Sunken lanes (hollow roads):an important element in hydrological connectivity

Runoff and sediments in Sunken Lanes

Runoff reaches SLs from various sources: arable and grazed fields, other SLs and tracks, woodlands and urban areas. Sediment may originate from these sources plus road verges and banks and floors of SLs (especially if unmetalled). Flow into SLs may be through field boundaries, gateways (Fig 2) and animal burrows, natural pipes and field drains (Fig 3). Mass movements on SL banks may be important and lead to the development of bank gullies (Fig 4) (Poesen,1989).

John Boardman

Environmental Change InstituteUniversity of Oxford, UK

Deep loessic soils are particularly vulnerable to erosion by traffic. Many SLs appear to be hundreds of years old; those in the Meerdaal Forest, Belgium, are probably Roman in origin (Vanwalleghem et al., 2003). The movement of animals, people, wheeled vehicles and the flow of water is responsible for the development of SLs. Some are associated with systems of transhumance. Most SLs are now metalled and therefore erosion is limited. Unmetalled SLs may be subject to rapid erosion 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 erosion.

The study area: Midhurst, West Sussex, UK

SLs trend north to south from higher wooded areas to the valley floor of the Western 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 dominated by arable crops (maize, potatoes, salad crop and cereals). Soils are erodible sandy loams and many fields are classified as at high risk of erosion.

Impacts

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.

Further research

Very little is known quantitatively about the origin of sediments in SLs. Observations suggest that in some areas major contributions are from arable fields (e.g. Fig 2). In 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.

References

Boardman J. 2013. The hydrological role of ‘sunken lanes’ with respect to sediment mobilization and delivery to watercourses with particular reference to West Sussex, southern England. Journal Soils and Sediments 13(9), 1636-1644

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

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

River Catchments. Wiley, Chichester, pp 453-461 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

Table 1. Selected examples of ‘sunken lanes’ from non-UK countries

Area Local name Notes Reference

Rhineland-Palatinate; German wine regions;Kaiserstuhl region(Germany)

hohlweg Stanjek (1993);Straßmann (2004)Wilmanns (1991)

Galicia and Asturias (Spain)

Flows of polluted water (with animal droppings) along eroded tracks fed into fields as fertiliser

Personal communication, Professor Daiz-Fierros Viqueira

Mediterranean vineyards;Orne (France)

chemin creux

Levavasseur (2012).

Lublin region (Poland) road gullies Rejman et al. (2005)

Loess Belt (Belgium) sunken lanesroad gullies

Development of bank gullies in sunken lanes

Poesen (1989)Poesen (1993)Poesen et al. (1996)Vanwalleghem et al. (2003)

Deep Hill Ruts, Oregon Trail, Guernsey, Wyoming (USA)

Developed in less than 20 years

Kreutzer (2008)

Introduction

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 significant elements of landscape connectivity.

Figure 1. A sunken lane in deep loess, near Lublin, Poland (Photo: R. Wawer)

Figure 3. Flow through pipes and burrows, Somerset, UK

Figure 2: Runoff from arable field through gateway into sunken lane, Somerset, UK (Photo: Environment Agency)

Figure 4. Mass movement on sunken lane bank, Flanders (Photo: Karel Vandaele)

Figure 5. The study area

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 damage to the formerly gravel-bedded river with its valued trout fishing. Observed entry points for sediment into the river are shown on Fig 5. Most of these are associated with sediment transfer via SLs. The drainage density of the area is roughly trebled if SLs are taken into account. In wet winters sediment from rills and gullies on arable fields adds greatly to the overall connectivity by feeding into SLs (Boardman et al., 2009).

Figure 6. Muddy flow in sunken lane, Somerset, UK (Photo: Graham Colborne)