1. Assoc. Prof., Delft Univ. of Technol., Facu. of Civ. Engrg. and Geosciences, Fluid Mech., Delft, The Netherlands. E-mail: W.Uijttewaal @citg.tudelft.nl
2. Student, Univ. of Karlsruhe, Inst. for Hydromechanics, Karlsruhe, Germany.
3. Assoc. Prof., Delft Univ. of Technol., Facu. of Civ. Engrg. and Geosciences, Hydrol. and Ecology Sect., Delft, The Netherlands.