Investigating skimming flow conditions over stepped spillways using particle image velocimetry

Turbulent flow over stepped spillways can be considered the most significant flow that can cause severe problems at the downstream side, near the toe of the structure, such as sediment erosion which normally occurs due to the high amount of water energy. The presence of turbulent flow over the steps can cause cavitation damages due to pressure differences over the steps. The turbulent flow, which is induced at certain times of the year, especially during the flooding seasons, is examined experimentally and numerically in this study. Flow measurements were conducted using Particle Image Velocimetry (PIV) system in a hydraulic flume where the dam break condition is applied in order to achieve the skimming turbulent flow. Two cases of stepped spillways were tested, normal stepped spillways and gabion stepped spillways. For each case, measurements of the instantaneous turbulent velocity field were taken at different locations of the physical models of a slope of (1V:2H). A comparison has been conducted between the gabion and normal steps to assess the required time to attach skimming flow. The results indicated that the presence of the porous media could increase the required time to attach skimming flow

Natural dynamics overshadow anthropogenic impact on marine fauna at an urbanised coastal embayment

Understanding vulnerabilities of coastal ecosystems facing anthropogenic use is a precondition for management decisions and development planning. This can be challenging in urbanised areas with multiple activities affecting different faunal communities. The aim of this study was to provide a holistic understanding of the relative importance of anthropogenic and natural variables for macroinfauna, epifauna and fish in a heavily modified waterbody (HMWB) designated under the EU Water Framework Directive (WFD). The study area, Swansea Bay (Wales, UK), had two regularly dredged industrial ports, three estuaries, a wastewater discharge point and a dredge-spoil disposal site. Wave and tidal current models were constructed, and environmental data were gathered by field studies. Biota were assessed by grab sampling and dredging. Modelled and empirical data were combined in a Distance-based Linear Model (DistLM) that quantified how much of the faunal variation was explained by wave exposure and tidal currents, sediment characteristics and other environmental factors, and by anthropogenic usage. Wave and tidal current parameters explained over 50% of the variation in all biota. Infauna communities were further linked with sediment properties and epibenthos with distance to estuaries. Fish and epibenthos were affected by a dredge-spoil disposal site, but none of the faunal communities was affected by the wastewater outfall. Biota were predominantly driven by the natural hydrodynamic regime while anthropogenic factors had secondary influence. The study highlighted that ecosystems driven by a strong hydrodynamic regime can be relatively resistant to human activities

The shape and residual flow interaction of tidal oscillations

Tidal flows are seldom exactly sinusoidal, leading to a small discrepancy, or residual, over each tidal cycle. Although residuals are generally small in comparison with instantaneous currents, their cumulative effect is important for sediment transport and dispersion of contaminants. The meso-scale characteristics of tidal residual currents are investigated with a computational model of the Irish Sea. The role of the tidal oscillations, or eddies, in forcing the residual flow is considered first through theoretical considerations and secondly by calculating time mean flow quantities directly from the computational model. The tidal eddies contribute to the time mean vorticity balance through the tidal stresses which can be written in the form of a divergence of an eddy vorticity flux. In regions where the tidal flows are approximately horizontally non-divergent the anisotropy of the tidal eddies is strongly linked to their contribution to driving the residual flow. A measure of eddy anisotropy is proposed and this mirrors the shape and orientation of the tidal ellipses of the main tidal constituent. The vorticity balance of the residual flow is dominated by the frictional torque and the eddy vorticity flux divergence, with vorticity advection and vortex stretching by the residual flow generally being of secondary importance

Experimental study of wave trains generated by vertical bed movements

Laboratory experiments were conducted to explore the wave trains generated by vertical bed movements. The investigation consisted of 32 cases, involving four different water depths with unimodal and bimodal bed movements. Water surface displacement was measured using gauges positioned along a 30m long tank. A PIV system was set up to provide detailed measurement of the fluid velocity field in the vicinity of the bed movement. Generally, a unimodal movement generated a solitary-like wave, followed by a trailing sequence of waves. A bimodal bed movement induced a more complex flow field, with both the first and second extrema being significant. New analytical solutions have been derived, enabling the calculation of velocity fields. The nature of the wave generation and propagation were characterised using the disturbance-amplitude scale (α) and disturbance-size scale (δ). The applicability of linear theory was investigated, by validating the linear solutions of the generated waves against the experimental observations. For α ≤ 0.25, the analytical solutions were in good agreement with observations of the free surface shape, flow field and wave elevation history. For α ≥ 0.5, non-linearity became more pronounced, and the analytical solutions were only capable of reasonably estimating the amplitude of the first extremum in the vicinity of the moving bed. The initial crest maintained its amplitude and shape more effectively in crest-leading waves, whereas the leading trough decayed significantly in trough-leading waves. Non-linear phenomena were observed, such as wave breaking, air entrapment and twisted free surface. Bimodal bed movements with α ≥ 0.5 generated large, steep crests immediately following the initial trough in trough-leading waves

