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A Bayesian method for improving probabilistic wave forecasts by weighting ensemble members
No full textNew innovations are emerging which offer opportunities to improve forecasts of wave conditions. These include probabilistic modelling results, such as those based on an ensemble of multiple predictions which can provide a measure of the uncertainty, and new sources of observational data such as GNSS reflectometry and FerryBoxes, which can be combined with an increased availability of more traditional static sensors. This paper outlines an application of the Bayesian statistical methodology which combines these innovations. The method modifies the probabilities of ensemble wave forecasts based on recent past performance of individual members against a set of observations from various data source types. Each data source is harvested and mapped against a set of spatio-temporal feature types and then used to post-process ensemble model output. A prototype user interface is given with a set of experimental results testing the methodology for a use case covering the English Channel
Hydraulic study on scouring downstream of Funagira Dam spillway apron (Japan)
Get PDFFunagira Dam is a concrete gravity dam having a height of 24.50 m and a dam crest length of 220.00 m, and is located approximately 30 km upstream from the mouth of the Tenryu River. The Funagira Dam is owned by the Ministry of Agriculture, Forestry and Fisheries, Shizuoka Prefecture, and Electric Power Development Co., Ltd. (J-POWER), and its objectives include power generation and supply of agricultural water, public water, and industrial water. For energy dissipation, the downstream area of the dam was designed as a hydraulic jump-type dissipater using bed protection blocks. The dam has experienced several large storm events in the 40 years after its completion. The protection blocks were damaged and the scouring downstream of the dam began soon after completion. About 6,300 tetrapods were added stage by stage to the bottom. In recent times (2011), however, flood events have been more intense and of longer duration, with maximum spillway releases of up to approximately 6,396 m3/s. This resulted in increased scouring at the foot of the dam. Emergency measures were taken just downstream of the dam apron from 2011 to 2013. The scouring process is a function of several variables, such as water discharge, water level downstream, bed protection, bed materials, transport, morphological riverbed conditions, and gate operations. Experimental studies with field observations, mathematical model tests, and vertical 2-dimensional and fully 3-dimensional hydraulic scale model tests have been carried out to examine the scouring processes and to determine long-term measures. These studies indicate that scouring was caused by a horizontal secondary flow with non-uniform discharge releases, and by changing the types of hydraulic jump because of long-term morphological riverbed changes. The study especially indicates that severe scouring would occur not only for extreme planned discharge but also for small discharges, because of the downstream water level of the dam
The Influence of Physical Cohesion on Scour around a Monopile
Get PDFWe present experiments that systematically examine how the addition of physically cohesive clay to sand affects scour evolution around a monopile in a current. Repeated centreline transects are used to show the changes in scour depth and excavated material over time. Combined with 3D plots of the final equilibrium morphology, the results conclusively prove that clay content causes a progressive reduction in the equilibrium depth, excavated area and that timescales of scour increase with clay content. Winnowing of clay particles from the sand matrix is a pre-requisite for scour and differences in clay content influence the rate and extent of winnowing, ultimately controlling equilibrium scour morphology. The strong linear relationships between clay content and equilibrium scour parameters offers a simple index on which to modify existing scour prediction methods. It follows that improved predictions of scour development can reduce manufacturing costs and related logistical expenses of structure operations in fluvial, coastal or offshore environments
Calibration of erodibility testing devices for bridge design support
Get PDFIn order to promote bridge safety under flood conditions, the U. S. Federal Highway Administration (FHWA) has been developing more reliable bridge scour evaluation methods that take into account bridge site specific flow conditions and stream bed material properties. The FHWA is currently developing two such devices: the Ex-situ Scour Testing Device (ESTD) and the laboratory version of an In-situ Scour Testing Device (Lab-ISTD), both test erodibility of stream bed material under specific flow conditions. To maintain portability and robustness, the Lab-ISTD has evolved into a compact cylindrical device that uses a radial flow pattern towards the center of the device to mobilize the foundation material and carrying it away.
