Remo Cossu

Global (latitudinal) variation in submarine channel sinuosity

Current classifications of submarine channels and fans link channel sinuosity to gradient, and in turn to sediment caliber, with end members being high-sinuosity, low-gradient, fine-grained systems and low-sinuosity, high-gradient, coarse-grained systems. However, the most sinuous modern submarine channels, such as the Amazon, Bengal, Indus, and Zaire, along with ancient sinuous submarine channels, are located in equatorial regions. Here we quantitatively compare slope versus latitude controls on submarine channel sinuosity and show that the latitudinal control is strong, while that of slope is weak. Variation in sinuosity with latitude is shown to occur uniquely in submarine channels; no comparable relationship is observed for terrestrial river channels. Possible causal mechanisms for this latitudinal variation are explored, focusing on the influence of the Coriolis force, flow type, and sediment type. Although climate does not vary straightforwardly with latitude, climatic controls on flow and sediment type may explain some of the latitudinal variation; Coriolis force, however, varies with latitude alone and produces an excellent fit to the observed sinuosity-latitude distribution. Regardless of which control predominates, latitudinal global variation in channel sinuosity should have changed over geologic time. Since deposit architecture and facies are linked directly with sinuosity, submarine channel deposits should also systematically vary in space and time.

Coriolis forces influence the secondary circulation of gravity currents flowing in large-scale sinuous submarine channel systems

A combination of centrifugal and Coriolis forces drive the secondary circulation of turbidity currents in sinuous channels, and hence determine where erosion and deposition of sediment occur. Using laboratory experiments we show that when centrifugal forces dominate, the density interface shows a superelevation at the outside of a channel bend. However when Coriolis forces dominate, the interface is always deflected to the right (in the Northern Hemisphere) for both left and right turning bends. The relative importance of either centrifugal or Coriolis forces can be described in terms of a Rossby number defined as Ro = U/fR, where U is the mean downstream velocity, f the Coriolis parameter and R the radius of curvature of the channel bend. Channels with larger bends at high latitudes have ∣Ro∣ < 1 and are dominated by Coriolis forces, whereas smaller, tighter bends at low latitudes have ∣Ro∣ ≫ 1 and are dominated by centrifugal forces.

The Interaction of Large Amplitude Internal Seiches with a Shallow Sloping Lakebed: Observations of Benthic Turbulence in Lake Simcoe, Ontario, Canada

Observations of the interactions of large amplitude internal seiches with the sloping boundary of Lake Simcoe, Canada show a pronounced asymmetry between up- and downwelling. Data were obtained during a 42-day period in late summer with an ADCP and an array of four thermistor chains located in a 5 km line at the depths where the thermocline intersects the shallow slope of the lakebed. The thermocline is located at depths of 12–14 m during the strongly stratified period of late summer. During periods of strong westerly winds the thermocline is deflected as much as 8 m vertically and interacts directly with the lakebed at depth between 14–18 m. When the thermocline was rising at the boundary, the stratification resembles a turbulent bore that propagates up the sloping lakebed with a speed of 0.05–0.15 m s−1 and a Froude number close to unity. There were strong temperature overturns associated with the abrupt changes in temperature across the bore. Based on the size of overturns in the near bed stratification, we show that the inferred turbulent diffusivity varies by up to two orders of magnitude between up- and downwellings. When the thermocline was rising, estimates of turbulent diffusivity were high with KZ ∼10−4 m2s−1, whereas during downwelling events the near-bed stratification was greatly increased and the turbulence was reduced. This asymmetry is consistent with previous field observations and underlines the importance of shear-induced convection in benthic bottom boundary layers of stratified lakes.

Latitudinal variations in submarine channel sedimentation patterns: the role of Coriolis forces

Turbidity currents transport clastic sediments from the continental margin to deep ocean basins and along their pathways they erode large submarine channels. The driving mechanisms for submarine channel evolution are highly complex, reflected by recent debates about the formation and global distribution of sinuosity in turbidite channels. We present novel experiments on channelized gravity currents running over an erodible bed, where the magnitude of Coriolis forces is changed to reproduce conditions at low and high latitudes. We find a striking systematic change in deposition and erosion patterns as Coriolis forces become dominant at high latitudes so that erosion and deposition occur only on opposite sides of channels; in contrast, at low latitudes significant inner-bank intra-channel bars form on alternate sides of sinuous channels. Our observations show very good agreement with sedimentation patterns in Coriolis-dominated contourite drift systems and with deposits in modern and ancient turbidity current channels. We hypothesize that Coriolis forces are a key parameter for submarine channel evolution and sedimentary architecture at high latitudes but not at low latitudes; this proposal offers a new approach to interpret deep-sea architectural features at high latitudes.

The possible role of Coriolis forces in structuring large-scale sinuous patterns of submarine channel-levee systems

Submarine channel–levee systems are among the largest sedimentary structures on the ocean floor. These channels have a sinuous pattern and are the main conduits for turbidity currents to transport sediment to the deep ocean. Recent observations have shown that their sinuosity decreases strongly with latitude, with high-latitude channels being much straighter than similar channels near the Equator. One possible explanation is that Coriolis forces laterally deflect turbidity currents so that at high Northern latitudes both the density interface and the downstream velocity maximum are deflected to the right-hand side of the channel (looking downstream). The shift in the velocity field can change the locations of erosion and deposition and introduce an asymmetry between left- and right-turning bends. The importance of Coriolis forces is defined by two Rossby numbers, RoW=U/Wf and RoR=U/Rf, where U is the mean downstream velocity, W is the width of the channel, R is the radius of curvature and f is the Coriolis parameter. In a bending channel, the density interface is flat when RoR∼−1, and Coriolis forces start to shift the velocity maximum when |RoW|<5. We review recent experimental and field observations and describe how Coriolis forces could lead to straighter channels at high latitudes.

