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Flocculation: What the floc is that all about?
Get PDFAs flocs grow, their settling velocities increase, despite their density decreasing.
The increased settling velocity improves water clarity as particles settle out faster from the water column.
In shallow water, as flocs have higher settling velocities than their individual particles, they settle faster and are therefore transported shorter distances, resulting in a decrease in overall sediment transport.
In deep water, increased settling velocities mean that particles sink to greater depths before they are remineralised, providing a vital food source to the deep ocean in the form of marine snow
Pressure fluctuations on the scour surface before prismatic pier
Get PDFThe results of experimental research of the wall-pressure fluctuation field acting on the scour surface upstream of the prismatic pier model are presented. Experiments were carried out in the hydraulic flume with an open water surface and sandy bed. The spatial and temporal characteristics of the field of pseudosound the wall-pressure fluctuation were determined on the equilibrium scour surface upstream of the prismatic pier model, as well as the sources of their generation. Two quasistable large-scale horseshoe vortex formations occurred inside the scour hole in front of the bluff body. The first of them were generated in the separation of the boundary layer with the front edge of the scour hole and it formed the upper slope of the scour. A second smaller horseshoe formation were formed by the interaction of the shear layer beyond the scour hole and the down flow along the front surface of the prismatic model and it formed the lower slope of the scour. The highest intensity and level of spectral components of the wall-pressure fluctuation occurred inside the scour hole upstream of the prismatic pier model
The consistency of laboratory jet erosion tests performed on undisturbed samples
Get PDFThe U.S. Army Engineer Research and Development Center performed 75 laboratory Jet Erosion Tests (JETs) on undisturbed samples collected from the riverbank along the Lower American River (LAR) near the city of Sacramento, California, to identify the erosion resistance of soil along the riverbanks and riverbed of the LAR through the erosion parameters of erodibility coefficient, Kd, and critical stress, τc. Thirty-nine of the samples were Shelby tube samples and the other forty-two were 4 in.-diameter Plexiglas tube samples. The soil type varied from weak sandy silt to stiff silt with the JET results varying from Very Erodible (VE) to Very Resistant (VR). For many of the samples, the variations in results were due to changes in the quality of the undisturbed samples; however, the variations may also be caused by the changes in the soil type from silt/sand to clay in some of the tested samples and natural variability of the soils. The method of sampling tube and the process of cutting the tube may also add to the variability in test results. To use the erosion parameters for specific soil types outside of the samples tested, the erosion parameters were related to index properties of the soil: particle diameter, percent clay content, and Plasticity Index. Erosion parameters were found to be most significant correlated to the particle diameter
Optimised generation and Absorption for 3D numerical wave and current facilities
No full textDimakopoulos, A., Cuomo, G. and Chandler, I., A., Cuomo, G. and Chandler, I
Believe it or not? The challenge of validating large scale probabilistic risk models
No full textThe National Flood Risk Assessment (NaFRA) for England and Wales was initially undertaken in 2002 with frequent updates since. NaFRA has become a key source of information on flood risk, informing policy and investment decisions as well as communicating risk to the public and insurers. To make well informed decisions based on these data, users rightfully demand to know the confidence they can place in them. The probability of inundation and associated damage however cannot be validated in the traditional sense, due the rare and random nature of damaging floods and the lack of a long (and widespread) stationary observational record (reflecting not only changes in climate but also the significant changes in land use and flood defence infrastructure that are likely to have occurred). To explore the validity of NaFRA this paper therefore provides a bottom-up qualitative exploration of the potential errors within the supporting methods and data. The paper concludes by underlining the need for further research to understand how to robustly validate probabilistic risk models
A discrete numerical description of the mechanical response of soils subjected to degradation by suffusion
