Five common mistakes in fluvial morphodynamic modelling

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Five common mistakes in fluvial morphodynamic modelling

Erik Mosselman & Le Thai Binh

Session HS9.5-GM7.11 “Numerical modelling and experiments in river morphodynamics” EGU General Assembly, Vienna, 2 May 2014

Five common mistakes in fluvial morphodynamic modelling Rationale

• Increased availability of morphodynamic models • Proliferation of new morphodynamic codes • Increased use of models by non-experts

Approach

• Computations for EGU 2014 (mistakes 1 and 2) • Examples from existing studies (mistakes 3, 4 and 5)

Reference computation

Loosely based on: Crosato, A., E. Mosselman, F. Beidmariam Desta & W.S.J. Uijttewaal (2011), Experimental and numerical evidence for intrinsic nonmigrating bars in alluvial channels, Water Resources Research, Vol.47, W03511, doi:10.1029/2010WR009714.

10 000 m 90 m 30 m i = 0.1 m/km L = 10 km B = 90 m Q = 180 m3_/s h0= 3 m D = 0.2 mm C = 42.84 m1/2_/s Sediment transport formula of Engelund & Hansen

Bed slope effect on sediment transport direction: f(0) = 0.50.5 Morphological factor = 10 Reference computation 3 4 5 6 7 0 1000 2000 3000 4000 5000 6000 B ed le ve l( m ab o ve d at u m ) Distance (m)

Mistake 1: New codes without essential physics 3 4 5 6 7 0 1000 2000 3000 4000 5000 6000 B ed le ve l( m ab o ve d at u m ) Distance (m)

Mistake 2: Imposing of solutions by so-called “fine-tuning” of calibration

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Mistake 2: Imposing of solutions by so-called “fine-tuning” of calibration

3 4 5 6 7 0 1000 2000 3000 4000 5000 6000 B ed le ve l( m ab o ve d at u m ) Distance (m)

Mistake 3: Inadequate upscaling

active layer mix

bed

T

T h

Meuse: graded sediment with active-layer thickness = 0.1 m

Result: no morphological response to giving more space to the river

Mistake 4: Confusion of physical and numerical phenomena Observed oscillation suppressed in computation: • due to roughness submodel? (Lesser et al, 2004) • due to numerical scheme as analytical model predicts less damping?

Lesser, G.R., J.A. Roelvink, J.A.T.M. van Kester & G.S. Stelling (2004), Development and validation of a three-dimensional morphological model. Coastal Engineering, Vol.51, Nos.8-9, pp.883-915.

Mistake 5: Belief that 2D and 3D models require more data than 1D

Implications for validation … … and review!