Effects of urban expansion on gully landform evolution in the Dongzhiyuan loess tableland of the Chinese loess plateau

Gully erosion has been recognized as one of the most impactful processes in sediment production and land degradation in a wide range of environments. To quantitatively characterize the dynamics of gully erosion and elucidate the impacts of urban expansion and rainfall, a typical eroded gully near the urban area of Qingyang City (35°44'24"N, 107°39'03"E) in the Dongzhi tableland of the Chinese Loess Plateau was selected. Seven aerial and satellite imageries between 1964 and 2017 were obtained and the techniques of GIS and RS were applied to analyze the geometry of the gully landform based on these images. Moreover, the relationships between gully erosion, urban expansion and rainfall were analyzed. Results showed that: (1) The studied gully (Huoxiang gully) advanced 3.1 m annually during the period of 1964~1969, and that change was mostly caused by agricultural activities. During the period of 1970~2002, the gully head retreated 359.8 m, which caused by backfilling the gully channel. In 2003~2017, the gully channel was further backfilled due to urban development, which led to the gully head retreated 178.3 m; (2) Although gully headcut has been prevented through human intervention, the gully banks have continuously expanded on both sides. In the period of 1964~1966, the average rate of gully bank expansion was 1.1 m/a, and it was 1.4 m/a in 1967~1969. In 1970~2002, the gully bank expansion rate was 1.1 m/a and then increased to 2.9 m/a in 2003~2008. In 2009~2014, the expansion rate increased to 8.0 m/a, and it was 2.7 m/a in 2015~2017; (3) There was a significant positive relationship between gully bank expansion and urban area development. In 1964, the urban area of the Xifeng District was only 2.1 km² and increased to 19.8 km² in 2017, which was a 9-fold increase during the 54-year period. Moreover, the contact area between the gully head and the urban was small in 1964, while the gully was totally surrounded by the urban area in 2017, which indicates that the increase of impervious land surface in the urban area would profoundly increase the surface runoff and thus increase the erosion of the gully banks; (4) Based on the current data, we did not find significant relationship between rainfall and gully bank erosion, while acquiring high-resolution time series of gully bank erosion and rainfall data is crucially important in the future research. Results of this study have improved our understanding of gully landform evolution and the effects of urbanization on the tableland area of the CLP.