Rena Mizutani

A Study on the Treatment of Riverbed Elevation and Riverbed Slope in Natural Rivers

Minjiao Lu, Toshiro Kumakura, Hongxuan Yang, Tokuzo Hosoyamada

This study investigates how different treatments of riverbed elevation and riverbed slope in numerical analyses of natural rivers influence water level calculations and riverbed slope evaluations. Although these parameters are fundamental to hydraulic analyses such as water surface profile and riverbed variation analyses, their definitions and calculation methods are not uniquely determined in natural rivers with irregular cross-sections, leading to variability in analytical results.
Actual river data from the Shinano River and an idealized channel were analyzed to compare several definitions of representative riverbed elevation: cross-sectional average elevation, deepest riverbed elevation, the MLIT standard, and hydraulically defined representative riverbed elevation. Their effects on longitudinal riverbed elevation distributions and riverbed slope evaluations were examined.
The results indicate that geometrically defined representative riverbed elevations offer high uniqueness and reproducibility but do not always reflect actual flow conditions. The cross-sectional average elevation tends to overestimate riverbed elevation, while the deepest riverbed elevation is strongly affected by localized scour, producing unstable longitudinal distributions with frequent adverse slopes. In contrast, the hydraulically defined representative riverbed elevation, based on water level and discharge, shows relatively stable longitudinal distributions and riverbed slope trends, suggesting its effectiveness in representing flow conditions, despite its dependence on discharge.
Further analysis using an idealized channel revealed that under subcritical flow, the hydraulically defined representative riverbed elevation increases with discharge, although this does not directly correspond to an overall increase in evaluated riverbed slope. Under supercritical flow, its dependence on discharge becomes weak. Calculations using trapezoidal cross-sections also showed that, except for rectangular cross-sections under uniform flow, the hydraulically defined representative riverbed elevation varies with water depth, with smaller variations at lower discharges.
These results demonstrate that representative riverbed elevation cannot be uniformly defined. Both geometric and hydraulic definitions have advantages and limitations, and appropriate selection and interpretation according to flow conditions are essential.