Enhancing scour protection in river bends: a novel slotted bank-attached vane

One of the most effective approaches for bank control erosion is using bank-attached vanes. In spite of the superiority of the bank-attached vanes to spur dikes, the vanes’ tips are still vulnerable to local scour caused by flow–structure interaction. In this study, slotted bank-attached vanes are proposed to reduce local scour at the tip of the triangular submerged vane. For this, a rectangular slot is created parallel to the chord of the vane with an area of ten percent of the effective area of the vane surface. Two types of conventional vanes and slotted vanes were installed at different angles of attack of 23, 30, 40 and 60 degrees in an arch flume. Experiments were carried out in clear water conditions with different flow regimes with Froude numbers of Fr1⁄4 0.287, 0.304 and 0.322. The results show that the slotted vane outperforms the conventional vane by reducing maximum scour depth by about 70, 20, 17 and 54 percent for different angles of attack of 23, 30, 40 and 60 degrees, respectively. The proposed slotted vane also resulted in reduction of scour hole volume around the vane and formed the scour hole away from the outer bank.


INTRODUCTION
The main drawback associated with using submerged vanes is the local scour around their tips which may cause destruction of the structure. This paper aims at introducing a novel approach to reduce the local scour around the vane's tip and to increase the effectiveness of the triangular submerged vane in protecting riverbank erosion. This is achieved through creating a slot in the structure to modify the flow pattern around the submerged vane. Although using a slot to control local scour around a bridge pier was first introduced by Chiew (), to the best of our knowledge, there is no study in the literature investigating the effect of the slot on scour around a submerged vane.
In this study, the effect of a slot in a submerged vane on bed topography and its skew angle is experimentally investigated. Results show that the proposed slotted vanes are superior to conventional vanes in protecting a bank from erosion by reducing the maximum scour depth.

MATERIALS AND METHODS
The experiments were conducted in a single-bend flume All experiments were carried out for both cases of conventional and slotted triangle vanes. The conventional and proposed slotted triangular vanes are shown in Figure 2. As shown in Figure 2 (a 2 ), a rectangular slot is created on the vane parallel to the chord. The slot surface is equal to ten percent of the effective surface of the vane, which is defined as the surface that is exposed to the approaching flow. The ratio of length to width of the slot is considered as 4 a b ¼ 4 . As illustrated in Figure 1, the effective length is the perpendicular distance between the vane's tip to the outer bank, which is taken as hours to be completed and the water is then gradually drained out of the flume to avoid any disturbance in the bed topography. The bed profile is then measured by using a laser distance meter (Bosch GLM 30) with an accuracy of 1 mm, which is installed on a rail on the top of the flume.

Scour morphology
The bed morphology and contours of the scoured beds for both cases of slotted and conventional vanes, for a constant  indicates that the slotted vane has superior performance to conventional vanes in decreasing scour, regardless of angle of attack. The superiority of slotted vanes to conventional vanes was observed not only for Fr ¼ 0.322, but also for all hydraulic conditions considered. It is also obviously seen from Figure 4 that the maximum scour depth is reduced by using the slotted vane for all cases.

Characteristics of scour hole
Determining maximum scour depth is of great importance in design of in-stream structures. This, therefore, necessitates the careful investigation of the location and the depth of maximum scour. However, prediction of maximum scour depth and its location is complicated especially in river bends where helical flow is formed due to the presence of centrifugal forces. Since the maximum scour depth plays an important role in the stability of the in-stream structures, it could be concluded that the slotted vane is much more stable than the conventional vane.
The location of maximum scour depth and its distance from the bank of the river are also crucial parameters needing to be considered besides the maximum scour depth.

Characteristics of deposition zone
The process of scouring the outer banks in river bends is always associated with deposition of point bars on the      values. For this, the performance of the vane is defined as the ratio of maximum deposition (H max ) to maximum scour (Z max ), which can be mathematically formulated as The higher the value of P v , the better the performance of the vane.
The results of performance evaluation of the proposed slotted vane are presented in Table 3 and compared with those obtained for conventional vanes, for different angles of attack (α) and various hydraulic conditions. The table indicates that the proposed slotted vane performs more efficiently than conventional vanes regarding the value of P v for all cases considered in this study. In other words, the slotted vanes have been able to deposit the eroded sediments with proper height at the outer bank.

DISCUSSION
The presented results show that by using the slotted vane the scouring potential is significantly reduced. The performance of the slotted vane in decreasing scour potential can be attributed to either diverting downflow away from the bed, or creating an upward flow pattern.
There are three types of approaching streamlines to the vane illustrated in Figure 7. The streamlines with higher elevation than slot elevation collide with the vane forming a downflow which has the main impact on scouring potential.
A part of such streamlines is deflected to the downstream of the vane through the slot, before hitting the bed, and the