1642 research outputs found
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Wave Spectra Revisited – New guidelines based on observations
No full textMetocean studies often experience a lack of wave observation data at sites of interest. This leads to assumptions generally accommodated in maritime engineering but not necessarily accurate or correct. This is the case for the wave spectral shape and peakedness, often approximated to a standard JONSWAP spectrum with gamma (peakedness) of 3.3 (Hasselman et al., 1973) when no spectral data are available. This assumption leads to a set of spectral parameter ratios widely used in coastal engineering. Offshore peakedness variation however leads to different Tm02/Tp and Tm-1,0/Tp ratios (Goda, 2010).
This paper presents the findings arising from the analysis of observed wave energy spectra at a number of offshore wave buoys, and compares the results against JONSWAP 1D (frequency) spectral profiles. Resulting averaged fitted gamma values at each location remain within a range between 1.4 and 2.5, lower than the standard 3.3. A Deviation Index (D.I.) proposed by Liu (1983) and Pires Silva (1988) to evaluate the goodness of spectral profile fits against observations is also discussed
A review of the understanding of uncertainty in a flood forecasting system and the available methods of dealing with it
Get PDFThe increased availability and application of probabilistic weather forecasts in flood forecasting means that the uncertainty arising from the precipitation forecast can be assessed. This has led to a wider interest in how uncertainty is affecting flood forecast systems. In literature there are general techniques and principles available on how to deal with uncertainty. However, there are no of well-accepted guidelines on the implementation these principles and techniques. There is neither coherent terminology nor a systematic approach which means that it is difficult and perhaps even impossible to assess the characteristics and limitations of uncertainty quantification methods. Selecting the most appropriate method to match a specific flood forecasting system is therefore a challenge. The main findings of this review are that there are remaining mathematical and theoretical challenges in uncertainty quantification methods and that this leads to the use of assumptions which in turn could lead to a misrepresentation of the predictive uncertainty
The development of Aberdeen Harbour expansion project
No full textThe Aberdeen Harbour Expansion Project is to be one of Europe’s largest greenfield port capital investments projects over the next few decades (Figure 1). With a project investment of over £300 million, the project involves the construction of two new breakwaters each 600m long, quay lengths of over 1.5km, 2 million m3 of dredging including 0.25 million m3 of rock dredge and circa 1 million m3 of reclamation. The site is situated on the east coast of Scotland with severe wave climate where design waves exceed Hs~8m requiring single layer concrete armour units of up to 16m3 to protect the Southern Breakwater.
This paper sets out the development of the port masterplan in respect to the key engineering design and environmental constraints and operational requirements utilising many of the principles set out in the forthcoming PIANC WG185 guide to site selection and masterplanning of greenfield ports. The paper sets out details of the context and background of the project, the masterplanning process, numerical and physical wave modelling studies, navigation simulation, aspects of the engineering design and the procurement process. The construction contract was awarded on 20 December 2016 with the project due to be complete in 2020. The construction is now fully underway
Numerical modelling of caisson breakwaters under sliding and overturning motion using the Proteus toolkit
No full textA 2D computational fluid dynamics (CFD) model is set-up using the computational toolkit Proteus (https://proteustoolkit.org) in order to simulate sliding and overturning motions of caisson breakwaters. The model solves the Navier-Stokes equations using the Finite Element Method and is able to calculate wave-induced pressures both around the caisson and inside the rubble foundation of the breakwater. Displacement and rotations of the caisson are calculated by coupling a mesh motion technique with a dynamic response model for the rubble mound. Results are compared against experimental data and other analytical and numerical methodologies. Comparison shows that the model is able to capture the significant processes associated with incipient sliding and rocking. Further sensitivity tests are performed, demonstrating the robustness of the approach and its ability to capture with the sensitivity of the wave-structure interaction to changing wave conditions and/or structural / soil dynamic properties
Paull Tidal defence optimisation using 2D and 3D physical modelling
No full textEast Riding of Yorkshire Council is undertaking the Hull and Holderness Flood Alleviation Strategy: Phase 1 - Paull Tidal Defences. The flood alleviation strategy envisages embankments, revetments and walls offering improved 200 year protection against flooding to 1,461 residential properties, with 962 of these currently at high risk.
The main defence includes an existing compound concrete seawall with a recurve wave return wall, to which has been added a 1m high glass wall to achieve the 200 year protection standard. As part of the performance assessment for these defences, it was apparent that there were no calibrated empirical methods available for predicting overtopping for this particular configuration.
