1. a Laboratory of Hydraulic Constructions, , Ecole Polytechnique Fédérale de Lausanne (EPFL) , Rue de Lac, 33, 1020, Renens, Switzerland E-mail:
2. b Laboratory of Hydraulic Constructions , Ecole Polytechnique Fédérale de Lausanne (EPFL) , Switzerland Fax:
3. c Laboratory of Hydraulic Constructions , Ecole Polytechnique Fédérale de Lausanne (EPFL) , Switzerland E-mail: