Long-term monitoring (1953–2019) of geomorphologically active sections on LIA lateral moraines under changing meteorological conditions

Abstract. We show a long-term erosion monitoring of several geomorphologically active gully systems on Little Ice Age lateral moraines in the central Eastern Alps covering a total time period from 1953 to 2019 including several survey periods in order to identify corresponding morphodynamic trends. For the implementation, DEM of Differences were calculated based on multitemporal high-resolution digital elevation models from historical aerial images (generated by structure-from-motion photogrammetry with multi-view-stereo) and light detection and ranging from airborne platforms. Two approaches were implemented to achieve the corresponding objectives. First, by calculating linear regression models using the accumulated sediment yield and the corresponding catchment area (on a log-log scale), the range of the variability of the spatial distribution of erosion values within the areas of interest is shown. Secondly, we use volume calculations to determine the total/mean sediment output (and erosion rates) of the entire areas of interest. Subsequently, a comparison is made between the areas of interest and the epochs of both approaches. Based on the slopes of the calculated regression lines, it could be shown that the highest range of the variability of sediment yield within all areas of interest is in the first epoch (mainly 1950s to 1970s), as in some areas of interest sediment yield per square metre increases clearly more (regression lines with slopes up to 1.5), which in the later epochs (1970s to mid-2000s and mid-2000s to 2017/2019) generally decreases in 10 out of 12 cases (regression lines with slopes around 1). However, even in the areas of interest with an increase in the variability of sediment yield over time, the earlier high variabilities are no longer reached. This means that the spatial pattern of erosion in the gully heads changes over time as it becomes more uniform. Furthermore, using sediment volume calculations and corresponding erosion rates, we show a generally decreasing trend in geomorphic activity (amount of sediment yield) between the different epochs in 10 out of 12 areas of interest, while 2 areas of interest show an opposite trend where morphodynamics increase and remain at the same level. Finally, we summarise the results of long-term changes in the morphodynamics of geomorphologically active areas on lateral moraines by presenting the "sediment activity concept", which, in contrast to theoretical models, is based on actually calculated erosion. The level of geomorphic activity depends strongly on the characteristics of the areas of interest, such as size, slope length and slope gradient, some of which are associated with deeply incised gullies. It is noticeable that especially areas with decades of dead ice influence in the lower slope area show high geomorphic activity. Furthermore, we show that system-internal factors as well as the general paraglacial adjustment process have a greater influence on long-term morphodynamics than changing external weather and climate conditions, which, however, had a slight impact mainly in the last, i.e. most recent epoch (mid-2000s to 2017/2019) and may have led to an increase in erosion at the areas of interest.