Abstract Coarse-grained deltas are often characterized by steep foreslopes (often more than 10°) that are traversed by delta-front channels. The channels thus erode into relatively steeply inclined bedding. In this context, the slopes flanking the channels can be steeper than the friction angle since they include a component of dip related to the delta-front slope as well as the channel-related erosion slope. In this study, part of the Busu River delta (Papua New Guinea) was imaged using a high-resolution multibeam bathymetry survey over an area where the angle of the slopes flanking the channels locally reaches 50°. A detailed analysis of the delta slope morphology has revealed an additional source of instability due to erosion within the main channels. In some places, erosion cuts into the channel flank forming a local knickpoint inclined in a direction approaching that of the bed dip. The cut can then initiate breaching or static liquefaction failure from that point up to the crest of the interfluve resulting in a V-shaped gully.

Abstract On the north shore of the St Lawrence Estuary (Québec, Canada), near the Betsiamites river delta, a large sub-aerial submarine landslide complex was mapped using multi-beam bathymetry and light detection and ranging (LiDAR) data. Previous analysis of this landslide complex revealed that, since 7250 cal years BP, at least four different landsliding events occurred to form the present morphology, in which over 2 km 3 of material have been mobilized. The 7.25 cal ka BP landslide is of particular interest here: this landslide is entirely submarine and mobilized about 1.3 km 3 of material, deposited over an area of 54 km 2 , which make this landslide the largest identified on the St Lawrence estuary seafloor. This landslide showed a runout distance of about 15 km. Landslide-generated tsunamis may be triggered by such a landslide, where a large volume of material is mobilized in a short time. Kinematic analysis of this landslide was previously performed, and here we use these analyses in order to perform tsunami wave generation and propagation modelling. It is shown in this analysis that, even if the mobilized volume is very high and there is a long runout, the tsunami generated is small with tsunami wave amplitudes of <1.5 m, except in the vicinity of the landslide. The highest tide elevation in this part of the St Lawrence Estuary is about 5.5 m, so the impact of such a tsunami wave may be limited.