In many engineering projects, submarine gravity flows may represent one of the aspects that require geohazard assessment according to which mitigation measures have to be implemented. Numerical modelling represents the used methodology for the analysis of the turbidity currents and the quantification of their associated flow parameters. To this purpose, we developed a proprietary fully 3D forward modelling tool for modelling the complex hydrodynamic behavior of bipartite (dense and turbulent) gravity flows, as well as the associated depositional aspects. In particular, the presented 3D modelling approach is able to quantify near the route of a planned seafloor structures either sedimentation/erosion processes, and flow velocity variation through time, event duration, direction and water density. A case is shown to provide an example of simulations and results that were used to select the best monitoring tools locations, improving the installation phases. First of all, morphological analysis of the seabed was performed to identify the principal structures and the instability prone areas.
Furthermore, slope stability simulations implemented on a selected profile allowed to define the initial sediment volumes to be used as input into the 3D forward modelling. Simulations were run on a high-resolution sea floor bathymetric map to test different flows and quantify the potential effects of their impact on the subsea facilities, allowing improvements on structures design. Beside the 3D analysis of the whole simulated event, velocity versus time and velocity versus depth diagrams allowed a detailed investigation of the potential effects of a bipartite gravity flow at specific locations, contributing to the selection of the proper installation layouts. When these simulations are performed during the early phase of a geo-hazard campaign, they can significantly contribute to the risks assessment and to the development project optimization.
Deep-water canyons often represent preferential pathways for turbidity currents or debris flows and, in some cases, a pipeline route may include a canyon crossing. Gravity flows may travel at velocities up to 19 m/s and can maintain velocities up to 10 m/s on slopes as low as 0.2° (e.g. Piper et al., 1999; Talling, 2014). They can flow for long distances (>100 km) over several days duration, causing damage over large areas of the seafloor. Even less powerful flows (1-2 m/s) can potentially damage seafloor equipment, or break submarine telecommunication cables (Clare at al., 2015). The consequences of turbidity currents affecting seafloor structures depend on flow characteristics such as velocity, duration, direction of impact, density and capability to erode the seafloor.
The ability to model properly the gravity flows, in order to evaluate the potential impacts against submarine facilities, represents a strong improvement in risk assessment. To this purpose, a 3D forward modelling tool was recently developed to reproduce the hydrodynamic behaviour of bipartite (dense and turbulent) gravity flows, as well as the associated deposits. We present here a case study along a submarine canyon hypothetically crossed by pipelines (Fig.1), where we tested gravity flows, either triggered by gravitational failures or associated to the downcurrent evolution from storm events affecting the shelf. In the former case, slope stability simulations implemented on a selected profile were used to define the initial sediment volumes representing an input into the 3D forward modelling. In the latter case, turbidity flows were assumed to feed continuously the canyon, from the canyon head, and allowed to propagate down the slope until reaching a quasisteady state.
