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

Morphodynamic evolution of an estuary inlet

This paper presents the application of a morphodynamic model based on 2D reduced-physics principles to investigate morphology change of a complex estuary inlet system in the United Kingdom. The model combines a simple governing equation with a set of measured bathymetry data in order to model morphology change. The modelling method suggests that this simplified approach is able to recognise principal medium term morphodynamic trends in the estuary. However, the length and quality of the estuary bathymetry data set limits the applicability of the model to inter-annual scale

Performance of a data-driven technique applied to changes in wave height and its effect on beach response

AbstractIn this study the medium-term response of beach profiles was investigated at two sites: a gently sloping sandy beach and a steeper mixed sand and gravel beach. The former is the Duck site in North Carolina, on the east coast of the USA, which is exposed to Atlantic Ocean swells and storm waves, and the latter is the Milford-on-Sea site at Christchurch Bay, on the south coast of England, which is partially sheltered from Atlantic swells but has a directionally bimodal wave exposure. The data sets comprise detailed bathymetric surveys of beach profiles covering a period of more than 25 years for the Duck site and over 18 years for the Milford-on-Sea site. The structure of the data sets and the data-driven methods are described. Canonical correlation analysis (CCA) was used to find linkages between the wave characteristics and beach profiles. The sensitivity of the linkages was investigated by deploying a wave height threshold to filter out the smaller waves incrementally. The results of the analysis indicate that, for the gently sloping sandy beach, waves of all heights are important to the morphological response. For the mixed sand and gravel beach, filtering the smaller waves improves the statistical fit and it suggests that low-height waves do not play a primary role in the medium-term morphological response, which is primarily driven by the intermittent larger storm waves

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

Two-dimensional reduced-physics model to describe historic morphodynamic behaviour of an estuary inlet

Understanding medium to long term morphodynamic change is important for sustainable coastal and estuary management. This paper analyses morphodynamic change of a complex estuary inlet which is subjected to multiple environmental drivers and proposes a reduced physics model to explain the historic medium term morphodynamic change of the inlet. The analysis shows that even though the estuary inlet undergoes multiscale morphological change, the changes that take place over a timescale of several years are more significant and important. The reduced physics model suggests that this simplified modelling approach is able to recognise principal historic morphodynamic trends in the estuary. However, the length and quality of the inlet bathymetry data set limits the applicability of the models and the quality of model outputs

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