Local scour calculation around bridge pier during flood event: Fid-Bau Portal

Local scour calculation around bridge pier during flood event

Tabarestani, Mojtaba Karimaee / Zarrati, Amir Reza

Abstract In the present study, local scour around bridge pier during flood event especially in flashy hydrographs is investigated, experimentally. The duration of tested hydrographs were deduced from different ephemeral rivers in nature using physical modeling concepts. Final scour depth as well as scour hole width and time development of scour depth were recorded by highresolution cameras. The effects of flood hydrograph rising as well as falling limb and peak discharge on local scour were studied. Results showed that, for hydrographs with similar duration and peak discharge, the influence of time for hydrograph rising limb (T r ) on final scour depth is not of great importance. On the other hand, the scour depth slightly decreased after the hydrograph peak due to the slide of the scour hole slopes and deposition of some of suspended sediments. The maximum decrease was about 5% of the pier diameter. In addition, it is shown that the effect of time for hydrograph falling limb (T f ) on scour depth and width is closely related to parameter T f /T r . For example, final scour depth for a typical hydrograph with T f /T r = 9.25 is about 42% larger than similar hydrograph with only a rising limb. Overall, experimental results revealed that hydrograph peak discharge plays a much more important role than other parameters in affecting the development of scour hole. Based on the present as well as the previous experimental results, a new modification factor is added to HEC-18 empirical equation to calculate final scour depth during a hydrograph event. Finally, by using stepped-hydrograph model, time development of scour depth was calculated for the present tested flashy hydrographs which showed acceptable accuracy of this method.