Probabilistic characterization of the vegetated hydrodynamic system using non-parametric bayesian networks

The increasing risk of flooding requires obtaining generalized knowledge for the implementation of distinct and innovative intervention strategies, such as nature-based solutions. Inclusion of ecosystems in flood risk management has proven to be an adaptive strategy that achieves multiple benefits. However, obtaining generalizable quantitative information to increase the reliability of such interventions through experiments or numerical models can be expensive, laborious, or computationally demanding. This paper presents a probabilistic model that represents interconnected elements of vegetated hydrodynamic systems using a nonparametric Bayesian network (NPBN) for seagrasses, salt marshes, and mangroves. NPBNs allow for a system-level probabilistic description of vegetated hydrodynamic systems, generate physically realistic varied boundary conditions for physical or numerical modeling, provide missing information in data-scarce environments, and reduce the amount of numerical simulations required to obtain generalized results-all of which are critically useful to pave the way for successful implementation of nature-based solutions.Hydraulic Structures and Flood RiskCoastal Engineerin

Quantifying Frontal-Surface Area of Woody Vegetation: A Crucial Parameter for Wave Attenuation

The last years, capacity of vegetation to reduce wave impact is receiving considerable attention. To predict wave attenuation processes within vegetation fields reliable estimates of vegetation parameters are needed. This proves to be difficult for woody vegetation as it consists of complex branch structures, characterized by varying branch densities, diameters and angles. State of the art physical and numerical models effectively use a single value for the diameter, b v and density, N of vegetation, which is unrepresentative for complex vegetation, such as trees. Trees can be better described by the projected frontal-surface area, A v. Hence, this work compares methods to quantify the A v in space for a pollard willow forest, and determines suitability of these methods for predicting wave attenuation using a spectral wave model (SWAN). We use data from manual measurements and Terrestrial Laser Scans (TLS), to estimate the vertical distribution of A v; and data from large-scale flume experiments performed on a willow forest to verify model sensitivity to A v inferences. As a baseline for comparison, tree models that describe the structure of the trees in various degrees of complexity are compiled. The most realistic tree model is used to quantify potential errors in TLS and basic manual measurements of N and b v. An initial comparison shows that the TLS data underestimates A v, which indicates that conducting manual measurements is more suitable to quantify a homogeneous forest. We found that the TLS suffers from shadowing effects (i.e., blockage of laser beams) and we recommend to apply a correction factor to improve its measurements. Furthermore, we identified the impact that the different methods to determine A v have on the estimation of wave attenuation using SWAN; in addition we verified the model results with data from large-scale flume experiments performed on the willow forest. The modeled sensitivity tests indicate large differences in wave attenuation and, consequently, a wide range (0.94–1.70) of bulk drag coefficients, (Formula presented.), for the various methods applied. This shows the variation of outcome between measuring methods and highlights the importance of stating the selected method for reliable frontal-surface area estimations, and consequently for reliable wave attenuation predictions. Coastal EngineeringHydraulic Structures and Flood Ris

Tidal flat-wetland systems as flood defenses: Understanding biogeomorphic controls

Coastal managers worldwide increasingly recognize the importance of conservation and restoration of natural coastal ecosystems. This ensures coastal resilience and provision of essential ecosystem services, such as wave attenuation reducing coastal flooding and erosion. In the continuum from unvegetated tidal flats to salt marshes and mangroves, fundamental physical controls as well as biotic interactions, and feedbacks among them, determine morphology and vegetation distribution. Although these processes are well described in established literature, this information is rarely applied to understanding the role of these ecosystems as coastal defense. The focus is often on specific elements of the complex system, such as vegetation structure and cover, rather than on their complex natural dynamics. This review examines whether and how the dynamic nature of tidal flat - wetlands systems contributes to, or detracts from, their role in coastal defense. It discusses how the characteristics of the system adjust to external forcing and how these adjustments affect ecosystem services. It also considers how human interventions can take advantage of natural processes to enhance or accelerate achievement of natural coastal defense.Coastal EngineeringEnvironmental Fluid Mechanic

Assessing safety of nature-based flood defenses: Dealing with extremes and uncertainties

Vegetated foreshores adjacent to engineered structures (so-called hybrid flood defenses), are considered to have high potential in reducing flood risk, even in the face of sea level rise and increasing storminess. However, foreshores such as salt marshes and mangrove forests are generally characterized by relatively strong temporal and spatial variations in geometry and vegetation characteristics (e.g., stem height and density), which causes uncertainties with regards to their protective value under extreme storm conditions. Currently, no method is available to assess the failure probability of a hybrid flood defense, taking into account the aforementioned uncertainties. This paper presents a method to determine the failure probability of a hybrid flood defense, integrating models and stochastic parameters that describe dike failure and wave propagation over a vegetated foreshore. Two dike failure mechanisms are considered: failure due to (i) wave overtopping and (ii) wave impact on revetments. Results show that vegetated foreshores cause a reduction in failure probability for both mechanisms. This effect is more pronounced for wave impact on revetments than for wave overtopping, since revetment failure occurs at relatively low water levels. The relevance of different uncertainties depends on the protection level and associated dike height and strength. For relatively low dikes (i.e., low protection levels), vegetation remains stable in design conditions, and plays an important role in reducing wave loads. In case of higher protection levels, hence for more robust dikes, vegetation is less important than foreshore geometry, because of expected stem breakage of the vegetation under these more extreme conditions. The integrated analysis of uncertainties in hydraulic loads, dike geometry and foreshore characteristics in this paper enables the comparison between nature-based flood defenses and traditionally engineered solutions, and allows coastal engineers to design hybrid flood defenses worldwide.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Coastal EngineeringHydraulic Structures and Flood Ris

