Influence of Characteristic Diameter on Downstream Hydraulic Geometry Relations for a High Gradient Gravel Bed River

Hydraulic geometry (HG) theory has been applied to characterize the morphological changes that a river undergoes as a result of changes in discharge. Furthermore, HG has been used in a variety of studies including indirect discharge estimates, habitat assessment, and flow resistance analysis. Since a river is always evolving towards a morphological equilibrium, HG relations are obtained based on measured data that relate top width, mean flow depth, and mean velocity with discharge. At-a-station and downstream HG have been defined to characterize river development at a cross-section and reach scales, respectively. Dimensionless forms of downstream HG relations have been proposed to capture the physics of river morphology. Median sediment diameter d₅₀has been used as the characteristic diameter to put HG relations in dimensionless form. However, for rivers with coarse bed material, it has been shown that the characteristic diameter may be greater than d₅₀. In the present study, field measurements from a high gradient (slopes from 2% to 10%) gravel bed river (d₅₀from 4 to 25 mm), the Tabacay River, are used to establish the downstream HG relations and to determine the impact of the characteristic diameter by considering d₈₄and d₉₀as characteristic diameters. Additionally, based on these relations, regime equations are obtained for the Tabacay River to compare the performance of each characteristic diameter. The results helped to define the appropriate characteristic diameter to make HG relations and regime equations more representative of measured data.