Experimental Study on Scour at 90° Horizontal Forced Bend and its Protection Using Riprap

Abstract

Experimental investigations were done to study the scour at bend and its protection using stone riprap. The experiments were carried out in a 90° horizontal bend flume with non-cohesive bed sediments having median diameter 0.34mm and 1.18mm. The cross-section of flows was made trapezoidal in shape with sediments at the bottom and sides. The results of the present study indicate that the scour occur mainly at the outer bank of the 90° horizontal bend. Maximum scour and deposition occur around 30° azimuthal section near the concave and convex bank respectively. Upstream at the 0° section, small scour near the inner bank was observed. These may be due to cross current towards the concave bank. Along downstream, the depth of scour increases near the concave bank and reaches its maximum value around the 30° section. Further downstream, the depth of scour decreases along concave bank but little scour was noticed along the convex bank downstream of 70° section. On the other hand, the deposition occur near the lower part of convex bank from 20° to 60°. There exists a transverse slope of the water surface across the section with higher level at the concave side. As a result of this transverse slope, the bottom water moves from the concave bank to the convex bank whereas, the surface water moves from convex to concave. It also carries with it the bed material and deposits it near the convex bank, Further, it is found that flexible riprap placed on the sloping bank extending little beyond the toe can be used to protect the bend erosion due to flow. The data obtained in the experimental investigation will be useful for the proper design of protection works at bends.