outbuildings were destroyed or rendered unusable, and communication routes were interrupted for a long time. In the north of the region (Lower Ob region) during catastrophic floods, fishing trade was almost stopped and the opportunities for cattle breeding in the flooded floodplain were sharply reduced. Floodplain agriculture fell into decay during high-water cycles in the southern boreal forest area. The population of coastal areas tried to protect themselves from flooding with storage dams, but they were not built everywhere and often could not withstand the pressure of water. The only effective means of flood defense was relocation to high river banks. Therefore, the floods in 1912 and 1914 years provoked the relocation of the Irtysh River low-cost residents of the Tobolsk province. The authorities facilitated this relocation. Assistance was provided to flood victims, even though not so often. In these conditions, the population often had to rely only on themselves and God’s help. Thus, for example, in the city of Berezov the cult of St. Epiphanius was formed. On his Memorial Day people asked the higher forces for help in eliminating the consequences of the flood. But the most effective tool in combating floods was folk natural science knowledge. Over the long history of life on the river, the Russian population has developed omens, which helped them to judge the level of the upcoming flood. Among the enlightened part of the local population, there were ideas about the cyclical nature of catastrophic floods.

Flood risk mitigation through self-floating amphibious houses - Modelling, analysis, and design

Flood resilient designing is now generally acknowledged as an integral component of the comprehensive approach to flood risk management, which is centered on flood threat assessment concepts and adaptive flood living. Although resilient technology has been around for decades, it was only recently acknowledged as a component of this larger flood risk management plan. Amphibious architecture is indeed a flood mitigation method that enables a typically standard structure to rise to the top surface of surging floodwater instead of being submerged. Kerala just experienced the most devastating flood in its history, affecting about 20 percent of the state's total population. It has become essential to look for more effective flood mitigation strategies in Kerala's flood-prone districts. The research examines floating architectural technology by researching floating structures, materials, working models, and foundation design. This research will result in a flood-resilient sustainable design solution that can move up and down with the water. The floating deck is constructed using fiberglass cylinders, where fiberglass is one of the materials widely used in the construction of boats. This research aims to introduce a new strategy for flood resilience by giving out an economical solution for the flood-prone areas to thrive out of the existing flood disasters. The buoyancy theory underpins this solution for flood-prone locations with an automated flood defense mechanism.

Otpornost grada Smedereva na smanjenje rizika od bujičnih poplava i njen uticaj na kritičnu infrastrukturu

The resilience of the city of Smederevo in the area of reducing the risk of torrential floods is not at a satisfactory level. About thirty torrents have been recorded on the territory of the city of Smederevo, and their general condition is alarming. With the development of urbanization, some of them have been introduced into sewage systems and rainwater collectors, which are mostly of insufficient dimensions for receiving large waters. In order to reduce the damage caused by floods, it is necessary to envisage and implement certain works every year, the General Urban Plan of Smederevo specifies flood zones, prohibits any type of construction in them and determine areas for future retentions. Torrential floods cause great damage to settlements and the economy, so the degree of threat from flooding must be analyzed as well as the impact of natural and anthropogenic factors. The influence of anthropogenic factors on the state of order of torrent watercourses in the area of Smederevo is manifested in two ways - passive and active. The city of Smederevo implements integral flood defense on the entire territory of the city, and the aim of this paper is to point out the basic problems in the current flood protection and the possible directions towards which the local self-government of Smederevo should be directed.

Lessons learned in the application of formal methods to the design of a storm surge barrier control system∗

The Maeslantkering is a key flood defense infrastructural system in the Netherlands. This movable barrier protects the city and harbor of Rotterdam, without impacting ship traffic under normal circumstances. Its control system, which operates completely autonomously, must be guaranteed to work correctly even under extreme weather conditions, although it closes only sporadically. During its development in the 1990’s, the formal methods Z and Spin were used to increase reliability. As the availability of industrial expert knowledge on these formal methods declines, maintaining the specifications defined back then has become cumbersome. In the quest for an alternative mathematically rigorous approach, this paper reports on an experience in applying supervisory control synthesis. This formal method was recently applied successfully to other types of infrastructural systems like waterway locks, bridges, and tunnels, with the purpose to ensure safe behavior by coordinating hardware components. Here, we show that it can also be used to coordinate several (controller) software systems. Additionally, we compare the lessons learned from the originally used formal methods and link Z to supervisory control synthesis.

