Impact of gabion sill on scouring depth downstream grade control structures

• Gabion sill (GS) at L g /h=1 reduce scour length by 51% and depth by 44% under free tailwater depth (TwD). • GS by relative spacing of L g /h=1.5 is the most effective to reduce scour depth at a high flow discharge. • By increasing TwD to 2y′ t reduces scour depth by 25% on average. • GS accelerate stabilization, by falling scour equilibrium time from 6 to 2 hours. • Experimental relationships assess scour hole for grade control structures with GS. Grade control structures (GCS) stabilize riverbeds and reduce sediment movement, but downstream scour threatens their stability. This experimental research investigates the effects of gabion sills (GSs) on scour at three relative spacings (L g /h=1, 1.5, & 2) under five discharges and three tailwater depths (TwDs) (freely=y′ t , 1.5y′ t , & 2y′ t ). A series of experiments conducted in an 18 m-long, 0.3 m-wide tilting flume with a discharge range of 7–13 L/s. The observations demonstrated that the placement of gabions significantly accelerated the bed stabilization process and effectively reduced both the depth and length of the scour hole. The results establish that positioning the GS at a relative spacing of L g /h=1 reduces the scouring length and depth by an average of 51% and 44%, respectively, compared to the GCS without a GS under free TwDs conditions. For a case of L g /h=1.5, those reductions were 44% and 49%, respectively, while for the case of L g /h=2, they were 36% and 7%, correspondingly. The L g /h=1 configuration is recommended for controlling scour length, while L g /h=1.5 has the most effective among cases in reducing scour depth at high discharge. The findings indicated that an increasing TwD contributes to reducing scouring dimensions. Specifically, in the absence of GS, increasing TwD to 2y′ t led to an average reduction of 19% in scour length and 25% in scour depth. A comparison of TwD effects among different GCS models with GS showed that increasing TwD to 2y′ t in the L g /h=2 GS configuration was the most effective, achieving a 30% reduction in scour depth.