Get PDFInternal erosion is a major cause of the failure of hydraulic earthen structures. A particular case of such an erosion process is suffusion which constitutes a strongly coupled fluid-solid interaction problem. It is a selective erosion of fine particles from an unstable soil structure leaving behind the granular skeleton which possibly leads to deformations. Such a process may cause modification in the mechanical behavior of the soil. To study this problem numerically, a model is established based on the discrete element method implemented in Yade software (Smilauer et al. 2015). Periodic boundary conditions are adopted and the soil is represented by a 3D assembly of spherical discrete elements. Such an oversimplified particle’s shape leads to excessive rolling. To overcome this obstacle, rolling resistance was taken into consideration in the inter-particle contact law. Bearing in mind that numerical modeling of suffusion can constitute a difficult task requiring important computational resources due to the direct description of interactions between solid and liquid phases, a one-way coupling with a fluid phase is considered here. However, effects on the soil due to the loss of a fraction of fine particles is investigated either by modeling soils with different grains size distribution and different initial fines content to characterize its influence on the soil microstructure, or by mimicking the suffusion process by defining an extraction criterion of potentially erodible particles. This extraction criterion is based on the size of the particles, constriction sizes, and the velocity of particles under the effect of fluid forces. From these two approaches, we were able to specify the fines content from which their erosion may have a significant influence on the microstructure. Moreover, the defined extraction criterion was able to describe the effect of erosion on the stability of the soil structure
Alleviation the scouring problem downstream of dam spillways through a reversed cross-jet flow dissipator
Get PDFIn this paper, a reversed cross-jet flow is developed to dissipate the energy of flow over an ogee weir spillway. The problem is handled analytically and experimentally. Both measured and calculated data indicate a great effect on the forced hydraulic jump characteristics compared to those describing the characteristics of the free jump condition. The obtained results revealed that, the reversed flow, can speed up the transition from supercritical to subcritical flow by creating a forced perfect jump starting at the contracted section and consequently, shortening the protection length needed to counter the problem of scouring downstream of the channel, since the length of the perfect hydraulic jump was reduced by 19%, while reduction in the length of the stilling basin amounted to 79%, in comparison to the case without a dissipator. The efficiency of such energy dissipation methods can have a significant impact on the overall cost of hydraulic structures projects
Assessing the efficiency of filters protecting base soil subject to erosion
Get PDFDams filters are mainly designed using filter criteria based on the grain size distribution (Sherard & Dunnigan, 1985). This paper reports experimental results obtained on the soil-filter system behaviour subject to different hydraulic and geometrical conditions. A silt soil and three sandy gravels were used as the core and different filters (F1, F2 and F3), respectively. The objective of this study was to determine the effectiveness of the filter to protect the silt submitted to erosion under controlled water flow (horizontal and vertical configurations). Particles transport and filtration through each granular filter were analysed as regards to filter retention capacity, particles size selection and grains shape. This study was achieved by conducting a comparison of the behaviour of the three filters against the silt erosion. A comparison of the efficiency of the filters is assessed toward the required usual relationship criterion and the most appropriate for the dam filters
Storm peak validation and analysis of uncertainty in estimates of extreme sea states
No full textStorm peaks are often under-estimated in numerical models, as is widely acknowledged. Yet, for many sites where estimates of extreme storm conditions are needed for engineering design, numerical models are the best – if not only – source of information. We discuss uncertainty in estimates of extreme sea states based on numerical model datasets . A method of validation for extreme conditions is presented based on matched pairs of independent storm peak events between modelled time series and buoy based observations, and found to be preferable to exceedance based validation techniques. Systematic biases in the storm peaks of the CFSR, ERA-Interim and NORA10 datasets, when compared to buoy data, are discussed. Sea states at the peaks of storms are compared and contrasted to the general population of sea states. We demonstrate a calibration scheme designed to remove bias from estimated storm peaks using a minimal set of parameters, in order that parameters may be mapped and estimated at sites where no observations are available.
Bias corrected model estimates of storm peaks have a remaining uncertainty associated with model precision. Probability distributions of extreme sea states are estimated using Markov Chain Monte Carlo and Bayesian techniques. Estimates of model precision, based on peak-focussed validation, are used to predict extreme sea states with associated uncertainty representing both model precision and sampling of the long-term distribution. The analysis allows investigation of the relative contribution to estimate uncertainty of model precision and sampling/length of record. Once uncertainty is considered in the estimation of extreme sea states, a contribution to the mean estimate from precision based uncertainty in the source data becomes apparent.
