1642 research outputs found
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Hydraulic cable protection system stability on concrete and rock armour in lateral flow
No full textThis paper describes the results of large-scale laboratory flume experiments on the stability of submarine cable-Cable Protection Systems. The data was collected in a 1.6 m wide flume channel with flow velocities ranging from as low as 0.1 m/s up to as fast as 1.9 m/s. The cable-CPS was installed in the test section and the flow velocity was increased in a controlled fashion to remove the effects of acceleration. Results were obtained for the onset of motion of two different CPS systems. The results reveal the added stability against movement arising from increased cable-CPS mass and differences in CPS external shape, and the enhanced stability when the CPS is placed on a layer of typical scour protection rock
Grand Bahama and Eleuthera hurricane surge pilot study
No full textIn a pilot study for The Bahamas Department of Meteorology, a storm surge model and atlas were developed covering the islands of Grand Bahama and Eleuthera.
The hydrodynamic model, TELEMAC2D, is used to simulate the response of water level to tide and the wind and atmospheric pressure fields of hurricanes and subsequent inundation over land. The model is used by The Bahamas Department of Meteorology to predict storm surge and flooding over the islands of Grand Bahama and Eleuthera for incoming hurricanes to assist in preparation for and management of hurricane surge events. The model has been validated for its representation of water level and against tide gauge measurements during four historical hurricanes: Irene, Sandy, Matthew and Dorian. The comparisons show that the model is capable of representing hurricane surge with good accuracy.
The storm surge model was also applied in a hurricane surge and inundation risk assessment for both Grand Bahama and Eleuthera to create an atlas of flood scenarios. 10,000 years of synthetic hurricane tracks were generated based on a statistical analysis of historical hurricanes from 1979 to 2020 inclusive. Predictions of flood extents and depths were mapped for return periods of up to 1000 years for present day conditions and three climate change scenarios to 2100. The flooding experienced over Grand Bahama during Hurricane Dorian in 2019 was estimated to have a return period of up to approximately 450 years. Under the climate change scenarios the likelihood of flooding similar to Hurricane Dorian was estimated to be approximately 1.7 times more likely in 2050 and up to 3.4 times as likely in 2100 under a high greenhouse gas emissions scenario. The storm surge maps can be used by the Bahamas Department of Meteorology and other government agencies for emergency management, planning of infrastructure and building resilience in response to climate change.
The pilot study, including storm surge model and atlas, provide a template for further expansion to cover other inhabited islands of The Bahamas
On the role of organic matter composition in fresh-water kaolinite flocculation
No full textOrganic matter has long been understood to affect fine sediment flocculation, yet the specific effects of different types of organic matter remain only partially understood. To address this knowledge gap, laboratory tank experiments were conducted in fresh water to investigate the sensitivity of kaolinite flocculation to varying organic matter species and contents. Three species of organic matter (xanthan gum, guar gum and humic acid) were investigated at varying concentrations. Results revealed a significant enhancement in kaolinite flocculation when organic polymers (xanthan gum and guar gum) were introduced. In contrast, the addition of humic acid had minimal influence on aggregation and floc structure. Notably, the nonionic polymer guar gum demonstrated greater efficacy in promoting the development of floc size compared to the anionic polymer, xanthan gum. We observed non-linear trends in the evolution of mean floc size (Dm) and boundary fractal dimension (Np) with increasing ratios of organic polymer concentration to kaolinite concentration. Initially, increasing polymer content facilitated the formation of larger and more fractal flocs. However, beyond a certain threshold, further increases in polymer content hindered flocculation and even led to the break-up of macro-flocs, resulting in the formation of more spherical and compact flocs. We further quantified the co-relationships between floc Np and Dm and found that larger Np values corresponded to larger Dm. These findings highlight the significant impact of organic matter species and concentrations on floc size, shape and structure, and shed light on the complex dynamics of fine sediment and associated nutrients and contaminants in fluvial systems
Wind effects on wave overtopping at a vertical sea defense
No full textThe wind effect on the efficiency of a coastal defense structure is studied in this paper. It is normally assumed that the strength of the wind impact is characterized by the impulse parameter. If it is lower than a certain value, the wind is expected to have a dominant effect on the wave overtopping rate. In contrast to the regular observation, this study reports a new regime of wave overshoot when a low value of the impulse parameter does not lead to increased importance of wind. It is argued that the new regime appears due to the triplet instability previously studied by others. The variation of the standing wave height and the overshooting jet between the sequential cycles results in independence of the overtopping rates of the wind speed
Storms, bi-modal seas and our changing understanding of shingle beach response at Eastoke, Hayling Island, UK
No full textBeach management through nourishment and annual recycling has been applied for over 37 years to manage flood and coastal erosion risk to 1700 properties at Eastoke, Hayling Island, UK. This form of natural flood risk management has proved successful, in avoiding the annual flooding that blighted this area previously. However there have been unexpected storm events which have caused beach erosion and localised flooding.
