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Investigating the formation of a filter cake in column experiments, for combinations of filter and fine sand in a coarse sand barrier
The coarse sand barrier (CSB) is a single granular filter used to retrofit an existing structure, making it more stable against backward erosion piping. The barrier material should be chosen carefully to retain particles from the sand layer upstream of the barrier, yet provide optimal resistance against backward erosion piping. This means the particles in the barrier should be large, thus difficult to transport, and the barrier should have a high permeability in order to reduce the local hydraulic gradient inside the barrier. However, transport of the fine sand upstream of the barrier into the barrier, may result in less permeable filter cake just inside the barrier. Therefore, a column set-up was designed and experiments were conducted with various sand types, using the same materials as used in small-scale and medium-scale backward erosion piping experiments with at CSB. The aim was to compare the results of different tests and check if the criterion for the formation of a filter cake is the same as the well-known filter rules. Consequently, this paper presents the results of different column experiments. These materials were carefully selected and fulfilled the selected filter rules but one soil composition caused the filter cake formation. This indicates that to avoid filter cake formation for the conditions tested stricter rules apply than the filter rules considered here.
Analytical groundwater flow calculations for understanding the flow and erosion in a coarse sand barrier
The coarse sand barrier (CSB) is a promising method to avoid ongoing backward erosion piping resulting in increased safety of a dike for this failure mechanism. Experiments are performed at different scales at Deltares, the Netherlands. These experiments show a significant increase in the critical head for structures with a CSB compared to structures without a CSB. The increase of critical head cannot only be ascribed to the lower erodibility of the coarser particles in the barrier, but also to the reduction of hydraulic load on these coarse particles in the barrier, resulting from the permeability contrast of barrier material and surrounding sand. To investigate the influence of a CSB on the flow pattern numerical and analytical calculations have been performed. This paper focusses on the analytical calculations. It will be shown that these can explain the increase in strength and the measured scaling effects.
Databases for backward erosion piping laboratory experiments and field observations
Backward erosion piping is a failure mechanism which involves the formation of shallow pipes in a sandy foundation layer and is considered to be a major risk for levees. For understanding this mechanism and the development of prediction models, laboratory experiments are essential. In addition, due to scale effects and heterogeneity in field conditions, field observations and case histories are indispensable for validation of models and delineation of piping sensitive conditions. However, both experiments and field observations are often not easily utilized for this purpose. Piping experiments have been conducted in various research programmes, countries, and in a variety of configurations making the experiments difficult to compare due to inconsistent observations and differing configurations. Case histories are often poorly documented and like experiments, described in different sources and different levels of detail, due to which their full potential is often not reached. Given the importance of experimental and field data for the prediction of backward erosion piping, a need exists for a centralized organization of data. Two different databases are presented here, for laboratory experiments and field observations respectively, each combined with a web application for viewing and exporting the data. The laboratory experiment database is populated with 332 experiments. The field observation database is currently populated with 3 failure cases and 2840 sand boils located in the Netherlands and the United States. Future work will focus on a more complete population of the databases, user-friendliness of the web viewer, and analysis of the gathered data for improvement of prediction models.
Scale effects in coarse sand barrier experiments
The coarse sand barrier is considered as a promising measure to prevent backward erosion piping from causing failure of embankments. A pipe is allowed to progress backwards until it encounters the coarse sand barrier, which prevents it from progressing unless the head difference over the embankment is significantly increased. A three stage experimental programme supported by groundwater flow modelling is carried out to investigate the feasibility of this method. The hypothesis is that the strength of the barrier is characterised by a local gradient at the interface between the barrier and the pipe. Major questions are: can the horizontal gradient as measured in laboratory tests be used to characterise the strength of the barrier material, over which distance should a horizontal gradient be determined, and is this distance the same for models at different scales? This paper presents the background theory and demonstrates the effects using scale dependent criteria. Preliminary results of small- and medium-scale experiments are used to compare the two approaches.
A first collective validation of global fluvial flood models for major floods in Nigeria and Mozambique
Global flood models (GFMs) are becoming increasingly important for disaster risk management internationally. However, these models have had little validation against observed flood events, making it difficult to compare model performance. In this paper, we introduce the first collective validation of multiple GFMs against the same events and we analyze how different model structures influence performance. We identify three hydraulically diverse regions in Africa with recent large scale flood events: Lokoja, Nigeria; Idah, Nigeria; and Chemba, Mozambique.We then evaluate the flood extent output provided by six GFMs against satellite observations of historical flood extents in these regions. The Critical Success Index of individual models across the three regions ranges from 0.45 to 0.7 and the percentage of flood captured ranges from 52% to 97%. Site specific conditions influence performance as the models score better in the confined floodplain of Lokoja but score poorly in Idah’s flat extensive floodplain. 2D hydrodynamic models are shown to perform favourably. The models forced by gauged flow data show a greater level of return period accuracy compared to those forced by climate reanalysis data. Using the results of our analysis, we create and validate a three-model ensemble to investigate the usefulness of ensemble modelling in a flood hazard context. We find the ensemble model performs similarly to the best individual and aggregated models. In the three study regions, we found no correlation between performance and the spatial resolution of the models. The best individual models show an acceptable level of performance for these large rivers.
