بررسي آزمايشگاهي الگوي فرسايش و رسوبگذاري در محل تلاقي رودخانه اي غيرهمكف

Experimental Investigation of Erosion and Sedimentation at Unequal Bed Level River Confluence

Introduction Due to the three dimensional flow structure at river channel confluences, especially at the downstream flow mixing zone a deep scour hole is formed. At the separation zone downstream of the junction comer a point bar is also developed. The seour hole and the point bar can cause changes in river morphology and can be the main reason for bank instability or failure of the structures near the confluence area. In nver confluences SIX different regions are distinguished, as shown in Fig (I). Locations and dimensions of regions 1 and 3 and flow characteristics in regions 2 and 4 depend on many factors. As region 3 becomes larger due to sedimentation, the flow velocity in region 4 will be increased and the scour dimensions become larger. This will cause bank failure and create a meander at this location. The flow vortex developed in this region can also cause many difficulties for navigation. According to these phenomena, the study of flow pattern and scour hole in river confluences, has attracted the attention of many researchers. Most of these studies have however been focused on river confluence of equal bed level. Although an unequal river bed level at the river junctions are common in nature, the scour and sedimentation studies at these junctions have not received the attention of researchers due to the complexity of the experimental setup. It has been assumed that the bed level has no significant effect on the scour and the sedimentation patterns. However it has been reported by a few investigators that the flow patterns and the zone of maximum shear stress in junctions of unequal bed level is different from that of equal bed level cases, (Best, 1987; Ghobadian and Shafai Bejestan, 2007). Objective The main objective of this study is to investigate the dimensions of the separation zone at river confluences of unequal bed levels and to develop relations for prediction of these dimensions compared to the case of equal bed level Melhodology An extensive experimental study was conducted in the hydraulic laboratory of the Shahid Chamrao Uoiversity. By applying the dimensional analysis theory, a dimensionless relation is developed as shown in Eq.(I): ~=f(&,-.!...-) d,o QI B] where cD is a symbol for maximum scour depth (H,) and height of sediment deposition downstream of scour hole (Hb) , d,o is the mediao sediment size, Q, aod Q, are respectively the discharges in the main and lateral channel, z is the difference of the bed levels at the river confluence, and 8] is the river bed level downstream of the confluence. Results and Discussion Effect of Ihe discharge ratio: The variation of (H,/d,o) and (Hi/d,,) was plotted against the discharge ratio Q, for the experimental data. The results indieated that both these dimensionless parameters are directly proportional to the diseharge ratio. Effect of Ihe river bed level: Variation of (Hjdso) and (Hi/d,o) against the ratio of ZIB, showed that the bed level can generally decrease both the scour depth and the height ofsediment.