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Contribution of 3D coupled hydrodynamic-ecological modeling to assess the representativeness of a sampling protocol for lake water quality assessment
This study deals with the impact of spatio-temporal heterogeneities on the assessment of lake ecological status according to the European water framework directive (WFD). A method, based on threedimensional coupled hydrodynamic and ecological modeling, is presented to assess the variability of lake ecological status, and to locate the most representative sampling station of Lake Geneva (France/Switzerland). Five variables used in the lake ecological status evaluation were simulated by using the free software Delft3D. The numerical simulation results showed that the simulated ecological status based on chlorophyll a and total phosphorus concentrations measured at the regulatory monitoring station depend on the choice of the sampling date. Results also indicated a strong spatial heterogeneity in ecological status that varies from “poor” to “good” along an East-West gradient. Finally, the numerical simulation results showed that the most representative point of a mean theoretical ecological quality for Lake Geneva would be located in the center of the upper basin, close to the historical sampling station.
Is density enough to predict the rheology of natural sediments?
Mud is a cohesive material which contains predominantly clay minerals,water, organic matter and some amounts of silt and sand. Mud samples can have complex rheological behaviour, displaying viscoelasticity, shear-thinning, thixotropy and yield stress. In this study, influence of organic matter on the rheological behaviour of different mud samples having similar densities is investigated. Four samples, collected from different locations and depths of Port of Hamburg (Germany) were selected. Two samples with the density of about 1210 kg/m3 and two samples with the density of about 1090 kg/m3 were analysed by different rheological tests, including stress ramp-up tests, flow curves, thixotropic tests, oscillatory amplitude and frequency sweep tests. Two yield stress regions (with two yield stress values stated as “static” and “fluidic” yield stresses) were identified for all the samples, and these regions, corresponding to a structural change of the samples were significantly different from sample to sample due to the differences in organicmatter content. For lower density samples, the ratio of fluidic to static yield stress increased from3 to 4.4 while it increased from 4.4 to 5.2 in case of higher density samples, by increasing the organic matter content. The thixotropic studies showed that the mud samples having lowest organic matter content (VH and KBZ) exhibit a combination of thixotropic and anti-thixotropic behaviours. The results of frequency sweep tests revealed the solid-like character of the mud within the linear viscoelastic regime. Mud samples having higher organic matter content (RVand RT) had a higher complex modulus (417 Pa and 7909 Pa) than the ones with lower organic matter content (13 Pa and 1774 Pa), for a given density. This study demonstrated that the density only is not a sufficient criterion to predict the rheology of different mud. Furthermore, even small amounts of organic matter content change significantly the mud rheological behaviour.
Uncertainties in offshore wind turbulence intensity
In the development, design and operation of wind farms the knowledge of wind turbulence intensity is of paramount importance, particularly at hub height. Given that measurements are rare, turbulence intensities are often determined using simplified formulations. These formulations only account for a dependence on wind speed and include a neutral stability assumption. There are, therefore, large uncertainties associated with the results of such formulations. In order to quantify these uncertainties we determine the dependence of turbulence intensity at different heights on the surface wind velocity, wave conditions and vertical temperature gradients from offshore LiDAR wind observations in the North Sea. The turbulence intensity is shown to depend strongly on the atmospheric stability and less strongly on the sea surface roughness. The lower turbulence intensity values are observed under stable atmospheric conditions. The dependence of the turbulence intensity on the surface roughness is higher at the lower levels, with the significant wave height being the sea surface roughness parameters with the stronger correlation with the turbulence intensity.
Invited perspectives : how machine learning will change flood risk and impact assessment
Increasing amounts of data, together with more computing power and better machine learning algorithms to analyse the data are causing changes in almost every aspect of our lives. This trend is expected to continue as more data becomes available, computing power increases and machine learning algorithms improve. Flood risk and impact assessments are also being influenced by this trend, particularly in areas such as the development of mitigation measures, emergency response preparation, and flood recovery planning. Machine learning methods have the potential to improve accuracy as well as reduce calculating time and model development cost. It is expected that in the future more applications become feasible and many process models and traditional observation methods will be replaced by machine learning. Examples of this include the use of machine learning on remote sensing data to estimate exposure or on social media data to improve flood response. Some improvements may require new data collection efforts, such as for the modelling of flood damages or defence failures. In other fields, machine learning may not be suitable or should be applied complementary to process models, for example in hydrodynamic applications. Overall, machine learning is likely to drastically improve future flood risk and impact assessments, but issues such as applicability, bias and ethics must be considered carefully. This paper presents some of the current developments on the application of machine learning for flood risk and impact assessment, and highlights some key needs and challenges.
