East Yorkshire coastal erosion

migration chart


Page created by Brian Williams in February 2015.
Reorganised in July 2016 as an appendix
to the sediment segmentation page.



The period covered by the chart is sixteen seasons over eight years from September 2008, the start of beach contour data, to March 2016. No survey was conducted in autumn 2016.

go straight to the migration chart
observations




chart layout

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 read from right to left, to mimic east to west, the general direction of cliff recession. Each season, autumn or spring, is served by a pencil line in grey descending the chart.


cliff loss

At the left of the pencil line 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.


sediment

Profiles 8 to 18, and 115 to 123
To the right of the pencil line, a strip in 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. Isolated instances also occur.

Profiles 19 to 114
Strips in columns are run together in order to obtain an idea of the distribution of full upper beach deposits over seasons. A portrayal of the entire coast without the fill is available as a strip chart (image may require a double click to open to size).

In the context of the migration chart, a full upper beach is regarded as lying within a contour minimum of +3.5 metres OD, extending for a few metres from the base of the cliff.

Sharing the same alignment as yellow strips are strips in grey. These show where the upper beach is low against the base of the cliff, contours having a maximum of +1.5 metres OD.




 migration chart 2008-2015 fill


observations

According to the sediment segmentation model, beach sediment starts a journey south from the ‘ord 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. Masses of sediment (in yellow) are seen to work down the chart. Configuration becomes less distinct in the approach to Spurn.

Where defence structures interrupt movement, sediment tends to accumulate on the updrift side and is depleted on the downdrift side. For a description, see the terminal groyne effect page.

Almost all stretches of full upper beach away from defence structures could be regarded as salients. A salient is the part of a sediment segmentation body (SSB) that occupies the upper beach. The chart tentatively identifies salients and numbers them according to age. Since a salient represents the upper beach component of a sediment segmentation body, then SSBs themselves may so be numbered.

From a salient and in a southerly direction, an SSB begins to curve – or pull – gradually seaward, resulting in a drop in beach level at the cliff base. A reduced upper beach, shown in the chart as grey strips within the white space between positions of yellow, allows wave energy to have a greater impact on the fabric of the cliff.

Around two-thirds of instances of cliff loss (red boxes) are found in sections between salients. Some loss, however, is seen to occur within the leading edge of a sediment mass. Much of this may be due to delayed erosion.


 East Newton, 3 February 2017
A sand bar (right) pulls seaward from a salient (centre distance, under the cliffs) to create a runnel (foreground). Landward of the runnel is an erosion event (left), characterised by thin beach and mudballs [SSB-12, East Newton, 3 February 2017]. The feature is sometimes described as an ord system.


direct and delayed erosion

Within 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.


wave angle and length of sediment body

Distances between salients can be used to estimate SSB lengths. The longshore extent of a sediment segment is in the main determined by the angle an incoming wave makes with the shoreline. As the angle of wave approach (Θ in the diagram) increases, contact with the shore (and cliff line) decreases, and vice versa, while the width of the beach (from cliff to low water mark) remains the same.


 wave approach triangle

In the figure below, a comparison goes some way to confirm the relationship.

A bar chart shows average wave approach angles between Bridlington and Neck of Spurn (the longer the bar, the greater the angle). Ranges change wherever the coast has curvature or a shift in alignment – e.g. Bridington to Barmston, Holmpton to Spurn, and south of Mappleton.

Appearing above the bar chart is an inverted line chart (graph) which plots distances between the black lines used for identifying salients in the sediment chart (the lower the point, the greater the distance). The graph is positioned to match locations on the bar chart. A credible fit is evident.


 comparison of wave approach angle and salient spine intervals
Angle of wave approach data from K. Pye & S. Blott (2015) – see references.

Over many seasons, as a sediment body migrates south from Barmston (B), length should increase steadily until Mappleton (M), be relatively stable at about 3000 metres to Holmpton (H), before decreasing towards Spurn.




cycles of cliff loss

A straight path projected horizontally across the chart from the lower tip of a numbered black line will meet with a second line bearing another number. 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, 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 reducing the potential for cliff erosion.

Less cliff loss means a smaller contribution to the sediment budget. Beach level drops, allowing wave contact with the cliff base to increase. And so another cycle begins.

Again, the process does not explain patterns in migration. Although the stated source of sediment is important, it is not the only one, and is arguably insuffient to influence changes in erosion activity to the extent observed.




forecasting erosion

The pace of cliff loss migration is essentially the same as that for sediment movement, estimated to be on average a touch over 500 metres per year (the angle of the black lines on the chart is set to reflect this).

With allowance for variation depending on seasonal energy, it is possible by using the migration chart to forecast where – but not by how much – erosion events and consequential loss at the cliff top are likely to occur in forthcoming seasons. Such locations may manifest as dynamic hotspots.




top

migration chart image – with fill
migration chart image – without fill
migration chart image – with fill (fieldwork)

sediment segmentation model

more on East Yorkshire coastal erosion
home