This contribution is also relevant to estimates of extreme conditions based on buoy data with measurement and short-term sampling uncertainties, and provides a baseline level of achievable uncertainty in extreme conditions. Validating uncertainty in estimates requires analysis at a large number of sites, and depends to a great extent on the probability associated with the most extreme events in a series, the so-called plotting position. The formulation of the plotting position has been debated for many years. Numerical experiments are presented that suggest that the correct formulation lies within a narrow subset of the debated schemes
Coupled hydrodynamic-meteorological models for Gulf coastal outfall studies
No full textIntroduction. The Arabian Gulf is a complex hydrodynamic environment with currents and water levels driven by a combination of tidal, meteorological and buoyancy effects. As a region of rapid industrial and economic development, the coastal waters of the Gulf are under increasing pressure from the effects of marine outfalls and other pollutants. It is important that environmental impact assessments and planning studies for proposed coastal discharges use the best available technology to model pollutant mixing and dispersion. The authors present the findings of their latest studies using advanced coupled hydrodynamic-meteorological models in the prediction of coastal flow and pollutant dispersion assessment in the Arabian Gulf.
Hydrodynamics of the Arabian Gulf. The hydrodynamics and circulation of the Arabian Gulf have been the subject of many studies over the years. Reynolds (1993) estimated that over much of the Gulf the relative influence of tides, winds and buoyancy effects on the kinetic energy in currents are approximately 100:10:1 respectively. In general, therefore, the strongest influence on currents and water levels is the tide. However, the Gulf contains a number of amphidromic points, or amphidromes, where one or more constituents of tidal water level fall close to zero. Two amphidromes offshore in the Gulf are shown in Figure 1. Near these points, the relative influence of winds can be much more important, and the authors have observed that tidal effects on direction and circulation of coastal currents can be overcome by stronger winds for periods of several days. For coastal outfalls and other sources of pollution, especially those near amphidromes, it is vital that meteorological effects are accurately represented in hydrodynamic models of mixing and dispersion.
Pollutant transport models for environmental impact assessment. The potential dispersion of effluent is most commonly assessed using hydrodynamic models. A summary of model techniques is given by Roberts et al (2010) and Wood et al (2014). Models of flows in the Arabian Gulf are usually driven by tidal boundary conditions and meteorological forcing.
Meteorological forcing. Meteorological information is typically available in the form of in situ measurements (e.g. airport data or site-specific meteorological station), or reanalysis datasets such as ERA-Interim (Dee et al, 2011). These provide either very local scale winds (perhaps valid for a few kilometres around a particular site), or larger scale circulations on the scale of 100 km or more. Local effects such as land-sea breeze can clearly be seen in station-based observations throughout the Arabian Gulf, and often dominate the local atmospheric circulation. Land-sea breeze is a thermally driven convection cell driving winds that are typically oriented perpendicular to the coastline. Modelling this requires a high resolution atmospheric model and an accurate coastline. The authors demonstrate an application of the Weather Research and Forecast model (WRF, Skamarock et al. 2008) to downscale winds over the Arabian Gulf. ERA-Interim boundary conditions – with an effective resolution of approximately 75km – are downscaled to 8km over the Arabian Gulf. The downscaled winds are validated against in-situ and satellite observations and found to be consistent with the regional circulation, while also providing good agreement with local coastal observations. Sea breeze signals are reproduced well and the local orientation of the wind to the coast at many sites is captured.
Coupled modelling approaches. Accurate multi-scale effluent plume dispersion simulations should include winds effects over a range of scales. In the author’s experience, studies encountered in the Arabian Gulf typically only use either local scale winds, or coarser regional wind fields. To support important environmental impact assessments and planning studies for coastal discharges, the authors have applied the downscaled wind techniques described above to generate accurate hydrodynamic and pollutant dispersion simulations. This has the advantage over typical modelling approaches that multiple wind scales and effects can be resolved within a single model. For example, the improvement in orientation of the wind to the coastline in the downscaled winds shows benefits in modelled long-shore drift. The authors are able to include local sea breezes that may affect movement of plumes towards or away from sensitive coastal habitats, as well as the regional prevailing wind patterns that influence wider-scale hydrodynamics and transport.
Applications. We show an application of the new techniques for a safety-critical outfall development in Abu Dhabi