Assessing the impacts of lithology on short-term gully evolution within the karst Mediterranean area

Abstract

Gullies exhibit rapid short-term changes, with lithology serving as a primary predisposing factor influencing their gradual evolution. However, the relationship between lithology and gully evolution in complex karstic environments remains poorly investigated. This manuscript focuses on assessing the impact of lithology on short-term gully evolution, with special emphasis on detection of one-year gully erosion induced spatio-temporal changes (STCs). The study was carried out within the lithologically versatile karst area of gully Santiš, located within the Pag Island, Croatia. Repeat UAV photogrammetry was used for aerial surveys carried out on December 17th, 2019 and December 17th, 2020. Detection of gully erosion induced STCs was based on creation of a very-high resolution (VHR) digital elevation model of difference (DoD) with 1 cm spatial resolution. Additionally, a detailed lithological map (1:50) was created through comprehensive field lithological mapping. Nine different lithological units were mapped, with DoD enabling the detection of sub-decimeter STCs. Short-term gully evolution (2019–2020) resulted in an eroded material rate of 13.46 m3·yr−1. The highest erosion rates (6.80 m3·yr−1) were detected within Carbonate sandstones and sands, where gradual selective erosion led to the collapse of a large part of the sandstones. Significant erosion rates (4.84 m3·yr−1) were also detected within Calcocambisol soils, primarily due to the mass wasting and gradual uphill retreat of the gully headcut. The other seven mapped lithological units (e.g., Limestone, Quaternary breccia and Talus) accounted for only 13 % of all detected erosion, with an average erosion rate of 0.22 m3·yr−1. Highest relative erosion rates were observed within Carbonate sandstone and sands (6.38 10−3 m3 yr−1 m−2), Calcocambisols (2.01 10−3 m3 yr−1 m−2) and Talus (7.29 10−4 m3 yr−1 m−2). This study confirms that short-term gully evolution within karstic relief is highly influenced by lithology, directly controlling the intensity and distribution of erosion processes. The combined use of repeat UAV photogrammetry derived VHR DoD and detailed field lithological mapping demonstrates great potential for interpreting short-term gully evolution. Therefore, this methodology can be applied to monitor short-term gully evolution in other lithologically complex areas.