Seasonal change of tensile crack morphology and its spatial distribution along gully bank and gully slope in the Mollisols region of Northeast China

Gully bank expansion is a critical process of gully development, with tensile cracks development being a necessary condition for gully bank instability and collapse. However, there has been a lack of research on the morphological characteristics of tensile cracks and their distribution along gully bank and slope, which hinders our understanding of gully expansion and its underlying mechanisms. This study aims to address this knowledge gap by investigating the temporal changes in the morphology and spatial distribution of tensile cracks in the Mollisols region of Northeast China. We conducted field investigations at three times: 2020-10 (before freeze--thaw), 2021-05 (after freeze-thaw), and 2021-10 (after rainy season). Morphological and topographical in-dicators, as well as the locations of each crack, were taken into determination, and an unmanned aerial vehicle (UAV) was employed to capture images of the gullies for clarifying the distribution of tensile cracks. A total of 284 cracks were found with 66.9 % of them on the gully bank and 33.1 % on the gully slope, with their average length, width, and depth of 4.69 m, 8.88 cm, and 20.55 cm, respectively. The length of cracks after freeze-thaw increased by 12.8 %, while the width and depth decreased by 31.2 % and 32.5 %, respectively, and the width of cracks increased by 16.5 % but their length and width decreased by 14.8 % and 24.8 % after rainy season, respectively. The cracks in the rainy season were mainly extinction, and the total length of the cracks is 86.3 % higher than that in the freeze-thaw period, while the freeze-thaw period is dominated by the new formation of cracks with the length of the newly formed cracks being 28.3 % higher than that in the rainy season. Most cracks were distributed in the middle and lower part of gully, and the cracks were closer to the gully head and the gully channel centerline after freeze-thaw. The length, width, and depth of cracks showed a significantly negative correlation with upstream slope, and they were significantly and positively related with gully slope and gully-slope length. Our results provide critical insights into the nature and severity of cracks, lending vital support to the study of gully bank expansion.