Study of Local Scour around Vertical Bridge Pier Groups in Presence of Debris Accumulation

One of the most important issues in scour depth at bridge piers is effect of debris raft which convened by flood and by blocking all or a part of the bridge opening, changes the flow pattern and increases the maximum scour depth around bridge pier considerably. In this paper, the effect of debris dimensions on a maximum scour depth around vertical bridge pier groups was investigated. The studied bridge piers consisted of two rectangular piers with 2.5×3.5 cm dimensions, which installed on the foundation with 10cm × 16cm width and length, respectively. The experiments performed consisted of relative depth (the depth to width pier ratio) 3.7, 5.2 and 6.6, debris relative width (the debris width to pier width ratio), 6, the relative length (debris length in flow direction to pier width), 2 and 3, the relative thickness (debris thickness to pier width), 0.5 and 1 and different levels of foundation. All experiment conducted under clear water condition. Comparison of results indicated that increase of flow depth did not considerably affect the local scour depth. By increasing debris relative thickness, the maximum scour depth increased about 42% in comparison with no debris raft. Furthermore,with increase of relative debris length, the effect reduced and increase of the maximum scour depth reached up to 17%. As compared to without debris raft condition. Comparison of results indicated that by increasing the level of foundation, increase of the maximum scour depth was more in presence of debris raft.