The most robust approach is to use physical model studies to determine the overtopping performance of the structure; especially as rudimentary predictions had demonstrated uncertainties of the order of ±x100. 2D and 3D Physical Models were therefore commissioned to investigate both normal and oblique wave attack. The seawall cross-sections were optimised in 2D and the results were used to construct the 3D model. 3D results showed local increases in overtopping at the seawall junctions / corners and at sections under oblique wave attack that could not have been assessed in 2D. Discharges were unlikely to cause flooding nor hazards that could not be managed for the worst conditions. It provided the owners with performance specifications and confirmation of the design not available by other means
Equilibrium scour-depth prediction around cylindrical structures
No full textOffshore gravity base foundations (GBFs) are often designed with complex geometries. Such structures interact with local hydrodynamics, creating an adverse pressure gradient that is responsible for flow and scour phenomena, including the bed shear stress amplification. In this study, a method is presented for predicting clear-water scour around cylindrical structures with nonuniform geometries under the force of a unidirectional current. The interaction of the flow field with the sediment around these complex structures is described in terms of nondimensional parameters that characterize the similitude of water-sediment movement. The paper presents insights into the influence the streamwise depth-averaged Euler number has on the equilibrium scour around uniform and nonuniform cylindrical structures. Here, the Euler number is based on the depth-averaged streamwise pressure gradient (calculated using potential flow theory), the mean flow velocity, and the fluid density. Following a dimensional analysis, the controlling parameters were found to be the Euler number, pile Reynolds number, Froude number, sediment mobility number, and nondimensional flow depth. Based on this finding, a new scour-prediction equation was developed. This new method shows good agreement with the database of scour depths acquired in this study (). Measurements of the equilibrium scour depth around nonuniform cylindrical structures were used to show the importance of the Euler number in the scour process. Finally, the importance of the remaining nondimensional quantities with respect to scour was also investigated in this study
Acoustic scattering from flocculating suspensions
No full textAcoustic backscatter from sediment suspensions in the marine environment has been limited in application by the lack of understanding of how sound scatters from flocculating particles. To support theoretical development of sound scattering, combined measurements of high frequency acoustic backscatter and particle population characteristics are presented over a range of flocculating suspensions, from natural in-situ muddy suspensions to laboratory controlled pure clay flocs. Field measurements of cohesive suspended sediments were made in the meso-tidal Tamar Estuary, Devon, UK over several tidal cycles during spring tides. Controlled laboratory experiments were conducted using oscillating grid turbulence to suspend kaolin, oxidized, and natural marine sediments sieved below 63 microns. In both field and laboratory cases distributions of floc size and settling velocity were acquired using video techniques (from which effective density was derived) and acoustic backscatter measured over frequencies of 1-4 MHz. Particle measurements were complimented by pumped suspension samples later analyzed for mass and organic content. Measured scattering properties from the various sediments are compared against each other and with predictions from a hybrid elastic-fluid sphere model. Initial results suggest a good agreement with the model for both in-situ field suspensions and oxidized natural sediments from the laboratory
Analysis of the uncertainty in flood predictions of GloFAS forecasts for Piura in the Pacific region of Peru
Get PDFGloFAS is a global flood awareness system based on a distributed hydrological model forced with numerical
ensemble weather predictions (Alfieri et al. 2013). Results are published on a password-protected website.
Forecasts from the GloFAS are currently limited in resolution and quality, but are nonetheless being used by
humanitarian and aid organisations and a small number of forecasting agencies. One such agency is SENHAMI
in Peru. To get around the limited accuracy issue, SENHAMI are applying a simple bias correction to the initial
conditions of the GloFAS forecasts. This process is reliant on in situ measurements being available and reliable,
therefore limiting the locations which can be corrected. Also, the uncertainties of the initials conditions are
reduced but the remaining uncertainties will continue limiting the predictability of the forecasts. This research
aims to understand and quantify the inaccuracy and uncertainties in the GloFAS forecasts for the Pacific region of
Peru. The work will explore ways of improving the predictability of the forecasts within the GloFAS framework.
The research will start with looking at the performance of the three main components of the GloFAS forecasting
system: the forcing data, the runoff component and the flow routing component. The forcing data, consisting of
the ERA-Intrim (Dee et al. 2011) and Variable Resolution Ensemble Prediction System (Miller et al. 2010),will be
validated. The starting point will be finding if the weak rainfall along the Pacific coast caused by the large-scale
mid tropospheric subsidence over the southeaster subtropical Pacific Ocean and enhanced by the coastal upwelling
of cold air (Garreaud, Rutliant, and Fuenzalida 2002), is present in the forcing data. The representation of the
hydrological processes, as done by HTESSEL, will be analysed focussing on the surface runoff, subsurface runoff
and soil moisture. The results of the flow routing model, LisFlood-Global, will be validated, focussing on the
channel and subsurface flow components. Performance of the model will be quantified with as a starting point
using the performance indicators available in the Ensemble Verification System (Brown 2010). The uncertainty
will be quantified and alternatives to the bias correction methods reliant on in situ measurements will be trialled,
starting with the nonparametric data-based approach (Van Steenbergen, Ronsyn, andWillems 2012). The increased
understanding of the flow predictability in the Pacific region of Peru will allow a widening of the use of forecasts
to catchments which do not contain in situ measurements and potentially to catchments without any measurements
at all
The influence of scale on the air flow and pressure in the modelling of Oscillating Water Column Wave Energy Converters
No full textIn this work, air compressibility effects are investigated during wave interaction with an Oscillating Water Column (OWC) Wave Energy Converter (WEC). Mathematical modelling includes a thermodynamic equation for the air phase and potential flow equations for the water phase. A simple three dimensional OWC geometry with a linear Power Take Off (PTO) response is considered and both the thermodynamic and potential flow equations are linearised. Analysis of the linearised system of equations reveals a nondimensional coefficient which we name “compression number”. The flow potential is decomposed into scattering and radiation components, using an analogue of spring-dashpot response and taking into account the additional effects of air compressibility to wave interaction processes. We use these concepts to characterise the relative importance of the air compressibility effects inside the OWC and to derive novel scaling relations for further investigation of scaling effects in OWC physical modelling. The predictions of the methodology are validated against large scale experimental data, where compressibility effects are evident and further application of the methodology to a realistic OWC geometry is used to demonstrate the importance of these effects to prototype scale
Orphan breakwaters – what protection is given when they collapse?
No full textAround the UK, many coastal harbours have reduced in importance and/or lost the original sources of income against which to defray maintenance or refurbishment. Their breakwaters may however still protect harbour-side properties against wave overtopping, and thus flooding. This paper presents results from an exploratory study to identify how blockwork breakwaters common in many smaller UK coastal harbours may collapse due to storm action, and in this paper, how much wave protection is given by collapsed breakwaters. The companion paper by Pearson & Allsop (2017) describes initial work to estimate the failure of blockwork walls, and presents results of wall collapse tests