Using long term real-time nearshore wave datasets and state of the art statistical methods, extreme wave conditions for both unimodal and bi-modal wave events were generated, which have illustrated some concerning patterns. These extreme events have been applied in Shingle-B, a new parametric model, which has indicated a more extreme beach response to bi-modal sea states compared uni-modal (which the beach was originally design for). The model was then applied to sensitivity test alternative beach profile designs.
This has important consequences for frontages like Eastoke, which depend on maintaining a healthy beach crest to avoid erosion and flooding. The Shingle-B results suggest that a beach designed for uni-model conditions may not provide the expected standard of protection for certain bi-modal wave conditions in terms of crest cut back
On numerical modeling of flocculation and cohesive sediment transport in the bottom boundary layer
No full textCohesive sediment transport is crucial in aquatic systems since many processes (e.g., dissolved oxygen transfer, light extinction, fate of spilt oil, morphodynamics) can be altered via the flocculation, causing deterioration in ecosystems (Ye et al., 2020). Hence, many cohesive sediment transport models with flocculation capabilities have been developed using turbulence closures such as Reynolds-averaged models (Sherwood et al., 2018) or Large Eddy Simulation (LES) (Liu et al., 2019) to represent the physics of the aggregation/breakup process. These models use the computed turbulent shear rate () in the entire computation domain by calculating the turbulent dissipation rate () and the kinematic viscosity (). However, our general understanding on flocculation effect of cohesive sediment transport in bottom boundary layer is limited. This study focuses on developing a modelling framework for a statistically averaged turbulent flow in dilute conditions, which incorporates the Population Balance Equation (PBE) and predict the weight-averaged floc property profiles such as floc density, floc diameter and settling velocity. To obtain high-fidelity turbulence statistics in bottom boundary layer, we use Direct Numerical Simulation (DNS) data (Yue et al., 2020) to establish the initial conditions (i.e., turbulent shear rate and fine sediment profiles). The DNS model uses a constant particle settling velocity () and a flocculation effect on () is not included
Inundation, runup and flow velocity of wavemaker generated bores on a planar beach
No full textUndulating and breaking bores are generated in the laboratory using a programmable long-stroke wavemaker. By changing the stroke length and the speed of the wavemaker, both non-decaying and decaying bores are generated and studied. Bore strength, height and duration are measured and compared with the solutions derived by using the method of characteristics, with excellent agreement. The measurements for inundation depth, runup height and flood duration are checked with the formulas presented in Barranco & Liu (J. Fluid Mech., vol. 915, 2021). The comparisons show that the formulas are also accurate for the non-decaying bores generated by the wavemaker. The maximum inundation depth predicted by the formula for zero bore length at the beach toe agrees with the laboratory observations for decaying bores. Using a high-speed particle image velocimetry system, the ensemble-averaged velocities and fluctuating velocities under undulating bores and breaking bores are measured in constant water depth and in the vicinity of the still water shoreline. Detailed analyses of the velocity fields are presented and discussed. For the undulating bore a long quiescent flood duration is observed, while for the breaking bore the up-rush flow changes into down-rush flow almost linearly
Synoptic observations of sediment transport and exchange mechanisms in the turbid Ems Estuary: the EDoM campaign
No full textAn extensive field campaign, the Ems-Dollard Measurements (EDoM), was executed in the Ems Estuary, bordering the Netherlands and Germany, aimed at better understanding the mechanisms that drive the exchange of water and sediments between a relatively exposed outer estuary and a hyper-turbid tidal river. More specifically, the reasons for the large up-estuary sediment accumulation rates and the role of the tidal river on the turbidity in the outer estuary were insufficiently understood. The campaign was designed to unravel the hydrodynamic and sedimentary exchange mechanisms, comprising two hydrographic surveys during contrasting environmental conditions using eight concurrently