Monitoring Coastal Changes in Saltmarshes: Maps or Aerial Photography?

A key aspect of the recent changes in planning legislation is that development should be preferred provided it is sustainable. On coastal margins there is an additional issue of the potential impact of sea-level change on increased development. Development behind existing coastal defences, both human and natural, seems to be encouraged within the new planning legislation as it makes use of investment already sunk into defence and implies that any future sea-level rise will be accompanied by increasing investment these defences.

Given the importance of these defences it is essential to know how the coastline has responded in the past. The rate of past change is an important indicator as to how dynamic the coast is and how it is likely to respond to increasing sea-level. A long-term, over 50 or 100 or more years, view of rates of change often makes use of historic maps to establish baselines from which change is measured. A recent paper by Brian Baily and myself ( I made the coffee again!) published in the Journal of Coastal Conservation looks at how maps have been used as sources of evidence of coastal change in the Solent (specifically Lymington, Beaulieu River, Calshot Spit, Eling, Portsmouth Harbour, Langstone Harbour and Pagham Harbour). The paper  is entitled Assessing historical saltmarsh change; an investigation into the reliability of historical saltmarsh mapping using contemporaneous aerial photography and cartographic data, a long title but a very accurate description of what the paper does. The paper assesses how these maps have been used to identify and quantify changes in saltmarsh, an important coastal ecosystem and a natural protective barrier. In current terms a key ecosystem service. Importantly, the locations and rates of change these maps suggest are compared to the rates of change that an analysis of aerial photography provides. 

The mapping of saltmarsh is full of problems that limit the reliability of the changes measured. Surveyors in the mid-nineteenth century, for example, did not have a clear and specific set of instructions about what to map above the low water mark. Inconsistencies in the accuracy and precision of saltmarsh identification and mapping are bound to arise when a surveyor was confronted with the practical and often hazardous task of trying to get into a saltmarsh and survey it. Similarly, this ecosystem was not viewed as a particularly valuable resource in the nineteenth and early twentieth centuries and so the incentive to get into the mud and accurately survey was not really there. In addition, the growth of Spartina spp. in this period would have made the identification and mapping of this ecosystem tricky at best in some locations. Often saltmarshes were only indicated by some symbol on a map covering a vaguely defined area of land – not the best baseline from which to accurately measure changes.

The aerial photography of the same areas provided a useful control from which to assess the accuracy of the location and rates of change in saltmarshes derived from maps. The aerial photography shows that large areas of saltmarsh were excluded from the OS maps – so major losses of a valuable coastal ecosystem can not be quantified. These areas seem to be the newer salt marshes and so areas that are likely to have provided coastal protection in the recent past as development has occurred in these coastal regions. Given that salt marshes can change in extent rapidly this suggests that analysis of rates of change in this important protective coastal ecosystem needs to be gauged against the accurate data provided by aerial photography which is only available from the early twentieth century onwards rather than from the potentially more inaccurate figures provided by historic mapping in the nineteenth century.

Using St Paul’s Erosion Data to Predict Future Stone Decay in Central London

In a recent blog I mentioned a research project just completed on a 30-year remeasurement of stone decay on St Paul’s cathedral in central London. A second paper looks at how this data might be used to model decay into the future (http://www.sciencedirect.com/science/article/pii/S1352231012007145  - you need to have an account to get access to the full paper in Atmospheric Environment). Modelling erosion rates into the future tends to use relationships derived from erosion data of small (50x50x10mm) stone tablets exposed in different environmental conditions. Using such data and regression analysis a statistical relationship can be derived between stone loss and changing environmental conditions. These relationships are often referred to as dose-response functions.


Two equations stand out – the Lipfert and the Tidblad et al. does response functions. Using these two equations for the decades 1980-2010, they predict an erosion rate of 15 and 12 microns per year as opposed to the measured losses on St Paul’s of 49 and 35 microns per year. The ratio between the measured and the dose-response erosion rates varies from 3.33 in the decade 1980-1990 to 2.75 in the decade 2000-2010, so fairly consistent. The difference between the two measures of decay may result from differences in what they are actually measuring. The dose-response function uses small stone tablets, exposed vertically in polluted environments. The weight loss of these tablets is measured and then converted to a loss across the whole surface of the tablet. The micro-erosion meter sites measure the loss of height of a number of points across the same surface on a decadal time scale. Both measures are changes in height but derived in different ways. What is important is that both methods indicate the same patterns of change in relation to declining sulphur dioxide levels. Both measures of erosion show a decline and both show it in the same direction and, by and large, in proportion to each other. Interestingly, when the dose-response functions are used to work out erosion on the cathedral since it was built the long-term erosion rate (as measured by lead fin heights relative to the stone surface) is only 2.5 times greater than that predicted by the does-response functions. This is a similar ratio, more or less, to those indicated over the last three decades.

The St Paul’s data does not imply that dose-response functions do not work – if anything it confirms the patterns in decay they indicate – but the St Paul’s data does suggest that using these dose-response functions to model decay into the future may require a correct function, equivalent to the ratio of about 2.5-2.75 to convert the losses to those that will be found on St Paul’s Cathedral.