18 March 2021
Retrogressive breach failures
Earlier this week I posted a piece about the remarkable failure at Knappensee in eastern Germany, which was caught on video. The failure, on the banks of a flooded open cast coal mine, was retrogressive, generating a substantial displacement wave. Loyal reader Konrad Beinssen from Australia posted a description of this type of failure, which is termed a “retrogressive breach failure”. As I wasn’t aware of this particular failure type, I thought I would post his comment here.
This type of soil failure now known as a ‘retrogressive breach failure’ is not uncommon in coastal, river and lake settings, worldwide. Notable locations are the lower Mississippi, Inskip and Amity Points in Eastern Australia and in the sandy estuaries of the Netherlands.
A small triggering event in submerged (saturated) sand starts the process of ‘breaching’, well known to operators in the dredging industry. A near vertical wall of sand (the breach) migrates upslope. This wall is temporally stabilised by dilation of the sand skeleton immediately behind the breach causing suction (strain hardening). Granular material (such as sand) is shed from the breach as it migrates upslope and this generates a density current which transports grains offshore before they loose momentum and settle to the bottom.
As the breach arrives at the shore and migrates further inland, it undermines material above the water table, which topples in and adds further to the density (turbidity) current.
Konrad points us to a paper (Mastbergen et al. 2019), which is open access, which describes this failure mechanism in more detail. It includes this explanatory diagram:
The paper includes a number of other examples of these types of failure. It includes the Inskip landslide in Australia, which I also featured on this blog.
Mastbergen, D.R.; Beinssen, K.; Nédélec, Y. 2019. “Watching the Beach Steadily Disappearing: The Evolution of Understanding of Retrogressive Breach Failures” Journal of Marine Science and Engineering, 7 (10):368. https://doi.org/10.3390/jmse7100368.