1642 research outputs found
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Comparison of deep-water-parameter-based wave overtopping with wirewall field measurements and social media reports at Crosby (UK)
No full textWave overtopping formulae, which often underlie coastal hazard early warning systems, are typically parameterised using wave conditions at the toe of the structure. For very shallow conditions where significant wave breaking occurs over the foreshore, this usually requires computationally-demanding numerical models—and practitioners skilled in their application—to accurately transform offshore waves to the structure toe. An additional concern is that overtopping formulae are scarcely validated in the field due to the very limited availability of in-situ overtopping data obtained at actual structures. Here, we validate a set of deep-water-parameter-based formulae for mean overtopping discharge () at smooth slopes, which remove the need for nearshore measurements or additional numerical modelling but require that a single representative foreshore slope angle () be defined. The validation is carried out against field data gathered at Crosby (UK) using two novel approaches: i) a new overtopping measurement system called “WireWall”; and ii) crowd-sourced data in the form of overtopping images obtained from a community Facebook page (social media). A method is introduced to define for irregular bathymetries, based on the location where the local water depth is equal to the offshore significant wave height. The overtopping formulae proved accurate—with estimates of being within a factor of 4 of observations—when compared to both 1-h averaged and 15-min averaged overtopping data, suggesting that the approach can be used for both design and assessment and now-casting hazard information. Finally, hindcasts made using the newly validated formulae for the events reported by the community indicate that can exceed 10 l/s/m under yearly winter conditions, posing a serious hazard to pedestrians. This highlights the pressing need to update the current hazard warning system at Crosby, which estimates to be a factor of 3 lower than the deep-water-parameter-based approach, on average
Evolution of nature-based dredging solutions at Harwich, UK
Full text linkHarwich Harbour, in the Southeast of the UK, is the location of the Port of Felixstowe and the confluence of the highly protected Stour and Orwell Estuaries. The maintenance of the approaches to the Port of Felixstowe is carried out by the Harwich Haven Authority and entails the dredging of up to 3 Mm3 of mud by trailer suction hopper dredger, almost all of which is placed offshore. Deepening of the approach channel to the Port of Felixstowe in 1998/2000 resulted in regulatory agreements for an innovative sediment recycling strategy (or mud engine) to offset the perceived effects of the deepening. This sediment recycling involves the release of a proportion of the material dredged from the maintenance areas of the Port of Felixstowe within both estuaries to enhance the sediment supply to the intertidal areas within each estuary. The present sediment recycling methodology consists of the release into the water column of around 50,000 tonnes/year of fine sediment from small trailer suction hopper dredgers, releasing on the flood tide in a number of campaigns throughout the year. Survey evidence and detailed numerical modelling shows that this strategy is effective for improving habitat, but it is still not optimal, either environmentally or on grounds of dredging efficiency, because of the current requirement for offshore disposal of most of the dredged material from the Harbour. As a result, Harwich Haven Authority intend to develop their dredging and sediment recycling strategy further. To this end they have patented and trialled an agitation dredger which removes the need for offshore disposal, greatly reduces production of CO2 and dredging costs, and takes a leap forward in providing a more nature-based dredging solution providing the recycling of sediment within the estuarine system
Quantifying the 3D structure and function of porosity and pore space in natural sediment flocs
No full textPurpose: Flocculated cohesive suspended sediments (flocs) play an important role in all aquatic environments, facilitating the transport and deposition of sediment and associated contaminants with consequences for aquatic health, material fluxes, and morphological evolution. Accurate modelling of the transport and behaviour of these sediments is critical for a variety of activities including fisheries, aquaculture, shipping, and waste and pollution management and this requires accurate measurement of the physical properties of flocs including porosity.
Methods: Despite the importance of understanding floc porosity, measurement approaches are indirect or inferential. Here, using μCT, a novel processing and analysis protocol, we directly quantify porosity in natural sediment flocs. For the first time, the complexity of floc pore spaces is observed in 3-dimensions, enabling the identification and quantification of important pore space and pore network characteristics, namely 3D pore diameter, volume, shape, tortuosity, and connectivity.
Results: We report on the complexity of floc pore space and differentiate effective and isolated pore space enabling new understanding of the hydraulic functioning of floc porosity. We demonstrate that current methodological approaches are overestimating floc porosity by c. 30%.
