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An efficient modelling approach for probabilistic assessments of present-day and future fluvial flooding
Flood risk management and planning decisions in many parts of the world have historically utilised flood hazard or risk maps for a very limited number of hazard scenarios (e.g. river water levels), mainly due to computational challenges. With the potentially massive increase in flood risk in future due to the combination of climate change effects (increasing the hazard) and increasing population and developments in floodplains (increasing the consequence), risk-informed flood risk management, which enables balancing the risk with the reward, is now becoming more and more sought after. This requires a comprehensive and quantitative risk assessment, which in turn demands multiple (thousands of) river and flood model simulations. Performing such a large number of model simulations is a challenge, especially for large, complex river systems (e.g. Mekong) due to the associated computational and resource demands. This article presents an efficient modelling approach that combines a simplified 1D hydrodynamic model for the entire Mekong Delta with a detailed 1D/2D coupled model and demonstrates its application at Can Tho city in the Mekong Delta. Probabilistic flood hazard maps, ranging from 0.5 yr to 100 yr return period events, are obtained for the urban centre of Can Tho city under different future scenarios taking into account the impact of climate change forcing (river flow, sea-level rise, storm surge) and land subsidence. Results obtained under present conditions show that more than 12 % of the study area is inundated by the present-day 100 yr return period water level. Future projections show that, if the present rate of land subsistence continues, by 2050 (under both RCP4.5 and RCP8.5 climate scenarios), the 0.5 yr and 100 yr return period flood extents will increase by around 15-fold and 8-fold, respectively, relative to the present-day flood extent. However, without land subsidence, the projected increases in the 0.5 yr and 100 yr return period flood extents by 2050 (under RCP4.5 and RCP8.5) are limited to between a doubling to tripling of the present-day flood extent. Therefore, adaptation measures that can reduce the rate of land subsidence (e.g. limiting groundwater extraction), would substantially mitigate future flood hazards in the study area. A combination of restricted groundwater extraction and the construction of a new and more efficient urban drainage network would facilitate even further reductions in the flood hazard. The projected 15-fold increase in flood extent projected by 2050 for the twice per year (0.5 yr return period) flood event implies that the do nothing management approach is not a feasible option for Can Tho.
Multi-scale experiments for a coarse sand barrier against backward erosion piping
Backward erosion piping poses a severe threat for embankments founded on sandy deposits. The physical mechanism by which a pipe progresses upstream beneath the embankment is conceptually understood, and assessments indicate high failure probabilities in the Netherlands. Rising sea levels and economic growth increase the risk of flooding, and the need for remediation measures. Conventional measures have undesired side effects, in terms of costs, maintenance, space requirements and sustainability. The coarse sand barrier (CSB) is a novel nature-based solution lacking these side effects. The CSB is a trench filled with coarse sand that is placed below the downstream toe of the embankment, which prevents the pipe from progressing upstream. The effectiveness of the CSB is based on its higher resistance against erosion and its effect on groundwater flow. This paper presents experimental work that was conducted to test the feasibility of a CSB for application at a pilot site in the Netherlands. This involved experiments with seepage lengths of 0.34, 1.34 and 15 m, using different barrier materials and background sands. A conceptual model is presented for the physical mechanism of pipe formation in the presence of a barrier. As the resistance of the barrier can be characterized by a local scale-independent strength criterion, laboratory results are applicable for design practice.
Breaching flow slides and the associated turbidity current
This paper starts with surveying the state-of-the-art knowledge of breaching flow slides, with an emphasis on the relevant fluid mechanics. The governing physical processes of breaching flow slides are explained. The paper highlights the important roles of the associated turbidity current and the frequent surficial slides in increasing the erosion rate of sediment. It also identifies the weaknesses of the current breaching erosion models. Then, the three-equation model of Parker et al. is utilised to describe the coupled processes of breaching and turbidity currents. For comparison’s sake, the existing breaching erosion models are considered: Breusers, Mastbergen and Van Den Berg, and Van Rhee. The sand erosion rate and hydrodynamics of the current vary substantially between the erosion models. Crucially, these erosion models do not account for the surficial slides, nor have they been validated due to the scarcity of data on the associated turbidity current. This paper motivates further experimental studies, including detailed flow measurements, to develop an advanced erosion model. This will improve the fidelity of numerical simulations.
