An Experimental Study Of The Hydrodynamic Properties Of The Dam-break Induced Bore Run-up Process

Taking into account the breaking types,bores can be classified as undular bores and breaking bores.Undular bores consist of a series of short waves which are smooth and non-breaking without white caps.Breaking bores take the form of a highly turbulent bore front followed by a stream which is basically flat although covered by irregular disturbances.The bore propagation process to neashore area is smilar to that of tidal bore and tsunami.Therefore,studies of the hydrodynamic characteristics of these two types of bores are crucial for the in-depth understanding of the neashore propagation of tidal bore and tsunami,which are important for the coastal disaster prevention and mitigation efforts.Using a horizontal dam-break flume with a 1:7.5 slope,a series of experiments were conducted.Different ratios of upstream to downstream water level were applied to generate undular bores and breaking bores with different propagation velocities.Particularly,dry-bed conditions were also conducted for validation.Wave gauges,Electromagnetic current meter(ECM)and High speed video camera(HSVC)were used to investigate bore hydrodynamic and run-up characteristics,e.g.,the wave height,wave profile,propagation velocity,run-up duration,maximum run-up height,instantaneous bore front velocity and position.This study shows that undular bores present similar hydrodynamic and run-up characteristics as the solitary wave,which,in terms of physical mechanisms,are clearly different from the breaking bores.Under the downstream wet bed condition,the breaking bore propation velocity accords well with the theoretical result.Meanwhile,energy conservation was confirmed during the breaking bore run-up process from the beach toe(kinematic energy)to the run-up limit(potential energy).Under the same initial water head,water depth of breaking bore front at the shoreline of wet bed is much larger than that of dry bed case.As for the breaking bore run-up process under the wet and dry bed conditions,a linear relationship with the uniform gradient is found between the maximum run-up height and the initial water head for both conditions.Two successive bores were observed under the wet bed run-up process,while multiple bores(three bores in general)were generated during the dry bed run-up process due to the significant bottom friction effect.Under the same initial water head,although the dry bed run-up process takes a longer duration than that of wet bed cases,the maximum run-up height is smallest for the dry bed case.Under the wet bed conditions,temporal variation of the bore front run-up velocity can be classified into two stages,i.e.,the acceleration stage induced by the relatively large incident bore front water depth(resulting in onshore hydrostatic pressure gradient)and the deceleration stage governed by the offshore-directed gravity force and bottom friction.Nevertheless,due to the small incident bore front water depth,run-up process under the dry bed conditions does not show the acceleration stage.