Conclusion: These new data have implications for our understanding of the controls on floc dynamics and the function of floc porosity and can improve the parameterisation of current cohesive sediment transport models
Scour depth development at piles of different height under the action of cyclic (tidal) flow
No full textThe impact of cyclic (tidal) flow on scour at cylindrical monopiles in the live-bed regime has been examined on the basis of results from mobile sand bed flume experiments. Four tests were conducted with four 0.114 m diameter piles of differing stickup height placed across the 4 m wide test section of the Fast Flow Facility. The tidal tests Tide01, Tide02 and Tide03 had the same peak current speed in forward and reverse direction with different cycle periods to examine the role of cycle length on scour. The period in Tide01 was approximately halved in Tide02 and doubled in Tide03. A unidirectional test Uni01 was run for comparative purposes. The results have shown how the cycle length adopted in testing has a key controlling effect on the scour achieved in a fixed number of cycles. The scour data measured on two sides of the piles at 1 Hz are plotted both in time-series format and pairwise to illustrate the cyclic nature of the scour development and recycling of sediment within the scour hole. The effective work method of Link et al. (2016) provides a good correlation for the dimensionless scour depth. The cyclic behaviour of the scour difference across the pile diameter has been analysed in terms of a basal slope and the magnitude of the scour depth, akin to geotechnical testing of monopile response to cyclic loads. Comparison with field measurements of scour shows that the cyclic tests can achieve non-dimensional scour depths found in the field. A progressive reduction in the slope angle is demonstrated for increasing number of cycles but further investigation of the different angle observed in field and laboratory is required. Finally the effect of pile stickup height is evaluated using both published and the current data, and a modified value of the Sumer and Fredsøe coefficient is proposed for undirectional and tidal scour
Multiple-scales analysis of wave evolution in the presence of rigid vegetation
No full textThe study of free-surface flows over vegetative structures presents a challenging setting for theoretical, computational and experimental analysis. In this work, we develop a multiple-scales asymptotic framework for the evolution of free-surface waves over rigid vegetation and a slowly varying substrate. The analysis quantifies the balance between the competing effects of vegetation and shoaling, and provides a prediction of the amplitude as the wave approaches a coastline. Our analysis unifies and extends existing theories that study these effects individually. The asymptotic predictions are shown to provide good agreement with full numerical simulations (varying depth) and published experimental results (constant depth)
Large-scale experiments on tsunami inundation and overtopping forces at vertical sea walls
No full textTsunami are very long gravity waves that may cause significant damage to coastal sea walls. The majority of relevant design codes and research papers that describe methods for predicting tsunami loads on coastal walls consider the scenario of transitory force from a bore-led wave. This does not relate to tsunami that do not form bore waves. Bore fronts generally cause short term spikes in force, which may have little effect on the vulnerability of massive structures. Post disaster accounts suggest that most coastal walls show damage that implies failure modes that occur over moderate to long durations. Therefore it is likely that the bore front assumption gives an overly conservative prediction of maximum force, and may not capture the full timescale of tsunami loading. This paper uses a pneumatic tsunami generation facility to determine the force loading on two vertical coastal sea walls during tsunami inundation. Two sea-wall models, 0.15 and 0.25 m high, with crown widths of 0.1 m (7.5 and 12.5 m at a nominal prototype scale of 1:50) are tested. It is shown that bore fronts only occur for short period waves over the bathymetry tested. Bore fronts cause a very short period spike in force, which is followed by a transitory force approximated by the hydrostatic pressure equation. The loading of tsunami length waves of periods 40 s (280 s prototype at 1:50 scale), which do not break is not greater than 1.2 times the hydrostatic force. Overtopping volume is positively correlated to the time duration of positive upstream head over the crest, rather than its maximum value. Overtopping causes a small increase in the horizontal load due to the addition of a drag and momentum load. The magnitude and time of these effects are small and short-lived in comparison to the hydrostatic load. The results compare well with available equations based on hydrostatic force and the engineer may apply a desired multiplying coefficient of a factor of at least 1.2 to account for any added pressure and momentum, and the factor of safety intended
Sticky stuff: biological cohesion for scour and erosion prevention
No full textThis study examines the potential for biological cohesion to arrest scour erosion at marine infrastructure. Biological cohesion occurs naturally in sedimentary environments, and is caused by extracellular polymeric substances (EPS) which result from the life cycles of microorganisms. EPS is known to dramatically increase the resistance of natural biomediated sediment to erosive hydrodynamic forces. In this study, we test, for the first time, whether EPS can be deliberately added to a sediment to mitigate against scour erosion – a process we term ‘biostabilisation’. A systematic laboratory experiment is used to investigate the effects of an EPS additive on scour erosion around a monopile in a sand substrate. Results show that increasing EPS content causes a progressive reduction in equilibrium scour depth, the volume of excavated material and the timescale required to reach equilibrium scour morphology. These parameters are linearly related to EPS content, showing that the effects of EPS on the physical processes required for erosion to occur are concentration dependent. It can be concluded that biostabilisation offers a potential new ecologically engineered, nature-based solution to a range of scour and erosion scenarios. The economic and environmental advantages are discussed, and a methodology for biostabilisation use in individual erosion mitigation scenarios is proposed
The effects of meteorological factors on dengue cases in Malaysia
