Introduction • Land claim • Coastal squeeze

Coastal squeeze

In the 1980's concern was raised about the extent and apparent acceleration of the rate of saltmarsh loss on many parts of southeast and southern England. A study by Burd (1992), utilizing air photographs of different dates, concluded that significant saltmarsh area loss had indeed occurred in the estuaries of Essex, Suffolk and north Kent (see table below). The conclusions of this study were largely confirmed by a wider study of marshes in England and Wales (Pye and French, 1993), although this study also demonstrated that in some parts of the country, most notably the estuaries of northwest England, significant increases in saltmarsh area have occurred in recent decades. Net saltmarsh loss at most sites in southeast England was also confirmed by Cooper et al. (2000).

Loss of saltmarsh in the estuaries of Essex and Kent 1973-1988

This documented loss of saltmarsh habitat has been attributed (at least partially) to the process of 'coastal squeeze' (or in this context 'saltmarsh squeeze'), which represents the result of interaction between the reclamation of former saltmarsh habitat and sea-level rise, (see figure below).

Illustration of the concept of Coastal Squeeze (a) before construction of Sea Wall and (b) after construction (from French, 1997)

The Essex and North Kent saltmarshes provide an illustration of the process. The saltmarshes in these areas have been enclosed by embankments for many years, with the creation of considerable areas of low quality agricultural land. At the same time there has been a relative rise in sea-level, resulting from a rise in mean sea level and the isostatic adjustment which is taking place in southeast England. Because the upper limit of the saltmarsh is constrained by a sea wall, the intertidal habitat is squeezed between it and the rising sea level. If the rate of sea level rise increases as a result of climate change, then the saltmarsh zone could ultimately disappear, as it has already done in some localities in Essex.

A fuller description of recent historical changes in saltmarshes in England and Wales, and discussion of the possible contributory factors, is provided in Carpenter and Pye (1997). More detailed consideration is given to the nature and causes of saltmarsh erosion in southeast England by Pye (2000) and van der Wal and Pye (2004).

Left to themselves, natural systems can typically accommodate changes in sea level by moving either landwards or seawards, depending on the direction of relative sea level. Under these circumstances there is no need for intervention. However, together, the reclamation of estuarine areas for agricultural purposes and the construction of flood defences and embanked enclosures have effectively prevented the landward movement of saltmarsh in response to sea-level rise. The natural habitats are therefore ‘squeezed’ (see figure above) between the embankments (which have moved progressively seawards in the past) and the rising sea level which attempts to move the intertidal zone landward. It is thought that these two effects may be partly responsible for the loss of saltmarshes observed between 1973 and 1988, as described in Burd (1992).

Historically, it was assumed (e.g. in The Wash) that new saltmarsh would develop in front of a new enclosure. However, the coasts in Essex and Kent have shown that this is unlikely to be the normal progression, particularly in areas where relative sea level is rising and where the natural transition between tidal waters and the land has been truncated by an artificial barrier.