Repetitive surveys to assess sand ridge movement offshore Sable Island

A considerable amount of seabed information has been collected over the years on the Scotian Shelf near Sable Island. The seabed around Sable Island is composed predominantly of sands and its morphology is characterized by numerous large sand ridge features and smaller megaripple fields. It has been speculated that the larger sand ridge bedforms are storm generated and are possibly moving, at least on an intermittent basis. The objective of this paper is to present comparative measurements to shed light on how much the seabed has changed over a time period of engineering significance. The fact that the top surface sands are actively mobile from storm to storm is not disputed nor is it the subject of this paper. The focus of this paper is to evaluate the change of the underlying bedforms based on repetitive surveys. In the past it was difficult to evaluate bedform migration due to navigation uncertainty and the large line spacing (and resulting low data density) of bathymetric measurements. With the availability of differential global positioning systems (DGPS) and swath bathymetry in recent years, a more accurate evaluation of sand ridge movement can now be made. Coincident swath surveys from 1995 and 1996 are compared to each other, and to various data sets from the 1980s to evaluate sand ridge movements over a maximum of 16 years. The amount of potential sand ridge movement is important to pipeline design. If the sand ridges are moving then buried pipelines may become exposed over time creating free-spans and possible pipe overstress conditions. Thus it is important to understand the magnitude of seabed migration over the design life of the pipeline.