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Wondermateriaal olivijn vangt CO2 uit de lucht
Het mineraal olivijn is het meest voorkomende gesteente in de aardmantel. Olivijn verweert langzaam tot kalk en gebruikt daarvoor water en CO2 uit de lucht. Er zijn succesvolle kleinschalige proeven uitgevoerd met olivijn om het broeikasgas weg te vangen. Jos Vink (Deltares) heeft een model gemaakt dat de snelheid van de verwering berekent, de hoeveelheid vastgelegde CO2 laat zien en uitrekent hoeveel kalk, magnesium en nikkel daarbij vrijkomt. De komende twee jaar volgen Vink en zijn collega’s de verwering van het mineraal onder verschillende omstandigheden, op twaalf proefveldjes op het terrein van Deltares.
Morphological computation of dune evolution with equilibrium and non-equilibrium sediment-transport models
This paper presents an exploratory study that comprises the implementation and comparison of different approaches and parameterization of sediment transport mechanisms in a process-based morphological model for simulating river dunes. The purpose of this study was to assess the underlying physical processes associated with sediment transport and dune evolution simulated by the model with two different bedload transport models: a nonequilibrium approach considering saltation distance and an equilibrium approach with the incorporation of the effect of the local bed slope. The advancement of this study is that it comprised detailed analysis of the bed shear stress and sediment transport over a dune during time varying flows, comparing both transport models and revealing distinctive transient features of bedload transport and dune evolution. We also improved a non-equilibrium transport model by incorporating a formulation for sediment saltation distance as a lengthscale. Using both sediment transport approaches, the morphological model was applied to replicate a large-scale field experiment. Notably, the model yielded the quasi-equilibrium dune feature reasonably well using either sediment-transport formulation. However, detailed analysis of simulated spatial and temporal features of sediment transport and the dune evolution process were found to be noticeably different. Even though both sediment transport mechanisms appear to work adequately at a basic level, the major difference between the two approaches is the underlying transport process over the dunes and time-scale of the dune evolution process.
Risicobenadering voor droogte : lessen uit 4 jaar onderzoek
De afgelopen jaren is een risicobenadering voor droogte ontwikkeld. Hiermee is invulling gegeven aan de wens om net als voor waterveiligheid, ook voor zoetwatervoorziening beslissingen te baseren op informatie over het risico. De risicobenadering kwantificeert zowel de variabiliteit van de droogtecondities (neerslagtekort en afvoertekort) als het economisch effect van droogte op de belangrijkste watergebruikers. Op basis van de inzichten uit de toepassingen concluderen we dat de risicobenadering meerwaarde biedt door de gestructureerde manier waarop de gevolgen van droogte voor verschillende gebruiksfuncties en gebieden worden afgewogen.
Future response of the Wadden Sea tidal basins to relative sea-level rise : an aggregated modelling approach
Climate change, and especially the associated acceleration of sea-level rise, forms a serious threat to the Wadden Sea. The Wadden Sea contains the world’s largest coherent intertidal flat area and it is known that these flats can drown when the rate of sea-level rise exceeds a critical limit. As a result, the intertidal flats would then be permanently inundated, seriously affecting the ecological functioning of the system. The determination of this critical limit and the modelling of the transient process of how a tidal basin responds to accelerated sea-level rise is of critical importance. In this contribution we revisit the modelling of the response of the Wadden Sea tidal basins to sea-level rise using a basin scale morphological model (aggregated scale morphological interaction between tidal basin and adjacent coast, ASMITA). Analysis using this aggregated scale model shows that the critical rate of sea-level rise is not merely influenced by the morphological equilibrium and the morphological time scale, but also depends on the grain size distribution of sediment in the tidal inlet system. As sea-level rises, there is a lag in the morphological response, which means that the basin will be deeper than the systems morphological equilibrium. However, so long as the rate of sea-level rise is constant and below a critical limit, this offset becomes constant and a dynamic equilibrium is established. This equilibrium deviation as well as the time needed to achieve the dynamic equilibrium increase non-linearly with increasing rates of sea-level rise. As a result, the response of a tidal basin to relatively fast sea-level rise is similar, no matter if the sea-level rise rate is just below, equal or above the critical limit. A tidal basin will experience a long process of ‘drowning’ when sea-level rise rate exceeds about 80% of the critical limit. The insights from the present study can be used to improve morphodynamic modelling of tidal basin response to accelerating sea-level rise and are useful for sustainable management of tidal inlet systems.