No full textDengue is a vector-borne disease affected by meteorological factors and is commonly recorded from ground stations. Data from ground station have limited spatial representation and accuracy, which can be overcome using satellite-based Earth Observation (EO) recordings instead. EO-based meteorological recordings can help to provide a better understanding of the correlations between meteorological variables and dengue cases. This paper aimed to first validate the satellite-based (EO) data of temperature, wind speed, and rainfall using ground station data. Subsequently, we aimed to determine if the spatially matched EO data correlated with dengue fever cases from 2011 to 2019 in Malaysia. EO data were spatially matched with the data from four ground stations located at states and districts in the central (Selangor, Petaling) and east coast (Kelantan, Kota Baharu) geographical regions of Peninsular Malaysia. Spearman's rank-order correlation coefficient (ρ) was performed to examine the correlation between EO and ground station data. A cross-correlation analysis with an eight-week lag period was performed to examine the magnitude of correlation between EO data and dengue case across the three time periods (2011-2019, 2015-2019, 2011-2014). The highest correlation between the ground-based stations and corresponding EO data were reported for temperature (mean ρ = 0.779), followed by rainfall (mean ρ = 0.687) and wind speed (mean ρ = 0.639). Overall, positive correlations were observed between weekly dengue cases and rainfall for Selangor and Petaling across all time periods with significant correlations being observed for the period from 2011 to 2019 and 2015 to 2019. In addition, positive significant correlations were also observed between weekly dengue cases and temperature for Kelantan and Kota Baharu across all time periods, while negative significant correlations between weekly dengue cases and temperature were observed in Selangor and Petaling across all time periods. Overall negative correlations were observed between weekly dengue cases and wind speed in all areas from 2011 to 2019 and 2015 to 2019, with significant correlations being observed for the period from 2015 to 2019. EO-derived meteorological variables explained 48.2% of the variation in dengue cases in Selangor. Moderate to strong correlations were observed between meteorological variables recorded from EO data derived from satellites and ground stations, thereby justifying the use of EO data as a viable alternative to ground stations for recording meteorological variables. Both rainfall and temperature were found to be positively correlated with weekly dengue cases; however, wind speed was negatively correlated with dengue cases
Development of the CSOMIO coupled ocean-oil-sediment biology model
No full textThe fate and dispersal of oil in the ocean is dependent upon ocean dynamics, as well as transformations resulting from the interaction with the microbial community and suspended particles. These interaction processes are parameterized in many models limiting their ability to accurately simulate the fate and dispersal of oil for subsurface oil spill events. This paper presents a coupled ocean-oil-biology-sediment modelling system developed by the Consortium for Simulation of Oil-Microbial Interactions in the Ocean (CSOMIO) project. A key objective of the CSOMIO project was to develop and evaluate a modeling framework for simulating oil in the marine environment, including its interaction with microbial food webs and sediments. The modeling system developed is based on the Coupled Ocean-Atmosphere-Wave-Sediment Transport model (COAWST). Central to CSOMIO’s coupled modeling system is an oil plume model coupled to the hydrodynamic model (Regional Ocean Modeling System, ROMS). The oil plume model is based on a Lagrangian approach that describes the oil plume dynamics including advection and diffusion of individual Lagrangian elements, each representing a cluster of oil droplets. The chemical composition of oil is described in terms of three classes of compounds: saturates, aromatics, and heavy oil (resins and asphaltenes). The oil plume model simulates the rise of oil droplets based on ambient ocean flow and density fields, as well as the density and size of the oil droplets. The oil model also includes surface evaporation and surface wind drift. A novel component of the CSOMIO model is two-way Lagrangian-Eulerian mapping of the oil characteristics. This mapping is necessary for implementing interactions between the ocean-oil module and the Eulerian sediment and biogeochemical modules. The sediment module is a modification of the Community Sediment Transport Modeling System. The module simulates formation of oil-particle aggregates in the water column. The biogeochemical module simulates microbial communities adapted to the local environment and to elevated concentrations of oil components in the water column. The sediment and biogeochemical modules both reduce water column oil components. This paper provides an overview of the CSOMIO coupled modeling system components and demonstrates the capabilities of the modeling system in the test experiments
Measurements of suspended sediment concentration in the deep sea – challenges based on a previous measurement campaign and how those challenges might be addressed
No full textDeep-sea mining can involve the extraction of poly-metallic nodules, ferromanganese crusts and sulphide deposits from the ocean floor and transportation of the mined material to the water surface. The mining of these resources (and potential discharge of unwanted material also excavated from the seabed) will potentially result in adverse ecological effects arising from the formation of sediment plumes that could deposit fine sediment on sensitive species. Identifying the behaviour of such sediment plumes is therefore a critical part of designing deep-sea mining operations that minimise impacts on ecology. Monitoring of the plumes has particular importance given the very low background levels of suspended sediment concentration that the sensitive receptors often exist in. In coastal waters sediment plume characterisation is traditionally carried out using optical (optical backscatter sensor (OBS)) and acoustic (ADCP) technology. Here we present some of the challenges of performing sediment plume monitoring in the deep-sea and how those challenges may be addressed. The information presented is based on experience gained during a previous sediment plume monitoring campaign undertaken on the Tropic Seamount, 550km SSW of the Canary Islands. The monitoring undertaken included the deployment of an ADCP and OBSs in water depths of >1000m