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Human-caused avulsion in the Rhine-Meuse delta before historic embankment (The Netherlands)
Although the shifting of deltaic river branches (avulsion) is a natural process that has become increasingly influenced by humans, the impact of early human activities as a driver of avulsion success has remained poorly explored. This study demonstrates how two important avulsions in the downstream part of the Rhine-Meuse delta, The Netherlands, were stimulated by human activities in the first millennium CE, before historic embankment constrained the river courses. Peatland reclamation induced land subsidence in the lower delta. This effect, together with a human-induced increase in suspended fluvial sediments and tidal backwater effects, allowed for a gradual ingression of tidal creek channels and progradation of fluvial crevasse channels into human-occupied and drained peatlands, where they eventually connected. We reconstructed the initial situation and identified the feedback loops among overbank sedimentation, tidal incursion, and land drainage subsidence that led to avulsion success. The processes and feedbacks resulting from human activities are generic and hence relevant to many other deltas today where human-induced subsidence results in tidal ingression, potentially connecting to rivers and causing unexpected avulsions.
Low-hanging fruits in large-scale fluvial landscaping measures: trade-offs between flood hazard, costs, stakeholders and biodiversity
Adapting densely populated deltas to the combined impacts of climate change and socioeconomic developments presents a major challenge for their sustainable development in the 21st century. Decisions for the adaptations require an overview of cost and benefits and the number of stakeholders involved, which can be used in stakeholder discussions. Therefore, we investigated the balance between multi-faceted costs and benefits of common landscaping measures to compensate for changes in discharge and sea level rise on the basis of relevant, but inexhaustive, quantitative variables for physical, ecological and societal costs and benefits. We modelled the largest delta distributary of the Rhine River with adaptation scenarios driven by (1) the choice of seven measures, (2) the areas owned by the two largest stakeholders (LS) versus all stakeholders (AS), and (3) the ecological or hydraulic design principle. We evaluated measures by their efficiency in flood hazard reduction, potential biodiversity, number of stakeholders as a proxy to governance complexity, and measure implementation cost. We found that only floodplain lowering over the whole study area can offset the altered hydrodynamic boundary conditions; for all other measures, additional dike raising is required. LS areas comprise low hanging fruits for water level lowering due to the governance simplicity and hydraulic efficiency. Measures implemented in LS areas are 3 to 74% more efficient than in AS areas. Clear trade-offs were revealed between evaluation parameters, but no single measure represented the optimal combination on all aspects. The multidimensional evaluation space provides a frame for the co-creation of adaptation paths for climate-proofing deltas.
Urban water security dashboard : systems approach to characterizing the water security of cities
Urban water security is a major concern in the context of urbanization and climate change. Water security goes beyond having good infrastructure or good governance. Systems thinking can help in understanding the mechanisms that influence the long-term water security of a city. Therefore, we developed a dashboard of 56 indicators based on the pressure-state-impact-response (PSIR) framework. We applied the dashboard to ten cities to capture different characteristics of their water security and ranked the cities based on their overall water security index score. We found the highest levels of water security in wealthy cities in water-abundant environments (Amsterdam and Toronto), in which security is determined by the ability of the city to mitigate flood risks and the sustainability of hinterland dependencies for water supply. The lowest security was found in developing cities (Nairobi, Lima, and Jakarta). Here, the combination of large socio-economic pressures (e.g., rapid population growth, slums, low GDP, polluting industries) and an inadequate response (weak institutions, and poor planning and operational management) leads to inappropriate fulfilment of all functions of the urban water system.
How to model algal blooms in any lake on earth
Algal blooms increasingly threaten lake and reservoir water quality at the global scale, caused by ongoing climate change and nutrient loading. To anticipate these algal blooms, models to project future algal blooms worldwide are required. Here we present the state-of-the-art in algal projection modelling and explore the requirements of an ideal algal projection model. Based on this, we identify current challenges and opportunities for such model development. Since most building blocks are present, we foresee that algal projection models for any lake on earth can be developed in the near future. Finally, we think that algal bloom projection models at a global scale will provide a valuable contribution to global policymaking, in particular with respect to SDG 6 (clean water and sanitation).
Lessons learned from dike failures in recent decades
The paper describes five different dike failures that occurred in recent decades. The case histories were located on different rivers and involved different loading conditions. The observed failure mechanisms involved erosion and stability problems. The types of erosion observed were both internal erosion due to extreme groundwater flow and external erosion caused by the river flow and wave action. The case involving instability was caused by uplift, i.e., increased hydraulic head led to a sharp reduction in maximum shear strength between subsoil layers. Two cases demonstrate the importance of dike management and maintenance. Despite the variations in the loading conditions and failure mechanisms, all the cases show that the strength of a dike depends not only on the material used to build the dike but also on the strength of the subsoil.
Een Nederlandsch Waterbouwkundig Laboratorium
Waterloopkundig Laboratorium te Delft (architect ir. H. Kammer)
Mogelijke gevolgen van versnelde zeespiegelstijging voor het Deltaprogramma : een verkenning
De zeespiegel kan mogelijk (veel) sneller gaan stijgen dan tot nog toe is aangenomen in het Deltaprogramma. Deze extra versnelling heeft te maken met recente inzichten over het mogelijk versneld afbreken en smelten van het landijs op Antarctica. In het Deltaprogramma 2018 is opgenomen dat de mogelijke consequenties van de resulterende extra versnelde zeespiegelstijging nader onderzocht gaan worden. Dit rapport beschrijft de resultaten van een eerste verkenning naar de mogelijke gevolgen van deze extra versnelde zeespiegelstijging voor het kustfundament (inclusief de Wadden en zuidwestelijke delta), de waterveiligheid, en de zoetwatervoorziening in Nederland en de implicaties voor de voorkeursstrategieën van het Deltaprogramma.
Modelling tidal-induced sediment transport in a sand-silt mixed environment from days to years : application to the Jiangsu coastal water, China
In the present study a new multi-fractional, depth-averaged sediment transport module was developed and embedded into a morphodynamic model for a sand-silt mixed shallow water environment. Subsequently, the model was applied to the case of the Jiangsu coast, which features a silt enriched sedimentary environment bordered by two large-scale geomorphological units: the Old Yellow River Delta (OYRD) in the north and the Radial Sand Ridge Field (RSRF) in the south. Based on this case, the predictive abilities of the present model are assessed on both the short-term and the long-term. Comparisons with measurements over two successive tidal cycles indicate that the present model produces very good results on short-time scales. The model performance is extended and further validated by comparing the overall annual Suspended Sediment Concentration (SSC) pattern, the annual morphological changes, the annual sediment budget and the evolution trend of the bed composition. Also, these long-term results agree well with existing observations over the past several decades. Hence, an essential feature of the present modelling approach is the ability to simulate sediment transport and morphological changes over a relatively long time span (i.e., time scale of years) in a sand-silt mixed sedimentary environment, based on its validated short-term performance.
Physical model tests to determine the roughness of stair shaped revetments
Stair shaped revetments are considered as an attractive alternative for traditional revetments since the surface roughness reduces wave overtopping resulting in a lower crest height. The specific shape enables easy access from and to the watar surface, enhancing the attractiveness for tourists and residents. To implement stepped revetments in the design of a seawall there is a need to quantify the roughness of a stair shaped reventment to predict wave overtopping rates. This paper describes research in which 2D physical model experiments were conducted in Deltares' Scheldt Flume. Based on the obtained data a method to quantify the roughness coefficient of such structures is suggested.