EXTRACTING FEATURES OF ALLUVIAL FAN AND DISCUSSING LANDFORMS EVOLUTION BASED ON HIGH-RESOLUTION TOPOGRAPHY DATA: TAKING ALLUVIAL FAN OF LAOHUSHAN ALONG HAIYUAN FAULT ZONE AS AN INSTANCE
Range-front alluvial fan deposition in arid and semiarid environments records vast amounts of climatic and tectonic information. Differentiating and characterizing alluvial fan morphology is an important part in Quaternary alluvial fan research. Traditional method such as field observations is a most important part of deciphering and mapping the alluvial fan. Large-scale automatically mapping of alluvial fan stratigraphy before traditional field observations could provide guidance for mapping alluvial fan morphology,thus improving subsequent field work efficiency. In this research,highresolution topographic data were used to quantify relief and roughness of alluvial fan within the Laohushan. These data suggest that mean surface roughness plotted against the size of the moving window is characterized by an initial increase in surface roughness with increased window size,but it shows no longer increase as a function of windows size. These data also suggest that alluvial fans in this study site smooth out with time until a threshold is crossed where roughness increases at greater wavelength with age as a result of surface runoff and headward tributary incision into the oldest surfaces which suggests the evolution process of alluvial fan. Researchers usually differentiate alluvial morphology by mapping characteristics of fan surface in the field by describing surface clast size, rock varnish accumulation, and desert pavement development and analysis of aerial photographs or satellite imagery. Recently,the emergence of high-resolution topographic data has renewed interest in the quantitative characterization of alluvial and colluvium landforms. Surface morphology that fan surface initially tends to become smoother with increasing age due to the formation of desert pavement and the degradation of bar-and-swale topography and subsequently,landforms become more dissected due to tectonics and climatic change induced increased erosion and channelization of the surface with time is widely used to distinguish alluvial fan types. Those characteristics would reflect various kinds of morphology metrics extracted from high-resolution topographic data. In the arid and semiarid regions of northwestern China,plenty of alluvial fans are preserved completely for lack of artificial reforming,and there exists sparse surface vegetation. In the meantime,range-front alluvial fan displaced by a number of active faults formed a series of dislocated landforms with different offsets which is a major reference mark in fault activity research. In this research,six map units( Qf6Qf1 ),youngest to oldest,were observed in the study area by mapping performed by identifying geomorphic features in the field that are spatially discernible using hill-shade and digital orthophoto map. Alluvial fan relief and roughness were computed across multiple observation scales( 2m*2m to 100m*100m) based on the topographic parameters of altitude difference and standard deviation of slope,curvature and aspect. In this research,mean relief keeps increasing with increased window size while mean surface roughness is characterized by a rapid increase over wavelengths of 6 ~ 15m,representing the typical length scale of bar-and-swale topography. At longer wavelengths,surface roughness values increase by only minor amounts,suggesting the topographic saturation length is 6 ~ 15m for those fan surfaces in which saturation length of standard deviation of curvature is less than 8m.