Abstract:
Bottom vanes are small foil type flow training structures mounted vertically on the
riverbed with an angle to the prevailing flow. The effectiveness of vane is dependant on
the height, length, shape, water depth and angle of attack of the approach flow. The
development and magnitude of local scour for different angle of attacks are still
undefined, though local scour is of utmost importance to the design of a vane. The
foundation should be able to sustain the maximum scour depth around the vane. In
Bangladesh, failure of submerged vanes often occurred due to excessive scouring.
Regarding the complexity mentioned above, the present study was performed under
mobile bed conditions with the aim of investigating the general pattern of local scour
around bottom vanes with variable vane height and angle of attack and an attempt was
made to develop an expression using dimensional analysis for predicting local scour
around vanes. Dimensional analysis was done using Reyleigh method. The expression
developed from the approach was compared with available scour predictors for
rectangular bridge pier and performance of the expression was evaluated by computing
the discrepancy factor. For this purpose a total number of 20 test runs were performed
with four different vane heights (6cm, 9cm, 12cm and 18cm) and five different angle of
attack (10°,15°,20°,30° and 40\
In evaluating the general behavior of local scour around bottom vane, sand with a grain
size of dso of 0.18mm was used which is similar to the major rivers of Bangladesh. The
length of the vane was chosen to be 40cm. For the total experiment the discharge and
water level were kept constant to a value 200 liter/sec and 30 em respectively. Under this
fixed boundary conditions the equilibrium scour depth for each test run was achieved
after a minimum of 16 hI'S running depending on the angle of attack. The flow field and
bed level measurements were then carried out to analyze the scour pattern around bottom
vane.Scour contour maps were constructed and analyzed to investigate the extent and location
of maximum scour hole. Longitudinal and cross sectional profiles of bed topography
were also prepared to critically analyze the scour pattern in this regard. Velocity contour
maps and vector plotting were also prepared to observe the change in flow velocity due to
formation of scour around bottom vanes.
On completion of these experiments and analysis, it appeared that the maximum scour
depth occurred at the leading edge of vane at pressure side. The scour depth was found to
be increasing with increasing vane height and angle of attack. At the pressure side of
vane, the presence of horseshoe vortex was found which is responsible for the formation
of local scour around the vane. At the suction side near the vane tip the vortex core was
found. This core indicates the presence of tip vortex that travels downstream of the vane
was responsible for the formation of erosion gully at the downstream. At further
downstream the vortex core was found to be decayed. Moreover it was also found that
vane has no impact on the upstream velocity.
