Experimental study of local scour around subsea caissons in steady currents

Local scour of sediments around subsea caisson structures was investigated experimentally by carrying out flume tests. The horizontal shape of the caissons is rectangular and the incident directions of the flow were 0°, 45° and 90°, with 0° representing flow parallel to the long boundary of the caisson and 90° representing flow parallel to the short boundary of the caisson. The study was focused on the low caissons whose vertical dimensions were equal to or less than their horizontal dimensions. It was found from the test results that the horseshoe vortex played a less important role compared with the velocity amplification at the sharp corners of the caisson if its height is smaller than its horizontal dimension. As the incident angle of flow is either 0° or 90°, scour started at the two upstream corners of the caisson, while scour at the centre of the upstream boundary did not start until the scour pits at the side corners extended there. This was also true for the case with 45° incident angle of the flow. The development of the scour depth was fitted according the exponential function and the hyperbolic function. It was found that the hyperbolic function fits the experimental data better than the exponential function because its correlation factor is larger.