This page follows on from a description of the sediment segmentation model.
The period covered by the migration chart is twenty seasons, autumn and spring, beginning September 2008, the start of beach contour data, to May 2018. Nominally, the ten years represented are 2008 to 2017.
Data for autumn 2016 are absent as a result of adverse weather preventing a physical survey from being conducted. Measurements taken in spring 2017 were for a full year. In the chart the results are split 2:1 over two seasons.
Vertically, the chart represents the East Yorkshire coastline. In the interests of simplicity and space, the characteristic ‘S’ outline is made schematically straight.
Numbers at the edges of the chart are those of monitoring profiles, which intersect the coastline at intervals of 500 metres (there are exceptions – all locations listed at data summary, coordinates).
The chart is updated from right to left, to mimic east to west, the general direction of cliff recession. Each season, autumn or spring, is served by a column.
At the left of the column for any one season, erosion in terms of land lost at the cliff top is indicated by a red box opposite the appropriate profile number. Width of the box indicates relative severity of the erosion. Actual values are available from spreadsheet or pdf.
Across the bottom of the chart is an activity index – the higher the number the greater the total cliff loss for the season.
Profiles 8 to 18
To the right of the column, a strip in gold-yellow depicts sections of full upper beach opposite the appropriate profile number. A continuous length signifies ample evidence of a full upper beach. Less well defined situations are marked by broken sequences.
Profiles 19 to 123
Strips in columns are run together in order to present an idea of the distribution of full upper beach deposits over seasons.
In the context of the migration chart, a full upper beach is regarded as lying within a contour minimum of +2.5 metres OD, extending for a few metres from the base of the cliff.
Sharing the same alignment as gold-yellow strips are strips in silver-grey. These show where the upper beach is low against the base of the cliff, contours having a maximum of +1.5 metres OD.
According to the sediment segmentation model, beach sediment starts a journey south from the ‘nursery’ of Bridlington Bay, north of Barmston. Movement, driven by longshore drift, takes place in the form of a succession of segments.
In the chart, the process becomes apparent below Barmston. Bodies of sediment (in gold-yellow) are seen to work down the chart. Configurations become less distinct in the approach to Spurn.
Where major defence structures interrupt movement, sediment tends to accumulate on the updrift side and is depleted on the downdrift side. For more information, see the terminal groyne effect page.
Stretches of reduced upper beach, represented in the chart by silver-grey, migrate at the same pace as sediment bodies.
Almost seventy percent of instances of cliff loss (red boxes) are found in sections between bodies of sediment, and tend to be associated with stretches of depleted beach, migrating at a similar pace.
Some cliff loss, however, is seen to occur within the leading edge of a sediment mass. Much of this may be due to delayed erosion, as follows.
direct and delayed erosion
During an erosion event, a depletion of beach sediment may bring about an entire collapse of the cliff, with loss of land at the cliff top which is captured in the erosion data for the same season. This could be considered direct erosion.
Alternatively, instead of full vertical failure, the cliff might be undercut or the base otherwise weakened. At the cliff top, there is no indication of removal of material below, and no retreat is recorded for that season.
After a while, acted on perhaps by a variety of processes (see cliffs), the degraded lower cliff is no longer able to support the weight above. Failure ensues, usually in the form of a slump or rotational slide. The resultant loss at the cliff top appears in data for a season later than that of the initial weakening, and therefore lags behind a beach situation that has moved on. This could be considered delayed erosion.
The two types are by no means mutually exclusive. A cliff possibly undergoes partial failure at the time of maximum wave exposure to become once more unstable as the loosened material is removed, leading to a subsequent slump or slide. In such a case, cliff top recession at a particular location will appear in the data over two or more seasons.
cycles of cliff loss
A straight path projected horizontally across the migration chart especially within the area from Mappleton to Withernsea, where segments are well defined, will meet with two separate spreads of gold-yellow. This is equivalent to someone at a single location perceiving that, over time, beach conditions and therefore incidences of erosion are repeated.
Cyclicity in cliff recession along the East Yorkshire coast is well noted. One long-held view is that material from a major event protects the cliff base for a time. The sea eventually removes the material, when the cliff is once more exposed to wave action.
Although the mechanism is certainly to be seen, on its own it does not account for an overall pattern to lateral movement of erosion. Also, cliff fall can be removed in a relatively short time, sometimes a matter of a few tides, rather than to the scale of years that separate cycles.
In a variation of the above view, clay is washed away relatively quickly, but sandy material and boulders from the cliff remain on the beach for longer. Sand constitutes about a third of the coast’s glacial deposits, while stony content is abundant (the tills were once known as boulder clay). All are added to the sediment budget to produce a rise in beach level. A higher beach absorbs some wave energy thereby decreasing the potential for cliff erosion.
Less cliff loss means a smaller contribution to the sediment budget. Beach level drops, allowing more wave contact with the cliff base. And so another cycle begins.
Again, the process does not explain patterns in migration. The stated source of sediment is important, but it is not the only one, and is arguably insuffient to influence changes in erosion activity to the extent observed.
As noted, the pace of cliff loss migration is essentially the same as that for sediment movement, estimated to average close to 500 metres per year.
With allowance for variation depending on annual and seasonal energy, it is possible by using the migration chart to forecast where – though not to what extent – erosion events and consequential loss at the cliff top are likely to occur in forthcoming seasons. Such locations may manifest as dynamic hotspots.
chart as above (jpg) (png) (pdf)
chart without fill (jpg) (png) (pdf)