Experimental investigation of the effect of height and crest slope of bendway weirs on flow pattern in a 90 degree bend

Bendway weirs are an important tool in multi-purpose erosion control and are implemented to control erosion, restore streams, and improve habitat. These structures create a hydraulic effect that reduces erosion on the outer bank of a bend by reducing flow velocities near the outer bank and reduce the concentration of currents on the outer bank creating better current alignment through the bend. In this research the effect of bendway weirs on flow pattern in a 90 degree mild bend has been investigated experimentally. Experimental tests were conducted in a curved flume with relative radius of 3.3 (R/b=3.3) and with 24 m length and 0.92 m width at three flow discharges (55, 66 and 77 l/s). Series of bendway weirs (7 weirs) were constructed at the outer bank side of the bend at three weir height ratios (0.3, 0.5 and 0.7 of water depth), three crest slopes (0%, 5% and 10%), while inclination angle (60°) and distance between weirs (equal to 3 times of weir length) were kept fixed. Velocity measurements around weirs were performed in two levels, near the bed and close to the water surface. The research results showed that the velocity magnitude decreased in the outer bank by more than 70% and increased in the inner bank side by more than 50%, after bendway weirs installation. Weirs with height of 0.5y and with crest slope of 5% were more effective in deceleration of velocity at the outer bank side. On the other hand, weirs with height of 0.7y and also weirs with flat crest slope were more efficient in velocity magnitude increment at the inner bank side, especially in the head of the bend. Generally, the present research results revealed that the bendway weirs with crest slope of 5% and with height of 0.5y are the most suitable design options in such bends.