| Summary: | Maximum equilibrium scour depths, equilibrium scour volumes,
and induced equilibrium movements at isolated obstacles on river
beds were investigated in clear-water scour. The research was
achieved by a systematic flume study based on the results of
dimensional analysis of local scour.
The main tested obstacles for local scour were cubes. Local
scour at a circular cylinder of aspect ratio 1 and a sphere were
also examined for comparison with scour at cubes. Each obstacle was
placed on a flat movable bed and oriented normal to the incident
flow. Non-cohesive uniform medium sand and very fine gravel were
used as movable beds. The tested flow conditions were controlled to
give many combinations of the relevant dimensionless parameters for
local scour: the flow Froude Number, relative obstacle size, and
relative roughness of bed material.
Water surface profiles and longitudinal velocity profiles along
the plane of symmetry, as well as the horizontal velocities near the
bed around isolated obstacles on a fixed bed were examined in
several runs. For a slightly submerged cube, the path of maximum
velocity downstream of the cube moves from the water surface into
the lower part of the flow. At a greater distance downstream, the
path of maximum velocity tends to move upward toward the water
surface. An increase of flow Froude Number leads to a hydraulic
jump behind the cube. The relative longitudinal velocity near the
bed is increased in this region. The decrease of relative water
depth or the increase of flow Froude Number can lead to increased
average relative horizontal velocity (V/u) near a cube. The cube
has the highest average V/u value, as compared to flow near a
circular cylinder and a sphere.
The local scour processes and equilibrium scour patterns at
isolated cubes on movable beds were also examined. There is not a
unique equilibrium scour pattern for clear-water scour at a cube on
a movable bed. Equations were developed to predict the maximum
equilibrium scour depth and scour volume and the critical condition
for incipient motion of bed material at a cube in clear-water scour.
As the water becomes relatively deep, the shape effect of the
circular cylinder and the sphere become negligible. The major
equilibrium movement of a cube on a movable bed due to scour
involves longitudinal displacement, vertical settlement, and
longitudinal inclination. Equations were developed to predict the
equilibrium movement and the critical condition for incipient
upstream movement of a cube in clear-water scour. === Graduation date: 1991
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