Seagrass roots strongly reduce cliff erosion rates in sandy sediments

Vegetated coastal ecosystems such as saltmarshes, mangroves and seagrass beds are increasingly promoted as sustainable storm and flood defence solutions by reducing wave energy. Yet, there is still intense debate on the ability of root mats to mitigate erosion, with some studies arguing that the direct contribution of roots in preventing sediment erosion is minor, while others consider them of major importance. Here, we hypothesized that the contrasting findings on the role of seagrass root mats in preventing erosion may stem from differences in sediment type. To test this idea, we investigated how root mats of seagrass that thrives in both sandy and muddy sediments mitigate wave-induced cliff erosion using Zostera marina in manipulative flume experiments. Results demonstrate that roots are very effective in reducing cliff erosion rates in sandy sediments. Cliff erosion rates were reduced up to 70% in sandy sediment with high seagrass root biomass. In contrast, cliff erosion rates in cohesive muddy sediments were low and unaffected by seagrass roots. This highlights the important role of seagrass roots in erosion mitigation, which has been overlooked compared to the role of canopies, which has received more attention. We suggest that management strategies should be developed to enhance the stabilization of sandy sediment, such as (1) using species with high belowground biomass, (2) using fast-growing pioneer species and (3) applying temporary stabilising measures.

Evolution of Flood Defense Strategies: Toward Nature-Based Solutions

Flood defense strategies have evolved from hard-engineered systems to nature-based solutions that advocate for sustainability to meet today’s environmental, social, and economic goals. This paper aims to analyze the historical progression and evolutionary trends in flood control strategies that have led to nature-based solutions. An evaluative literature review was conducted to narrate the evolution of nature-based flood management approaches for different flood types, river floods, coastal floods, and stormwater run-offs. The analysis reflected three evolutionary trends: the transformation of hard measures to soft measures; secondly, the increase in society’s attention to ecosystems and their services; and, finally, divergence from single-function solutions to multi-function solutions. However, continuous monitoring and evaluation of the previous projects and adapting to the lessons learned are the key to progress towards sustainable flood management strategies and their societal acceptance.

Cooperating for added value: Using participatory game theory in implementing nature-based flood defences

The increasingly active role of stakeholders in the development of innovative nature-based solutions calls for appropriate instruments to support and realise added value from their involvement. In this paper we apply a newly developed instrument “Cooperation for Added Value” (Co-Add) to a study area on the Dutch coast. The instrument draws on participatory game theory and policy analysis to provide a theoretically sound structure for facilitating interactions aimed at identifying shared opportunities and potential coalitions for cooperation. The application in the case study Noard-Fryslân Bûtendyks affirmed that the Co-Add instrument systematically facilitated stakeholders in exploring potentially promising opportunities and gaining insight in the added value of engaging in diverse cooperations. Stakeholders came to understand which solutions were more achievable than others and what was needed to enable implementation in terms of collaboration, including the role of their own organization. Furthermore, social dilemmas in which a particular nature-based solution is attractive to a coalition of stakeholders but is not the most beneficial solution for a particular individual stakeholder, became clear. This represents a practical contribution to the range of participatory instruments that can be applied in societally challenging complex problems that require collaboration for their resolution.

Collaboratively planning climate action: What would it mean for Dublin? An analysis of the Clontarf flood defences

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Modeling of a compound flood induced by the levee breach at Qianbujing Creek, Shanghai, during Typhoon Fitow

Abstract. Levee-breach-induced flooding occurs occasionally but always causes considerable losses. A serious flood event occurred due to the collapse of a 15 m long levee section in Qianbujing Creek, Shanghai, China, during Typhoon Fitow in October 2013. Heavy rainfall associated with the typhoon intensified the flood severity (extent and depth). This study investigates the flood evolution to understand the dynamic nature of flooding and the compound effect using a well-established 2D hydro-inundation model (FloodMap) to reconstruct this typical event. This model coupled urban hydrological processes with flood inundation for high-resolution flood modeling, which has been applied in a number of different environments, and FloodMap is now the mainstream numerical simulation model used for flood scenarios. Our simulation results provide a comprehensive view of the spatial patterns of the flood evolution. The worst-hit areas are predicted to be low-lying settlements and farmland. Temporal evaluations suggest that the most critical time for flooding prevention is in the early 1–3 h after dike failure. In low-elevation areas, temporary drainage measures and flood defenses are equally important. The validation of the model demonstrates the reliability of the approach.

