Flow resistance in a subtropical canyon river

Abstract

We conducted several field surveys to measure velocity and channel geometry in 15 cross-sections distributed along three reaches in the Boi River (Southern Brazil), a canyon river that belongs to the most extensive canyon chain in South America. The three reaches present, respectively, canyon, transition, and floodplain landscapes. The resulting data were used to compute the flow resistance. A downstream hydraulic geometry analysis indicates that the exponent describing the velocity increases with the discharge and that these values were between 0.3043 and 0.3507. A downstream hydraulic geometry analysis indicates that changes in velocity with discharge are greater than the rate of change of width or depth. It is typically found in steep mountain streams laterally confined, and it was verified in the canyon reach. However, despite the transition presents a considerable lateral constriction and the floodplain reach presents mountain river characteristics, both reaches did not present such behavior. The findings are coherent with other mountain river datasets, although the Boi River presents distinct geomorphic units such as glide-runs. In contrast, other available datasets are usually characterized by step-pool channels. Flow resistance in the canyon and transition reaches differ in many ways from floodplain reach, indicating larger resistance inside the canyon than outside, probably related to the landform and grain size in the canyon. Regression analysis of the combined field dataset indicated that dimensionless unit discharge is an important independent variable to explain variations in velocity and flow resistance. We also verified that the use of Rickenmann's Equation can be helpful to estimate velocity in canyon rivers. Our findings include: i) canyon landscape presents higher friction factors than floodplain landscape; and ii) canyon and transition landscapes are similar in terms of flow resistance during high-flow conditions, while floodplain present remarkably differences.