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A study of scale effects in experiments of monopile scour protection stability
No full textSmall scale hydraulic experiments are widely used to model the stability of a scour protection around an offshore monopile foundation. Knowing the associated scale effects is important for evaluating the validity of the obtained experimental and design results. This paper provides a quantification analysis of scale effects that exist in monopile scour protection experiments, with a focus on the shear damage of a dynamically stable scour protection. Large scale models (scale ratio 1:8.33 and 1:16.67) and similar small scale models (scale ratio 1:50) have been adopted in the experiments with waves against current hydrodynamic conditions applied. For the waves and current conditions, the Froude scaling rule is used; for the armour stones, the so-called Best Model scaling rule suggested by Hughes (1993) is used. The scaling scheme achieves similarities between large and small scale models with regard to Shields number, relative density, geometry and settling velocity of particle. The scour protection damage patterns are measured and the three dimensional damage numbers are analysed. For better comparing the small and large scale test results, the small scale tests are performed repeatedly to obtain reliable damage results with associated range of deviation. Visual assessment of the damage patterns shows some agreements between small and large scale tests with regard to the damage and accretion locations. However, detailed analysis shows that the small scale tests introduce higher global and subarea damage numbers compared to large scale tests. The damage areas in small scale tests are larger than that in large scale tests. Significant lee-side damage due to the presence of lee-wake vortices is found in small scale tests. The dissimilarities of pile Reynolds number (), ratio between Shields numbers () and vortex shedding frequency in the different scaled models are believed to be the primary reasons of obtained scale effects
Industrial projects in hydraulics and hydro-environmental engineering
No full textHistorically, coastal engineering sediment transport models can be divided into short-term small-scale storm response models and one-line models to simulate long-term large-scale coastal changes. Only relatively recently large scale sediment transport models have become more relevant for engineering project, proving an intermediate between the two, modelling detailed processes over longer length- and timescales.
HR Wallingford have started using large-scale coastal models commercially in the past 5 years. With the advancement of high performance computing, these area models are becoming ever larger, modelling shorelines up to hundreds of kilometre. These projects have shown that large scale modelling of sediment transport and morphodynamics adds value. It helps to deal with complex situations; provides detailed information over large areas; and has shown to change the perception on the mechanisms driving coastal change in specific locations.
Area models simulate the complex interactions between winds, waves and currents on the sediment transport. Waves currents and sediment transport are computed fully coupled at a time scale of seconds to minutes over periods of years, sometimes using a morphological factor to increase the simulation period to up to 20 years. The mesh resolutions at the shoreline were about 10m, increasing to 5km elements offshore. The models are driven by water levels, currents and wave spectra on the boundary and spatially varying wind fields over the domain. Boundary conditions were taken from global models: e.g TPXO/HYCOM for currents ERA5 for waves).
The modelling showed some requirements. It is necessary to put the offshore boundary well away from the shoreline of interest, in one case in the English Channel even about 100km away, to eliminate errors due to the poor resolution of the driver models. Spectral wave boundary condition can be important even if the sediment transport predictors use monochromatic wave input. Primarily, this is due to our inability to objectively determine in advance which wave period will be dominating the nearshore sediment transport.
For a nourishment project, area modelling provided offshore losses to calibrate a 1-line model. Although the area modelling does not account for cross-shore processes in the breaker zone, it does provide reliable estimates of cross-shore processes around the depth of closure for wave driven transport. The 1-line model was then used for the nourishment option analysis. Finally, the preferred option was verified with the area modelling. Elsewhere, the modelling provided stark warning about the necessity of a long groyne protecting a soft-cliff headland. The area model identified clear transport pathways due to the currents with wave stirring, often in the opposite direction of the wave driven littoral drift, explaining unexpected losses of sediments and disappearing nourishments.
