Analytical Methods:Erosion Monitoring

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1 Monitoring Soil Erosion

[edit] Monitoring Soil Erosion

[edit] Degradation processes linked to erosion

Processes of erosion include wind, water, mass movements, and tillage erosion. It is the combined effects of these on the pattern of sediment redistribution that is most relevant to archaeological preservation.

Soil erosion may be directly physically damaging to buried archaeological remains. Progressive lowering of the ground surface also exposes previously preserved remains to plough damage, bioturbation, oxidation, the effects of surface contamination etc., as well as negatively affecting the landscape setting through the silting of lakes, the development of erosion features and the degradation of soil quality. The redistribution of sediment may also lead to burial of archaeological remains and deposits. It is also important to note that in some circumstances erosion processes may be directly (tillage erosion) or indirectly linked to human activity and hence the erosion surfaces and deposits formed are themselves part of the cultural record.

Estimates of surface lowering under arable cultivation in the UK are up to 3.9 mm/year on cropmark sites (Wilkinson et al., 2006), and peat wastage can produce between 2 and 3.8 m surface lowering over 100 year period (French and Pryor, 1993; Hutchinson, 1980). Over time this can have a significant effect, particularly in combination with ploughing and other cultivation processes. Over a period of 10 years observable changes should be in the order of a few centimetres for arable cultivation or tens of centimetres for peat wastage.

[edit] Approaches to monitoring soil erosion

Direct observation of the number and size of erosion features may be appropriate for monitoring rabbit or plough damage to upstanding monumnets, but doesn’t monitor ground surface lowering or sediment deposition, and hence is less relevant to other monitoring studies.

Peat depths and the effects of peat wastage have traditionally been monitored through the use of fixed posts such as at Holme Fen, UK, however, data based on a single pole are very localised and should not be extrapolated over large areas. Direct topographic survey using theodolite, EDM or Lidar are all sensitive to monitor changes in the shape of upstanding features and slopes.

Gamma spectrometry can be used to monitor Caesium 137 activity; this approach has been used successfully to map patterns of post-1950 soil erosion and deposition at the field scale (Walling et al, 2005; Wilkinson et al., 2006) and could aid site management.

Radar based measurements (Lidar and IfSAR) using airborne or satellite imagery have proved highly successful at creating relative topographic maps with a vertical accuracy of a few centimetres. However, monitoring change in surface elevation requires high resolution absolute topographic mapping so that changes over time can be accurately assessed. Currently these techniques are not sensitive enough to detect absolute changes with a resolution of a few centimetres as would be required for this monitoring project. However, developments and improvements to Lidar and IfSAR may make this feasible in the near future.

Changes in the risk of erosion may also be monitored particularly at a regional or national level. Erosion risk is influenced by soil properties such as organic carbon content, aggregate stability, soil texture, and slope shape as well as land management practices. A modelling approach based on these soil properties can provide an indirect means of mapping erosion risk in order to highlight monuments at risk, target sampling and help interpret direct erosion data. In the UK erosion risk in arable areas has been mapped by Oxford Archaeology (2002) and Lilly et al., (2001).

[edit] Sampling

The frequency of measurement must be relative to the likely scale of change, the sensitivity of the measurement technique and the likely impact on the archaeological remains.

[edit] References

French, C.A.I. and Pryor, F. (1993) The south-west Fen Dyke survey project, 1982-1986. East Anglian Archaeology, 59.
Hutchinson, J.N. (1980) The record of peat wastage in the East Anglian Fenlands at Holme Post, 1848-1978 AD. Journal of Ecology, 68, 229-49.
Lilly, A., Birnie, R.V., Hudson, G., Horne, P.L., Gordon, J.E. and Puri, G. (2001) Inherent geomorphological risk of soil erosion by overland flow in Scotland. SNH Conference Publication: Earth Science and the Natural Heritage.
Oxford Archaeology (2002) The management of archaeological sites in arable landscapes. Research report to DEFRA, BD1701.
Walling, D., Zhang, Z., and Parker A. (2005) Documenting soil erosion rates on agricultural land in England and Wales. Research report to DEFRA, SP0411, Exeter University.
Wilkinson, K., Tyler, A., Davidson, D., Grieve, I. (2006) Quantifying the threat to archaeological sites from the erosion of cultivated soil. Antiquity 80, 658-670.