Abstract: Turbulent flow past series of groins in a shallow open channel is studied by large eddy simulation (LES) in this paper. A direct-forcing immersed boundary method is proposed to approximate complex boundaries around groins with round heads. Time-averaged velocity and turbulence intensities at the water surface obtained by a PIV experiment are used to validate the LES model, and the numerical results from LES agree well with the PIV measurements. Moreover a comparing case for groin flow is set up to investigate the effect of the groin aspect ratio L/D on flow characteristics, involving flow patterns, turbulent intensities, vorticity distribution and vortex dynamics. The distance D between two groins is smaller when the groin aspect ratio L/D is bigger and L is constsant. And this a ects the strength of turbulence intensities and vorticity in the mixing layer and flow patterns in the groin field. After producing around the groin heads, the shape of the vortex group varies as the vortices being transported downstream by the flow.