1. Laboratory of Hydraulics, Hydrology and Glaciology (VAW), Swiss Federal Institute of Technology (ETH) Zürich, VAW E 35, Gloriastrasse 37/39, 8092 Zürich, Switzerland; formerly, School of Engineering, Fraser Noble Building, Univ. of Aberdeen, AB24 3UE Aberdeen, UK (corresponding author).
2. Laboratory of Environmental Hydraulics (LHE), Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland.