|Whitbread, K. 2014. The geomorphic impact of road construction: a case study of the A9 in Scotland. British Geological Survey Internal Report, OR/14/051.|
This preliminary study of the geomorphic impact of road construction utilises site investigation records held in the BGS archive and recently acquired DTMs to provide an independent estimate of ground surface change and material transfer.
To assess the accuracy of the different methods used to quantify these geomorphic impacts, comparison of these results with import/export volumes recorded by the road construction contractors could be made if this data is available. In addition, further assessment of different interpolation methods, potentially using non-excavated areas to test how well interpolations reconstruct terrain along narrow route lines, would be useful to determine the optimal interpolation algorithms with which to reconstruct the pre-construction ground surface.
The analysis indicates that construction of the A9 in the two test areas has been associated with a net ‘erosion’ of rock along the road route that equates to an average ground surface lowering of between 0.5 and 2.5 m over the road lengths studied. Individual cuttings are on average over 4 m deep, but most are locally considerably deeper, whilst embankments are on average 1.5–3.4 m high and are less extensive than cuttings in the study areas. The depths and relative proportion of cuttings and embankments are likely to vary with the form of the landscape; the deeper and more extensive cuttings in the study areas likely reflect the test area locations in upland terrain.
Equivalent erosion rates calculated to compare road construction with natural fluvial erosion processes indicate that erosion associated with road construction occurs at rates that are 2–3 orders of magnitude faster than even the most rapid erosion in natural streams, and 3–4 orders of magnitude faster than previously measured erosion rates in Scottish streams.
Unlike rivers, road construction may be considered as a one-off erosion event relating to the initial period of road construction. However, in regard to the A9, a programme of works to widen and dual the road between Perth and Inverness is ongoing and several phases of upgrades have been recently completed or are currently in progress. The ongoing development of new roads and widening of existing trunk roads means that ground surface change associated with road construction may not be limited to the initial construction phase.
Ground surface change associated with road construction typically results in excavation of cuttings into hillslopes and the construction of embankments where roads cross valleys or run along flood plain areas. The removal and transport of materials from road developments is an expensive component of construction. Quantification of the mass transfer of material and consideration of the role of topographic factors in determining net material fluxes during road construction may assist the development of planning and design strategies to minimise construction costs. Furthermore, an understanding of the surface change associated with road construction may be used to help mitigate the impacts on runoff and river flows (hydrology), and potentially geomorphic process systems (erosion and sediment transport).
This study focuses on a major trunk road in a rural, upland area. Comparison of the geomorphic impact of road construction in other types of terrain, including lowland areas and urban to semi-urban settings is needed to better understand the factors that control how much geomorphic work is done during road construction. For instance, the role of relief and terrain roughness in affecting the balance between excavation and embanking along road corridors, and the influence of road type (trunk road, local road) on the degree of ground surface change and net material transfer.
There are nearly 250 thousand miles of road within the UK, with 31 thousand miles of major road (motorway and ‘A’ roads). Consideration of the impact of this road construction on our landscape is needed is if we are to understand the how, and by how much, human activity influences our geomorphic systems.