Large-scale physical modelling of static liquefaction in gentle submarine slopes

Planning a monitoring campaign for a natural submarine slope prone to static liquefaction is a challenging task due to the sudden nature of flow slides. Therefore, gaining a better insight by monitoring the changes in pore pressure and acceleration of the soil mass, prior to and at the onset of static liquefaction, of submerged model slopes in the laboratory, helps in quantifying the minimum required triggering levels and ultimately the development of effective margins of safety for this specific failure mechanism. This study presents a set of physical model tests of submarine flow slides in the large-scale GeoTank (GT) of Delft University of Technology, in which a tilting mechanism was employed to trigger static liquefaction in loosely packed sand layers. Novel sensors were developed to locally monitor the hydro-mechanical soil responses acting as precursors of the onset of instability. The measurements indicated that soil instability can initiate at overly gentle slope angles (6–10°) and generate significant excess pore water pressures that intensify the deformations to form a flow slide. Moreover, it was observed that the onset of instability and its propagation are highly dependent on the rate of shear stress change and the state of the soil. The obtained data can be used for the future validation of numerical models for submarine flow slides.

What mechanisms trigger coastal flow slides?

<p>Some beaches regularly experience a rapid decrease in volume due to ‘coastal flow slides’. These events visually resemble subaerial landslides, but are subaqueous and located along river or tidal channels. Along a steeper shoreface, material eroded from the upper beach can be stored in deep water. In some cases, these events can remove thousands of cubic meters (m<sup>3</sup>) of beach sand in a few hours.<br><br>On several occasions in recent years, a flow slide has formed at Seabrook Island, South Carolina (USA). As of January 2021, there have been five events observed since July 2016. Surveys of a January 2017 event show the slide displaced ~25,000 m<sup>3</sup> into deep water (15–20 m) along North Edisto River Inlet. This volume is comparable to hillside-scale slides observed in mountainous regions like the Blue Ridge, and similar-scale failures have been observed in the Netherlands, France, and Australia (Mastbergen, 2019).<br><br>The Seabrook flow slide is consistently located along a marginal flood channel of a relatively large ebb-dominant inlet, just below a quarrystone revetment protecting an upland development. In this particular location, erosion of the dry beach could cause undermining of the revetment. Historical charts suggest a small inlet was located along this portion of the beach as recently as ~1920. Reviews of available rainfall and water level data suggest exceptional (ie – near-record daily total) rainfall events and spring tide levels may coincide with observed flow slide events.<br><br>This study analyzes available meteorological, water level, geotechnical, and historical shoreline data to identify mechanisms affecting repeat coastal flow slide events at Seabrook Island (SC). A combination of excessive rainfall, spring tidal currents, and sediment characteristics all appear to affect these events. Because of the unpredictability of these events, and the dynamic nature of the inlet channel adjacent to this portion of the island, it is difficult to observe events in situ and identify specific mechanisms triggering flow slides. While a hard structural solution is unlikely to effectively mitigate the hazard in this location, providing an excess of beach sand may help maintain a shallower shoreface slope and mitigate future flow slides.</p>

Experimenting with Underwater Sediment Slides

Sediment-laden currents caused by breaching flow slides are hazardous to flood defenses and seabed infrastructure. New research shows that these phenomena must be accounted for in erosion simulations.

Modeling of Breaching-Generated Turbidity Currents Using Large Eddy Simulation

Breaching flow slides result in a turbidity current running over and directly interacting with the eroding, submarine slope surface, thereby promoting further sediment erosion. The investigation and understanding of this current are crucial, as it is the main parameter influencing the failure evolution and fate of sediment during the breaching phenomenon. In contrast to previous numerical studies dealing with this specific type of turbidity currents, we present a 3D numerical model that simulates the flow structure and hydrodynamics of breaching-generated turbidity currents. The turbulent behavior in the model is captured by large eddy simulation (LES). We present a set of numerical simulations that reproduce particular, previously published experimental results. Through these simulations, we show the validity, applicability, and advantage of the proposed numerical model for the investigation of the flow characteristics. The principal characteristics of the turbidity current are reproduced well, apart from the layer thickness. We also propose a breaching erosion model and validate it using the same series of experimental data. Quite good agreement is observed between the experimental data and the computed erosion rates. The numerical results confirm that breaching-generated turbidity currents are self-accelerating and indicate that they evolve in a self-similar manner.

Bank Protection Structures along the Brahmaputra-Jamuna River, a Study of Flow Slides

The planform of the Brahmaputra-Jamuna River followed its natural path in Bangladesh until the construction of bank protection works started to save Sirajganj from bank erosion since the 1930s. Several so-called hardpoints such as groynes and revetments were constructed in the period 1980–2015 and the Jamuna Multipurpose Bridge was opened in 1998. The Brahmaputra Right Embankment and other projects had saved the western flood plain from inundation during monsoon floods. These river training works experienced severe damage by geotechnical failures, mostly flow slides. A flow slide is an underwater slope failure because of liquefaction or a breaching process in the subsoil or a combination of both. The design of most of these training works did not consider the risk of damage by flow slides. All descriptions of the observed damages show that scour phenomena in the channel close to a river training work are a cause of flow slides, besides pore water outflow. The research question was: how can the design of river training works be improved to reduce the risk of damage by flow slides? The main part of the investigation was focussed on reducing local scour holes near river training works. The most promising results are river training works with gentle bank slopes, permeable groynes, bed protections in dredged trenches with gentle side slopes, and methods to increase locally the bearing capacity of the subsoil. It is recommended to increase the knowledge of the failure mechanisms in the Brahmaputra-Jamuna River by improved monitoring in the field, the setup of a database with descriptions of all observed flow slides and the circumstances in which they occur. In addition to these recommendations, a field test facility is proposed to verify the knowledge of the failure mechanisms in that river. These activities will optimise the design of new river training structures with a very low risk of damages by flow slides and geotechnical instabilities and they will contribute to an improvement of the current design guidelines for river training structures.

Breaching Flow Slides and the Associated Turbidity Current

This paper starts with surveying the state-of-the-art knowledge of breaching flow slides, with an emphasis on the relevant fluid mechanics. The governing physical processes of breaching flow slides are explained. The paper highlights the important roles of the associated turbidity current and the frequent surficial slides in increasing the erosion rate of sediment. It also identifies the weaknesses of the current breaching erosion models. Then, the three-equation model of Parker et al. is utilised to describe the coupled processes of breaching and turbidity currents. For comparison’s sake, the existing breaching erosion models are considered: Breusers, Mastbergen and Van Den Berg, and Van Rhee. The sand erosion rate and hydrodynamics of the current vary substantially between the erosion models. Crucially, these erosion models do not account for the surficial slides, nor have they been validated due to the scarcity of data on the associated turbidity current. This paper motivates further experimental studies, including detailed flow measurements, to develop an advanced erosion model. This will improve the fidelity of numerical simulations.