Dynamic Characteristics of Break Debris Flow and its Numerical Simulation State Key Laboratory of Geohazard Prevention and Geoenvironment Protection Chengdu University Chengdu University of Technology Le Maohua () (Environmental Geology) Le Maohua () (Environmental Geology) Jan. 10, 2014 Supervisor: Professor Tang Chuan

OUTLINE Introduction Introduction Case Study Case Study Discharge Process Discharge Process Simulation Methods Simulation Methods Further Works Further Works

Introduction 1.A lot of collapses and landslides induced by the Wenchuan Earthquake have formed a lot of barrier dams. When these dams outburst, it tend to form Break Debris Flow. 2.Owing to the serious blockage in the gully, The flow discharge calculated by the flood method was obviously smaller than that calculated by morphology investigation method. 3.If an incoming mountain torrent collapses a series of landslide dams, large debris flows can form in a very short period. Moreover, the torrent can amplify the scale of the debris flow in the flow direction. 4.The Break Debris Flow is highly destructive, such as Hongchun gully debris flow, Qipan gully debris flow.

OUTLINE Introduction Introduction Case Study Case Study Discharge Process Discharge Process Simulation Methods Simulation Methods Further Works Further Works

Case Ⅰ :The Zhouqu debris flows, Aug. 8,2010 Ref: P. Cui, et al. Geomorphology,2013

Case Ⅰ :The Zhouqu debris flows, Aug. 8,2010

Six obvious debris-flow scale amplification phases between point L G and A’ Case Ⅰ :The Zhouqu debris flows, Aug. 8,2010 erosion of the channel bed The cascading failure of landslides dams in the gully

Case Ⅰ :The Zhouqu debris flows, Aug. 8,2010 Four stages of debris-flow scale amplification can be seen in the above figure The cascading failure of landslides dams in the gully erosion of the channel bed

Case Ⅱ :The Qipan gully debris flow, Jul. 10,2013

UpstreamDownstream

Case Ⅲ :The Hongchun gully debris flow, Aug. 14,2010 Ref: Chuan Tang, et al. Landslides,2011

Case Ⅲ :The Hongchun gully debris flow, Aug. 14,2010

OUTLINE Introduction Introduction Case Study Case Study Discharge Process Discharge Process Simulation Methods Simulation Methods Further Works Further Works

Dam-break induced debris flow A dam-break wave starting out as clear water is likely to rapidly pick up sediment both from the dam body and from the valley floor. In many instances, it is possible for the wave to entrain enough sediment to turn into a fully developed debris flow. (Costa, et al.1988) A dam-break wave starting out as clear water is likely to rapidly pick up sediment both from the dam body and from the valley floor. In many instances, it is possible for the wave to entrain enough sediment to turn into a fully developed debris flow. (Costa, et al.1988) Dam-break waves can in many instances entrain and deposit large volumes of sediment. This bulking and debulking process can significantly affect (i) the flow rheology, (ii) the wave hydrodynamics, (iii) the valley morphology. (H. Capart et al., 2001) Dam-break waves can in many instances entrain and deposit large volumes of sediment. This bulking and debulking process can significantly affect (i) the flow rheology, (ii) the wave hydrodynamics, (iii) the valley morphology. (H. Capart et al., 2001)

Discharge Process  Prediction of potential peak discharge and resulting hydrograph is necessary for the management of dam-break flood hazards and to decide appropriate mitigation measures including evacuation.  Basically, there are two methods to predict probable discharge from potential failure of landslide dam. One method relies on regression equations that relate observed peak discharge of landslide dam failure to some measure of impounded water volume. The other method employs computer implementation of a physically based mathematical model. (Walder and O’Connor, 1997)  The final purpose of research on dam break wave is: Calculate the process of the wave propagation speed, water level and flow characteristics with time in the detention basin and downstream areas.

Discharge Process Ref: X. Q. Chen, et al. Iranian Journal of Science & Technology, April 2007  Overall failure and Partial failure  Sudden failure and gradual failure

Flood/Debris flow hydrograph due to landslide dam failure Governing Equations Neglecting the effects of friction Considering the slope angle At the outlet of the failed dam Initial and Boundary conditions for the dam-break problem Ref:1.C. Ancey, et al. Water resources research, X. Q. Chen, et al. Iranian Journal of Science & Technology,2007 Note: k is a coefficient of debris flow discharge

The wave propagation in the downstream areas Governing Equations The flow below the dam Initial and Boundary conditions for the dam-break problem Note: k G is a parameter related to gully shape Ref:1.C. Ancey, et al. Water resources research, X. Q. Chen, et al. Iranian Journal of Science & Technology,2007 Neglecting the effects of friction Considering the slope angle

OUTLINE Introduction Introduction Case Study Case Study Discharge Process Discharge Process Simulation Methods Simulation Methods Further Works Further Works

Simulation Methods  Understanding the breaching process and the possible distribution of debris deposition is crucial to effective hazard mitigation and timely emergency response.  To reasonably facilitate the simulation of debris flows induced by the landslide dam breach, The CFD software FLUENT will be applied to generate the landslide dam break hydrograph which is then used as input to drive the FLO-2D model. A systematic integrated numerical approach is proposed.

Dam break routing and overland flood routing Flowcharts of the integrated models approach  Model of seepage flow  Model of slope stability  Model of dam surface erosion and flow Failure?

Dam break routing and overland flood routing  The FLUENT model provides comprehensive modeling capabilities for a wide range of incompressible and compressible, laminar and turbulent fluid flow problems. Steady-state or transient analyses can be performed. It also provides the volume-of-fluid, mixture and Eulerian models, as well as the discrete phases, including coupling with the continuous phase. Ref: Ansys Fluent 12.0 Documentation Ref: Ansys Fluent 12.0 Documentation  The FLO-2D model is a physically based, distributed flood-routing model using the fully dynamic wave momentum equation and a central finite- difference routing scheme. It also provides modules to simulate two- dimensional sediment transport, hyperconcentrated debris and mud flow. Ref: Flo-2D reference manual Ref: Flo-2D reference manual

Part Ⅰ : Dam break routing simulation Case: Partial breach modeling with FLUENT ↑ Computational domain Initial conditions ↓

Part Ⅰ : Dam break routing simulation ↑ Water surface profile at t = 7.2 s Velocity vectors ↓

Part Ⅱ : Overland flood routing simulation 1.Digital elevation data 2.Hydrograph at the outlet of the failed dam 3.Rheology parameter 4.Resistance parameter 5.Manning’s roughness coefficient. Note: Field investigation

OUTLINE Introduction Introduction Case Study Case Study Discharge Process Discharge Process Simulation Methods Simulation Methods Further Works Further Works

Further Works 1.Calculate the impact force of the typical cases. 2.Considering the model of dam surface erosion and flow, The dissertation will put forward the calculation methods of k, k G. k can be estimated from the size of grains remaining along the debris flow path, and k G can be estimated from the channel shape. In other words, the attention will turn into the changes in debris flow density along the flow path. k G depends on the geometry of the cross-section below the dam. Deposition will occur where the channel is wide (k G is small), and temporary damming may occur at a bend or where the channel narrows (k G is large). 3.The systematic integrated numerical approach will be validated in the typical cases.

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