The thermal regime around buried submarine high voltage cables

The expansion of offshore renewable energy infrastructure and the need for trans-continental shelf power transmission require the use of submarine High Voltage (HV) cables. These cables have maximum operating surface temperatures of up to 70°C and are typically buried 1–2 m beneath the seabed, within the wide range of substrates found on the continental shelf. However, the heat flow pattern and potential effects on the sedimentary environments around such anomalously high heat sources in the near surface sediments are poorly understood. We present temperature measurements from a 2D laboratory experiment representing a buried submarine HV cable, and identify the thermal regimes generated within typical unconsolidated shelf sediments—coarse silt, fine sand and very coarse sand. We used a large (2 × 2.5 m) tank filled with water-saturated spherical glass beads (ballotini) and instrumented with a buried heat source and 120 thermocouples, to measure the time-dependent 2D temperature distributions. The observed and corresponding Finite Element Method (FEM) simulations of the steady state heat flow regimes, and normalised radial temperature distributions were assessed. Our results show that the heat transfer and thus temperature fields generated from submarine HV cables buried within a range of sediments are highly variable. Coarse silts are shown to be purely conductive, producing temperature increases of >10°C up to 40 cm from the source of 60°C above ambient; fine sands demonstrate a transition from conductive to convective heat transfer between c. 20°C and 36°C above ambient, with >10°C heat increases occurring over a metre from the source of 55°C above ambient; and very coarse sands exhibit dominantly convective heat transfer even at very low (c. 7°C) operating temperatures and reaching temperatures of up to 18°C above ambient at a metre from the source at surface temperatures of only 18°C. These findings are important for the surrounding near surface environments experiencing such high temperatures and may have significant implications for chemical and physical processes operating at the grain and sub-grain scale; biological activity at both micro-faunal and macro-faunal levels; and indeed the operational performance of the cables themselves, as convective heat transport would increase cable current ratings, something neglected in existing standards

Fine particle retention and deposition in regions of cyclonic tidal current rotation

Benthic sediments in continental shelf seas control a variety of biogeochemical processes, yet their composition, especially that of fine sediment, remains difficult to predict. Mechanisms for mud or fine sediment deposition and retention are not fully understood. Using sediment data and a hydrodynamic model of the Northwest European shelf seas, a relationship is shown to exist between fine benthic sediment composition and regions of cyclonic tidal current rotation. The reduced thickness of cyclonic tidal benthic boundary layers compared with the anticyclonic case promotes deposition of fine sediment and trapping of resuspended material. Adding the effects of the benthic boundary layer thickness, as influenced by ellipticity or not, sheds some light on the limitations of approaches only focusing on bed shear stress and sediment pathways to predict the location of mud deposits. A tidal boundary layer predictor that includes ellipticity alongside tidal current magnitude and depth was shown to spatially agree with maps of mud deposits

The impact of mobile disarticulated shells of Cerastoderma edulis on the abrasion of a cohesive substrate

An annular laboratory flume was used to investigate the effect of mobile cockle shells on the erosion of a cohesive sediment bed. A standard clay bed was created and shells of differing sizes placed upon it. Flow in the flume was increased in increments and the onset of motion and transport pattern of the cockles was monitored. The release of bed material to the water column was monitored and compared to releases in the absence of shells (due only to the flow). The shells moved in traction; firstly as surface load (dragging) and then by rolling. The motion of the shells was found to be directly related to their motion settling rate in still water. The fluid induced stresses were unable to cause any detectable erosion of the bed. The addition of even single shells induced significant erosion. The erosion was found to be the result of abrasion rather than corrosion, as the shells never entered into saltation. There was a linear increase in erosion rate with increasing shell size, and an exponential increase in the suspended sediment concentration with time. The presence of large numbers of cockle shells in areas such as Southampton water and Lymington have suggested that the processes investigated here may be responsible for the erosion regimes in these areas

The effect of sand movement on a cohesive substrate

Flume experiments investigated the effect of mobile sand on the erosion of cohesive beds. The fluid-induced stress alone was not enough to cause erosion, and sand motion as bed load was needed. Erosion rates and suspended sediment concentration were found to increase with increasing sand transport and to decrease with increasing median grain size. The erosion rate was found to be at a maximum during saltation, intermediate during creep, and lowest during suspension

Wave-induced coherent turbulence structures and sediment resuspension in the nearshore of a prototype-scale sandy barrier beach

The suspension of sediments by oscillatory flows is a complex case of fluid–particle interaction. The aim of this study is to provide insight into the spatial (time) and scale (frequency) relationships between wave-generated boundary layer turbulence and event-driven sediment transport beneath irregular shoaling and breaking waves in the nearshore of a prototype sandy barrier beach, using data collected through the Barrier Dynamics Experiment II (BARDEX II). Statistical, quadrant and spectral analyses reveal the anisotropic and intermittent nature of Reynolds’ stresses (momentum exchange) in the wave boundary layer, in all three orthogonal planes of motion. The fractional contribution of coherent turbulence structures appears to be dictated by the structural form of eddies beneath plunging and spilling breakers, which in turn define the net sediment mobilisation towards or away from the barrier, and hence ensuing erosion and accretion trends. A standing transverse wave is also observed in the flume, contributing to the substantial skewness of spanwise turbulence. Observed low frequency suspensions are closely linked to the mean flow (wave) properties. Wavelet analysis reveals that the entrainment and maintenance of sediment in suspension through a cluster of bursting sequence is associated with the passage of intermittent slowly–evolving large structures, which can modulate the frequency of smaller motions. Outside the boundary layer, small scale, higher frequency turbulence drives the suspension. The extent to which these spatially varied perturbation clusters persist is associated with suspension events in the high frequency scales, decaying as the turbulent motion ceases to supply momentum, with an observed hysteresis effect

