Sunken lanes - Development and functions in landscapes

Abstract Sunken lanes are roads or tracks, 2 m or more wide, that are incised at least 0.5 m, but often by several meters, below the general level of the surrounding land surface. They are formed by the passage of people, animals, vehicles and erosion by water and gravity. Although these anthropogenic landforms are quite common worldwide they received limited interest by the international scientific community. This comprehensive review analyzed all available information on their characteristics, development and functions in landscapes. Most research on sunken lanes has been conducted in Europe, whereas sunken lanes have been occasionally reported in other continents as well. Major topics addressed are spatial distribution, origin and development, morphology, erosion rates, hydrology, ecology, management, geotourism and research needs. Mean dimensions of sunken lanes reported for various regions vary widely: i.e. 30–2300 m for their length; 0.6–12.5 m for their depth and 2–36 m for their top width. Typical sunken lane densities in European regions (10 to 100 km2 large) characterized by such landforms range between 0.2 and 0.5 km km−2 but for smaller areas ( Sunken lane formation results from interactions between natural factors (i.e. lithology and soils, topography, climate, vegetation) and anthropogenic factors (i.e. traffic, land use and management). Rock type, weathering status and soil types control the erodibility of hillslope materials and hence the development and preservation of sunken lanes. Sunken lanes have been reported in several lithologies but most have been studied in loess regions. Sunken lanes, can be initiated at topographic landscape positions with a much lower slope gradient and corresponding contributing area than those needed to initiate classical gullies, due to the combined action of natural (i.e. concentrated flow erosion and mass movements) and anthropogenic processes (i.e. erosion by animal and human trampling, wheel traffic and digging). Once formed, medium to long-term average incision rates of unpaved bare sunken lanes are 1 to 5 cm year−1 often exceeding erosion rates on nearby cropland by at least one order of magnitude. Sunken lanes perform many functions in landscapes i.e. microclimatological, hydrological, geomorphological, ecological, transport, aesthetic, geotouristic, educational, scientific, strategic, and historical functions. Sunken lanes represent long-standing heritage of past agricultural landscapes and, taking into account their natural and cultural assets, justifies their protection. Unfortunately in several regions, sunken lanes are threatened by urban sprawl, agricultural intensification or land consolidation programs. It remains a challenge for environmental planners to conserve this characteristic geomorphosite for the Anthropocene and to reconcile its competing functions.