Assessing the Impacts of Lithology on Short-term Gully Evolution using the Very-high Resolution UAV Photogrammetry

In this study we focus on the assessment of impacts of lithology on short-term gully evolution, with special emphasis on detection and quantification of one-year gully erosion induced spatio-temporal changes (STCs) within different lithological units. Study was implemented within the lithologically versatile wider area of gully Santiš (13 881.613 m²), located within the karst relief of the Pag Island (Croatia).

Repeat UAV photogrammetry was used for creation of very-high resolution (VHR) digital surface models (DSMs) of study area, where identical aerial surveys were carried on December 17^th, 2019 and December 17^th, 2020. Detection of gully erosion induced STCs was based on creation of digital elevation model of difference (DoD) from two created multi-temporal DSMs, using the Geomorphic change detection 7 software. Detailed lithological map (1:50) of whole study area was created through the comprehensive field lithological mapping, that was carried on September 16^th, 2020.

In total nine different lithological units were mapped within the chosen karstic study area and VHR DoD with spatial resolution of 1 cm was created, allowing the detection of sub-decimeter STCs. Short-term gully evolution (2019-2020) resulted with the 13.46 m³ of eroded material, with very high variability of erosion intensity between certain lithological units. Highest erosion rates (6.80 m³) were detected within carbonate sandstones and sands, where gradual selective erosion has caused collapse of larger part of the more resistant sandstone. Significant erosion rates (4.84 m³) were also detected within Calcocambisol soil sediments, which were mainly caused by the mass wasting and gradual uphill retreat of the main gully headcut. Other seven mapped lithological units (e.g. limestone, quaternary breccia, talus, etc.) accounted for only 13% of all detected erosion (average erosion rate = 0.22 m³), thus confirming the lower erodibility to gully erosion of these more resistant units.

Carried study has confirmed that short-term gully evolution within specific karstic relief is highly affected by lithology, which directly controlls the intensity and distribution of erosion processes. Combined use of repeat UAV photogrammetry derived VHR DoD and detailed field lithological mapping has shown great potential for detection and monitoring of complex STCs and interpretation of short-term gully evolution. Due to that this methodology can be applied for monitoring of short-term geomorphic evolution in other lithologically complex areas.