Influence of Bed Roughness on Turbulence Characteristics in a Gravel-Bed Reach of the Mapocho River

Abstract

This study characterized the vertical distribution of turbulence intensity (TI) and its relationship with bed roughness in a reach of the Mapocho River at the Los Almendros gauging station, characterized by straight morphology, fine gravel bed, and subcritical flow conditions. Two field campaigns were conducted in September 2025, measuring instantaneous velocities in three cross sections by wading using a SEBA FlowSens electromagnetic sensor following the ISO 748:2007 standard, complemented by bed granulometric characterization using the Wolman pebble count method. The riverbed was dominated by moderately sorted fine gravels (D50 ≈ 4.8 mm, σg = 1.40). TI decreased from the bed toward the surface in all sections, with values ranging between 4.7% and 22.3%. In verticals over zones of higher local roughness, TI at 40% of the flow depth exceeded the value recorded near the bed, which is consistent with the propagation of turbulent structures from the bed toward intermediate layers of the water column. The Shields parameter exceeded the critical threshold in all three sections, confirming active sediment transport under the measured discharges.