Imaging scours in straightened and braided gravel-bed rivers with ground-penetrating radar
The genesis of alternate gravel bars in straightened rivers and braided rivers is closely coupled to the formation of scours. Scours can lead to riverbank failure, initiate lateral river dynamics and strongly impact the interaction between surface and subsurface flow and transport. It is, therefore, critical to account for scours in the design of flood protection measures. However, there is still little knowledge on the formation and characteristics of such scours, especially on the dynamic relationship between the riverbed morphology and the scours. Bathymetric riverbed surveys conducted at medium to low discharge may underestimate the real scour sizes and shapes if scours filled with sediments during waning discharge. Furthermore, the literature suggests that gravel bars and scours can jointly migrate on the riverbed, resulting in cut-and-fill sedimentary structures in the near subsurface. These cut-and-fill structures cannot be observed from the surface. We investigate with ground-penetrating radar the presence of scour fills below the riverbed of two gravel-bed rivers and study any indications of bar and scour migration. Thus, two ground-penetrating radar surveys were conducted, one on the emerged part of an alternate bar of the channelized Alpine Rhine River (Switzerland) and the other on a flat gravel surface in a braided reach of the Tagliamento River (Italy) that is near the pristine state. At both sites, a scour fill could be clearly identified below a 2-m-thick sediment layer. The imaged part of the scour of the Alpine Rhine River (30 × 100 × 4.5 m) is located at the front end of the gravel bar next to the riverbank. This scour was only partially imaged by ground-penetrating radar and is in reality significantly deeper and larger. The scour of the Tagliamento River (20 × 30 × 2.5 m) shows a clear internal structure consisting of inclined, planar cross-beds that merge tangentially with the lower-bounding erosional surface of the scour. At the light of the literature, we compare these two scours in terms of sedimentary processes resulting from the complex interplay between scours and gravel bars. The study findings offer a promising research avenue on the dynamic relationship between discharge, sediment transport, scour and bar formation and migration.