The Floodstone Project: compiling a database of historic flood markers
Photo credit: Wells-next-the-Sea – Mark Bateman
The risk posed by coastal flooding is often considered according to the relationship between the magnitude of flood events (best expressed as the elevation to which the water rises) and how often such events occur. This relationship is termed a magnitude-frequency relationship (or recurrence interval) and is often expressed as a curve. These curves allow flood experts and coastal managers to determine the extent of defences required to provide to provide a certain level of protection.
To construct magnitude-frequency curves requires data on historic floods through time. In the UK such data is provided by the the UK tide gauge network. The UK tide gauge network continuously monitors sea level around the UK. Much of the network was established following the North Sea disaster of 1953, though many records do extend as far back as the early 19th century. Combined with reliable Met Office hindcast data this gives coastal managers a reasonable understanding of storm surges in the last century. However, this data is usually only large scale and lacks the nuanced understanding of local impacts and the degree of flood inundation required to properly understand local flood risk. Further, the brevity of tide gauge datasets can also be a problem.
To create meaningful and high quality magnitude-frequency curves it is necessary to have data from as long a time frame as possible. The relatively short term nature of the tide gauge record is problematic in that;
- It raises into question the statistical validity of determining the likelihood of large events (1 in 100y or even 1 in 1000y) from such a short dataset
- It may miss significant events associated meteorological processes which lie outside the range of the dataset
Thus it is necessary to attempt to supplement and extend the range of instrumental data with data from other sources, which is where http://www.floodstones.co.uk/ attempts to help.
Given the often catastrophic nature of large floods for coastal communities it is not surprising that the height of coastal floods may be locally recorded. Often the elevation of floods may be recorded on a floodstone; a horizontal mark of the water level, usually carved into the stones of buildings, during a significant water level. These floodstones may represent an additional means of determining the heights of historic floods if each is manually surveyed and assessed critically.
However, the location of floodstones and other historic flood markers has not been recorded en masse to date and are therefore difficult to find. The role of http://www.floodstones.co.uk/ is to help find and catalogue such floodstones so that their elevations may be surveyed and used to enhance instrumental records of coastal floods.
It is here that the role of people such as yourself becomes critical. With no formal record of the location of floodstones yet in place it necessary for members of the public, such as yourself, to help contribute to putting this a database together. Do you know of a floodstone local to you? Please consider visiting the http://www.floodstones.co.uk/ and let us know about it.
Any contributions will be very gratefully received.
This article was written by Greg Rushby – PhD student in the Department of Geography at The University of Sheffield.