Sunken Lanes (Hollow Roads): an Important Element in Hydrological Connectivity
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Sunken lanes (hollow roads): an important element in hydrological connectivity John Boardman Environmental Change Institute University of Oxford, UK Introduction The study area: Midhurst, West Sussex, UK Sunken lanes (SL) are widespread in Europe especially in areas of soft rocks and long-term human occupation (Table 1). They have, however, been neglected as SLs trend north to south from higher wooded areas to the valley floor of the Western significant elements of landscape connectivity. Rother river (Fig 5). The lanes probably developed as part of a transhumance system of movement of animals and products from high ground to villages of Saxon origin along the river valley. Lower ground adjacent to, and including the river flood plain, is Area Local name Notes Reference dominated by arable crops (maize, potatoes, salad crop and cereals). Soils are Rhineland-Palatinate; hohlweg Stanjek (1993); German wine regions; Straßmann (2004) erodible sandy loams and many fields are classified as at high risk of erosion. Kaiserstuhl region Wilmanns (1991) (Germany) Galicia and Asturias Flows of polluted water Personal communication, (Spain) (with animal droppings) Professor Daiz-Fierros along eroded tracks fed Viqueira into fields as fertiliser Mediterranean vineyards; chemin Levavasseur (2012). Orne (France) creux Lublin region (Poland) road gullies Rejman et al. (2005) Loess Belt (Belgium) sunken lanes Development of bank Poesen (1989) road gullies gullies in sunken lanes Poesen (1993) Poesen et al. (1996) Vanwalleghem et al. (2003) Deep Hill Ruts, Oregon Developed in less than Kreutzer (2008) Trail, Guernsey, Wyoming 20 years Figure 5. The study area (USA) Table 1. Selected examples of ‘sunken lanes’ from non-UK countries SLs cut deeply into Lower Greensand beds of Cretaceous age. The SLs provide an efficient network of routes for runoff and sediments from higher ground to the River Rother. Ecological impacts of sediment into the river are of great concern due to Deep loessic soils are particularly vulnerable to erosion by traffic. Many SLs appear damage to the formerly gravel-bedded river with its valued trout fishing. Observed to be hundreds of years old; those in the Meerdaal Forest, Belgium, are probably entry points for sediment into the river are shown on Fig 5. Most of these are Roman in origin (Vanwalleghem et al., 2003). The movement of animals, people, associated with sediment transfer via SLs. The drainage density of the area is roughly wheeled vehicles and the flow of water is responsible for the development of SLs. trebled if SLs are taken into account. In wet winters sediment from rills and gullies on Some are associated with systems of transhumance. Most SLs are now metalled arable fields adds greatly to the overall connectivity by feeding into SLs (Boardman et and therefore erosion is limited. Unmetalled SLs may be subject to rapid erosion al., 2009). e.g. Poland (Fig 1). Detailed studies of the development of SLs were carried out by Froehlich (1995) in the Polish Carpathians where logging activities greatly enhanced Impacts erosion. In several areas of western Europe muddy flooding of villages has been associated with the presence of SLs – most notably in Flanders (Boardman et al., 2006). In other areas impacts on freshwater systems are of concern e.g. West Sussex, UK. Figure 6. Muddy flow in sunken lane, Somerset, UK (Photo: Graham Colborne) Further research Very little is known quantitatively about the origin of sediments in SLs. Observations Figure 1. A sunken lane in deep loess, near Lublin, suggest that in some areas major contributions are from arable fields (e.g. Fig 2). In Poland (Photo: R. Wawer) other areas mass movements on lane banks or contributions from woodland tracks may be important. Observation and measurement during storms and fingerprinting approaches are needed to quantify the various contributions. Runoff and sediments in Sunken Lanes Runoff reaches SLs from various sources: arable and grazed fields, other SLs and References Boardman J. 2013. The hydrological role of ‘sunken lanes’ with respect to sediment mobilization and delivery to watercourses with particular tracks, woodlands and urban areas. Sediment may originate from these sources reference to West Sussex, southern England. Journal Soils and Sediments 13(9), 1636-1644 plus road verges and banks and floors of SLs (especially if unmetalled). Flow into Boardman J, Shepheard M, Walker E, Foster IDL. 2009. Soil erosion and risk assessment for on- and off-farm impacts: a test case in the Midhurst area, West Sussex, UK. Journal Environmental Management 90, 2578-2588 SLs may be through field boundaries, gateways (Fig 2) and animal burrows, natural Boardman J, Verstraeten G, Bielders C. 2006. Muddy floods. In: Boardman J, Poesen J (eds) Soil Erosion in Europe. Wiley, Chichester, pp 743-7 Froehlich W. 1995. Sediment dynamics in the Polish Flysch Carpathians. In: Foster IDL, Gurnell AM, Webb BW (eds) Sediment and Water Quality in pipes and field drains (Fig 3). Mass movements on SL banks may be important and River Catchments. Wiley, Chichester, pp 453-461 lead to the development of bank gullies (Fig 4) (Poesen,1989). Poesen J. 1989. Conditions for gully formation in the Belgian loam belt and some ways to control them. Soil Technology 1, 39-52 Vanwalleghem T, Van Den Eeckhaut M, Poesen J, Deckers J, Nachtergaele J, Van Oost K, Slenters C. 2003. Characteristics and controlling factors of old gullies under forest in a temperate humid climate: a case study from the Meerdaal Forest (Central Belgium). Geomorphology 56, 15-29 Figure 2: Runoff from arable field through Figure 3. Flow through pipes and burrows, Figure 4. Mass movement on sunken lane gateway into sunken lane, Somerset, UK Somerset, UK bank, Flanders (Photo: Karel Vandaele) (Photo: Environment Agency).