Near-bed flow dynamics and bed shear stress in a mangrove-vegetated estuary

Abstract

Understanding sedimentation in riverbanks requires investigating near-bed flow velocity and bed shear stress. Studying flow in vegetated rivers presents complexities due to vegetation acting as a drag force that slows d o w n water flow, especially near the bed. In tidal rivers, aside from river discharge influence, flow patterns are also affected by tides. This study investigates near-bed flow velocity and bed shear stress in the Ender River estuary, Cirebon, West Java, Indonesia. The Ender River estuary is a micro-tidal estuary bordered by mangrove vegetation on its riverbanks with a dominant clay sediment substrate. High-frequency measurement of near-bed flow velocity were conducted using an Acoustic Doppler Velocimeter (ADV) from October 4 to 10, 2023. The data were analyzed to determine the m e a n near-bed flow velocity (Ü) and flow fluctuation, which was then used to estimate bed shear stress using the Log Profile (LP) method. The result revealed that the average near-bed flow velocity pattern is dominated by river flow and varies with tides. Strong seaward currents occur during ebb tides and weaken during flood tides. M a x i m u m near-bed flow velocity occurs during ebb and flood conditions (4x10- 3 to 6x10-3 m / s) and m i n i m u m at peak flood (1x10- 3 m / s) . Bed shear stress ranged from 0 to 0.07 N / m 2 , with m a x i m u m values coinciding with ebb tide conditions. Given the calculated shear stress values, theoretically, the settled sediment is unlikely to be eroded or resuspended. This study provides insights into near-bed flow patterns in mangrove-vegetated tidal rivers, essential for understanding sedimentation and erosion processes in riverbanks with clay sediment substrates. However, the specific implications of these findings m a y vary depending on site-specific conditions.