Breach flow slides

Breach flow slides

Peelen, G.P.

van Rhee, C. (mentor)

Mechanical, Maritime and Materials Engineering

Marine and Transport Technology

Offshore and Dredging Engineering

2016-07-19

The stability of subaqueous slopes is often studied to determine the probability of failure of hydraulic structures. Slope failures have been widely studied to increase the understanding of this complex failure mechanism. However, after a failure it is often hard to figure out the type of failure mechanism: static liquefaction or breach flow. Densely packed soil show dilatant behavior when its subjected to shear, and therefore its stability is temporarily guaranteed. The slope gradual loses grains, which may initiate turbidity currents. This current can be accelerated over the entire slope, until far downstream the flow decelerates (decreased gravitational force and the bed friction) and settles eventually. 1D-equations are derived from conservation of mass and momentum along slopes, which results in an adaptation with respect to the equations of Mastbergen (2013) in the model HMBreach, namely the system is extended to non-stationary conditions. The 1D equations are the framework of the numerical model: BreachFlow. A first order upwind scheme is applied and the transition equations from Eke, Viparelli, & Parker (2011) form the boundary at the breach. This boundary provides initial conditions of thickness, velocity and density and are based on breach height, like in HMBreach and therefore change when the breach retrogrades in time. Due to the chosen numerical scheme hydraulic jumps cause errors when super critical flows decelerate and become sub-critical. Therefore, sedimentation cannot be modelled and the final slope is not found. However, several time steps at the toe of the breach can be modelled and provide information about the development of the breach height, which is important for the initial condition at the first grid point. In conclusion, with BreachFlow is not yet possible to model the entire final slope, when the breach height reduces to zero. However, it is compared to HMBreach/HMTurb and Retrobreach a promising model, which is easy to adapt to different parameters (including profile) and more accurate regarding the calculation of three variables instead of one (transport of sand). Future work should be based on the implementation of a higher order explicit scheme or even an implicit scheme, in order to model the sedimentation. Then the model can be compared to several case studies including tidal flat of Walsoorden (Van den Ham et al, 2015) in order gain insight what type of mechanism occurred and what parameters correspond to the final slope and retrogressive length.

breaching
erosion
retrogression

http://resolver.tudelft.nl/uuid:03e9e98b-dd70-4dec-ad16-c08d7f6d7dc2

Student theses

master thesis

(c) 2016 Peelen, G.P.