In summary, area modelling provides a valuable tool for coastal engineers next to the storm response and shoreline models
Quantification of 3-dimensional structure and properties of flocculated natural suspended sediment
No full textNatural sediment flocs are fragile and highly heterogeneous aggregates of biogenic and minerogenic material typically with high porosity and low density. In aquatic environments dominated by fine, cohesive or mixed sediments they can dominate suspended sediment flux. Consequently, monitoring and modelling the behaviour, transport and distribution of flocs is very important for many aquatic industries, maintenance of waterways and conservation and management of aquatic waterbodies. Mathematical models that predict the behaviour of flocs rely on the accurate assessments of the size, shape, density, porosity and fractal dimension of flocs. These inherently 3-dimensional (3D) characteristics are typically derived from 2-dimensional (2D) data, largely due to the challenges associated with sampling, capturing, imaging and quantifying these fragile aggregates. We have developed new volumetric microscopy techniques which can quantify 3D internal and external structures and characteristics of sediment flocs. Here, these techniques were applied to quantify the 3D size (volume), shape and fractal dimension of natural and artificial sediment flocs and compare them to standard 2D approaches. Our study demonstrates that 2D approaches are under-estimating shape complexity and over-estimating the size and mass settling flux of flocs by up to two orders of magnitude, and the discrepancy between 2D and 3D is most marked for natural, organic rich macroflocs. Our study has significant implications for estimations of sediment flux at local to global scales within in aquatic environments. These new data and approaches offer the potential to improve the current parameterisation of sediment transport models and to improve the accuracy of current field-monitoring techniques
Exorcising Malthusian ghosts: Vaccinating the Nexus to advance integrated water, energy and food resource resilience
No full textWater-Energy-Food (WEF) Nexus interactions vary from seemingly negative and intractable wicked problems to opportunities for enhanced sustainability. The aim of this paper is to review the current state of understanding on WEF resource interactions and to provide a roadmap to enhance integrated resource management. A qualitative perspective based on expert insight and experience was supported by a more quantitative systematic analysis of the literature to define Nexus interactions, describe the nature of different challenges, and explore the factors that influence them. We found that Nexus challenges, and associated interactions (e.g. trade-offs and synergies), vary with complexity and spatial and temporal scale, and biases in research and culture act as barriers to progress. An interdisciplinary approach is needed to develop technical solutions employed through the use of orchestrated shocks (e.g. historic analogues, predictive modelling, experimentation, and scenario planning) to “Vaccinate the Nexus” and improve system resilience. To achieve this, multidisciplinary capability should be developed to solve interdisciplinary challenges, while protecting specialism. It is recognised that through embracing complexity and “Nexus (or Systems) Thinking”, future integration of resource management may be facilitated through holistic education, informed by interdisciplinary research, and ingrained in cross-sector policy and governance
Characterization and classification of estuarine suspended particles based on their inorganic/organic matter composition
No full textAn 11 hours survey was performed on the 17th of September 2014 in the Rhine Region Of Freshwater Influence (Rhine-ROFI) about 10 km downstream of the mouth of the Rotterdam Waterway during calm weather conditions. Suspended Particle Matter (SPM) measurements were performed during a full tidal cycle, near the seabed, at neap tide, and samples were taken at 0.6 meter above bed for on-board analysis. The measurements were performed with (a) LISST 100X, a submersible particle size analyzer, (b) LISST-HOLO, a submersible digital holographic camera, (c) a home-made underwater camera and (d) an on-board LabSFLOC2 video microscopy equipment that used in-situ collected samples. The first aim of the present study was to compare the results obtained from the different monitoring techniques and to characterize the different types of suspended particles found in-situ. It was found that that the highly anisotropic particles present in the water column lead to multiple peaks in the Particle Size Distributions (PSD) found using the LISST 100X. Using the LISST-HOLO, underwater camera and LabSFLOC2 camera these particles could properly be imaged and meaningful PSD’s were obtained using these techniques in the size range > 20 μm. LabSFLOC2, LISST-HOLO and the underwater camera moreover provide information on the size and aspect ratio of particles. On the other hand, LISST 100X can be used to detect the fine fraction ( 200 μm. A large range of settling velocities (0.1 - 10 mm/s) and aspect ratios between 1 and 10 were recorded by video