Does the topographic threshold concept explain the initiation points of sunken lanes in the European loess belt?

Sunken lanes or ‘road gullies’ are a common geomorphic feature in the European loess belt. These landforms are usually created by various processes over time (water erosion, mass movements and traffic). They are important for ecology and biodiversity, for recreation and transport, as well as from a hydrological and geomorphological perspective (e.g. by enhancing the runoff and sediment connectivity of the landscape). Nonetheless, little is known about their origin and evolution. The formation of sunken lanes is typically only understood in qualitative terms, while no studies so far have provided a quantitative analysis of their initiation conditions. To address this research gap and to improve our geomorphic understanding of this landform, we investigated to what extent sunken lanes can be treated and described as a gully erosion process. More specifically, we explored to what extent their position in the landscape and their characteristics can be explained by the topographic slope-area (SA) threshold that typically controls gully initiation: S=kAb, with k and b empirical constants. Based on field surveys and LIDAR data analyses, we determined the slope gradient of the soil surface (S) and contributing area (A) for 132 representative sunken lane heads in the Belgian loess belt and collected data on several other characteristics (position of initiation and sedimentation points, width, depth and length of each sunken lane). Our results show a very large scatter on the S and A values. Moreover, the morphological characteristics of sunken lanes (e.g. width and length) showed no clear relation with these values. However, a very weak SA-threshold relation with a low b-exponent (−0.016) and a low k-coefficient (0.022) could be identified. Overall, our results show that the formation of sunken lanes is not in line with the topographic threshold conditions that govern the initiation of gullies across different environments. The low k-coefficient and the lack of a strong correlation between initiation slope and contributing area point towards an overall higher susceptibility to the formation of sunken lanes than the formation of gullies. This supports the conclusion that not only concentrated flow detachment and transport, but especially human activities (mainly traffic erosion) play a major role in the initiation and development of sunken lanes.