The shallow water equations in geomorphic modeling Guy Simpson Earth Science University of Geneva - Switzerland Guy.Simpson@unige.ch
Water-induced erosion ….. in the absence of water! (drainage area - water discharge proxy) Stream-power type model (detachment-limited) Transport-limited model A (drainage area) Flux = c An Introduce poorly constrained parameters Neglect flow dynamics of water and other important physics
Model of coupled shallow water flow and erosion-sedimentation Simpson and Castelltort (2006) Computers and Geosciences
Shallow water flow over a mobile bed water mass balance x- force balance y- force balance Sediment mass balance Kinematic relationship between bed fluxes and bed elevation c sediment concentration in fluid z bed topography E entrainment flux (between bed and water) D deposition flux f porosity r density of water-sediment mixture (rw(1-c)+rsc) ro density of saturated bed (rwf+rs(1-f)) rw density of water rs density of sediment Additional forces in x direction (e.g., bed friction, bed slope)
Empirical functions for E and D (for cohesionless material) If If Shields parameter Particle Reynolds number Friction velocity
Solution obtained with finite volume method using a Riemann solver
Dam break calculation in 1-D
Dam break calculation in 2-D
Computed water surface (in meters) after 60 seconds
Velocity field of water and water depth contours after 60 seconds
Dam break over mobile bed (1-D simulation)
Water reservoir dam
Coastal sediment transport due to a Tsunami Wave : period = 30 min. amplitude = 5 m Gently sloping coast Ocean
What are the (dis/)advantages relative to alternative formulations Improved (richer) physics - wide range of applications - Flexible non-capacity sediment transport formulation Avoid ad hoc parameters Readily testable (bench-marking) Disadvantages More complicated to program (development time) Computationally more expensive (longer model run time) Numerical stability issues (paricularly for small water depths) Other issues Well developed numerical methods to solve the SWE Easy to parallelise Unstructured meshes Other numerical techniques (e.g., Lattice Boltzmann)
Vector formulation
Finite volume method
Riemann solver