Surely covered in the article (paywalled) or literature, but sounds similar to a critical angle of repose https://wikipedia.org/wiki/Angle_of_repose https://wik

Surely covered in the article (paywalled) or literature, but sounds similar to a critical angle of repose https://wikipedia.org/wiki/Angle_of_repose https://wikipedia.org/wiki/Angle_of_repose where a pile of sand subsides if too steep. Not applicable to hard rock (non-granular), but a river bank profile with erosion due to flow granular particles, may behave as if granular.