Understanding Flow Slides in Flood Defences

During  flow  slides  thousands  of  cubic  meters  of  sediments  move  within  seconds  to  During flow slides thousands of cubic meters of sediments move within seconds to hours downwards along a submerged slope. Being able  able to estimate the risk that flow slides pose to flood defences is vital for the safety of low-lying, densely populated coastal regions.

This requires understanding  understanding the involved complex  complex processes within  within the eroding  eroding slope and the turbulent water flow above its dissolving surface. A numerical method, the material point method (MPM)   which  is  able  to  model  these  processes  in  a  uniform  framework  is  enhanced  in  this interdisciplinary  project  for  the  analyses  of  flow  slides.  This  requires  new  solutions  for  which is able to model these processes in a uniform framework is enhanced in this interdisciplinary project for the analyses of flow slides. This requires new solutions for the modelling of turbulent water flow, of soil erosion, transport and deposition and of heterogeneities of the subsoil. Models will be derived from laboratory and field experiments and translated into robust,   efficient  numerical  efficient numerical models.   The  obtained  The obtained MPM-based  based solution,   thoroughly  thoroughly validated through  through experiments,   will  be  provided  to  experts  in  will be provided to experts in industry,   consultancy,   academia  academia and government agencies.

Research Summary

During a flow slide, large amounts of soil move down an underwater slope. A flow slide is able to remove an entire dike or dune section which poses a severe threat to the water safety of low-lying countries. The ability to predict flow slides is an important asset for the design of flood defence measures, their  their construction,   maintenance  and  safety  assessment;  even  more  so  in  view of  maintenance and safety assessment; even more so in view of intensifying land use and the impact of climate change on low-lying coastal areas worldwide.

Flow slides are not yet well understood. Their study requires an integrated approach of fluid and soil mechanics; soil  soil movement induces turbulent  turbulent water motion  motion which in  in turn interacts with the eroding soil surface. Currently, such an integrated approach is lacking. Studies so far mostly rely on  empirical  approaches  that  apply  to  specific  circumstances  only  and  use  on empirical approaches that apply to specific circumstances only and use considerable simplifications. Physical experiments  involve  high  experiments involve high costs as scale effects necessitate large test facilities and such tests often only allow predictions for specific projects. This makes the safety assessment of flood defences and the development of measures to prevent flow slides difficult and costly.

In  the  In the proposed interdisciplinary project, an integrated numerical solution for the simulation  simulation of underwater flow slides from initiation up to deposition of sediments will be developed through enhancement  of  a  numerical  enhancement of a numerical method,   the  the so-called  material  point  method  called material point method (MPM).   Laboratory experiments will be performed to gain deeper insight into soil  soil and fluid mechanical  mechanical processes that  occur at  the  onset  of  and  during  that occur at the onset of and during flow slides.   They further  serve  for  the  validation  of  They further serve for the validation of the developed numerical solution method. New physics-based models for soil-water interaction, soil heterogeneity and turbulent flow as relevant to flow slides will be formulated and existing models will be extended. They will be translated into purpose-built, efficient algorithms to be integrated into available Anura3D MPM software.

Utilisation

Measures  taken  in  the  Netherlands  in  recent  years  to  counteract  flow  slides  involved  Measures taken in the Netherlands in recent years to counteract flow slides involved costs amounting to M€ 100.

Results of  of this project will allow an accurate and site-specific evaluation of the vulnerability of flood defences to flow slides. This enables integrated probabilistic safety assessments of flood defences - and also an estimation of the post-failure ability of a flood defence to prevent flooding. Results   of   this  project  will   thereby  allow  for  much   more   refined  and   thus  Results  of  this project will  thereby allow for much  more  refined and  thus economical maintenance works.

