2-D numerical modeling of water flow over a gravel bar

The paper presents the application of two-dimensional shallow-water numerical model Rubar20 to simulate water flow over a complex topographic configuration in an energetic river. The main aim is to simulate a flushing dam event. The study site is a straight reach of the Arc River, a highly regulated river located in the French Alps. The reach is about 1,5 km long and includes a bar at the left bank side. The model requires as input the river bed topography, initial water surface level, bed roughness and inflow discharge, and predicts the spatial distribution of water surface level, velocity and bed friction. An adapted mesh was designed to well reflect different topographic features, especially secondary and transverse channels and bar margins. A dam flushing event that took part in June 2006 was monitored and simulated.Water surface level, flow velocity and discharge were directly measured on the field. Computational results were compared to field measurements. Simulated water surface levels were compared to monitored data with bubble pipe devices in three locations and showed a good agreement of model predictions where differences were below 20 cm. Predicted velocities in two cross-sections were also compared with field measurement done by Large-Scale Particle ImageVelocimetry (LSPIV) and indicated a possible amelioration of the bed roughness distribution between main channel and gravel bar surface. Overall results agreed with field measurements and conveyed good quality analysis of water flow pattern. Topographic boundary condition specification and bed roughness are critical for flow simulations in term of flow pattern and velocity magnitude.