Estimation of maximum scour depth downstream of stilling basin (Case study: Masonary check dam of Ziarat basin)

Introduction
Masonry check dams are one of the common structures which used to regulate slope and control erosion in the watersheds. Due to the hydraulic head difference of water flows upstream and downstream of these structures, the flow has a lot of kinetic energy after passing through the structure that should be dissipated in the settling basin and before entering the river in downstream. Experience has shown that this phenomenon can make continuously erode the substructure and destroy it. One of the methods to control scour of downstream of the structures is using the stilling basin with appropriate length. The stilling basin is usually designed in such a way that the hydraulic jump is completely formed inside the stilling basin. However, due to the high energy of jet passing over the crest of check dam, scouring may still occur downstream. Looking to design of check dam shows that the there are two rows of pipes which can act during low flow condition in order of connectivity of upstream and downstream flow in rivers. During high flow condition the flow passes from the series of pipes and over the crest of check dam structures. Due to the fact that flow over the crest can interact with flow through pipes during high flow condition, therefore the flow pattern is complex and causes the scouring downstream of stilling basin.

Methodology
A physical hydraulic model of Tul Beneh with scale of 1:20 was used for simulating of scouring during different hydraulic conditions. The check dam height in prototype is 6 m and the check dam width is 20m. There are two rows of pipes with diameter of 0.6 m inside the body of checkdam structure. The experiments were conducted in a recirculating channel flume 9 m length, 1m width and 1m height. The side wall is made with glass to facilitate observing the scouring and flow pattern from side view. the flow depth was adjusted by tail gate. The sediment sample was taken from surface and subsurface of the river bed and size distribution of sediment was obtained with sieve analysis. The experiments were design in such condition that the flow can passes the series of pipes installed in two rows and over the weir structure. The flow discharge was measured by ultrasonic flow meter. The water surface profile and scouring pattern were also measured by 3D bed profiler instrument. The experimental results were also compare with different empirical formulas such as Borman and Julien (1991),Scurlock et al. et al. (2012), Fahlbusch(1994), Catakli et al et al. (1973), Novak(1961), Farhoudi and Smith (1982, 1985) and Dargahi (2003).

Results and Discussion
For assessing the time of experiments, a test was done for 48 hours and the temporal evolution of scour depth was measured. The results showed that 75 and 94 percent of maximum scour depth was occurred during 1 and 8 hours from the starting of experiments, respectively. Comparison of results with available empirical formulas showed that the Farhoudi and Smith (1985,1982) and Dargahi (2003) give more close results with the present results. The results also showed that the maximum scour depth occurred when the flow pass over the check dam structure. The comparison of results showed that when the flow pass through only the pipes the hydraulic jump formed in stilling basin and the energy of flow can be dissipated along the stilling basin. By increasing the flow, the flow pass through the pipes and over the crest of check dam structure. The sedimentation upstream of check dam causes the pipes filled with sediment, so the flow passes over the check dam and causes major scouring downstream of stilling basin. The results showed that there is sedimentation bar downstream of scouring region which can extend more when the flow passes both through the pipes and over the crest of check dam structure.

Conclusion
Complex flow pattern was observed when the flow passes through the pipes and over the crest of check dam structure. The scouring pattern showed that the scouring expanded both vertically and laterally when the flow pass over the crest of check dam. The ratio of maximum scour depth to downstream flow depth is 2.2 when the flow passes through the pipes and crest of check dam. This ratio is 0.66 when the flow only passed through the pipes. The maximum scour depth in prototype was 0.86 m which is 66 percent of downstream flow depth in river and shows that the flow passes through the pipes during the operation of check dam structure due to sand mining upstream of check dam structure.