Author(s): Simon Rusjan; Klaudija Lebar; Sebastian Gegenleithner; Josef Schneider

Linked Author(s):

Keywords: High-gradient streams; Discharge conditions; Flow velocities; Roughness characteristics

Abstract: In natural, high-gradient streams, variable and dynamic channel morphology on relatively small spatial scales can be found. This makes quantification and hydraulic analysis of flow conditions in such streams very demanding task. Proper quantification of flow characteristics in high-gradient stream is important for solving many hydraulic engineering problems (e.g. analysis of flow characteristics during contrasting discharge conditions, calculating the expected water levels, accounting the flow velocities for designing stream restoration measures, etc.). In section of the natural high-gradient torrential stream we performed analysis of flow resistance characteristics and flow velocities by combining field measurements and hydrodynamical simulation results. The studied step-pool and cascading torrential stream section with channel bed slopes ranging between 20 and 30% is located in the headwater part of a larger Gradaščica river catchment, which is one of the major tributaries of the Ljubljanica river, Slovenia. Based on detailed DTM (5 cm cell size) obtained by terrestrial Lidar scanning of a torrent stream channel and field measurements of water velocities during different discharge conditions along the studied channel section, we analyzed the roughness characteristics of the torrential channel by an iterative procedure of simulation runs using the Hec-Ras 2D hydrodynamical model. Simulation results indicate considerable decrease in the hydraulic roughness of the torrent channel with increasing discharge. The power-law relationship between the discharge and the iteratively defined mean stream section Manning roughness coefficient was defined. The mean stream section Manning roughness coefficient decreased from 0.61 for the low-flow discharge conditions down to 0.07 for the high discharge conditions. Further analysis of the influence of channel slope and bedform characteristics on the flow resistance and simulated flow velocities indicate limited influence of channel slope on the flow velocities in high-gradient streams. The influence of the stream channel bedforms on the flow resistance during high discharges is reduced as many of the bedforms become submerged when the flow depth increases. Our results clearly show strong dependence of the high-gradient streams flow resistance on the discharge conditions which should be properly accounted in various hydraulic engineering applications. The calculated Manning roughness coefficient values are generally well above the values that are usually used by engineers while performing the hydraulic analysis of natural high-gradient streams.

DOI: https://doi.org/10.3850/IAHR-39WC2521711920221033

Year: 2022