Linking turbulent flow and bank erosion with experiments in a field-scale meandering channel

Abstract:

Alluvial meandering rivers evolve through a complex interplay between channel shape, turbulent flow and sediment transport. These interactions cause bank erosion and bar construction, which collectively shift the channel and grow meander bends. A rich variety of theoretical models have been proposed to represent channel migration, and their predictions have been compared to natural rivers in some cases. However, isolating the relationships between turbulent flow fields and bank erosion patterns in natural channels is complicated by heterogeneities in meander planform shape and in bank cohesion due to non-uniform stratigraphy and vegetation. To control for these effects, we conducted experiments in the Outdoor StreamLab (OSL) at St. Anthony Falls Laboratory (SAFL). The OSL is a field-scale experimental stream (2.5 m wide and 0.3 m deep) encompassing a full meander wavelength with a mobile sand stream bed (D50 = 0.7 mm). We designed the experiments to quantify stream bank erosion in relation to the complex local velocity field that varies with depth and distance along a meander. We ran each experiment at bankfull conditions using precisely controlled water discharge and sediment feed. Topography of the water surface and channel morphology were collected throughout a meander at cm-scale spatial resolution, and flow and turbulence measurements were collected at high spatial and temporal resolution in targeted traverses. At five locations within this meander, synthetic streambanks (55 cm tall by 70 cm long by 2.5 cm thick) were installed over the existing streambanks to control for cohesion of the bank materials. Streambank composition was designed to enable measurable erosion by fluid shear over an experimental flood (~4 hours),. and consisted of a compacted, uniform mixture of sand and bentonite clay (90% and 10% of dry mass, respectively), and water (15% of wet mass). The experimental banks were surveyed before and after the experiment and compared to near-bank velocity and turbulence measured during each flood. Erosion patterns varied both with depth and along the meander. We will use these carefully controlled, high-resolution data to re-examine linkages between complex flow fields, near bank turbulence and the spatial distribution of bank erosion that form one of the essential behaviors of alluvial meanders.