Abstract
Open-channel two-phase flow above a granular mobile bed is studied experimentally and theoretically. In the two-phase flow, water serves as a carrying liquid for plastic grains transported as collisional contact load in the upper-stage plane bed regime. The investigation evaluates friction- and transport characteristics of the flow under the condition of intense collisional transport of grains and links them with the internal structure of the two-phase flow. The paper focusses on the effect of bimodal solids (mixed two fractions of grains of similar density and different size and shape) on the flow characteristics and internal structure. Hence, experimental results obtained for the bimodal mixture are compared with results for individual grain fractions. The experiments show that the bimodal character of the transported solids affects the layered internal structure of the flow as a result of fraction segregation due primarily to gravity (kinetic) sieving during transport. The segregation also affects the friction- and transport characteristics of intense bed load. In the paper, the effects are described and quantified.
Highlights
In natural hazard management of rivers and streams, sediment transport is one of the key elements to assess. It has direct consequences for morphology and stability of river channels and is responsible for infrastructural damages caused by extreme flood events [1]
Natural sediment forming a river bed and being transported during flood events is composed of grains of different sizes
As a first step in investigating the complex process of size-affected granular interactions, some authors study transport of bimodal sediments [1,3]
Summary
In natural hazard management of rivers and streams, sediment transport is one of the key elements to assess. It has direct consequences for morphology and stability of river channels and is responsible for infrastructural damages caused by extreme flood events [1]. Natural sediment forming a river bed and being transported during flood events is composed of grains of different sizes. As a first step in investigating the complex process of size-affected granular interactions, some authors study transport of bimodal sediments (sediments composed of two grain fractions of two different sizes) [1,3]. The vertical sorting is usually associated with armouring of a channel bed. The erosion/armouring phenomena preferably occur at bed shear conditions suitable for a development of bed forms ( dunes)
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