Benchmarking flexible meshes and regular grids for large-scale fluvial inundation modelling
Damage resulting from flood events is increasing world-wide, requiring the implementation of mitigation and adaption measures. To facilitate their implementation, it is essential to correctly model flood hazard at the large scale, yet fine spatial resolution. To reduce the computational load of models, flexible meshes are an efficient means compared to uniform regular grids. Yet, thus far they have been applied only for bespoke small-scale studies requiring a high level of a priori grid preparation. To better understand possible advantages as well as shortcomings of their application for large-scale riverine inundation simulations, three different flexible meshes were derived from Height Above Nearest Drainage (HAND) data and compared with regular grids under identical spatially explicit hydrologic forcing by using GLOFRIM, a framework for integrated hydrologic-hydrodynamic inundation modelling. By means of GLOFRIM, output from the global hydrologic model PCR-GLOBWB was passed to the hydrodynamic model Delft3D Flexible Mesh. Results show that applying flexible meshes can be beneficial depending on the envisaged purpose. For discharge simulations, similar model accuracy was obtained between flexible and regular grids, with the former generally having shorter run times. For inundation extent simulations, however, the coarser gridding of flexible meshes in upstream areas results in a poorer performance if assessed by contingency maps. Moreover, while the ratio between minimum and maximum spatial resolution of flexible meshes has limited impact on discharge simulations, water level estimates may be stronger influenced by the application of larger grid cells. . As this study presents only a small set of possible realizations, additional research needs to unravel how the data and methods used as well as the choices for discretizations influence model performance. Generally, the application and particularly discretization process of flexible meshes involves more options, bringing more responsibilities for the user. Once an a priori decision is made on the model purpose, flexible meshes can be a valuable addition to modelling approaches where short run times are essential, facilitating large-scale flood simulations, ensemble modelling or operational flood forecasting.
Development of an Integrated Biophysical Model to represent morphological and ecological processes in a changing deltaic and coastal ecosystem
Deltaic and coastal ecosystems are changing in response to natural and anthropogenic forces that require ecosystem-level restoration efforts to avoid habitat degradation or loss. Models that link ecosystem components of hydrodynamics, morphodynamics, nutrient and vegetation dynamics to represent essential processes and feedbacks are advancing the field of environmental modeling and are vital to inform coastal restoration decisions. An Integrated Biophysical Model was developed by creating a new vegetation dynamics component and linking it to other primary ecosystem components that included essential feedbacks. The model performance was evaluated by applying it to a deltaic ecosystem that included marshes and estuaries. The Integrated Biophysical Model output captured the general temporal and spatial environmental trends of key variables. This integrated model is capable to perform long-term simulations to assess responses of deltaic and coastal systems to global change scenarios and can be used to inform restoration strategies in ecosystems worldwide.
Internal erosion in earthdams, dikes and levees : proceedings of EWG-IE 26th annual meeting (Milano, Italy, September 10 to 13,2018)
Scenariostudie natuurperspectief Grevelingenmeer
In de Rijksstructuurvisie (RSV) Grevelingen en Volkerak-Zoommeer zijn diverse oplossingsrichtingen verkend om de waterkwaliteit te verbeteren als voorbereiding op een Rijksbesluit. In de planvorming is voor het Grevelingenmeer gekeken naar kansen vanuit verschillende invalshoeken zoals waterkwaliteit, getijdenenergie en waterveiligheid. Natuurorganisaties waren van mening dat hierbij vooral aandacht is geweest voor effecten op huidige beschermde natuurwaarden (in relatie tot Natura 2000-wetgeving), maar dat er slechts beperkte aandacht is geweest voor kansen voor het verzilveren van nieuwe (estuariene) natuurwaarden en optimalisatie mogelijkheden van toekomstig waterbeheer ten bate van natuurwaarden in een situatie met gedempt getij. Het ministerie van Landbouw, Natuur en Voedselkwaliteit (LNV) vindt het van belang dat het perspectief en de kansen voor natuur in het Grevelingenmeer grondiger worden uitgediept met betrokkenheid van verschillende experts en dat natuurpartijen (lokaal en landelijk), beheerders (Staatsbosbeheer en RWS) en provincies hierbij worden betrokken. Daarom heeft zij Wageningen Marine Research gevraagd om dit nader te onderzoeken. Er is aangesloten op het voorkeursalternatief van de RSV (50 cm getij), maar daarnaast zijn ook breder de gevolgen voor de natuur van (1) een kleinere getijslag en (2) een aangepast peilbeheer onderzocht en er is een doorkijk gegeven naar de lange termijn gevolgen waarbij tevens het effect van zeespiegelstijging is meegenomen.
Influence of El Niño Southern Oscillation on global coastal flooding
Anomalous atmosphere-ocean conditions in the tropical Pacific associated with the El Niño Southern Oscillation (ENSO) drive interannual variations in mean and extreme sea levels. Climate change may lead to more frequent extreme ENSO events in the future. Therefore, it is important to enhance our understanding of ENSO's influence on coastal flood impacts. We assessed ENSO’s influence on extreme sea levels using a global reanalysis of tides and storm surges. This allows for a full coverage of the global coastline from 1979 to 2014. A mean sea level component is added to account for steric effects. This results in a substantial improvement in the representation of the seasonal and interannual variability. Our results show significant correlations across the Pacific between ENSO and extreme sea levels (expressed as 95th annual percentiles), which is consistent with previous studies based on tide gauge observations. Average anomalies in the annual percentiles over El Niño years compared to neutral years show similar patterns. When examining total sea levels, results are largely statistically insignificant. This is because in many regions large tidal variability dominates over the other components. Combining sea levels with an inundation and impact model shows that ENSO has a significant but small effect on the number of people potentially exposed to flooding at the globally aggregated-scale. Our result demonstrate that a model-based approach allows for an assessment of the influence of ENSO on coastal flood impacts, and could be used to assess impacts of future changes in ENSO.