Abstract.
In this report, some classical and new simplifications in mathematical and numerical models for river morphology are compared for conditions representing rivers in mountainous areas (high values ofFroude numbers and relatively large values of sediment transport rates).
Options for simplification are summarized based on time scale of hydrographs and length scales of river geometry. This results in concepts based on quasi-steady and/or quasi-uniform flow assumptions. Additionally, the behaviour of frictionless, critical flow with a mobile bed is considered.
The non-linear interaction between changing flow and morphology is investigated for different values of the Froude number. Neglecting this interaction in numerical solution procedures appears to affect the solution. Also, mass and momentum contributions of sediment in transport on the mixture of water and sediment are analyzed.
It is shown that errors due to simplification in numerical models for river morhology vary with the different up- or downstream propagating waves that are part of the solution.
Conclusions further refer to the importance of wave non-uniformity (wave length, dominance of friction), Froude number and bed mobility on the error made when using simplified modelling concepts. Application of simplified modelling concepts based on subcritical low-land rivers in the modelling of transcritical and supercritical flows can result in significant errors.
Acknowledgements
This report was in part supported by the EC Environment Research Programme (contract: EV5V-CT94-0462, Climatology and Natural Hazards), as a part of the FRIMAR project (Flooding Risks in Mountain Areas). Comments by and discussions with Prof. Dr. M. de Vries, Dr. Z.B. Wang, Dr. E. Mosselman and Mr. H.J. Barneveld are gratefully acknowledged.
