Sand-filled geotextile bags (geobags) are commonly used for riverbank protection, however, despite their deployment in a significant number of locations, the failure modes of such structures are not well understood. This paper describes a series of 1:10 physical model tests designed to improve our understanding of the basic failure modes of geobag revetments used as river bank protection. In particular, the aim of the experimental programme is to quantify the relationship between hydraulic shear stress on the revetment surface and geobag displacement. The aspect of the work reported is the use of two analytical techniques, the Conveyance Estimation System (CES) and the membrane analogy method developed by Chow to predict hydraulic shear stress on the geobag surface. It is concluded that both methods produce acceptable shear stress predictions for the initially washed away bags, although the CES is recommended for future use because of its better prediction and ease of application.