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EFFECTS OF UNDERSCOUR DEPTH AND HORIZONTAL SPACING BETWEEN TWO bed protection BLOCKS ON STABILITY OF FRONTAL BLOCK

Journal of Hydrodynamics 2012年第4期24卷 579-588页

作者： SHAH Prem TANAKA Norio YAGISAWA Junji Graduate School of Science and Engineering Saitama University Sakura-ku Saitama-shi Saitama Japan

The stability of a river-bed protection block in a flood event is affected by underscouring of the block. The effects of block spacing on block stability were investigated in different underscour conditions. Three blocks were placed at the bottom in a flume, and the underscour depth of the frontal block and distance between the first and second blocks were changed. Drag and lift forces were measured directly in each experimental condition. Particle Image Velocimetry （PIV） was used to visualize the flow and eva- luate the streamline pattern. Pressure measurements around the block to understand the pressure distribution show that the pressure distribution on the rear and bottom surfaces of the block is significantly affected by changes in block spacing and underscour depth. The experimental results show that for the same underscour gap, lift, and drag coefficients decreased with increasing space between the two blocks. In addition, if the underscour gap is increased with the same horizontal spacing, the lift coefficient increases and drag coefficient decreases. These results demonstrate that wider spacing enhance stability of the frontal block in the underscour condition. Moreover, analysis of block stability with changing underscouring and horizontal spacing shows that bed protection blocks seem to have more stability when the gap between two bed protection blocks was equal to height of the blocks.

关键词： bed protection block stability underscouring Particle Image Velocimetry (PIV)

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Critical movement of large rocks in currents and waves

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International Journal of Sediment Research 2019年第4期34卷 387-398页

作者： L.C.van Rijn LVR-Sediment Consultancy

Cobbles, boulders, and rocks often are used in a bed protection layer near a structure to protect the underlying sand bed against erosion by combined current and waves. The design of a bed protection layer consisting of loose rocks(rubble mound) requires knowledge of the stability and movement(as bed load) of very coarse materials. If some movement(or damage) is acceptable, the rock diameter can be designed to be smaller. This paper addresses the stability and movement of very coarse materials(cobbles, boulders, and rocks) based on the concept of the critical Shields mobility number. It is shown that the bed load transport of large cobbles, boulders, and rocks can be described by the equations of Meyer-Peter and Mueller(MPM) and Cheng. Both are valid for relatively small Shields mobility *** and general equations for the design of a bed protection layer(including some permissible damage)in conditions with a current with or without waves are proposed based on the Shields mobility parameter and the bed load transport equation of Cheng. Laboratory and field data of critical velocities for pebbles, cobbles, boulders, and rocks have been analyzed and compared to the computed results of the proposed equations. Practical applications are given to demonstrate the general applicability of the proposed equations.

关键词： Rock stability bed protection Critical movement bed load transport Rocks

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