Coastal structures 2019 - 8th International Coastal Structures conference (Hannover, Germany, 29 September - 2 October 2019)
Prediction method for wave overtopping and wave forces on rubble mound breakwater crest walls
At the crest of rubble mound breakwaters a crest wall is often present to reduce the consumption of material compared to breakwaters without a crest wall or to enable access to the breakwater. Several methods to design crest walls are available. These methods are developed for wave loading where waves approach breakwaters in perpendicular direction. Compared to perpendicular wave attack, oblique waves can significantly reduce the amount of wave overtopping and reduce the wave loads on crest walls on breakwaters. Based on physical model tests new design guidelines have been developed by Van Gent and Van der Werf (2019) for perpendicular wave attack and for oblique wave attack. Here, a summary of those design guidelines is presented together with a discussion of the most essential aspects of these design guidelines.
Large-scale testing of distributed temperature sensing for early detection of piping
Internal erosion is the cause of significant damage in dams and river embankments in many countries. In the last 20 years, the use of fiber-optic distributed temperature sensing (DTS) has proved to be an effective tool for the detection and quantification of leakages and internal erosion in dams. This work investigates the effectiveness of DTS for levee monitoring and focuses on the early detection of backward erosion piping. The latter is a failure mechanism that affects the foundation layer of structures resting on sandy soils. The paper presents data from a piping test performed on a large-scale experimental levee equipped with a DTS system together with a large number of accompanying sensors.With the help of numerical modeling, the effect of seepage and backward erosion piping on the pore pressure and temperature field is analyzed, eventually bringing the identification of the processes that promote the onset of thermal anomalies.
Experimental study on the mechanisms of bentonite slurry penetration in front of a slurry TBM
Penetration of bentonite slurry reduces the effective support pressure at the tunnel face and thus the face stability during slurry shield excavation in saturated sand. The reduction of effective support pressure depends on the penetration distance of the bentonite slurry but also the hydraulic characteristics of the slurry-penetrated soil. The penetration of bentonite slurry, pressurized against saturated sand, therefore, was investigated in a laboratory set-up to improve the understanding of the mechanisms of two stages of the penetration: mud spurt and filter cake formation. The permeability of sand for bentonite slurry (kb), the permeability of filter cake for water (kc) and the Peclet number (Pe) at initiation of filter cake formation were calculated from the experimental results. It appeared that the values of kb and kc depend on the bentonite concentration of the slurry. Adding bentonite to the slurry decreases both kb and kc. Due to the thixotropic nature of the bentonite, the permeabilities estimated from the viscosity of fluid are approximately a factor 2 to 3 higher than the measured ones. The estimated Pe is a bit higher than the expected value. It was also found that filter cake is only formed on the surface of the sand in front of the shield tunnel, thus is very vulnerable to damage. The excess pore water pressure caused by slurry shield tunneling will only be negligible due to filter cake formation during standstill.
Lateral stress measurement in DSS testing
This paper discusses the improvement of a large direct simple shear, DSS, box to measure the horizontal, out of plane, stress development during direct simple shear testing. Tests are conducted on remoulded Oostvaardersplassen-clay, OVP clay specimen with dimensions 0.26 (l) x 0.08 (h) x 0.22 (w) m. Perpendicular to the shearing direction 8 stress transducers, 4 on each sidewall are applied. Details of the set-up are described in the paper. Horizontal stress measurements are compared to finite element simulations, considering different material models. Conventional triaxial compression and triaxial extension tests have been conducted on the same material. The results of the conventional tests and the DSS tests, with horizontal stress measurement, are plotted in the deviator-plane, showing good agreement to the Matsuoka – Nakai criterion.
Conventional reinforcement as a potential prevention measure against piping
Piping is one of the main failure mechanisms that can affect the safety of water-retaining structures. A phenomenon that can occur when a local disruption of water structure caused sand erosion and concentration of seepage flow at that location. This entails sufficient hydraulic gradient resulting in the formation of shallow pipes in the sand layer. There are number of methods to increase the factor of safety against piping. An effective technique is soil reinforcement. The soil reinforcement can be performed by the inclusion of elements (strips, bars, etc) within the mass of soil in a preferred direction. Geogrids can be used as a convenient reinforcement material for improving the behaviour of the soil because of the high tensile resistance and significant friction in the soil. This paper presents laboratory experiments that were performed on unreinforced and reinforced soil samples. Reinforcement was done using different types of geogrid in different layers of the soil sample to inves-tigate the effect of this method and arrangement of the geogrid sheets on the critical hydraulic gradient and resistance against piping. Subsequently, the results demonstrate that reinforcement increased the critical hydrau-lic gradient up to 75% compared to the value in unreinforced soil. The amount of improvement of the critical gradient is dependent on the arrangement and type of the geogrid.