Scour at coastal structures

The subject of scour (and siltation) at coastal structures continues to receive much interest in the consulting and research fields. Coastal structures can be categorised as having four functions, either (1) to provide permanent protection against flooding (e.g. dykes and seawalls), (2) water level control during storm surges (e.g. barriers) or (3) benefits in coastal management such as preventing shoreline erosion (e.g. seawalls, breakwaters, groynes), or (4) for other industrial or economic functions such as harbour breakwaters and jetties, outfalls/intakes, and windfarms. Therefore the topic of scour at coastal structures can be said to cover structures built on the shoreline as well as structures built in tens of metres of water, and waves and currents operate in varying combinations and relative magnitudes

Scour risk assessment at river crossings

Transport networks are large infrastructure projects that traverse long tracks of land and normally include many stream crossings. One of the main natural risks to river crossings is scour of the riverbed. Scour may expose the foundations of bridges or other infrastructure, or buried assets, making them vulnerable to failure causing undesirable social, operational and environmental impacts. This paper presents the framework and methods to develop a probabilistic scour risk assessment using fragility curves to account for uncertainty in input variables, prediction methods and performance of structures. Understanding the risks associated with possible movements of the riverbed, both in the vertical and lateral directions, is fundamental to provide an evidence base to define future management actions and strategies. The analysis includes the assessment of existing protection works, such as bed sills, and their impact in reducing the risk of scour

Understanding and Assessing Scour Development at Offshore Structures

Scour around a marine structure is the removal of sediment such as silt and sand, which can result in the formation of scour holes and may compromise the integrity of the structure. A great amount of research has been undertaken in laboratory facilities to measure scour development at vertical piles, in unidirectional flow conditions. This has given scientists and engineers a broad understanding of the mechanisms for the development of scour at a marine structure. Conditions in the laboratory can never fully mimic the conditions present in the real-world leading to uncertainties about the scouring process. Considerable research has also been carried out, outside of the laboratory and this has helped to fill in some of the gaps however the field data analysed tends to be snapshots of what is going, which again leads to uncertainties. A powerful tool in scour analysis, monitoring and prevention is an online system which provides continuous measurements over an area around a marine structure. This type of system, will provide a continuous picture of what is going on at the seabed and consequently act as an early warning system for the integrity of the marine structure

Marine scour: lessons from nature’s laboratory

Scour associated with anthropogenic activities in the marine environment has exercised the minds of scientists and engineers for decades. Despite the advances in understanding there remain areas of uncertainty which require further examination and challenges that require further research. Whilst real-life scour problems often help formulate the questions for detailed laboratory experiments, the associated information is less often used to answer some of those questions and yet the available data can offer the chance of exploring the scour at full-scale using real marine soils, albeit with all the inherent uncertainty associated with measurements obtained in the field. It can be argued that through the interpretation of these data, it forces the scientist and engineer to not only explore in more detail the limitations of the measurements but to engage in the full range of processes, whether physical (e.g. hydrodynamics and meteorological forcing) or biological (e.g. marine growth, benthic organisms) that impact on structures placed in what are often, very challenging environments

Scour development around large-diameter monopiles in cohesive soils: evidence from the field

Despite the progress in scour research over the last three decades, seabed scour development in cohesive and nonuniform soils is still an area of great uncertainty and remains a challenge for designing structurally efficient and effective foundations in the offshore marine environment. The uncertainty is made greater by the timescale required for scouring and effects, such as sediment abrasion, pile installation impacts, and operationally and environmentally induced dynamic motions. The rapid growth in offshore wind, particularly in European waters, has led to a requirement for estimates for scour development in such soils. This becomes very pertinent for large-volume serial installation of foundations, such as those required for offshore wind farm developments, given that there is a limit to the amount of detailed geotechnical information that can be collected as part of a project, and soil erosion testing is not standard. There is a reliance in geotechnical data, such as undrained shear strength, derived from cone penetration tests, supplemented with borehole data collected at a limited number of locations across the wind farm site combined with laboratory analysis of soil samples. This paper reviews the present evidence from both field and laboratory measurements of scour potential and looks at possible approaches for determining scour magnitude in cohesive soils including hydraulic and mechanical effects

