What kind of gully can develop into benggang?

•Catchment area and slope drop have close relationship with benggang formation.•Two equations were constructed to estimate benggang formation.•Collapsing wall height is an important indicator of benggang formation.•Benggang is shown to differ from gully in catchment area, lithology and shape. Benggang is a special landform with serious erosion damage, resulting in intensive soil erosion and land degradation. However, few studies have been conducted on prediction and critical conditions of benggang formation. This study analyzed the topography, hydrogeological conditions, number and size of benggang in Yuankengshui watershed (Guangdong, China) by extracting a digital orthophoto map (DOM) and digital surface models (DSMs) based on unmanned air vehicle (UAV) aerial images. The benggang number was found to increase first and then decrease with the increase of elevation, slope gradient and slope length, in contrast to a decrease with the increase of benggang catchment area. The elevation distribution is consistent between the small watershed and regional benggang, with the maximum benggang density in the elevation range of 200–250 m. The maximum benggang number is in the area with a slope gradient and slope length of 25-26° and 61–70 m, respectively, and the average benggang catchment area is 0.3 ha. Benggang formation and development was mainly affected by benggang catchment area and slope drop, whose correlation coefficients with benggang area and collapsing wall height were 0.957, 0.633 and 0.581, 0.568, respectively (P