High Drag and Flow Separation in Curved Estuarine Channels

Abstract:

The drag force is a key element of the hydrodynamics of estuaries and tidal rivers, affecting tidal propagation, flooding potential and marsh inundation, mixing processes, and salinity intrusion. Drag typically has a quadratic dependence on velocity times a drag coefficient (C_d). In estuarine channels, C_d is often found to be around 0.003, and it is mainly attributed to bottom friction. However, observations of water level and velocity in a narrow, curved tidal channel (North River estuary, MA, USA) found much greater values, with C_d = 0.005 – 0.02. Processes that could contribute to this high drag include friction from marsh vegetation, lateral exchange with small tributaries, and flow separation at sharp channel bends. Flow separation has been found to increase flow resistance through form drag. To investigate the role of channel curvature and flow separation in increasing drag, we applied the Regional Ocean Modeling System (ROMS) to idealized estuarine channel simulations that are scaled to the geometry and forcing conditions from the North River. Through the numerical model study, we found that flow separation occurs in relatively sharp channel bends, where lee eddies are generated. We calculate the form drag directly, and show that this flow separation is the dominant source of momentum loss, increasing the total drag to more than twice that from bottom friction alone. The high drag coefficient shows an increasing trend with the water depth, contrary to the depth dependence of drag from bottom friction alone. We introduce a scaling based on theoretical boundary layer separation to explain the depth dependence of drag associated with flow separation. The increased drag due to flow separation decreases tidal amplitude along the estuary compared to similar straight channels. Flow separation could also affect sediment erosion and deposition in channel bends and result in morphological feedbacks.