ABSTRACT
Recent field studies suggest that bedform dynamics and turbulence in large rivers are poorly described by semi-empirical models, which are based on small-scale laboratory experiments. Whether this also applies to intermediate-scale rivers is unclear because field data sets are rare and coarse. This is mainly because measuring turbulent flows synchronously with fast moving bedforms is a proven difficulty, even in highly controllable laboratory setups. To increase the accuracy of field studies, we improved existing methods for high-resolution and quasi-synchronous profiling of turbulent flow and mobile riverbed, by adopting laboratory instrumentation and techniques for studies in natural streams. This instrumentation combines a micro echo sounder, an array of acoustic Doppler velocimeters, a realtime kinematic differential global positioning system, as well as a custom-made, non-intrusive and adaptable field setup. Field-testing was performed in the Swider River in Poland. In the study reach the river is 0.4 m deep, has a bulk velocity around 1 m s-1, and a sandy riverbed with fast moving dunes (on average 0.1 m high, 1.2 m long with celerity of 1 m h-1 ). The field tests demonstrated that the improved method permits the collection of riverbed profiles, as well as vertical profiles of turbulence within 12% of the time period that an average dune required to migrate a distance equal to its wavelength. Vertical and horizontal features of bed topography that were as small as 1% with respect to dune length and height could be resolved. The resolution of vertical turbulence profiling, scaled with river depth, ranged from 1% near the bed to 3% in the interior region the flow. The results of the field tests provide insight into various morpho- and hydrodynamic phenomena, such as bedform amalgamation/modulation, and dynamics of stacked-wake flow structures. In conclusion, the method is promising for detailed field measurements of turbulence over mobile bedforms in intermediate-scale rivers.