Seasonal variability in turbidity currents in Lake Ohau, New Zealand, and their influence on sedimentation

Layers of sediment that are deposited on the floor of Lake Ohau, New Zealand, offer a means to reconstruct past climate conditions in the Southern Hemisphere at subdecadal and annual resolution. A robust understanding of the modern physical processes that control the influx and dispersal of sediment in the lake is required to reconstruct climate from these sedimentary archives. In this study, water temperature and velocity measurements collected during 2012–13 were analysed to determine the primary physical processes that influence sediment transport in the lake. Sediment input from river inflow occurs throughout the year but exhibits strong seasonal variation. Large inflow events (Q>500m3s–1) that follow strong summer rainstorms trigger high-concentration turbidity currents, which are the main agents for sediment delivery and deposition. During winter, smaller turbidity currents also occur after rain events and contribute to annual sediment accumulation. In addition, large internal waves were observed during the summer and may influence sedimentation. In conclusion, several processes including river inflow, internal waves and convectively driven flows control sediment deposition and accumulation in the Lake Ohau system. We utilise these observations to establish a conceptual model to explain the observed infill stratigraphy in Lake Ohau and guide interpretation of the longer sedimentary record.

Autonomous underwater vehicle motion response: a nonacoustic tool for blue water navigation

Autonomous underwater vehicles (AUVs) use secondary velocity over ground measurements to aid the Inertial Navigation System (INS) to avoid unbounded drift in the point-to-point navigation solution. When operating in deep open ocean (i.e., in blue water—beyond the frequency-specific instrument range), the velocity mea- surements are either based on water column velocities or completely unavailable. In such scenarios, the velocity-relative-to-water measurements from an acoustic Doppler current profiler (ADCP) are often used for INS aiding. ADCPs have a blank- ing distance (typically ranging between 0.5 and 5 m) in proximity to the device in which the flow velocity data are undetectable. Hence, water velocities used to aid the INS solution can be significantly different from that near the vehicle and are subjected to significant noise. Previously, the authors introduced a nonacoustic method to cal- culate the water velocity components of a turbulent water column within the ADCP dead zone using the AUV motion response (referred to as the WVAM method). The current study analyzes the feasibility of incorporating the WVAM method within the INS by investigating the accuracy of it at different turbulence levels of the water column. Findings of this work demonstrate that the threshold limits of the method can be improved in the nonlinear ranges (i.e., at low and high levels of energy); however, by estimating a more accurate representation of vehicle hydrodynamic coefficients, this method has proven robust in a range of tidally induced flow con- ditions. The WVAM method, in its current state, offers significant potential to make a key contribution to blue water navigation when integrated within the vehicle’s INS.

Scour propagation beneath a subsea pipeline in high and low energy environments

Bottom seated subsea pipelines resting on an erodible bed are subjected to scour and transport processes. These processes potentially undermine pipeline integrity by increasing free-span lengths and resulting in bending stresses. Making predictions of scour is fraught with challenges including: (1) determining when pipeline burial or lowering will occur; (2) unexpected obstacles to scour propagation (e.g. rocks); and, (3) varying incidence angles of fluid flow. Due to these challenges, pipeline surveys remain a key part of ongoing asset management. Field observations of phenomena such as self-burial and the impact of flow incidence angles and localized water velocities also help us characterize the subsea pipeline environment. The validation of empirical formulas through comparison to field data is essential for high-risk regions. For this investigation scour beneath a relatively deep and relatively shallow section of the Tasmanian Gas Pipeline in south-eastern Australia was examined. Our data were collected by an Autonomous Underwater Vehicle (AUV), a Remotely Operated Vehicle (ROV) and Acoustic Doppler Current Profiler (ADCP). tour findings suggest to refine empirical formulas to better explain the coupled effects of waves and currents. An improvement on these formulas may reduce the frequency of monitoring surveys required. AUV and ROV surveys are powerful resources to better inform decision making of these assets and better understand scour at individual sites.

Estimating flow velocities of the water column using the motion response of an Autonomous Underwater Vehicle (AUV)

The WVAM method is a nonacoustic method to calculate the velocity components of a turbulent water column using the motion response of an Autonomous Underwater Vehicle (AUV) without the aid of an Acoustic Doppler Current Profiler (ADCP). This study analyses water velocity measurements estimated using the WVAM method as a function of the turbulence level of the environment by testing the method in an estuary that exhibits strong tidal currents (around 2 m/s). The uncertainty of this method at different water column conditions was computed by comparing the velocity measurements from the WVAM method with those obtained from the AUV mounted ADCP. The WVAM method determines the water velocities by comparing the motion response of the vehicle when operating within turbulent and calm water environments respectively. The motion of the vehicle in the calm water environment was obtained by conducting simulations of the vehicles in calm water under the same control commands executed during the field experiments in turbulent conditions. A reduction in the accuracy of the method in rougher water environments was observed due to the hydrodynamic coefficients of the simulation model reaching their nonlinear range limits. A possible strategy to overcome this limitation and improve the WVAM methods ability to accurately estimate the flow field in the vicinity of AUVs operating in highly turbulent environments is also provided.