HYDRAULIC ROUGHNESS IN ALLUVIAL CHANNELS EXPOSED TO WAVES AND CURRENTS

Abstract

Hydraulic roughness is an important part for sediment transport phenomena. Understanding the physical process of the hydraulic roughness in alluvial channels exposed to waves and currents is rather difficult. This difficulty comes from the non-linear interaction of waves and currents. The aim of this study is to have a better understanding of the wave- current interaction and its effect on sediment transport phenomena and to verify some of the existing models. In order to achieve the aim of the study experiments were carried out in the Large Oscillating Water Tunnel of Delft Hydraulics in the Netherlands. The tunnel is designed to simulate the near-bed orbital velocity at full-scale (1.:1). It is also provided by a recirculating flow system which enables the study of wave-current interaction. Three series of experiments were carried out (experiments series A, B and C). Experiments series A were focussed on the hydrodynamic studies in a fixed bed condition. Waves were sinusoidal. Net current profiles with and without waves were studied. Experiments series B were focussed on bed form studies using a sand bed with D, = 0.21 mm. Waves were sinusoidal and different net currents were imposed ( O o. 4 m/ s) . The behavior of bed forms were studied in case of only waves and waves superimposed on net currents. Experiment series C averaged suspended sediment transport superimposed on net were focussed on the study of time• concentration and net time-averaged rate under regular asymmetric waves currents. Laser Doppler and Electro Magnetic Velocity Meter were used for velocity measurements. Time-averaged concentration profiles were obtained using the Transverse Suction System. Net time-averaged transport rates were determined using mass conservation techniques. It was observed that the waves reduce the velocity of the net current close to the bed over that expected for a net current without waves. The physical bed roughness has been replaced by an increased apparent roughness as a result of adding waves.