Effect of submerged vane on scour around abutment

Abstract

Scour at bridge abutment is a senous concern for hydraulic engmeers. Although many methods are available to mitigate scour around abutment, riprap is by far the most widely used method. Using submerged bottom vane as flow altering device to reduce scour around abutment is relatively a new idea. The ability of such vane to alter the magnitude and direction of the bed shear stresses causes a change in the distribution of velocity and sediment transport. In the current research it was intended to utilize this ability of the submerged vanes in mitigating the scour around abutment. The study was undertaken to investigate the effectiveness of submerged bottom vanes to mitigate scour around abutment in a large mobile physical model facility (46m x 11 m) of BUET. For this purpose a total number of 28 test runs were performed. Out of these 28 test runs, one was the base test without vane condition and rest 27 tests were conducted using. different vane arrangements placed at upstream of the abutment by varying the vane number (single, two vanes, and three vanes), vane angle (20°, 30°, and 40°) and vane height (Hjh = 0.3, 0.4, and 0.6). The abutment model used in the test runs was of vertical wall type with protrusion length of 40 cm. All tests were conducted for 8 hours duration in clear water condition with a constant discharge of 200 lis and flow depth of 30 cm. In order to avoid the ripple formation, the coarse sand with dso = 0.75 mm was selected as bed material. The effects of various vane arrangements to reduce the maximum scour depth around abutment were determined on the basis of reduction of maximum scour depth by comparing with maximum scour depth obtained for without vane condition. This was expressed in percentage reduction of maximum scour depth. The maximum reduction occurred was 41.92% for two vanes with Hy/h = 0.4 having angle of attack 30°, whereas minimum reduction obtained was 4.79% for single vane with Hy/h = 0.3 and angle 20°. In general, for lower vane height (Hjh = 0.3) and lower vane angle (20») percentage reduction increased with the increase in number of vanes, while for intermediate (Hjh = 0.4) and higher (Hjh=0.6) vane height percentage reduction increased from single vane to two vanes but decreased for three vanes. Again, it was observed that for single vane the effect in percentage reduction increased with the increase of vane height for different vane angle. For two and three vanes it increased from Hy/h = 0.3 to Hv/h = 0.4 but decreased for Hv/h = 0.6. It was also found that for vane with HV/h= 0.3 percentage reduction increased with the increase of angle of attack but for Hy/h = 0.4 and Hy/h = 0.6, reduction increased from 20° to 30° angle but decreased for 40° vane angle. In general the vane arrangements with 30° angle of attack showed better performances than 20° and 40° vane angles in all combinations, while vane arrangements with Hv/h = 0.4 was better than Hv/h = 0.3 and Hy/h = 0.6, and two vanes combinations exhibited better reduction of scour depth than single and three vanes combinations. The bed morphology was analyzed critically by examining the 2D- contour map, lateral and longitudinal bed profiles for each test. It was found that placing of vanes in different arrangement not only reduced the maximum scour depth but also had changed the deposition pattern around abutment. In most of the cases large deposition occurred at a location situated at dwonstream comer of the abutment. Another observation from this investigation was that in rectangular channel with vertical wall abutment the vane arrangements could not move the maximum scour hole away from the abutment significantly. The flow field analyses were conducted by exammmg the variation of streamwise and transverse velocity along longitudinal, lateral and vertical direction. In general it was found that, different vane arrangements resulted a reduction in near bed velocity around the vicinity of abutment. In most of the cases, shifting away of maximum velocity was observed that caused shifting of maximum discharge flux away from the abutment. Analysis of velocity contour also revealed this trend of shifting m~ximum velocity away from the abutment. Changes in circulation pattern around abutment face were also discernible from the vector plotting of the resultant velocity in XY plane. It was concluded that all these changes in the velocity field had contributed to reduce the scour around abutment for different vane arrangement at different magnitude.