Dynamics of nonmigrating mid-channel bar and superimposed dunes in a sandy-gravelly river (Loire River, France)

- A non-migrating mid-channel bar was intensively surveyed on a sandy-gravelly river during floods.
- A conceptual model of bar dynamics details the major role of the upstream riffle.
- The role of morphological forcing parameters (obstacle vs. planform) is stage dependent.
- Sediment supply/availability (phasing/quantity/grain size) controls superimposed dunes growth.

A field study was carried out to investigate the dynamics during floods of a nonmigrating, mid-channel bar of the Loire River (France) forced by a riffle and renewed by fluvial management works. Interactions between the bar and superimposed dunes developed from an initial flat bed were analyzed during floods using frequent mono- and multibeam echosoundings, Acoustic Doppler Profiler measurements, and sediment grain-size analysis. When water left the bar, terrestrial laser scanning and sediment sampling documented the effect of post-flood sediment reworking.During floods a significant bar front elongation, spreading (on margins), and swelling was shown, whereas a stable area (no significant changes) was present close to the riffle. During low flows and falling limbs of floods, intense sediment reworking on the top of the bar and lateral scouring occurred. Hydrological variations controlled the sediment supply (in terms of phasing, quantity, and grain size) delivered by surrounding channels during floods and thus superimposed dune development. Their development was also linked to the sediment availability (armor layers, riffle proximity). Their relatively constant height highlights a preferential adaptation on dune length during floods.The role of each morphological forcing parameters (riffle vs. channel widening and curvature) on the bar dynamics and evolution is stage dependent; the shape, dynamics, and long-term morphological evolution of the bar and of the river reach (surrounding islands, channel translation) mainly depends on the presence of the natural riffle.