Spatial Variation in Coastal Dune Evolution in a High Tidal Range Environment

Coastal dunes have global importance as ecological habitats, recreational areas, and vital natural coastal protection. Dunes evolve due to variations in the supply and removal of sediment via both wind and waves, and on stabilization through vegetation colonization and growth. One aspect of dune evolution that is poorly understood is the longshore variation in dune response to morphodynamic forcing, which can occur over small spatial scales. In this paper, a fixed wing unmanned aerial vehicle (UAV), is used to measure the longshore variation in evolution of a dune system in a megatidal environment. Dune sections to the east and west of the study site are prograding whereas the central portion is static or eroding. The measured variation in dune response is compared to mesoscale intertidal bar migration and short-term measurements of longshore variation in wave characteristics during two storms. Intertidal sand bar migration is measured using satellite imagery: crescentic intertidal bars are present in front of the accreting portion of the beach to the west and migrate onshore at a rate of 0.1–0.2 m/day; episodically the eastern end of the bar detaches from the main bar and migrates eastward to attach near the eastern end of the study area; bypassing the central eroding section. Statistically significant longshore variation in intertidal wave heights were measured using beachface mounted pressure transducers: the largest significant wave heights are found in front of the dune section suffering erosion. Spectral differences were noted with more narrow-banded spectra in this area but differences are not statistically significant. These observations demonstrate the importance of three-dimensionality in intertidal beach morphology on longshore variation in dune evolution; both through longshore variation in onshore sediment supply and through causing longshore variation in near-dune significant wave heights

A classification system for global wave energy resources based on multivariate clustering

Better understanding of the global wave climate is required to inform wave energy device design and large-scale deployment. Spatial variability in the global wave climate is analysed here to provide a range of characteristic design wave climates. K-means clustering was used to split the global wave resource into 6 classes in a device agnostic, data-driven method using data from the ECMWF ERA5 reanalysis product. Classification using two sets of input data were considered: a simple set (based on significant wave height and peak wave period) and a comprehensive set including a wide range of relevant wave climate parameters. Both classifications gave resource classes with similar characteristics; 55% of tested locations were assigned to the same class. Two classes were low energy, found in enclosed seas and sheltered regions. Two classes were moderate wave energy classes; one swell dominated and the other in areas with wave action often generated by more local storms. Of the two higher energy classes; one was more often found in the northern hemisphere and the other, most energetic, predominantly on the tips of continents in the southern hemisphere. These classes match existing regional understanding of resource. Consideration of publicly available device power matrices showed good performance was primarily realised for the two highest energy resource classes (25–30% of potential deployment locations); it is suggested that effort should focus on optimising devices for additional resource classes. The authors hypothesise that the low-risk, low variability, swell dominated moderate wave energy class would be most suitable for future exploitation

Linkages between sediment composition, wave climate and beach profile variability at multiple timescales

The paper analyses, compares and contrasts cross-shore morphodynamic behaviour of four diverse beaches that have very different regional settings, wave climates and sediment characteristics, with the aid of rarely available long term measurements of beach profiles and incident waves. The beaches investigated are Narrabeen Beach, New South Wales, Australia; Milford-on-Sea Beach, Christchurch Bay, UK; Hasaki Coast, Ibaraki Prefecture, Japan; and Joetsu-Ogata Coast, Niigata Prefecture, Japan. A statistical analysis, equilibrium beach profile analysis and Empirical Orthogonal Function analysis are used to investigate, compare and contrast spatial and temporal variability of cross shore beach profiles of the selected beaches at short-, medium- and long-term timescales. All beaches show evidence of multi-timescale morphodynamic change. Narrabeen Beach profile has the highest sensitivity to local weather patterns. Milford-on-Sea, Joetsu-Ogata and Hasaki profiles are sensitive to seasonal variation of the wave climate however, they also show some correlations with regional climate variabilities. The nature of sediment exchange across the profile, which contributes to profile shape change with time, is found to be related to sediment characteristics across the profile. At Milford-on-Sea and Joetsu-Ogata, both of which have composite profiles, sediment exchange between the upper beach and the inter-tidal zone dominates profile change, irrespective of the distinct differences in sediment composition found in the two beaches. On the other hand in Narrabeen and Hasaki where beach sediment comprises medium to find sand, sediment exchange and hence profile change occur mainly in intertidal and subtidal zones

Data-driven and hybrid coastal morphological prediction methods for mesoscale forecasting

It is now common for coastal planning to anticipate changes anywhere from 70 to 100 years into the future. The process models developed and used for scheme design or for large-scale oceanography are currently inadequate for this task. This has prompted the development of a plethora of alternative methods. Some, such as reduced complexity or hybrid models simplify the governing equations retaining processes that are considered to govern observed morphological behaviour. The computational cost of these models is low and they have proven effective in exploring morphodynamic trends and improving our understanding of mesoscale behaviour. One drawback is that there is no generally agreed set of principles on which to make the simplifying assumptions and predictions can vary considerably between models. An alternative approach is data-driven techniques that are based entirely on analysis and extrapolation of observations. Here, we discuss the application of some of the better known and emerging methods in this category to argue that with the increasing availability of observations from coastal monitoring programmes and the development of more sophisticated statistical analysis techniques data-driven models provide a valuable addition to the armoury of methods available for mesoscale prediction. The continuation of established monitoring programmes is paramount, and those that provide contemporaneous records of the driving forces and the shoreline response are the most valuable in this regard. In the second part of the paper we discuss some recent research that combining some of the hybrid techniques with data analysis methods in order to synthesise a more consistent means of predicting mesoscale coastal morphological evolution. While encouraging in certain applications a universally applicable approach has yet to be found. The route to linking different model types is highlighted as a major challenge and requires further research to establish its viability. We argue that key elements of a successful solution will need to account for dependencies between driving parameters, (such as wave height and tide level), and be able to predict step changes in the configuration of coastal systems