New outdoor experimental river facility to study river dynamics

Abstract. The Outdoor Experimental River Facility (OERF) is a new large-scale, semi-natural research facility designed to study river dynamics at scales that bridge small laboratory models and natural rivers. The facility comprises a 50 m long, 20 m wide floodplain corridor and is designed to sustain discharges up to 800 L s−1, allowing subcritical, fully rough flow with field-like Reynolds numbers approaching 105 – beyond values typical of small-scale planform experiments constrained by Froude similarity. This paper reports the first of three planned experimental campaigns at the OERF, providing a foundational assessment of facility capabilities and operational constraints to guide subsequent sinuous and vegetated experiments with sediment recirculation. In an initial 338 h (∼ 14 d) straight-channel run without upstream sediment supply, a bi-modal gravel–sand bed (initial median diameter = 10 mm) progressively armoured to ∼ 22 mm, and reach-scale planform change remained modest despite a width-to-depth ratio of 12 and near-threshold mobility. A three-phase, mathematically designed inlet bar–pool perturbation increased local velocities by 8 %–27 % and produced limited lateral bank erosion (∼ 2.5–7.5 cm). The results delineate a narrow operational window for sustained bar growth and migration, long adjustment times, practical constraints of outdoor operation, and the moderating role of bank-material strength and toe armouring. Together, these findings show that field-like hydraulics are achievable within the facility while clarifying what limits mobility at this scale, and they motivate future experiments that couple hydrodynamic similarity with controlled sediment recirculation/feed and refined boundary controls to advance understanding of controls on bank erosion and planform evolution.