AI-based techniques for multi-step streamflow forecasts: application for multi-objective reservoir operation optimization and performance assessment

Abstract. Streamflow forecasts are traditionally effective in mitigating water scarcity and flood defense. This study developed an artificial intelligence (AI)-based management methodology that integrated multi-step streamflow forecasts and multi-objective reservoir operation optimization for water resource allocation. Following the methodology, we aimed to assess forecast quality and forecast-informed reservoir operation performance together due to the influence of inflow forecast uncertainty. Varying combinations of climate and hydrological variables were input into three AI-based models, namely a long short-term memory (LSTM), a gated recurrent unit (GRU), and a least-squares support vector machine (LSSVM), to forecast short-term streamflow. Based on three deterministic forecasts, the stochastic inflow scenarios were further developed using Bayesian model averaging (BMA) for quantifying uncertainty. The forecasting scheme was further coupled with a multi-reservoir optimization model, and the multi-objective programming was solved using the parameterized multi-objective robust decision-making (MORDM) approach. The AI-based management framework was applied and demonstrated over a multi-reservoir system (25 reservoirs) in the Zhoushan Islands, China. Three main conclusions were drawn from this study: (1) GRU and LSTM performed equally well on streamflow forecasts, and GRU might be the preferred method over LSTM, given that it had simpler structures and less modeling time; (2) higher forecast performance could lead to improved reservoir operation, while uncertain forecasts were more valuable than deterministic forecasts, regarding two performance metrics, i.e., water supply reliability and operating costs; (3) the relationship between the forecast horizon and reservoir operation was complex and depended on the operating configurations (forecast quality and uncertainty) and performance measures. This study reinforces the potential of an AI-based stochastic streamflow forecasting scheme to seek robust strategies under uncertainty.

The wave overtopping load on landward slopes of grass-covered flood defences: Deriving practical formulations using a numerical model

Overtopping waves exert a high hydraulic load on the landward slopes of flood defences leading to erosion of the grass cover and finally to a dike breach. The hydraulic load is an important variable in erosion models and a detailed description of the load is necessary to determine where and when the grass cover erodes. We use a numerical model to simulate the flow of a single overtopping event over a flood defence with a grass-covered crest and landward slope. The model results show that the flow velocity, the shear stress and the pressure are maximal at the landward toe and can be used to describe grass erosion by shear forces. For steep slopes, the flow separates at the crest line and impacts on the upper slope. The normal stress is maximal at the location of impact and describes the grass erosion by normal forces. Practical formulations are developed for the maximum flow velocity, the maximum pressure, the maximum shear stress, the maximum normal stress and the impact location using three main design parameters for the landward slope: the overtopping volume, the slope steepness and the slope length. The formulations are able to accurately predict the overtopping load with Nash–Sutcliffe model efficiency factors between 0.41 and 0.90. The model output and these new formulations are used to calculate the erosive power of the overtopping waves predicted by eight erosion indices to show how the simulated load can be used in erosion models.

Cutting the costs of coastal protection by integrating vegetation in flood defences

Exposure to coastal flooding is increasing due to growing population and economic activity. These developments go hand-in-hand with a loss and deterioration of ecosystems. Ironically, these ecosystems can play a buffering role in reducing flood hazard. The ability of ecosystems to contribute to reducing coastal flooding has been emphasized in multiple studies. However, the role of ecosystems in hybrid coastal protection (i.e. a combination of ecosystems and levees) has been poorly quantified at a global scale. Here, we evaluate the use of coastal vegetation, mangroves, and marshes fronting levees to reduce global coastal protection costs, by accounting for wave-vegetation interaction.The research is carried out by combining earth observation data and hydrodynamic modelling. We show that incooperating vegetation in hybrid coastal protection results in more sustainable and financially attractive coastal protection strategies. If vegetated foreshore levee systems were established along populated coastlines susceptible to flooding, the required levee crest height could be considerably reduced. This would result in a reduction of 320 (range: 107-961) billion USD2005 Power Purchasing Parity (PPP) in investments, of which 67.5 (range: 22.5- 202) billion USD2005 PPP in urban areas for a 1 in 100-year flood protection level.

Going home for tea and medals: How members of the flood risk management authorities in England construct flooding and flood risk management

AbstractThe construction of flooding and flood risk management are complex and there is potential for dissonance between individual and institutional understanding and experience of both. In this article, we start by investigating how flooding is managed and the change in paradigm from flood defence to more adaptive approaches, which embed resilience into flood risk management. Using analysis of semi‐structured interviews with members of the flood authorities in England, we explore how flood management authorities construct ‘flooding’ and establish that it is often defined by in‐the‐moment impacts. Whilst these in‐the‐moment impacts are understood to be devastating, there is less appreciation of long‐term human impacts of living at risk of flooding. We uncover how the construction of ‘flood risk management’ by the flood authorities is complicated by factors, such as the construction of resilience, availability of funding, technical expertise and responsibility fragmentation that the Floods and Water Management Act (2010) has created. We conclude that the differing constructions of flooding and flood risk management between flood management authorities in England hinder how flooding is managed. Therefore, we propose that a more nuanced understanding of flooding and flood risk management is essential for effective partnership working between flood risk management authorities and communities.