Nieuwe tools voor inschatten klimaatschade en selecteren van adaptatiemaatregelen
De onderzoekslijn Klimaatbestendige Stad van het Nationaal Kennis- en Innovatieprogramma Water en Klimaat heeft twee tools gelanceerd om decentrale overheden te ondersteunen in hun opgave klimaatbestendig te worden. De Klimaatschadeschatter geeft per gemeente een schatting van de schade door hitte, droogte en wateroverlast voor de periode 2018-2050 als er geen klimaatadaptatiemaatregelen worden genomen. De Toolbox Klimaatbestendige Stad helpt personen en teams te verkennen welke adaptatiemaatregelen waar in een projectgebied genomen kunnen worden, hoe effectief die naar verwachting zijn en hoeveel ze ongeveer zullen kosten. Daarmee ondersteunen beide tools de risicodialoog.
The influence of grain size distribution on the hydraulic gradient for initiating backward erosion
Backward erosion by piping is one of the processes that threaten the stability of river embankments in the Netherlands. During high river stages, groundwater flow velocities underneath the embankment increase as a result of the steepened hydraulic gradient. If a single outflow point exists or forms, the concentrated flow can entrain soil particles, leading to the formation of a subsurface pipe. The processes controlling this phenomenon are still relatively unknown due to their limited occurrence and because piping is a subsurface phenomenon. To study the initiation of piping, we performed laboratory experiments in which we induced water flow through a porous medium with a vertically orientated outflow point. In these experiments, we explicitly considered grain size variations, thus adding to the existing database of experiments. Our experiments showed that the vertical velocity needed for the initiation of particle transport can be described well by Stokes’ law using the median grain size. We combine this with a novel method to relate bulk hydraulic conductivity to the grain size distribution. This shows that knowledge of the grain size distribution and the location of the outflow point are sucient to estimate the hydraulic gradient needed to initiate pipe formation in the experiment box.
Modeling of breaching-generated turbidity currents using Large Eddy Simulation
Breaching flow slides result in a turbidity current running over and directly interacting with the eroding, submarine slope surface, thereby promoting further sediment erosion. The investigation and understanding of this current are crucial, as it is the main parameter influencing the failure evolution and fate of sediment during the breaching phenomenon. In contrast to previous numerical studies dealing with this specific type of turbidity currents, we present a 3D numerical model that simulates the flow structure and hydrodynamics of breaching-generated turbidity currents. The turbulent behavior in the model is captured by large eddy simulation (LES). We present a set of numerical simulations that reproduce particular, previously published experimental results. Through these simulations, we show the validity, applicability, and advantage of the proposed numerical model for the investigation of the flow characteristics. The principal characteristics of the turbidity current are reproduced well, apart from the layer thickness. We also propose a breaching erosion model and validate it using the same series of experimental data. Quite good agreement is observed between the experimental data and the computed erosion rates. The numerical results confirm that breaching-generated turbidity currents are self-accelerating and indicate that they evolve in a self-similar manner.
Solids dynamics in gully pots
Runoff entering urban drainage systems contains suspended solids, which carry pollutants and may cause blockages in downstream parts of the system (for example infiltration facilities). Suspended solids inflow should, therefore, preferably be controlled by solids removal at gully pots. This paper presents the results of lab experiments on the solids accumulation in gully pots in a scale 1:1 setup. The accumulation process is initially dominated by settling in the gully pot. When a substantial sediment bed is created, the bed starts to interact with the flow, the removal efficiency of solids decreases, and the bed eventually reaches an equilibrium level. The effects of the discharge, sediment size, and geometry on these processes are assessed. The accumulation rate and equilibrium bed level are strongly affected by the flow pattern which is influenced by the combination of the position the jets impinge on the water and the gully pot’s outlet position.