A comparison between fluid shear stress reduction by halophytic plants in Venice Lagoon, Italy and Rustico Bay, Canada - analyses of in situ measurements

A series of in situ experiments on bed stability were carried out at three sites across Venice Lagoon using the benthic annular flume—Sea Carousel. Turbulence measurements were made at a range of flow speeds over different vegetated beds as well as ‘smooth’ muddy beds. The drag induced by the various bed types was estimated using flow deceleration. Bed shear stress was also estimated using three methods, and the results were compared with the bed shear stress as determined over a smooth bed in a laboratory equivalent of Sea Carousel—Lab Carousel. The stress was found to increase with increasing bed roughness and with the addition of vegetation in the form of the sea grasses Cymodocea nodosa and Zostera noltii. The stress was also found to be affected by the bending of the sea grass blades under flow velocities exceeding 0.4 m s?1, the sea grasses became flattened and the shear stress was found to decrease to produce skimming flow. It was concluded that the presence of sea grasses decreases erosion due to (1) stress reduction and (2) stabilization of the bed, thus reduction of the distribution of sea grass beds in Venice Lagoon will likely enhance bed erosion and hence habitat destruction. Stress was also reduced by an increase in levels of turbidity level in the water column

Influence of Zostera marina canopies on unidirectional flow, hydraulic roughness and sediment movement

Seagrasses develop extensive or patchy underwater meadows in coastal areas around the world, forming complex, highly productive ecosystems. Seagrass canopies exert strong effects on water flow inside and around them, thereby affecting flow structure, sediment transport and benthic ecology. The influence of Zostera marina canopies on flow velocity, turbulence, hydraulic roughness and sediment movement was evaluated through laboratory experiments in 2 flumes and using live Z. marina and a mobile sand bed. Profiles of instantaneous velocities were measured and sediment movement was identified upstream, within and downstream of patches of different sizes and shoot density and at different free-stream velocities. Flow structure was characterised by time-averaged velocity, turbulence intensity and Turbulent Kinetic Energy (TKE). When velocity data were available above the canopy, they were fitted to the Law of the Wall and shear velocities and roughness lengths were calculated. When a seagrass canopy was present, three layers were distinguishable in the water column: (1) within canopy represented by low velocities and high turbulence; (2) transition zone around the height of the canopy, where velocities increased, turbulence decreased and TKE was high; and (3) above canopy where velocities were equal or higher than free-stream velocities and turbulence and TKE were lower than below. Shoot density and patch-width influenced this partitioning of the flow when the canopy was long enough (based on flume experiments, at least more than 1 m-long). The enhanced TKE observed at the canopy/water interface suggests that large-scale turbulence is generated at the canopy surface. These oscillations, likely to be related to the canopy undulations, are then broken down within the canopy and high-frequency turbulence takes place near the bed. This turbulence ‘cascade’ through the canopy may have an important impact on biogeochemical processes. The velocity above the canopy generally followed a logarithmic profile. Roughness lengths were higher above the canopy than over bare sand and increased with increasing distance from the leading edge of the canopy; however, they were still small (&lt;1 cm) compared to other studies in the literature. Within and downstream of the canopy, sediment movement was observed at velocities below the threshold of motion. It was likely caused by the increased turbulence at those positions. This has large implications for sediment transport in coastal zones where seagrass beds develop.<br/

The abrasion of modern and archaeological bone by mobile sediments: the importance of transport modes

Fresh, weathered, archaeological and fossilized bones were subjected to a series of abrasion experiments using fine sand in an annular flume in order to link bone-surface abrasion to flow regimes and sediment transport modes, compare these effects on bones of different states, and quantify the extent and types of wear occurring. Flow velocities were chosen to replicate the predominant transport modes of bedload, saltation and suspension.Comparative scanning electron microscopic image analysis was performed to assess the degree and type of wear occurring on each bone type for the different transport modes over a range of exposure periods from 24 to 72 h.These preliminary investigations have shown that both the amount and type of wear experienced was related to the bone type, duration of exposure and the mode of sediment transport with wear being the result of deformation, rather than cutting wear.The formation of scour pits in the sand bed on the upstream side of the bone samples significantly reduced wear, and appears to be an important control mechanism for impact related wear that has been overlooked until now

Shipboard measurements of sediment stability using a small annular flume—Core Mini Flume (CMF)

Estimates of bed stability in coastal environments are essential to physical, biological, and chemical investigations of cohesive sediments. The Core Mini Flume (CMF), a 200 mm diameter annular flume has been designed to undertake sediment stability experiments on collected intact sediment box cores. Bed properties were assessed for replicate box cores at 3 contrasting sites in UK coastal waters (Tyne [in 2011 and 2012], Plymouth and Celtic Deep), each covering a maximum area of 80 m2. No significant horizontal spatial variations were found for grain size, bulk density, porosity, or oxygen penetration at the sites. Resuspension experiments performed on replicate cores yielded highly replicable results for each site, giving average erosion thresholds of 0.33 ± 0.02 (Tyne 2011), 0.215 ± 0.03 (Tyne 2012), 0.23 ± 0.01 (Plymouth), and 0.09 ± 0.006 (Celtic Deep) Pa and erosion depths of 10.7 ± 1.7, 6.63 ± 1.10, 3.65 ± 0.95, and 4.6 ± 0.5 mm. Using an already established methodology, the CMF allowed detailed replicate experiments to be performed on-board ship rapidly after sediment collection, while minimizing the time spent at each station. The use of intact box cores minimized the disturbance to the bed often associated with recovering material to a laboratory or remoulding a bed. We have demonstrated that the convenience of laboratory-based methodologies can be combined with the benefit of prompt investigations on undisturbed beds complete with overlying in situ water to produce robust measurements of sediment stability