Medium-term development of discontinuous gullies

Abstract

Intense gully erosion has sculptured remarkable channels into the Moldavian Plateau of eastern Romania, especially in its most representative subunit, the Bârlad Plateau covering >8000 km2. The permanent gully types are: (1) discontinuous gullies, mostly located on hillslopes and (2) large continuous gullies in valley bottoms.

This study seeks to improve our understanding of the development of discontinuous gullies over variable time-scales (mostly 17–30 years, but also including data collected since 1961) by providing quantitative information on gully evolution and processes. Several methods were used to accurately measure and estimate gully growth. These include intensive field monitoring between 1978 and 2000 using the ‘stakes grid method,’ repeated levelling until 2019, analysis of aerial photographs and Caesium-137 analysis.

The discontinuous gullies occur as single, successive (chains) or clusters. These are associated with small catchments (usually <100 ha in area) and ephemeral peak runoff discharges are usually ≤2 m3 s−1. The ‘hydraulic radius at bankfull-channel’ tended to decrease downstream of the gully head, accompanied by a gradual increase in gully bottom width. A salient tendency observed during major runoff events was for the hydraulic radius of the flow to progressively decrease downslope, accompanied by decreasing sedimentation rates on gully floors. The mean linear gully head retreat for 31 gullies was 0.97 m yr−1, indicative of a relatively slow erosion rate. However, their ‘pulsatory’ development was mostly controlled by runoff accommodation when runoff enters and is conveyed through a gully. We especially discuss the changing runoff pattern or ‘variable-geometry flow.’ The R2 of the association between linear gully head retreat or areal gully growth and catchment area indicated a weak correlation for discontinuous gullies.