In this study the previously calibrated ESTD is used as a reference to characterize the performance of the Lab-ISTD, and therefore establish a calibration procedure that enables the Lab-ISTD to produce necessary parameters for better bridge scour evaluation and foundation design
Phenomenological interpretation of internal erosion in granular soils from a discrete fluid-solid numerical model
Get PDFInternal erosion in granular soils may involve different steps: the detachment of solid particles from the granular skeleton under the action of water seepage; the transport of the detached particles carried with the water flow in the pore space; and eventually, for some erosion processes, such as suffusion, the possible reattachment of some transported particles to the solid skeleton of the soil, acting as a filter. The first part of this paper is devoted to the description and interpretation of the first step about the particle detachment. The analysis is mainly based on direct numerical simulations performed with a fully coupled discrete element-lattice Boltzmann method. Dynamics of the solid granular phase is represented thanks to the discrete element method in which each solid particle is explicitly described, whereas dynamics of the interstitial water flow is solved with the lattice Boltzmann method. Interactions between the solid phase and the fluid phase are handled at the particle scale avoiding the introduction in the model of some phenomenological constituents to deal with fluid-solid interactions. Numerical modellings of hole erosion can be interpreted similarly to laboratory hole erosion tests where the erosion rate is linearly related to the hydraulic shear stress. Further investigations from the numerical results suggest that the erosion rate for hole erosion in granular soil, can also be interpreted as a function of the water flow power according to a power law. The latter interpretation is applied to experimental data from suffusion tests on a cohesionless soil and glass bead mixtures. Here again, if change of erosion rate due to filtration is discarded, erosion rate is correctly described by the water seepage power according to a power law. Finally, a simple phenomenological model is suggested to describe the whole suffusion process, based on the previous results, to describe the particle detachment, and completed to take also into account the transport and filtration phases. Predictions of this model are compared with experimental results from suffusion tests on glass bead mixtures
A computational approach to the study of the stability of pier riprap at the Middle Fork Feather River
Get PDFThis paper discusses the use of various technologies and advanced computational modeling techniques that were combined for monitoring the performance of pier riprap on the basis of a field case study – Pier 3 of a bridge over the Middle Fork Feather River – in northern California, USA. The first phase involved capturing the field condition of the bridge site using sonar instrumentation technology in order to obtain high resolution bathymetry data. The second phase entailed enhancement and transformation of the scanned bathymetric data into a 3D CAD model to be used as the initial geometry for numerical modeling. A Fluid Structure Interaction (FSI) numerical approach was applied to simulate the rock incipient motion i.e. shear failure by coupling Computational Fluid Dynamics (CFD) software STAR-CCM+ and a Computational Structural Mechanics (CSM) software LS-DYNA. Several coupled simulations have been performed with varying flow conditions to identify shear failure conditions for the riprap apron
The use of smart infrastructure in dams to protect communities from flooding
Get PDFThe RTIM system outlined in this paper arose out of the EU funded UrbanFlood research project (2009-2012), which looked at combining sensors, embedded in urban flood embankments, with predictive breach and flood consequence models. The system was piloted on 3 sites in Europe. This successful demonstration of a new approach to asset monitoring, using cloud computing, sensor technology and predictive models, was assessed as an ‘excellent’ project by the EU evaluators. The technology has since evolved into the RTIM system, as a joint initiative between HR Wallingford and Siemens, and is currently being trialled for a UK dam owner. The RTIM system contains a cascade of physical process models for levee reliability, breaching, flood spreading and life safety. This enables scenarios of levee or dam failure to be run based on real time information and the resulting flood consequence assessed for emergency management. The paper will describe the background of the various elements of the RTIM system, with examples from the Boston pilot site in England. The system makes use of the Life Safety Model (LSM) for evacuation planning. HR Wallingford, in partnership with BC Hydro, is now responsible for the development and maintenance of the model, which is being used for emergency planning of flood evacuations internationally, which are briefly described
Feature of the vortex and the jet flows around and inside the three-row pile group
