© 2016 Elsevier B.V.This paper summarises the results of a series of experiments performed to investigate the onset of tunnel scour below subsea pipelines in steady currents. The experiments were performed on a model seabed that extended different lengths upstream of the pipeline to assess the effects of sediment supply on sedimentation around the pipeline and the potential for onset of tunnel scour. In each experiment, the flow velocity and pipeline embedment were recorded continuously from inside and outside of the model pipe. In general, the results show that following evolution of the seabed profile around the pipeline due to sedimentation and changes in sediment supply, the onset of tunnel scour may still occur even when the initial embedment is larger than the critical value obtained by an existing empirical formula according to Sumer et al. (2001). This result suggests the potential for tunnel scour beneath deeply embedded pipelines in the field where the sediment supply may be interrupted by, for example, rock outcrops or upward-sloping seabed on the upstream side of the pipe. It also demonstrates that onset of tunnel scour is possible for a pipeline on a seabed that is not flat on either side of the pipe, provided that the flow conditions are sufficient to promote piping. To complement the experiments, a series of numerical simulations have been conducted to investigate the seepage flow and dynamic pressure difference upstream and downstream of the pipeline prior to the onset of tunnel scour. The numerical results show that despite significant alteration of the surrounding seabed topography due to local scour for a deeply embedded pipe with limited sediment supply, the pressure gradient across the pipe is still sufficient to cause piping compared with the flat seabed case, and the maximum pressure gradient at the downstream side of the pipeline is consistent with the breakthrough point observed in the physical experiments.