Erosion monitoring of coastal cliffs of geoheritage significance – an example from north east England.

The coastline of County Durham and Tyne and Wear, north east of England, is of geological interest as it displays extensive cliff and foreshore exposures of the dolomites and limestones of later Permian age. The coastline is covered by multiple designations ranging from being a Site of Special Scientific Interest for geological factors to other forms of national and international protection. However, the high cliffs and nature of the deposits means that it is subject to much erosion, rockfalls and exposure to storm activity. For this reason, a lengthy monitoring programme using terrestrial laser scanning (Westoby et al, 2018), photogrammetry (Westoby et al, 2012) and geophysical techniques has been taking place in order to characterise the erosion, recognise points of issue and recommend potential action.

 

The carbonate rocks, also known as the Magnesian Limestone, were deposited in the Zechstein Sea in a relatively shallow landlocked sea. Straddling latitude 30° north during Late Permian times, the Zechstein Sea was subjected to high evaporation rates leading to evaporate sequences being present. The Permian concretionary limestone is most common in the headlands, stacks and arches, whereas the bays are cut into a weaker dolomite. Marsden Bay includes beach, rock and cliff features and is a classic locality for beach process studies (King, 1953). Whitburn was previously a quarry and coal mine, later infilled and now subject to erosion, undercutting and sink hole appearance.

 

Rockfalls are often characterised as episodic and unpredictable events, leading to uncertainty and risk for infrastructure and people. As a result of the monitoring it is possible to demonstrate that there are lithologically distinct responses to the passage of the largest storm events.  Foreshore morphology is significant for modulating the relative importance of subaerial and marine erosion drivers. The influence of external environmental controls, notably storm activity, is clearly detectable through regression analysis of rockfall descriptors and environmental variables. Increased storminess, associated with increases in offshore wave heights and cumulative precipitation, corresponds with an increase in total and mean rockfall volume rockfalls at the cliff top during these periods.

 

The study demonstrates that it is possible to quantify links between environmental variables, in this case offshore wave heights, and erosion prediction. From a hazard and geosite management perspective this finding and framework is significant because it represents an effective new tool for quantifying temporal convergence in rockfall dynamics at lithologically complex rocky coasts over timescales that are relevant for hazard assessment.

 

King, C.A.M. (1953) The relationship between wave incidence, wind direction and beach changes at Marsden Bay, County Durham. Transactions of the Institute of British Geographers, 19, 13–23.

Westoby, M.J., Brasington, J., Glasser, N.F., Hambrey, M.J., and Reynolds, J.M., 2012, ‘Structure-from-Motion’ photogrammetry: A low-cost, effective tool for geoscience applications: Geomorphology, v. 179, p. 300-314, https://doi.org/10.1016/j.geomorph.2012.08.021 .

Westoby, M.J., Lim, M., Hogg, M., Pound, M.J., Dunlop, L., and Woodward, J., 2018, Cost-effective erosion monitoring of coastal cliffs: Coastal Engineering, v. 138, p. 152-164, https://doi.org/10.1016/j.coastaleng.2018.04.008.