The  devised  enhanced  Anura3D MPM  software  is  a  3D  generic  numerical  method  for  integrated  The devised enhanced Anura3D MPM software is a 3D generic numerical method for integrated geotechnical and hydraulic analyses that can also be applied to other erosion processes than flow slides. It will for example also be of benefit to the Dutch offshore industry. Numerical analyses will  help  to  raise  the  level  of  confidence  in  innovative  technologies,  will help to raise the level of confidence in innovative technologies, e.g.   for  scour  for scour protection, protection of offshore pipelines, dredging and the exploration of hydrocarbon reservoirs. Furthermore,   advanced  mathematical  solutions  developed  in  the  frame  of  this  project  are expected  to  find  their  way  into  other  commercial  software,  e.g.  the  Plaxis  FEM  software  advanced mathematical solutions developed in the frame of this project are expected to find their way into other commercial software, e.g. the Plaxis FEM software for geotechnical applications.

With regard to academia, this project prepares the ground for future high-level national and international  collaborations  between  applicants  and  academia  as  well  as  the  high-tech  international collaborations between applicants and academia as well as the high-tech industry.

Numerical solution

Much   progress   has   been   made   throughout   the   last   decades   in   numerical   analyses   of geotechnical  problems  including  problems  that  involve  groundwater  flow,  for  example  by  Much  progress  has  been  made  throughout  the  last  decades  in  numerical  analyses  of geotechnical problems including problems that involve groundwater flow, for example by the Plaxis 3D FEM software. Simulations of fluid flow in hydraulic engineering applications based on solution of the Navier-Stokes equations reached a high level of sophistication, for example with the Delft3D computational fluid dynamics (CFD) software. However, no integrated solution exists to date for the combined modelling of deformation of water-saturated soil and flow of free surface water, the transition between the two. Soil-water interaction, i.e. erosion and sedimentation, is currently  modelled  with  available  software  on  the  basis  of  empirical  relations  rather  than  currently modelled with available software on the basis of empirical relations rather than a consistent continuum mechanical description.

A  solution  which  is  based  on  interfacing  geotechnical  engineering  and  CFD  software  is  A solution which is based on interfacing geotechnical engineering and CFD software is not straight forward. Geomechanical problems require a Lagrangian description and the widely used finite  element  method  finite element method (FEM)   is  commonly  used  for  their  solution.  CFD  software  follows  an Eulerian  approach  and  commonly  uses  a  Finite  Difference  scheme.  Combining  such  is commonly used for their solution. CFD software follows an Eulerian approach and commonly uses a Finite Difference scheme. Combining such differing approaches renders numerical inaccuracies. In  the  proposed  project,  a  novel  integrated  numerical  solution  for  the  analyses  of  In the proposed project, a novel integrated numerical solution for the analyses of underwater flow slides from initiation up to deposition of  of sediments will be developed on the basis of present numerical state-of-research approaches.

Throughout  the  last  years  considerable  progress  has  been  made  in  numerical  analyses  of geotechnical  problems  involving  large  deformations  of  water-saturated  soil  by  means  of  Throughout the last years considerable progress has been made in numerical analyses of geotechnical problems involving large deformations of water-saturated soil by means of the Material Point Method (MPM). MPM is closely related to FEM. It combines the Lagrangian approach of FEM with the Eulerian approach of particle methods such as SPH   SPH  (smoothed   particle   smoothed  particle  hydrodynamics).   Equilibrium   equations   are   solved   on     Equilibrium  equations  are  solved  on  a background finite element mesh as with FEM. A cloud of material points that moves through the  mesh  is  used  to  model  arbitrary  large   deformations  of  soil,  or  flow  of  water.  the mesh is used to model arbitrary large  deformations of soil, or flow of water. Mass conservation is implicitly obeyed. A separation of material, gapping or erosion-like processes is implicitly  included   in   this  mixed  implicitly included  in  this mixed Lagrangian-Eulerian   Eulerian  approach.   It  furthermore  features  It furthermore features a straightforward soil-structure and water-structure contact  contact formulation. Several highly non-linear density dependent strain softening models are  are available. They are  are well suited to  to model sand. MPM  has  been  extended  for  coupled  MPM has been extended for coupled 2-phase  phase analyses.   Recently,   it  was  found  that  it was found that this numerical method is well suited to simulate problems of erosion and  and sediment transport. Soil with water flowing through its pores, fluidized soil and the transitions between the two states are modelled  modelled in an integral  integral numerical framework. Such an  an MPM suited for first simulations of flow  slides  of  homogeneous  sediments  is   currently  developed  at  Deltares   together  flow slides of homogeneous sediments is  currently developed at Deltares  together with the Anura3D MPM Research Community (www.Anura3D.com).

