Daniel Conley

Field observations of the fluid‐granular boundary layer under near‐breaking waves

Numerous synchronized time series from video cameras, pressure sensors, current meters, and hot film anemometers on natural beaches show that boundary layer development under the crest of near-breaking waves can be idealized as a process composed of three distinct regimes here referred to as streaking, roiling, and pluming. The roiling and pluming regimes fail to develop under the trough. As a consequence, there is a pronounced asymmetry in instantaneous sand transport and boundary layer phenomena between the wave crest and trough. However, laboratory waves with field scale periods and wave heights over thin sand beds do not exhibit this crest-trough boundary layer asymmetry, indicating that a critical element of similitude is absent in laboratory experiments. We suggest that wave induced boundary ventilation is responsible.

Ventilated oscillatory boundary layers

Abstract : A combination of field and laboratory experiments are made in order to expand our knowledge of naturally occurring oscillatory boundary layers. Chapter 1 describes field observations of the development of wave driven boundary layers at the fluid sediment interface. Under the crest of the wave, this development can be idealized as an identifiable sequence of three parts. The latter parts of this development are never observed to occur under the trough of the wave despite similarities in wave orbital velocity and acceleration. It is proposed that wave induced boundary ventilation, the oscillatory flow through the surface of a permeable bed, may be responsible for this apparent developmental asymmetry. In chapter 2, a laboratory study is presented of ventilated oscillatory boundary layers. These are boundary layers arising from a flow which oscillates parallel to a permeable bed which is subject to oscillating percolation of the same frequency as the bed parallel flow. Measurements of boundary layer velocities, bed stress and turbulent flow properties are presented. It is observed that suction (flow into the bed) enhances the near bed velocities and bed stress while injection (flow out of the bed) leads to a reduction in these quantities. As the ventilated oscillatory boundary layer experiences both these phenomena in one full cycle, the result is a net stress and a net boundary layer velocity in an otherwise symmetric flow. While production of turbulence attributable to injection is enhanced, the finite time required for this to occur leads to greater vertically averaged turbulence in the suction half cycle. Turbulence generated in the suction half cycle is maintained in a compact layer much closer to the bed. These effects appear to hold for Re ranging from 10(100,000) to 10(1,000,000) and for oscillations other than sinusoidal.

Assessing wave energy effects on biodiversity: the Wave Hub experience

Marine renewable energy installations harnessing energy from wind, wave and tidal resources are likely to become a large part of the future energy mix worldwide. The potential to gather energy from waves has recently seen increasing interest, with pilot developments in several nations. Although technology to harness wave energy lags behind that of wind and tidal generation, it has the potential to contribute significantly to energy production. As wave energy technology matures and becomes more widespread, it is likely to result in further transformation of our coastal seas. Such changes are accompanied by uncertainty regarding their impacts on biodiversity. To date, impacts have not been assessed, as wave energy converters have yet to be fully developed. Therefore, there is a pressing need to build a framework of understanding regarding the potential impacts of these technologies, underpinned by methodologies that are transferable and scalable across sites to facilitate formal meta-analysis. We first review the potential positive and negative effects of wave energy generation, and then, with specific reference to our work at the Wave Hub (a wave energy test site in southwest England, UK), we set out the methodological approaches needed to assess possible effects of wave energy on biodiversity. We highlight the need for national and international research clusters to accelerate the implementation of wave energy, within a coherent understanding of potential effects—both positive and negative.

Comprehensive Field Study of Swash-Zone Processes. II: Sheet Flow Sediment Concentrations during Quasi-Steady Backwash

AbstractSheet flow sediment concentration profiles were measured in natural conditions for the first time as part of a comprehensive field study on swash-zone hydrodynamics and sediment transport. Three conductivity concentration profilers (CCPs) measured the sediment concentration profile in the sheet flow layer with a 1-mm resolution in the swash zone of a dissipative beach. This paper focuses on sheet flow during quasi-steady backwash events generated by infragravity motion when the effects of phase lags, surface-generated turbulence, and accelerations are small. The sheet flow sediment concentration profile has a linear shape in the lower section of the profile and a power-law shape in the upper section, with the transition occurring at sediment volume fractions of 0.20–0.30. The shape of the concentration profile is self-similar for measured sheet flow layer thicknesses ranging from 6 to 18 mm. Because of the self-similarity, a single concentration profile curve can be used to describe the normalized...

The effects of flow stratification by non-cohesive sediment on transport in high-energy wave-driven flows

The two-way effects of the time-varying suppression of turbulence by gradients in suspended sediment concentration have been investigated using a modified form of the Generalized Ocean Turbulence Model (GOTM). Field measurements of fluid velocities and sediment concentrations collected under high-energy conditions (mobility number ≈ 900) have been simulated both including and neglecting the feedback between sediment and turbulence. The results show that, when present, this feedback increases the wave-coherent component of transport relative to the mean component of transport, which can even change the direction of transport. Comparisons between measured and simulated time series of near-bed sediment concentrations show great coherence (0.95 correlation) and it is argued that the differences in net transport rates may be partially explained by the use of a uniform grain size in the simulations. It is seen that the effects of sediment stratification scale with orbital velocity divided by sediment setting velocity, u m /w s , for all grain sizes.