Get PDFIn this article the results of experimental research of kinematics and the dynamics of horseshoe vortex structures, wake vortices and lateral jet flows, formed around and inside the pile group of three-row grillage, set on a flat rigid surface and sand soil, are presented. Visualization of flow is conducted using water soluble coatings and contrast substances, introduced into the stream. The fields of velocity, pressure and shear stresses around and inside the pile construction were investigated using the miniature sensors of the velocity, dynamic pressure and pressure fluctuations. The place of formation and features of development of large-scale horseshoe vortex structures and wake vortices, and the hydrodynamic characteristics of the vortex and jet flow were determined. The space-time correlation and spectral characteristics of the velocity and pressure fluctuations were measured. The scale of the coherent vortex structures, their frequencies of rotations and oscillations, convective velocities and direction of transfer were determined. The three-dimensional spectrograms and correlograms of vortex and the jet flow about and inside the three-row pile grillage were obtained
Effect of densimetric Froude number on local bridge pier scour
Get PDFOver the past half-century, distress and collapse due to scour and scour-related complications have been shown to account for the majority of bridge failures in North America. Accordingly, there are several design standards which contain provisions for design of bridge piers with respect to scour on the basis of several design equations. These empirical equations have shown a tendency towards over-prediction of scour depth, which yields uneconomical design. Over the past several years, there have been a number of experimental investigations into local scour around bridge piers carried out at the University of Windsor with the intention of improving current scour depth estimation practices. While there is a large amount of scour modelling data publically available, there are many uncertainties regarding the conditions under which these tests were performed; hence, results of the present investigation were obtained under controllable conditions in order to isolate the effects of various non-dimensional quantities and attempt to quantify their individual and collective influences on scour depth and geometry. The focus of the majority of these investigations has been on determining effects of blockage ratio, flow shallowness (h/D), and relative coarseness (D/d50) on maximum scour depth. Preliminary analysis indicated that D/d50 is of great importance in scour modelling, but variation in results indicated additional contributing parameters. Analysis of experimental results obtained indicated that an additional sediment-related non-dimensional parameter, the densimetric Froude number (Fd), also contributed to scour geometry
Marine buffer zones and hydrodynamic assessments for regional planning
No full textIntroduction. Important strategic facilities such as refineries, power stations and desalination plants are widespread along the coastline of the Arabian Gulf. These require a plentiful supply of clean seawater, which is usually abstracted through marine intakes. In many countries that border with the Gulf, desalination is the only native source of potable water for the population. As such, desalination plants are strategically important facilities that require protection against the effects of future development. The rapid pace of coastal development (dredging and reclamation activity, construction of new outfalls, etc) in GCC countries means that many sensitive marine intakes can be at risk of poor water quality.
Regional planning. Regional and national coastal development plans can be formed between government departments, industry owners, environmental regulators and consultants. The intentions are to optimise land use for both economic and social development, while ensuring minimal environmental impact. An example is Bahrain’s National Planning Development Strategy (2007). In recent studies, the authors have assessed the potential effects of national coastal development plans on sensitive coastal facilities to support strategic planning and to protect sensitive sites.
Hydrodynamic modelling assessments. In the authors’ experience, national plans rarely include precise landforms and detailed reclamation plans for a region. More often, indicative areas are marked to separate regions of ownership and identify future industrial zones, residential blocks, etc. Therefore hydrodynamic model assessments are required to investigate potential developments that might occur within a region, including any major blockages to the local and regional currents. This is done through close liaison with stakeholders, analysing existing bed and landforms to see where developments are likely, establishing baseline marine ecology and hydrographic data, and then building potential future layouts into computational models. The effects of the development on water quality, sediment transport and pollutant transport close to sensitive sites can then be determined. Both near- and far-field effects are important to consider, as developments near a sensitive site may also impact on facilities further away. Therefore, modelling must be carried out holistically, ideally using regional-scale models. Multi-scale modelling techniques used by the authors mean that large regional-scale models can be created with areas of high resolution around multiple sites of interest.
Mitigation and buffer zone recommendations.
After the potential effects of future developments have been identified using hydrodynamic models, mitigation measures are recommended where needed. These may include widening corridors or approach channels to increase flushing of an area, or reshaping proposed reclamations to reduce their effects on flow patterns. Where a particular stretch of water is essential to the local hydrodynamics and water quality (for example, providing a supply of clean seawater to an important intake), buffer zones can be established to protect these against future developments.
Example. The authors present the techniques developed for an example study in the Arabian Gulf