Experimental study on the breaching of a moraine lake by overflow
Glacial lake outburst floods(GLOFs)are a unique type of natural hazard in the cryosphere that may result in catastrophic fatalities and damages.The Himalayas and Nyainqentanglha are second in the world regard-ing the frequency and severity of the GLOFs after the Peruvian Andes.Slope movements such as ice avalanches,landslides,and rockfalls have been blamed for most GLOF triggers.Despite the intuitive simplicity of the idea behind catastrophic erosional incision by a large wave,this explanation has many possible difficulties.The mo-raine-dammed lakes are shallow-long lakes,and the solitary displacement wave is the only type wave may be produced by massflows.The overtopping of the moraine dam may be only the first solitary displacement wave because the amplitude damping of the wave is more than 63%during two reflections if the losses of reflections were not considered.However,the erosion is usually a slow process on the hydrodynamic timescale that charac-terizes the passage of a displacement wave,it is not sufficient that erosion could occur during the relatively fast traversal of the dam.The erosion caused by a single wave was too small to breach the dam.However,it was far easier for a constant water flow to breach the dam by overflowing than for erosion to breach the dam by wave overtopping.A giant wave must be generated by a giant volume of ice avalanches or landslides entering a lake,and result a large water level increasing.The overflowing high water level was the reason for the breaching of moraine dams.Six moraine lakes,drained or undrained,were investigated in Xizang,China.The particle size of armor layers on the surface and the slope angle of these moraine dams and their spillway were measured.A se-ries of laboratory experiments on armor initiated by overflow in the spillway of the moraine dam were conduct-ed.The four statuses of moraine lake were simulated during the experiments.The moraine dam drained when the armor was removed by overflow.The initial model and the critical water depth of the moraine dam breaching were obtained.The initial armor models were the fluvial transport and bed failure models according to the bed slope range.The initial armor model was the fluvial transport model when the bed slope was less than 19.6°.The bed failure model occurred when the bed slope was over 25.2°.When the bed slope was in the range of 19.6°～25.2°,the initial model was a mixed regime.The depth of overflow on the armor and the gradient differ-ences between the upstream and downstream of the knick point were the key parameters to initiate the armor in the fluvial transport model.The relative degree of exposure at the knick point was less and resulted in the first coarse particle initiated by the fluvial transport model occurring at the knick point.The larger the gradient differ-ence between the upstream and downstream of the knick point,the smaller the relative degree of exposure value was.The ratio of the moraine dam's critical water depth to the armor diameter was inversely proportional to the gradient differences between the upstream and downstream of the knick point in the fluvial transport model.For the bed failure model,the depth of overflow on the armor,the internal friction angle of saturated armor,the bed slope of armor,and the saturated density of the armor were the key parameters to initiate the armor.For the bed failure model of the armor initiation,the ratio of the moraine dam's critical water depth to the armor's diameter was a fixed value.The moraine dam breaching prediction model was validated reasonably well by the field data in Xizang,China,and British Columbia,Canada.
万方数据
维普
