| Floods caused by ice jams have caused a great damage to social economy and ecological environment.Ice jams on rivers usually occur in the springtime as the river ice begins to break up,but may also occur in early winter during freeze-up.Ice jams mostly occur in high latitude rivers that are located in cold regions due to thermal and hydraulic effect factors during the the period of river water freeze-up and break-up in the winter and spring.Ice jams are mostly formed in bend and narrow reaches due to special river channel morphology.The narrowness of bridge piers on river cross-section also increases the occurring probability of ice jams.Compared with the period of freeze-up,ice floods disasters are more destructive in the period of break-up.Ice production caused by heat loss between water surface and atmosphere determines the formation process of river ice and the blockage degree of ice jams.Ice jam floods are less predictable and potentially more destructive than open-water flooding because their formation time,location,accumulation position,and section of ice jams are hardly predicted.Thus,it has always been one of the academic research interests.The results showed that the upstream water level backwater caused by ice jams is related to the thickness and the internal resistance of ice jams.The Non-Darcy seepage is taken into account in the study of ice jam internal resistance for the first time,the mechanism of energy loss in ice jams is clarified by thermodynamics and ice dynamics,which can provide scientific basis for ice jam breaking and determining the relationship between water level and discharge in the river course.Because of its high latitude and special climate,Inner Mongolia reaches of the Yellow River has become the most serious reach of the ice regime.This study takes Inner Mongolia reaches of the Yellow River as the research object.Through Landsat remote sensing image and 3S technology,the change of the plane swing of the river course from Puweibu to Chahekou reaches of the Yellow River in the past 20 years is analyzed.The process of ice generation and thaw is monitored.Based on the non-linear and linear heat loss model,the heat loss coefficient of the Inner Mongolia reaches of the Yellow River during the period of freeze-up was calibrated,the ice production was estimated,and the ice jam process was simulated thermodynamically.Meanwhile,the formation process of the ice jams and the mechanism of energy loss inside the ice jam were studied through lab experiments.On the basis of revealing the formation mechanism of the ice jam in the period of freeze-up and break-up,a complete ice jam model in the period of freeze-up and break-up was improved.(1)The air temperature in Inner Mongolia reaches of the Yellow River has shown an overall upward trend in recent 56 years.The width of the river channel in the monitoring sections had been narrowed in fluctuation for nearly 20 years.Compared with 1995,the width of the main river channel in 2013 had narrowed by 32.24 m,with an average change rate of-9%.Sandy riverbed sediment erosion was intense with a change rate of-12%to-33%.The change rate of bend channel width was more obvious,such as the change rate of Shensifenzi bend reached-28%.The piers of river-crossing bridges located in the river channel not only narrowed the river section,but also changed the velocity distribution and hydraulic characteristics,increasing the probability of ice jam accumulation.(2)The heat loss coefficient was estimated as 21.38 W/(m2.℃)between water surface and atmosphere during the period of freeze-up.When the wind speed is greater than 4 m/s,there is a certain deviation from the results of evaporation and heat conduction radiation of Ashton’s non-linear heat loss model,and the contribution of wind speed to evaporation is greater than its effect on heat conduction.Based on the results of linear model,the heat loss from the upstream to the downstream showed an increasing trend during the ice freeze-up period in 2015.The daily open water area during freeze up period can be estimated based on ice run density and freeze up length,which error are less than 13%compared with remote sensing monitoring results.The total ice production of study area in 2015/2016 was 9.20×1010 kg.(3)Under the support of non-Darcy seepage theory,the laboratory ice jam experiments showed that the porosity in the ice jam was between 0.39 and 0.5,which was close to the field measurement result of 0.4.The energy loss in the ice jam was mainly caused by ice jam roughness,river bed roughness and seepage through the ice jam.The ice jam roughness had a non-linear correlation with the ice thickness.The seepage energy loss had a positive correlation with the ratio of the ice thickness to the water depth.The flow rate accounted for 10%to 19%of the total discharge flow,among which the seepage energy loss accounted for 65%of the total energy loss.(4)The seepage drag coefficient and the Reynolds number in the ice jam shows a approximate rational function of quadratic polynomials,and the best fitting range of Reynolds number is between 2300 and 3500The ice jam porosity is positively correlated with the seepage coefficient,the shape and size parameter for random accumulated cubic ice particles(3.81 x 3.81 x 0.64 cm)is 0.42.In conclusion,when the difference between water temperature and air temperature was between 6 and 8℃,floating ice appeared in the Inner Mongolia reaches of the Yellow River.Affected by the circumfluence of the channel bends,the initial ice jam accumulates at the contact zone of the main river channel and the floodland at the outlet of the curved channel.Influenced by the ice blocking effect of the piers,river ice might accumulate near piers,backup high water level,some reaches with high velocity water flow could form open water leads,where produce frazil ice due to heat transfer in the whole winter.The operation of the Haibowan Junction Project delayed the freeze-up time,reduced the ice volume in the downstream and decreased the ice disaster through flow regulation.The seepage resistance in the ice jam is important to the ice jam profile prediction and the water level determination.The relationship between seepage drag coefficient and Reynolds number is an important equation for determining seepage drag force according to turbulence intensity.Meanwhile,the particle coefficient of cubic provides a basis for simulating the seepage resistance by using Non-Darcy seepage law.Figure out the ice jam mechanism and the internal resistance of ice jam makes a contribution to the ice jam disaster prevention. |
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