Physical and numerical modelling of trench infill

Physical modelling of trench infill, for trenches perpendicular to the flow direction, was undertaken in the Fast Flow Facility at HR Wallingford. Parameters including trench width, flow velocity (including reversing tidal flow), and the presence of berms along the sides of the trench were varied. Following the experiments, a numerical model was constructed with the T0 bathymetry from the physical model used as the starting numerical model bathymetry, allowing direct comparison between models. Numerical model parameters were tuned to represent the physical modelling results, with the calibrated model then used to predict infill rates for other scenarios modelled in the Fast Flow Facility

Foundation scour as a geohazard

Carrying out a hazard assessment for offshore structures can entail the consideration of a number of different factors. Scour hazard assessments are routinely undertaken, and scour development at offshore structures should be considered a time-varying process. However, scour may take place within a morphologically dynamic environment, the combination of which will impact on the soil–structure–fluid response. This paper presents the analysis of an unique data set that shows the partial collapse of a scour hole at a large monopile foundation within a morphologically active site. The collapse suggests a slope failure mechanism, resulting in the movement of around 450 m3 of material within a period of about 75 min. The paper analyzes the processes involved regarding formation and development of the collapse

Scour hazard assessment for offshore wind developments – lessons learned from European waters

Poste

Settling velocity and mass settling flux of flocculated estuarine sediments

New formulations are presented for the settling velocity and mass settling flux (the product of settling velocity and sediment concentration) of flocculated estuarine mud. Physics-based formulae for these are developed based on assumptions of a two-class floc population (microflocs and Macroflocs) in quasi-equilibrium with the flow. The settling velocities of microflocs and Macroflocs are related to floc size and density via the Kolmogorov microscale as a function of turbulent shear-stress and sediment concentration, including height-dependence and floc-density-dependence. Coefficients in the formulae are calibrated against an existing large data-set of in situ observations of floc size and settling velocity from Northern European estuaries. Various measures of performance show that the resulting formulae achieve an improved level of agreement with data compared with other published prediction methods. The new formulae, with the original calibration coefficients, perform well in tests against independent measurements made in two estuaries

Morphological evolution of a barchan dune migrating past an offshore wind farm foundation

As the number of manmade structures installed on the seafloor is increasing rapidly, we seek to understand the impact of these immobile obstacles on marine geomorphological processes, such as the evolution of bedforms. A 5.8 m diameter monopile foundation was installed at the case study offshore windfarm approximately 30 m ahead of an approaching barchan (crescent‐shaped) dune. The impact of the monopile on the dune's evolution was analysed using six multibeam bathymetry surveys spanning 20 years. To substantiate this analysis, coupled three‐dimensional numerical modelling of flows and sediment was conducted in which the scour inducing bed shear stresses were calculated from the modelled turbulent kinetic energy. Following the installation of the monopile, the mid‐section of the dune accelerated and stretched in the direction of the wake of the monopile. Four years after the monopile's installation the rest of the dune had caught up, flattening out the slip face within half the dune's length downstream of the monopile. Due to the modified flow field, the dune was scoured deeply at the base of the monopile to a depth of 6.8 m (supported by the model results that predicted a scour depth exceeding 2 m over a period of just a few days). The surveyed volume of material scoured amounted to 8% of the total dune volume. Whilst the process of scouring occurs at a timescale of days to weeks, the dune migrated on average by 25 m/yr. The difference in the timescale of the two processes allowed the scouring to occur through the full thickness of the dune. The scoured dune profile recovered rapidly once the dune migrated downstream of the monopile. This article demonstrates how large geomorphological features can intercept and migrate past a monopile foundation without long‐lasting impacts on the integrity of the feature or the foundation