Including sediment in European river basin management plans : twenty years of work by SedNet
This paper describes the efforts made by SedNet (the European Sediment Network) to generate attention for the inclusion of sediment in River Basin Management Plans (RBMPs) under the European Water Framework Directive (WFD). The SedNet response to key WFD implementation events is described using the “three-streams and windows-of-opportunities model” published by John Kingdon in 1995. SedNet was initially a response to the realization that the WFD -which came into force in 2000- largely neglected sediment. For SedNet, it was clear from the beginning that the WFD objectives can be achieved only if sediment is included in RBMPs. The SedNet efforts inspired the establishment of a sediment management concept for the Elbe river basin. That concept was used as a basis for the full inclusion of sediment in the second Elbe RBMP (2015–2021). SedNet experts are currently involved in the drafting of the WFD Common Implementation Strategy (CIS) sediment document which will be completed in 2021 and that will provide guidance about how to include sediment in RBMPs. Since 2000, SedNet has persistently drawn attention to the need to include sediment in RBMPs. However, it was not until 2015 that the Elbe became the first European river basin to include sediment management fully in their RBMP. The 2021 WFD CIS sediment document and the focus on sediment in the Water Fitness Check in 2019 make it significantly more likely that the sediment will be included in the updates of WFD RBMPs in the near future.
Bank protection structures along the Brahmaputra-Jamuna River, a study of flow slides
The planform of the Brahmaputra-Jamuna River followed its natural path in Bangladesh until the construction of bank protection works started to save Sirajganj from bank erosion since the 1930s. Several so-called hardpoints such as groynes and revetments were constructed in the period 1980–2015 and the Jamuna Multipurpose Bridge was opened in 1998. The Brahmaputra Right Embankment and other projects had saved the western flood plain from inundation during monsoon floods. These river training works experienced severe damage by geotechnical failures, mostly flow slides. A flow slide is an underwater slope failure because of liquefaction or a breaching process in the subsoil or a combination of both. The design of most of these training works did not consider the risk of damage by flow slides. All descriptions of the observed damages show that scour phenomena in the channel close to a river training work are a cause of flow slides, besides pore water outflow. The research question was: how can the design of river training works be improved to reduce the risk of damage by flow slides? The main part of the investigation was focussed on reducing local scour holes near river training works. The most promising results are river training works with gentle bank slopes, permeable groynes, bed protections in dredged trenches with gentle side slopes, and methods to increase locally the bearing capacity of the subsoil. It is recommended to increase the knowledge of the failure mechanisms in the Brahmaputra-Jamuna River by improved monitoring in the field, the setup of a database with descriptions of all observed flow slides and the circumstances in which they occur. In addition to these recommendations, a field test facility is proposed to verify the knowledge of the failure mechanisms in that river. These activities will optimise the design of new river training structures with a very low risk of damages by flow slides and geotechnical instabilities and they will contribute to an improvement of the current design guidelines for river training structures.
Physics-based basin-scale modelling of water quantity and sediment dynamics using wflow
The wflow_sediment model was developed to address basin-scale geomorphological processes and problems. It is a distributed physics-based model that uses the results of the wflow_sbm hydrological model in order to estimate soil erosion, delivery to the river, transport and deposition. Both the hydrologic and sediment dynamics wflow models are open-source and use openly available global datasets and parameter estimation in order to limit calibration and be applicable even in data scarce environments. Terrestrial processes include splash and overland flow erosion, as well as transport over the grid using either a total flow transport capacity or a transport capacity with particle differentiation. In-stream routing and erosion/deposition processes are adapted from the semi-distributed SWAT model. The wflow_sediment model was first tested in the Rhine basin (Western Europe) at a daily resolution and on a 1 km (0.008333°) grid. Both the inland and instream parts of the model gave promising results, showing the potential of this new tool for a very diverse range of applications.