On the modelling of the unstable breaching process

Breaching is an important production mechanism for stationary suction dredgers. It is a process occurring in submerged sandy slopes, which mostly occurs in dense sandy soils with a low permeability. The process is initiated by the formation of a slope under water, whose angle is steeper than the internal friction angle, called the breach face. For dredging related breaching, this steep slope is created by a suction dredger, but it can also be formed after initial shear failure, caused by over steepening due to erosion, an earthquake, or an outwardly directed water flow. During breaching process, this steep slope is semi-stable due to negative pore pressure. Instead of a shear failure, particles are released one by one from the breach face, making it seem like the breach face is slowly moving backwards. The released particles form a density current that flows away from the breach face, and can be collected by a stationary suction dredger. When the size of the breach face increases over time, we have an unstable breach.Offshore and Dredging Engineerin

The interaction between bed-load transport and dune orientation

Bedforms play an important role in the sediment transport of a river. Because of their importance many experiments have been carried out to better understand bedforms. Most experiments concern transverse bedforms (with crests aligned perpendicular to the flow direction). However, bathymetry measurements show many instances of oblique dunes. Oblique bedforms induce flow and sediment transport in the transverse direction. Based on the flume experiments of Talmon [2009], Sieben & Talmon [2011] proposed formulae to determine the bed-load transport direction over oblique dunes. To apply these formulae the dune orientation has to be known. Therefore, Sieben & Talmon [2011] proposed to derive the dune angle by relating it to the relative dune migration rate along the dune crest. This was applied in the preliminary work of Weij [2012]. It was found that the current formulae give unrealistic results. The main objective of this thesis is to improve the prediction of dune orientation angle for large-scale modelling. The mechanisms involved in the formation of oblique dunes are investigated in a simplified environment. For the simplified environment we created a model based on the model created by Jerolmack & Mohrig [2005]. The model calculates the bed level change in one-dimensional slices, which are then coupled with transverse sediment transport to create a quasi 2D model. The results of show a dune orientation that eventually finds an equilibrium. Two explanations were given: (1) A gradient in transverse sediment transport can decrease or increase the dune migration rate, (2) Dune crests break up, and merge before reaching a larger angle. Based on these findings, the calculation of the dune angle is adapted in three ways: (1) The effect of transverse sediment transport on migration rate, (2) a reduced dune height for larger angles, (3) a limited dune orientation angle. The first measure is promising; it reduces the calculated dune orientation angles to more realistic values. However, the current implementation is problematic. The second measure is simple to implement and also reduces the dune orientation angle to more realistic values. However, the factor used to reduce the dune height has no physical basis. The third measure did not lead to satisfying results, but can be combined with the other measures. In the simplified environment cross-stream sediment transport was based on just the transverse slope. However, the flume experiments by Talmon [2009] show that sediment transport also depends largely on the changed flow pattern near the bed. We analysed the flume experiments carried out by Talmon [2009]. These experiments were carried out to quantify the transverse sediment transport over oblique dunes. Furthermore, we carried out detailed three-dimensional flow simulations. We used the detailed model of Nabi [2012] to carry out these flow simulations. Sieben & Talmon [2011] derived formulae for the mean sediment transport direction based on the experiments of Talmon [2009]. During our analysis of the flume experiments and three-dimensional flow simulations we found three main improvements of the formulae by Sieben & Talmon [2011]: (1) The length of the zones as proposed by Sieben & Talmon [2011] are inconsistent with the lengths seen in the flume experiments. (2) The relation between dune angle and crest parallel flow velocity appears to be exponential. (3) The near-bed flow direction on the stoss side of the dune (outside the transition zone) was assumed to be equal to the main flow direction. We found that there is a small but noticeable effect of the dune angle on the near-bed flow direction.River EngineeringHydraulic EngineeringCivil Engineering and Geoscience

An extension of the drift-flux model for submarine granular flows

To model submarine flows of granular materials we propose an extension of the drift-flux approach. The extended model is able to represent dilute suspensions as well as dense granular flows. The dense granwular flow is modelled as a Herschel–Bulkley fluid, with a yield stress that depends on the dispersed phase pressure. Qualitative numerical experiments show that the model is able to correctly reproduce the stability of submerged sand heaps with different internal angles of friction and initial slopes. When initially starting with heaps with an angle smaller than the internal angle of friction, the heaps are stable. When starting with heaps with angles larger than the internal angle of friction, a flow of solid material is initiated. The flow later stops when the bed is at an angle smaller than the internal angle of friction.Offshore and Dredging Engineerin

Numerical investigation of mode failures in submerged granular columns

In submerged sandy slopes, soil is frequently eroded as a combination of two main mechanisms: breaching, which refers to the retrogressive failure of a steep slope forming a turbidity current, and instantaneous sliding wedges, known as shear failure, that also contribute to shape the morphology of the soil deposit. Although there are several modes of failures, in this paper we investigate breaching and shear failures of granular columns using the two-fluid approach. The numerical model is first applied to simulate small-scale granular column collapses (Rondon et al., Phys. Fluids, vol. 23, 2011, 073301) with different initial volume fractions to study the role of the initial conditions in the main flow dynamics. For loosely packed granular columns, the porous medium initially contracts and the resulting positive pore pressure leads to a rapid collapse. Whereas in initially dense-packing columns, the porous medium dilates and negative pore pressure is generated stabilizing the granular column, which results in a slow collapse. The proposed numerical approach shows good agreement with the experimental data in terms of morphology and excess of pore pressure. Numerical results are extended to a large-scale application (Weij, doctoral dissertation, 2020, Delft University of Technology; Alhaddad et al., J. Mar. Sci. Eng., vol. 11, 2023, 560) known as the breaching process. This phenomenon may occur naturally at coasts or on dykes and levees in rivers but it can also be triggered by humans during dredging operations. The results indicate that the two-phase flow model correctly predicts the dilative behaviour and the subsequent turbidity currents associated with the breaching process. Offshore and Dredging Engineerin