Numerical models of landscape evolution Mikaël ATTAL Marsyandi valley, Himalayas, Nepal Acknowledgements: Jérôme Lavé, Peter van der Beek and other scientists from LGCA (Grenoble) and CRPG (Nancy) Eroding landscapes: fluvial processes

Summary of last lecture At least 7 different fluvial erosion laws. - 3 “stream power laws” (erosion = f (A, S)) - 4 laws including the role of sediment (f(Q s ) ≠ 1) Low amount of field testing but recent work strongly support that: - sediments exert a strong control on rates and processes of bedrock erosion (f(Q s ) ≠ 1); - sediments could have “tools and cover effects”.

Lecture overview I. Examples of models and applications II. The Channel-Hillslope Integrated Landscape Development model (CHILD)

Numerical models of landscape evolution (2002) Willett, JGR, 1999:

Numerical models of landscape evolution  3D 1) Examples of models and applications 2) The Channel-Hillslope Integrated Landscape Development model (CHILD)

Tectonics CHILD Geomorphic modelling systems: simulate the evolution of a topographic surface under a set of driving erosion and sedimentation processes (CHILD, CASCADE, etc.) Initial topography Flexural isostasy Climate parameters Hillslope transport + landslide threshold Fluvial sediment transport + deposition + bedrock erosion Additional parameters and algorithms: fluvial hydraulic geometry, bedrock and sediment characteristics, role of vegetation, etc.

Modelling landscape evolution The elevation of each node changes through time under the effects of: hillslope processes (erosion, transport, deposition), fluvial processes (erosion, transport, deposition), tectonics. Different models include different sets of parameters and/or treat the processes differently: no “universal” model of landscape evolution

Modelling landscape evolution Models = fantastic tools integrating the effects of a wide range of processes in space and time, but… Processes are SIMPLIFIED, and some of them are badly constrained (e.g. fluvial incision laws, role of vegetation, etc.)  Need FIELD DATA to constraint parameters and processes. Combination of numerical modelling studies + field studies  test and calibration of models,  hypothesis testing,  sensitivity analysis.

CASCADE (Braun & Sambridge, 1997, van der Beek and Braun, 1999) + Simple model (e.g. 1 fluvial incision law = under-capacity model, law 6) + Includes landsliding + User friendly  easily coupled with other models: thermal models for the lithosphere (van der Beek et al., 2002), tectonic models (Cowie et al., 2006) - Simple model (e.g. 1 fluvial incision law) - No rainfall variablility - Rigid mesh

CASCADE (Braun & Sambridge, 1997, van der Beek and Braun, 1999) Example 1: SE African margin (van der Beek et al., 2002) Margin formed during rifting 130 Ma ago. Height of topographic scarp ~1.5 km

CASCADE (Braun & Sambridge, 1997, van der Beek and Braun, 1999) Aim: discrimination between 2 end-member models for the evolution of the margin Example 1: SE African margin (van der Beek et al., 2002)

CASCADE (Braun & Sambridge, 1997, van der Beek and Braun, 1999) Example 1: SE African margin (van der Beek et al., 2002) Coupling with thermal model for the lithosphere Aim: discrimination between 2 end-member models for the evolution of the margin

CASCADE (Braun & Sambridge, 1997, van der Beek and Braun, 1999) Example 1: SE African margin (van der Beek et al., 2002) Coupling with thermal model for the lithosphere Aim: discrimination between 2 end-member models for the evolution of the margin

CASCADE (Braun & Sambridge, 1997, van der Beek and Braun, 1999) Example 1: SE African margin (van der Beek et al., 2002) Preferred model: plateau degradation + lithologic variations Aim: discrimination between 2 end-member models for the evolution of the margin

CASCADE (Braun & Sambridge, 1997, van der Beek and Braun, 1999) Example 2: morphological response to the growth and lateral propagation of fault-related folds; application to the Siwaliks hills (Champel et al., 2002; van der Beek et al., 2002) JGR, 2002 Question: can variations in detachment dip explain differences in drainage patterns along the Himalayan front?

CASCADE (Braun & Sambridge, 1997, van der Beek and Braun, 1999)  dip of the detachment exerts a major control on drainage development: variations in dip could explain differences in drainage patterns along the Himalayan front Example 2: morphological response to the growth and lateral propagation of fault-related folds; application to the Siwaliks hills (Champel et al., 2002; van der Beek et al., 2002) Question: can variations in detachment dip explain differences in drainage patterns along the Himalayan front?

CASCADE (Braun & Sambridge, 1997, van der Beek and Braun, 1999) Example 3: landscape development in response to fault interaction and linkage in extensional settings (Cowie et al., 2006)

CASCADE (Braun & Sambridge, 1997, van der Beek and Braun, 1999) Example 3: landscape development in response to fault interaction and linkage in extensional settings (Cowie et al., 2006) Eliet & Gawthorpe, 1995 Questions: are rivers flowing across active faults antecedent to tectonics? What is the effect of fault interaction on drainage patterns? What are the implications for sediment routing?

CASCADE (Braun & Sambridge, 1997, van der Beek and Braun, 1999) Example 3: landscape development in response to fault interaction and linkage in extensional settings (Cowie et al., 2006)

CASCADE (Braun & Sambridge, 1997, van der Beek and Braun, 1999) Example 3: landscape development in response to fault interaction and linkage in extensional settings (Cowie et al., 2006)  Drainage patterns result from fault interaction  tectonic control on sediment routing and sediment fluxes to adjacent basins