The hydrological role of 'sunken lanes' with respect to sediment mobilization and delivery to watercourses with particular reference to West Sussex, southern England

Roads in general and sunken lanes (SLs) in particular play an important hydrological role in connecting areas of runoff and sediment generation with valley floors and therefore with habitation and watercourses. These connections lead to damage and pollution. The role of SLs has been neglected as an important source of runoff and sediment and in hydrological and geomorphological discussion in the literature on connectivity. Published data on the role of roads and specifically SLs are reviewed. Fluxes of runoff and sediment have been mapped in a study area during a period of serious erosion (2006-2007). Routes to watercourses and entry points to a vulnerable river are recorded. SLs are historic features often hundreds of years old. Unmetalled SLs continue to incise whereas metalled ones generally act as routes for runoff and sediment with inputs from many sources. These include field gateways, other SLs, tracks and trails (some metalled, some unmetalled), bank gullies and mass movements on SL banks, burrows and pipes in banks, and erosion of roadside verges. The proportion of sediments from these sources is unknown and area specific. Impacts and costs include muddy flooding of villages, road clearance and ecological damage to rivers due to pollution and sedimentation. The location of SLs is influenced by two factors: long-term human occupation of the landscape and soft, easily eroded geology. They are an important element in the connectivity between hillslopes and valley bottoms. They play a role in the muddy flooding of villages and increased sedimentation in rivers in areas such as Flanders, the Polish Carpathians and the study area in southern England.