Flow structure caused by a local cross-sectional area increase and curvature in a sharp river bend

Horizontal flow recirculation is often observed in sharp river bends, causing a complex three-dimensional flow structure with large implications for the morphological and planimetric development of meanders. Several field observations in small-scale systems show that sharp bends are often found in association with a strong increase in cross-sectional area, the deposition of outer bank benches, and reattachment bars near the inner bank. Recent studies show that these bends can also occur in large-scale systems. In this study, we present field measurements of a sharp bend in the Mahakam River, East Kalimantan, Indonesia. The cross-sectional area increases by a factor of 3 compared with the reach-averaged cross-sectional area. Along a river reach of about 150 km, cross-sectional area correlates strongly with curvature. The field measurements are analyzed together with the results from numerical simulation with a three-dimensional finite element model, which yields a comprehensive view of the intricate flow structure. In turn, the model is used to validate a new equation that captures the water surface topography dependence on cross-sectional area variation and curvature. The results show the importance of the increase in cross-sectional area in the development of horizontal recirculation. Vertical acceleration of the flow into the scour causes the pressure to deviate from a hydrostatic pressure distribution. Strong downflow (up to 12 cm s^-1) advects longitudinal momentum toward the bed, causing the flow to concentrate in the lower part of the cross section. This increases the velocity magnitude throughout the cross section, which is expected to maintain the large scour depth found in several bends along the Mahakam River.