Mapping and Quantification of Gully Erosion Using UAV Photogrammetry in Imo State

Gully erosion in Imo State, South East Nigeria, has reached alarming proportions, with documented gullies exceeding twenty metres in depth and hundreds of metres in length threatening infrastructure and displacing communities, yet the spatial extent and volumetric dynamics of active gullies lack precise quantification for engineering remediation planning. This study maps and quantifies active gully erosion in three local government areas of Imo State using unmanned aerial vehicle photogrammetry and change detection analysis. Sequential UAV surveys at three-month intervals over a twelve-month period generated centimetre-resolution digital surface models and orthophotographs of selected gully systems. Structure from motion processing was conducted using Agisoft Metashape software, with ground control points established by differential GPS to achieve absolute accuracy. Multi-temporal digital surface model differencing was performed to compute volumetric erosion and deposition between survey epochs. Erosion rates were disaggregated by gully zone to identify headcut retreat, sidewall expansion, and floor incision as the dominant processes at each monitoring location. The influence of rainfall events on inter-survey erosion increments was analysed using daily rainfall data from proximate recording gauges. Gully stability classification criteria are applied to prioritise sites for engineering intervention. Results are communicated to the Imo State Erosion and Flood Control Authority and the Ecological Fund Office. Keywords: gully erosion, UAV photogrammetry, digital surface model, Imo State, volumetric change.