SPATIOTEMPORAL DETAILS OF FLUID TURBULENT STRUCTURES AND SEDIMENT TRANSPORT OVER BEDFORMS: LABORATORY EXPERIMENTS OVER TRIANGULAR RIPPLES

Despite numerous experimental and numerical studies investigating transport over ripples and dunes in rivers, the spatiotemporal details of the pattern of transport over bedforms remain largely unknown. Previous experiments assessing the affects of flow separation on downstream fluid turbulent structures and bedload transport suggest that localized, intermittent, high-magnitude transport events, called permeable splat events, play an important role in both downstream and cross-stream transport near flow reattachment. Here we report results from a set of flume experiments that assess the combined affects of flow separation/reattachment and flow reacceleration up the stoss side of the bedform. The flume was lined with 17, 30 cm long concrete ripples with 2 cm high crests. A high-speed camera observed sediment transport along the entirety of the bedform at 250 f/sec. Downstream and vertical fluid velocity was observed at 1mm and 3 mm above the bed using Laser Doppler Velocimetry at 15 distances along the bedform profile. As observed in our 2014 backward-facing step experiments, mean downstream fluid velocity increases nonlinearly with increasing distance along the ripple. Observed sediment transport, however, increases linearly along the ripple. This pattern ceases at the crest of the bedform, where both mean downstream fluid velocity and sediment transport decrease significantly due to flow separation. Previous experiments assessing only the affect of flow separation showed that calculating sediment transport as a function of boundary shear stress (derived from near-bed fluid velocity), using a Meyer-Peter Müller type equation, produced a zone of underestimated transport near flow reattachment. Results reported here show that calculating sediment transport in this way underestimates observed sediment transport along the entire profile of the bedform, not just near flow reattachment. Using Reynold’s stress to calculate sediment transport also underestimates bedload transport and does not exceed the critical shear stress until ~7 step heights downstream. Sediment transport time-series data show a zone of high-magnitude cross-stream transport near flow reattachment, suggesting that permeable splat events are still playing an important role in the pattern of sediment transport.