Measuring marine plastic debris from space : initial assessment of observation requirements
Sustained observations are required to determine the marine plastic debris mass balance and to support effective policy for planning remedial action. However, observations currently remain scarce at the global scale. A satellite remote sensing system could make a substantial contribution to tackling this problem. Here, we make initial steps towards the potential design of such a remote sensing system by: (1) identifying the properties of marine plastic debris amenable to remote sensing methods and (2) highlighting the oceanic processes relevant to scientific questions about marine plastic debris. Remote sensing approaches are reviewed and matched to the optical properties of marine plastic debris and the relevant spatio-temporal scales of observation to identify challenges and opportunities in the field. Finally, steps needed to develop marine plastic debris detection by remote sensing platforms are proposed in terms of fundamental science as well as linkages to ongoing planning for satellite systems with similar observation requirements.
Watching the beach steadily disappearing : the evolution of understanding of retrogressive breach failures
Retrogressive breach failures or coastal flow slides occur naturally in the shoreface in fine sands near dynamic tidal channels or rivers. They sometimes retrogress into beaches, shoal margins and riverbanks where they can threaten infrastructure and cause severe coastal erosion and flood risk. Ever since the first reports were published in the Netherlands over a century ago, attempts have been made to understand the geo-mechanical mechanism of flow slides. In this paper we have established that events, observed during the active phase, are characterized by a slow but steady retrogression into the shoreline, often continuing for many hours. This can be explained by the breaching mechanism, as will be clarified in this paper. Recently, further evidence has become available in the form of video footage of active events in Australia and elsewhere, often publicly posted on the internet. All these observations justify the new term ‘retrogressive breach failure’ (RBF event). The mechanism has been confirmed in flume tests and in a field experiment. With a better understanding of the geo-mechanical mechanism, current protection methods can be better understood, and new defense strategies can be envisaged. In writing this paper, we hope that the coastal science and engineering communities will better recognize and understand these intriguing natural events.
Foam soil interaction and the influence on the stability of the tunnel face in saturated sand
The face stability of an EPB-shield in saturated sandy soil is investigated. Recent research on the behavior of foam at the interface is combined with the knowledge on the influence of excess pore water pressures in the soil on the face stability. A simple model is described to illustrate this influence. It appears that the processes at the tunnel face for an EPB are quite comparable with the processes for a slurry shield. ‘Clean foam’ leads to more reduction of the permeability at the face than a foamy sand that can be expected during drilling. Micro instability is not very likely, except for large diameter tunnels and when drilling in a confined aquifer. Excess pore water pressures in front of the EPB can require a roughly 2 times higher pressure difference between the mixing chamber and the pore pressure far away from the tunnel than calculated with traditional methods.
Use of incipient motion data for backward erosion piping models
Backward erosion piping involves the gradual removal of granular material under the action of water flow from the foundation of a dam or levee, whereby shallow pipes are formed that grow in the direction opposite to the flow. This pipe-forming process can ultimately lead to failure of a water-retaining structure and is considered one of the most important failure mechanisms for dikes and levees in the Netherlands and the United States. Modeling of this mechanism requires the assessment of hydraulic conditions in the pipe, which are controlled by the particle equilibrium at the pipe wall. Since the pipe׳s dimensions are controlled by the inflow to the pipe from the porous medium, the flow through the pipe is thought to be laminar for fine- to medium-grained sands. The literature provides data for incipient motion in laminar flow, which is reviewed here and complemented with data from backward erosion experiments. The experiments illustrate the applicability of the laminar incipient motion data to determine the erosion pipe dimensions and corresponding pipe hydraulics for fine- to medium-grained sands, for the purpose of backward erosion piping modeling.
Pressure infiltration of sandy foam during EPB shield tunnelling in saturated sand
Experiments on infiltration of pressurised sandy foam into saturated sand have been carried out in a laboratory setup that provides a comparable hydraulic gradient as in real tunnels. The sandy foam used was comparable to the excavated soil (foam-water-sand mixture) that can be expected in the mixing chamber of an EPB (Earth Pressure Balance) shield. It appears that a higher FERm (effective foam expansion ratio) is more effective to form an impermeable layer in the sand. Furthermore, the permeability of sand for water flow through the foam decreases with the FIR (foam injection ratio). In practice, a FIR of 35 - 40 is recommended for a sandy ground and the situation with high sand fraction of the sandy foam should be avoided in the field because there will be no impermeable or low permeable layer formed at the tunnel face.
Calibration of DEM material parameters to simulate stress-strain behaviour of unsaturated soils during uniaxial compression
Discrete element method (DEM) is an appealing technique to simulate soil deformation. However, the calibration of the material parameters in the model remains challenging due to the high computational cost associated with it. In this study, the stress-strain relation of 125 unsaturated soil samples was simulated using DEM and material parameters were calibrated with Kriging. The Young's modulus and friction angle were found to be the most sensitive DEM material parameters, and they had a significant correlation with the bulk density, clay content and water content of the undisturbed soil samples. The DEM simulation showed that the deformation process was a combination of elastic and plastic processes, also at low stresses. Further improvement such as the coupling with (and calibration of) a fluid dynamics model might allow to more accurately simulate the dynamic behaviour of unsaturated soil compression.