Quantifying scour depth in a straightened gravel-bed river with ground-penetrating radar

In straight gravel-bed river reaches, riverbank erosion can be induced by alternate bars and their associated scours. The maximum scour depth is therefore a key information to design reliable flood protections. However, scour depth cannot be correctly assessed by bathymetric riverbed surveys if scours are filled with sediments at low discharge. In this work, scour depths in a straightened, gravel-bed river with alternate bars is quantified with ground-penetrating radar (GPR) and the riverbed morphology is linked to the subsurface structure. A 4.5 m deep buried scour with an extent of 30 × 100 m is partially imaged by GPR at the front end of the gravel bar next to the riverbank. The non-imaged part of the scour is expected to be much larger and therefore deeper. Additional research is needed to assess how scour location and depth relate with discharge magnitude and gravel bar dynamics.