Marine Renewable Energies: Perspectives and Implications for Marine Ecosystems

Countries with coastlines may have valuable renewable energy resources in the form of tides, currents, waves, and offshore wind. The potential to gather energy from the sea has recently gained interest in several nations [1–3], so Marine Renewable Energy Installations (hereinafter MREIs) will likely become very diffuse in the near future and determine a further transformation of our coastal seas.

Comprehensive Field Study of Swash-Zone Processes. I: Experimental Design with Examples of Hydrodynamic and Sediment Transport Measurements

AbstractA comprehensive study of swash-zone hydrodynamics and sediment transport was conducted on a macrotidal beach in Perranporth, United Kingdom. The unique study is the first to simultaneously measure suspended sediment and sheet flow sediment concentrations, water depth, near-bed velocity profiles, and high-resolution swash surface and bed-level changes on a natural beach. Data collected during the study are used to quantify the vertical profile of cross-shore and alongshore velocities and the importance of sheet flow sediment processes in the swash zone. The swash-zone boundary layer for cross-shore velocities is observed to generally occur over at least the lower 0.06 m of the water column. Alongshore velocities are often the same order of magnitude as the cross-shore velocities and are dominant near cross-shore flow reversal. Flows are often logarithmic in profile, but the instantaneous nature of the measurements renders application of the logarithmic model difficult. When valid, the logarithmic m...

Up-Wave and Autoregressive Methods for Short-Term Wave Forecasting for an Oscillating Water Column

The real-time control of wave energy converters (WECs) requires the prediction of the wave elevation at the location of the device in order to maximize the power extracted from the waves. One possibility is to predict the future wave elevation by combining its past history with the spatial information coming from a sensor which measures the free surface elevation up-wave of the WEC. As an application example, this paper focuses on the prediction of the wave elevation inside the chamber of the oscillating water column (OWC) for the Pico OWC plant in the Azores, and two different sensors for the measurement of the free surface elevation up-wave of the OWC were tested. The study showed that the use of the additional information coming from the up-wave sensor does not significantly improve the linear prediction of the chamber wave elevation given by a forecasting model based only on the past values of the chamber wave elevation.

BARDEX II: Bringing the beach to the laboratory - again!

ABSTRACT Masselink, G, Turner, I.L., Conley, D.C., Ruessink, B.G., Matias, A., Thompson, C., Castelle, B. and Wolters, G., 2013. BARDEX II: Bringing the beach to the laboratory – again! Proto-type laboratory experiments are particularly useful in coastal research when forcing parameters are modified in a way that is impossible to achieve in the field, and where installation and maintenance of instrumentation requires absence of waves. In 2008, the Barrier Dynamics Experiment (BARDEX) took place in the Delta Flume, the Netherlands. This project, funded by Hydralab III, focused on the effect of varying wave, sea level and beach groundwater conditions on a gravel beach (D50 = 10 mm). In 2012, a similar project was carried out, referred to as BARDEX II, this time funded by Hydralab IV and on a sandy beach (D50 = 0.42 mm). During the experiment, a 4.5-m high and 70-m wide sandy barrier was constructed in the flume with a lagoon situated to the landward. The barrier was instrumented with a very large number (> 200) of instruments and subjected to a range of wave conditions (Hs = 0.8 m; Tp = 4–12 s) and varying sea and lagoon water levels. Five distinct test series were executed over a 20-day period: series A focused on beach response due to accretionary/erosive wave conditions and a high/low lagoon water level; series B investigated the effect of a lower sea level on nearshore bar dynamics; series C simulated tidal effects; series D addressed the swash/overtopping/overwash threshold; and during series E the beach-barrier system was subjected to an extended period of energetic overwash conditions. This paper will describe the experimental design and the test programme during BARDEX II.

Assessment of WERA long-range HF-radar performance from the user's perspective

Since April 2006, long range (8.3MHz) WERA HF radars have been operated on the Southeastern United States coastline, as part of the U.S. Integrated Ocean Observing System (IOOS) and in particular the national HF Radar network. These radars measure currents operationally, and waves and winds experimentally across the wide continental shelf of Georgia (GA) and South Carolina (SC). Half-hourly data at 3km horizontal resolution are acquired to a range of approximately 200 km, providing measurements across the wide continental shelf and into the adjacent Gulf Stream at the shelf edge. Radar performance in range and quality is discussed. Ease in siting of these space and cable intensive systems along populated coastlines, and the feasibility of their operation by non-radar specialists is also briefly discussed.