Study On Mechanism Of Ice-cover Breakup And Criterion For Onset Of River-ice Breakup

The period of river-ice breakup is critical for controlling ice-flood. Study on the mechanism of mechanical river-ice breakup, and presenting the accurate and universally applicable criterion for onset of river-ice breakup, can provide valuable time for preventing ice-disaster and offer scientific basis for ice-flood control department. The disastrous flooding taken by ice-cover breakup is very serious and frequent in our country, so it is of great significance to study the mechanism of ice-cover breakup and the criterion for onset of river-ice breakup.Basing upon the mechanics principle, the mechanism of ice-cover longitudinal fracture during river-ice breakup is studied in this thesis by using the theory and calculating method of beams on elastic foundation. The result shows that the runoff increase and water-level rise will induce ice-cover deflection, and longitudinal cracks may be formed subsequently. There will be two longitudinal cracks near river-banks in most river, whose location mostly depends upon the thickness and elastic modulus of ice-cover.After the formation of longitudinal cracks, the central part of ice cover would be detached from banks. Then, the transverse cracks will be formed, which may be induced by horizontal bending or vertical bending of ice cover. The mechanism of ice-cover transverse fracture is investigated in this thesis. And the crack-spacing formed by the two kinds of bending is analyzed by calculating the bending moment and bending stresses of ice cover. The result shows that the transverse-cracks spacing is related to width, thickness and bending strength of ice cover and topography.According to the achievements of ice-cover fracture, based on the residual shear strength of ice cover at crack near river-banks, a side-resistance criterion is presented by introducing side-resistance condition, which can be used to appropriately forecast the onset of ice-cover breakup for straight reach; also, the boundary constraint criterion for onset of ice-cover breakup for crooked reach is presented by introducing the concept of boundary constraint, which can sufficiency reflect the effect of hydraulic condition, heat and topography factor. Subsequently, field observations of Yellow River at Hequ reach are further used to analyze and ascertain these two criterions. And the results show that the two criterions can appropriately forecast the onset of ice-cover breakup.In addition, a discrimination method for ice-breakup mode is presented in this thesis, which is in the form of water-level increment. Then, field observations of Zhao JunFen hydrology station are used to validate this method, and the results show that the method can appropriately discriminate the mechanical and thermal breakup of river ice cover.