microscopy (LabSFLOC2). This spreading in settling velocities and aspect ratio was due to the different properties (shape, effective density and size) of the particles in the water column. The second aim of the study was to reproduce the flocs found in-situ in the lab and investigate the kinetics of flocculation between inorganic and living organic matter. Laboratory experiments were conducted with grab samples obtained from Port of Rotterdam harbour and living microalgae (Skeletonema costatum). The results of these experiments showed a shift in effective density upon addition of living algae to the sediment, which confirmed the flocculation ability between sediment and microalgae. The flocculation occured on a timescale of minutes and lead to flocs having a large spread in density for a given size, due to the heteregeous inorganic/organic composition of the flocs. This spread in density was at the origin of the large range of settling velocities for a given floc size observed in-situ, which leads to conclude that organic matter should be an important input parameter in sediment transport models
The role of biophysical stickiness on oil-mineral flocculation and settling in seawater
No full textBiophysical cohesive particles in aquatic systems, such as extracellular polymeric substances (EPS) and clay minerals, play an important role in determining the transport of spilled oil contamination and its eventual fate, particularly given that suspended sediment and microbial activities are often prevalent and diverse in natural environments. A series of stirring jar tests have been conducted to understand the multiple structures characteristics of the oil-mineral aggregates (OMAs) and EPS-oil-mineral aggregates (EPS-OMAs)
Sediment Transport - Recent Advances
No full textThe effective governance and administration of many aquatic environments requires a detailed understanding of sediment transport and behavioral dynamics. This has both environmental and economic implications, especially where there is any anthropogenic involvement. Sediment Transport - Recent Advances examines sediment transport-related issues in estuarial, coastal, or freshwater environmen..
Numerical modelling of reservoir flushing: a review
No full textReservoir storage loss due to sedimentation is a major issue affecting sustainable use of water resources. Sediment deposits can be removed by hydraulic flushing. During the flushing operation, bottom outlets are open and water and sediment released. The efficiency of the operation is improved if the reservoir water level is drawn down and free flow reestablished throughout the reservoir. Successfulness of flushing depends on a number of factors, such as bottom outlets capacity, reservoir shape and water availability. Modelling can be used to assess whether flushing is a viable solution for the management of sediment in the reservoir, as well as to design the flushing operations and optimize its scheduling. This paper reviews and analyses various modelling approaches described in the relevant literature. One, two and three dimensional numerical models have been used, as well as particle methods, such as smoothed particle hydrodynamics (SPH), more recently. One-dimensional models are useful for long term simulations, assessments on of a large number of scenarios, and optimization studies. Two- and three-dimensional models can be used to understand local scouring near the gates and other details of operation, as well as to study reservoirs of more complex shapes. Particle methods are particularly suitable for applications where the area occupied by water and sediment changes rapidly, as is the case with flushing, however they are also computationally more demanding, which limits their application to study of details
Long-term monitoring of sub-surface change in earth embankment dams
No full textHere we describe the development of a novel characterisation and monitoring approach for embankments such as earth dams. The system, called PRIME (Proactive Infrastructure Monitoring and Evaluation), is based on time-lapse electrical resistivity tomography (ERT), which is a geophysical technique used to non-invasively image subsurface resistivity to depths of tens of metres. Resistivity is a useful property because it is sensitive to compositional variations, changes in moisture content, and ground movement.
PRIME is a low-cost system designed for remote operation, allowing resistivity images to be captured automatically and streamed via a web interface. It comprises four key elements: (1) low-power field instrumentation; (2) data telemetry and storage; (3) automated data processing; (4) and web dashboard information delivery. These elements form the basis of an asset condition monitoring approach that provides near-real-time spatial information on both subsurface processes and surface responses
The use of this approach is illustrated with reference to a series of studies relating to earthwork condition monitoring. These studies demonstrate that PRIME provides a means of spatially tracking complex subsurface moisture driven processes (such as leakage or settlement) that would be very difficult to characterise using other approaches (e.g. surface observations or intrusive sampling)