The Anura3D MPM software will be used in this project for the numerical analyses of flow slides and other problems of erosion. This however requires significant enhancement of the MPM code through integration of existing physics based models and new models developed in the course of the project.

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MPM simulation of slope deformation (Wieckoswki Wieçkowski 2013) showing soil “particles” in the presence of fluid.

Tasks

1) Validation Anura3D MPM

Extensive validation of the Anura3D MPM software will be performed in the course of this project. The planned simulations of laboratory and field tests as well as case studies provided by industry will be highly complex, requiring good understanding of the numerical method and the considered problem. As a starting point, simplified benchmark problems based on analytical solutions will be used for validation. Here, the available Anura3D MPM code can be used. As the enhancement of MPM proceeds, the complexity of analyses will be gradually increased from laminar to turbulent flow, from homogeneous to heterogeneous soil, from the available simplified state transition criteria to more sophisticated solutions.

2) Improvement numerical solutions

Anticipated long timeframes and large dimensions of flow slide analyses as well as the complexity of the considered equilibrium and transport equations requires highly advanced, purpose-built numerical solutions in order to obtain usable, i.e. stable, software allowing for analyses whose computational effort is affordable and computation time remains within a reasonable timeframe.

The numerical integration of the dynamic equilibrium and transport equations over time involves advancement of the solution in steps. Depending on the differential equation and time integration scheme, different constraints are placed on the time increments to ensure stable solution. For propagation of waves in soil described by hyperbolic differential equations a different criterion applies than for dissipation of pore water out of soil, described by a parabolic equation. For implicit and explicit schemes different parameters of a problem are known to limit the step size, e.g. stiffness of soil. The impact of other parameters such as soil permeability is not well understood yet.

In principle two methods are used for the analysis of the stability of time stepping methods: 1) methods based on the amplification factor of the integration method combined with an estimate of the eigenvalue of the corresponding (linearized) space discretisation method or 2) a Fourier analysis. Gained insights can be used to invent more stable integration methods.

Another problem lies in the transition between soil filled with pore water whose mechanical behaviour is described by equilibrium equations of the soil-water mixture and fluidized soil described by the Navier-Stokes equations along a moving interface. This is presently modelled in a simplified, discretised way. State transition is detected on the basis of a threshold porosity of the soil. Here, an accurate, robust solution has to be developed that takes into account a gradual transition between the two states.

3) Integration of results into MPM

Integration of results into the Anura3D MPM involves extension of the MPM code or development of libraries which are linked to the MPM code. Special attention will be paid to efficient parallelisation of the code to be able to make optimal use of available state-of-the-art computer facilities. Sets of benchmark tests will be assembled for respective code extensions to prove their proper working. Extensions will be thoroughly documented and coding guidelines will be applied to ensure a high degree of quality of the implemented solution.

Partners

• Royal Boskalis Westminster N.V.

• Van Oord Dredging and Marine Contractors

• Rijkswaterstaat Water, Verkeer en Leefomgeving

• Stichting IJkdijk
• Sibelco
• Deltares

• Delft University of Technology, Faculty of Civil Engineering and Geosciences, Department of Hydraulic Engineering, Section Environmental Fluid Mechanics

• Utrecht University, Faculty of Geosciences, Department of Earth Sciences, Eurotank Flume Laboratory

• Delft University of Technology, Faculty of Civil Engineering and Geosciences, Department of Hydraulic Engineering, Section Hydraulic Structures and Flood Risk

• Delft University of Technology, Faculty Electrical Engineering, Mathematics, Computer Science, Department of Applied Mathematics, Section Numerical Analysis

Results 2017

Results 2018

Results 2019