OR/14/051 Discussion

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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.

Road construction as a geomorphic process

Assuming that both sections of the road took approximately two years to construct, the average vertical lowering along the roads in the test areas is 1–1.25 m/yr in the northern test area, and 0.2–0.35 m/yr in the southern area (see Table 7). Similarly, the net loss of material from each of the road sections equates to net erosion fluxes of approximately 32 and 6 m3/m/yr for the northern and southern areas respectively. Localised erosion fluxes and vertical lowering rates during construction will have been substantially greater than these values in individual cutting areas.

In terms of the geomorphic impact of road construction, a comparison between road systems and rivers may be made, as both may be considered as linear systems associated with the erosion and deposition of rock and sediment. River erosion occurs along the lines of channel systems where the energy of the stream outweighs the sediment supplied to it. Excess stream power contributes to the erosion of sediments and rock along the line of the stream, with erosion rates related to the slope of the channel, discharge of the stream and the resistance of the underlying rock.

In a study of Scottish Highland streams in metamorphic rocks similar to those underlying the A9, Whitbread (2012)[1] recorded river gorges up to 10 m deep that have been excavated by streams since deglaciation approximately 12 000 years ago. Although much of this erosion is likely to have occurred in the first few thousands of years after deglaciation, time-averaged erosion rates for the postglacial period were found to range between 0.0004–0.001 m/yr (0.35–1.3 m/kyr). At these average rates, it would take a Scottish stream 4200–12000 years to cut a gorge of equivalent depth to the average depth of a cutting (4.2–4.3 m).

If the formation of these river gorges is assumed to have occurred during an extremely rapid period of erosion occurring within 3–4 thousand years of deglaciation, the average rate of erosion for that period would be within the range 0.001–0.004 m/yr, and the time taken to erode a gorge of equivalent depth to an average cutting would be 1100–4300 years.

Maximum recorded rates of fluvial erosion worldwide occur in tectonically active high mountain terrains such as the Himalaya where average erosion rates of 0.002–0.012 m/yr have been recorded over timescales of 102–103 years (Burbank et al, 1996[2]). In Taiwan, erosion rates of 0.002–0.006 m/yr have been recorded annually, with rates of up to 0.01 m/yr recorded in a single wet season (Hartshorn et al., 2002[3]). In rivers fed directly by glaciers recorded erosion rates over periods of several years are up to 0.02 m/yr (Vivien, 1970[4]). Even under these very high measured rates of erosion it would take rivers between 200 and 2000 years to excavate a cutting-equivalent gorge.

Only when the effects of cataclysmic floods are considered do erosion rates in streams become comparable with, or exceed the rate of erosion during road construction; Lamb and Fonstad (2010)[5] describe a 7 m deep, 1.2 km long, canyon excavated into limestone and marl bedrock over a period of 3 days during a dam release. However, there are limited records of catastrophic erosion events, and there is no published information on potential maximum instantaneous erosion rates in resistant bedrock geologies such as those occurring in the Scottish Highlands. In summary, road construction results in the formation of cuttings during a period of months to years which would take a river in comparable rocks several thousand years to form, even under conditions that are the most favourable for fluvial erosion (e.g. relatively soft rocks, high discharge and steep terrain). Worldwide, only rare cases of cataclysmic flooding due to lake or reservoir outbursts are likely to be capable of excavating gorges comparable to the A9 cuttings in timescales equivalent to or less than the time taken to construct the road.

Implications for mapping and modelling artifical ground

In this study, areas of MGR and WGR along the A9 were mapped independently of the borehole data. However, the findings indicate that the correspondence between mapped areas of MGR and WGR and positive and negative differences between the borehole start height and the DTM may be used to help determine the distribution and thickness of artificial ground in areas where borehole records pre-date road construction (i.e. no MGR is recorded in the borehole log).

The ground surface elevations of the pre-construction boreholes have been found to reflect the real ground surface level prior to road construction, and this should be accounted for when modelling artificial ground, superficial deposits and bedrock along road routes. In particular, boreholes drilled prior to developments should not be hung from the DTM during modelling.

References

  1. WHITBREAD, K. 2012. Postglacial evolution of bedrock rivers in post-orogenic terrains: the NW Scottish Highlands. Unpublished PhD thesis, University of Glasgow.
  2. BURBANK, D W, LELAND, J, FIELDING, E, ANDERSON, R S, BROZOVIC, N, REID, M R, AND DUNCAN, C. 1996. Bedrock incision, rock uplift and threshold hillslopes in the northwestern Himalayas. Nature, 379(6565): 505–510.
  3. HARTSHORN, K, HOVIUS, N, DADE, W B, AND SLINGERLAND, R L. 2002. Climate-Driven Bedrock Incision in an Active Mountain Belt. Science, 297(5589): 2036–2038.
  4. VIVIAN, R. 1970. Hydrologie et érosion sous-glaciaires. PERSEE.
  5. LAMB, M P, AND FONSTAD, M A. 2010. Rapid formation of a modern bedrock canyon by a single flood event. Nature Geoscience, Vol. 3, 477–481.