Advancing disaster risk reduction through the integration of science, design, and policy into eco-engineering and several global resource management processes

By the later part of the 21st Century, our planet will be faced with compelling climatic circumstances requiring tradeoffs to maintain viable environmental conditions and standards of living. The prognosis for people near coastlines and waterways is particularly dire without decisive actions that capitalize on shared strengths such as ecosystems. One clear opportunity is the regenerative services and co-benefits of natural infrastructure that reduce the impacts of environmental disasters as magnified by climatic change. Certainly, nature-based solutions are increasingly being viewed as critical actions to reduce societal risk. However, to advance the use of natural infrastructure through eco-engineering, there is a need to clarify the science regarding risk reduction effectiveness, develop agreeable principles, standards, and designs, and grow a demonstration site network responsive to circumstances faced by communities around the globe. In addition, there is a need to consider the legal, policy, and regulatory obstacles and opportunities for natural infrastructure within local to national contexts (i.e., science-based building codes, architectural design criteria, incentive policies, etc.). Ultimately, the integration of science, designs, and policy coupled with installation within several global resource management processes versus global resource frames (IWRM, ICZM, etc.) will help establish eco-engineering standards. Supportive coastal, river, and urban examples from around the world are used to illustrate the current state of knowledge, model this integration of science, design, and policy, serve as initial "benchmark site", and finally help define guiding principles for the emerging field of eco-engineering.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Coastal Engineerin

Editorial: Coastal protection provided by ecosystems: Observations and modeling across scales

Coastal Engineerin

Coastal and riverine ecosystems as adaptive flood defenses under a changing climate

Adaptation planning for flood risk forms a significant part of global climate change response. Engineering responses to higher water levels can be prohibitively costly. Several recent studies emphasize the potential role of ecosystems in flood protection as adaptive risk reduction measures while also contributing to carbon fixation. Here, we use a conceptual model study to illustrate the built-in adaptive capability of ecosystems to reduce a wide range of wave heights, occurring at different water levels, to a narrower range. Our model shows that wave height of waves running through a forested section is independent of initial height or of water level. Although the underlying phenomenon of non-linear wave attenuation within coastal vegetation is well studied, implications of reducing variability in wave heights for design of ecosystem and levee combinations have not yet been properly outlined. Narrowing the range of wave heights by a vegetation field generates an adaptive levee that is robust to a whole range of external conditions rather than only to a maximum wave height. This feature can substantially reduce costs for retrofitting of levees under changing future wave climates. Thereby, in wave prone areas, inclusion of ecosystems into flood defense schemes constitutes an adaptive and safe alternative to only hard engineered flood risk measures.Hydraulic Structures and Flood Ris

How to restore mangroves for greenbelt creation along eroding coasts with abandoned aquaculture ponds

Globally, erosion of muddy tropical coasts that are dominated by aquaculture ponds, is an increasing problem. Restoration of mangrove greenbelts may counteract such erosion, by restoring the sediment balance. Hence, we aim to unravel the processes controlling natural mangrove regeneration in both “landward” (i.e., into aquaculture ponds) and seaward direction, using the fast eroding coastline of Demak (Indonesia) as case study. Firstly, we investigated which physical and chemical factors drive landward mangrove expansion by relating them to the presence/absence of mangrove seedlings in abandoned aquaculture ponds. Secondly, we investigated which physical parameters control seaward mangrove expansion by relating them to expansion and retreat at the sea-side of mature mangrove stands. Landward mangrove expansion into abandoned aquaculture ponds was positively related to both emergence time (%) and sediment stability (i.e., shear strength), which are in turn both associated to bed level elevation and pond drainage. Surprisingly, there was no effect of soil chemistry. Seaward expansion of existing mangrove stands was strongly associated to foreshore morphology. Mangroves only expanded in the presence of an elevated mudflat, whereas the absence of a mudflat in combination with a concave (hollow) profile was associated with mangrove retreat. Our findings suggest that restoration of a mangrove greenbelt can be stimulated landward by improving drainage of abandoned aquaculture ponds. This enhances sediment stability and allows ponds to accrete. Seaward expansion can be induced by restoring foreshore morphology. Present results are discussed in the context of large-scale applications.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Coastal Engineerin

The benefits of coastal adaptation through conservation of foreshore vegetation

Due to rising sea levels and projected socio-economic change, global coastal flood risk is expected to increase in the future. To reduce this increase in risk, one option is to reduce the probability or magnitude of the hazard through the implementation of structural, Nature-based or hybrid adaptation measures. Nature-based Solutions in coastal areas have the potential to reduce impacts of climate change and can provide a more sustainable and cost-effective alternative to structural measures. In this paper, we present the first global scale assessment of the benefits of conserving foreshore vegetation as a means of adaptation to future projections of change in coastal flood risk. In doing so, we extend the current knowledge on the economic feasibility of implementing global scale Nature-based Solutions. We show that globally foreshore vegetation can contribute to a large decrease in both absolute and relative flood risk (13% of present-day and 8.5% of future conditions in 2080 of global flood risk). Although this study gives a first proxy of the flood risk reduction benefits of conserving foreshore vegetation at the global scale, it shows promising results for including Nature-based and hybrid adaptation measures in coastal adaptation schemes.Coastal EngineeringWater Resource

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.Coastal EngineeringWater ResourcesApplied Science