| Step-pool system is a typical bed micro-morphology in high gradient mountain rivers, which creates extremely high turbulence intensity and thus dissipates most of the flow energy. The hydraulic and structure features of step-pool system were studied through field investigation in mountain rivers of southwest China. An artificial step-pool system was constructed and a series of experiments were conducted to study the characteristics of turbulence and energy dissipation mechanism of step-pool system. The effects of the bed load transport on the energy dissipation of step-pool system were studied.Three types of step-pool system were commonly observed in field investigation in the Xiaojiang River and Nujiang River in Yunnan province and the Yalutsangpo River in Tibet, which were three-dimensional step-pool system, two-dimensional step-pool system and step-pool system in between. Structure characteristics of these three types of step-pool system were summarized. Three-dimensional step-pool system was choosen as the typical step-pool system. Results of the experiments showed that the three-dimensional step-pool system had higher energy dissipation ratio than the two-dimensional one at various discharges. The energy dissipation ratio of the two-dimensional step-pool system decreased rapidly with discharge, while the energy dissipation ratio the three-dimensional one decreased slower and maintained high energy dissipation ratio in high discharge.The energy dissipation ratio of step-pool system was high, which was 64%-91% in this field experiment. The flow field around the step-pool was very three dimensional. On the step, the mean streamwise velocity was high and the turbulence intensity was only 0.1 time of the mean velocity. In the pool, the mean velocity was low and the turbulence intensity was 8.0 times of the mean velocity. The vertical turbulence component was much greater than the streamwise and cross-stream components. The Reynolds stress in the pool was about 50 times higher than that on the step. A formula for calculation of energy dissipation ratio was deduced based on the research achievements of predecessors on the stepped spillway, plunge pool and hydraulic jump. The calculated values of the formula were accorded with the measuring values well.Analyses on the measured turbulence revealed that the flow energy was dissipated on the step and hydraulic jump in the pool. The energy transformation process can be divided into three processes:(1) as the water flows down the step, the potential energy transforms into the kinetic energy sharply;(2) as the water plunges into the pool, hydraulic jumps occurs, and the kinetic energy transforms into the turbulent energy dramatically;(3) as water flows out of the pool, the kinetic energy gradually regains, and the turbulent energy transforms into heat.It was showed from the energy spectrum analysis that, as water flowed over step-pool system, energy transformed from low frequency to high frequency, the turbulence intensity and energy dissipation rate increased with discharge in the pool. The step-pool system generated more turbulence than the mountain channel without bedforms, thus was more effective energy dissipators. The energy of the flow was dissipated mainly on the steps under low flow discharges, and the energy of the flow was dissipated mainly in hydraulic jumps in the pool under high flow discharges. The Froude number increased with the flow discharge in the experiment, therefore, the energy dissipation effectiveness of the hydraulic jump in the pool increased with the discharge. The step-pool system in mountain streams can dissipate most of the energy of the flow during both the dry season and the flood season because the combination of the step dissipation and the pool dissipation works well for both low flow and high flow discharges.Flow energy in mountain streams was mainly dissipated by bed structure and bed load transport. The increase of the bed load transport intensity changed the flow field. The turbulence intensity on the step increased and the turbulence intensity in the pool decreased sharply. The increase in bed load transport intensity also changed the energy dissipation distribution. The energy dissipated by bed load transport increased, thus the energy dissipation effect of the bed structure of step-pool system decreased and the pool of the step-pool system was filled up. |
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