operating ships and 10 moorings measuring for at least one spring-neap tidal cycle. All survey locations were equipped with sensors measuring flow velocity, salinity, and turbidity (and with stationary ship surveys taking water samples), while some of the survey ships also measured turbulence and sediment settling properties. These observations have provided important new insights into horizontal sediment fluxes and density-driven exchange flows, both laterally and longitudinally. An integral analysis of these observations suggests that large-scale residual transport is surprisingly similar during periods of high and low discharge, with higher river discharge resulting in both higher seaward-directed fluxes near the surface and landward-directed fluxes near the bed. Sediment exchange seems to be strongly influenced by a previously undocumented lateral circulation cell driving residual transport. Vertical density-driven flows in the outer estuary are influenced by variations in river discharge, with a near-bed landward flow being most pronounced in the days following a period with elevated river discharge. The study site is more turbid during winter conditions, when the estuarine turbidity maximum (ETM) is pushed seaward by river flow, resulting in a more pronounced impact of suspended sediments on hydrodynamics. All data collected during the EDoM campaign, but also standard monitoring data (waves, water levels, discharge, turbidity, and salinity) collected by Dutch and German authorities are made publicly available at 4TU Centre for Research Data (https://doi.org/10.4121/c.6056564.v3; van Maren et al., 2022)
Impacts of sea level rise on wave overtopping rates around the coast of England
No full textThere is unequivocal evidence that global sea levels are rising. It is therefore inevitable there will be socioeconomic impacts as a result of this. To aid mitigation, and the implementation of adaptation measures, it is vital the magnitude of the potential impact is quantified. Current approaches in the UK make simplifying assumptions regarding the relationship between present-day and future economic damage relating to coastal flood risk. The analysis undertaken here supports studies of an improved impact of sea level rise by providing national-scale estimates of changes in wave overtopping rates and flood defence overflow rates, as a result of different amounts of sea level rise. The analysis involves the application of components of an existing risk-based coastal flood risk analysis method. A subset of almost 600 flood defence assets around the country has been analysed for sea level rise rates up to 1 m. The resulting analysis shows that, on average, the wave overtopping rate increases by up to 150 times above present-day rates for lower return periods and by up to five times for higher return periods. This differential arises as a result of non-linearities in overtopping rates with increasing extreme sea levels
On the importance of temporal floc size statistics and yield strength for population balance equation flocculation model
No full textMany aquatic environments contain cohesive sediments that flocculate and create flocs with a wide range of sizes. The Population Balance Equation (PBE) flocculation model is designed to predict the time-dependent floc size distribution and should be more complete than models based on median floc size. However, a PBE flocculation model includes many empirical parameters to represent important physical, chemical, and biological processes. We report a systematic investigation of key model parameters of the open-source PBE-based size class flocculation model FLOCMOD (Verney, Lafite, Claude Brun-Cottan and Le Hir, 2011) using the measured temporal floc size statistics reported by Keyvani and Strom (2014) at a constant turbulent shear rate S. Results show that the median floc size d50, in terms of both the equilibrium floc size and the initial floc growth, is insufficient to constrain the model parameters. A comprehensive error analysis shows that the model is capable of predicting three floc size statistics d16, d50 and d84, which also reveals a clear trend that the best calibrated fragmentation rate (inverse of floc yield strength) is proportional to the floc size statistics considered. Motivated by this finding, the importance of floc yield strength is demonstrated in the predicted temporal evolution of floc size by modeling the floc yield strength as microflocs and macroflocs giving two corresponding fragmentation rates. The model shows a significantly improved agreement in matching the measured floc size statistics