The Morphological Evolution Process And Dynamical Interpretation Of Mobile Dunes During Seasonal Freezing Period

Seasonal frozen soil plays an important role in controlling dune motilities in northern China.Studying the dynamics of mobile dunes morphology is the basis for clarifying the contribution rate of dunes during the freezing period to the annual wind-sand activities,which will provide a theoretical reference for evaluating the response of wind-sand activities in northern China for climate change.The study took the mobile and fixed sand dunes located in the East Desert of the Yellow River in Ningxia and the mobile dunes in the Ulanbu desert(Dengkou).Then completed continuous observation of meteorological elements,morphology,erosion-deposition pattern,movement rate,wind-sand flow structure and physical properties of the frozen layer,et al.The results and conclusions were as follows:(1)The freezing period of dunes was from mid-November to early March next year,there wezre distinct differences in freezing-thawing date and thickness of frozen layers among geomorphic sites of mobile dunes in the East Desert of the Yellow River in Ningxia(leeward slope>windward slope>top of dune).During the seasonal freezing period,there was a surficial unfrozen layer on fixed dune(Yanchi)which protected the frozen layer,and the thickness of it was about 10 cm.The thickness of frozen layer of top and leeward slope on mobile dune affected by short-term(32 h)and long-term(15 d)average temperature respectively.The results from field observation and laboratory experiment indicated that the dune could not be frozen with sand moisture content under 1.6%,and the hardness increased slowly with temperature decreasing.(2)The study area were mainly barchan and pyramid dunes in the East Desert of the Yellow River in Ningxia(Baijitan).The average moving speed of the barchan dunes was 2.33 m·a-1,and the average moving speed of the pyramid dunes was 3.27 m·a-1.They were slow rate and reciprocating motion.The shape parameter values of sand dunes showed a increasing trend during the freezing period,and the surface of the sand dunes was more stable compared to the unfrozing period.The moving rate of the ridge line in the freezing period was less than that in the unfrozing period.The top of dune erosion-deposition was strongest on pyramid dune.During the freezing period of the barchan dunes,the top of dune was erosion intension,and the middle,middle-upper,middle-lower of windward slope were in a state of accumulation,the toe of the windward slope is close to erosion-deposition equilibrium.During the unfreezing period of the barchan dunes,the top of dune,the middle and toe of the windward were accumulated,other parts were in a state of wind erosion.(3)The study area belongs to a low wind energy environment in the East Desert of the Yellow River in Ningxia(Baijitan),and its annual sediment discharge capacity was weak.The annual resultant sand-transporting direction was 347.15°,the sand drift potential was 24.72 VU,the synthetic sediment discharge direction was 345.69°,and the sediment discharge potential was 24.45 VU.The directional variability index was small(0.22),and the wind direction was variable,the synthetic sand transport direction during the freezing period was 299.83°,and the sand transport potential was 0.34 VU,accounting for 1.39%of the annual sand transport potential.The directional variability index was large(0.59).This showed that the wind direction during freezing period was single,the potential sediment discharge capacity was lower than the unfrozing period.The total sediment dischargeed by the 100 cm height sand collector during the observation period was 575 kg,of which the frozen period accounted for 17%of the total sediment dischargeed during the observation period.(4)The observation results of the wind-sand flow structure showed that the sediment discharge rate in the freezing and unfreezing periods follows the law of decreasing with height in Dengkou County,Inner Mongolia province,and the relationship between the sediment discharge rate and height conformed to the exponential function,power function,and logarithmic function.The sediment discharge rate at both the unfrozen period and the non-frozen period showed stratification at a height of 0.04 m.The sediment discharge rate during the freezing period showed a logarithmic increase with height,and the sediment discharge rate during the unfrozing period showed a power function increase with height.During the freezing period,the sand transport height is lower than that in theunfreezing period,the sand particles moved closer to the land surface.(5)Among the dynamic factors of dune morphology evolution,wind regime are the common factors that affect the dune morphology changes during the freezing period and the unfrozing period.The appearance of the frozen layer during the freezing period limited the amount of potential erodible sand sources,which made the sand in the wind sand flow closer to the land surface migration and saturation rang became longer.At the same time,due to the spatial difference of the frozen layer on the dunes,erosion-deposition pattern and morphological evolution of the dunes are different from those in the unfrozing period.However,in areas where the sand drift potential is small in winter,the freeze didn't limit the total amount of sand dunes transported.After wind erosion of the unfrozen layer on the land surface,the thawing cycle of the frozen layer was the main reason for maintaining a large proportion of total sediment discharge.