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Image from Thieler, E. R., Martin, D., and Ergul, A., 2003. The Digital Shoreline Analysis System, version 2.0: Shoreline change measurement software extension for ArcView. USGS Open-File Report 03-076.

Digimap Case Study:
An analysis of soft cliff recession adjacent to coastal defences at Holderness

Authors

Sally Brown

Title

An analysis of soft cliff recession adjacent to coastal defences at Holderness

Date

2004-2008

Application Area

Coastal engineering

Application to other subject areas

Geography

Project type

PhD thesis

Summary

Coastal defences, such as groynes and seawalls have been built to protect eroding coastlines for hundreds of years. Groynes are beneficial as they inhibit longshore drift, allowing sediment to build and form a beach, hence slowing erosion. However, they have a negative impact in that they create a sediment deficit down drift of the groynes, leading to increased erosion and shoreline setback. This is known as the terminal groyne effect.

This research focused on the cliff top retreat along Holderness, where substantial defences were first built up to 130 years ago and where retreat rates range from 1.0m/yr to 2.5m/yr. The aim was to establish how much the down drift coastline was setback with respect to the defended section of the coast, and subsequently the increase in erosion rate after defence construction. The research was undertaken by mapping and analysing cliff top positions taken from historical maps, aerial photographs and surveys. From this, retreat and retreat rates were calculated before and after defence construction.

Datasets Used

• Historic map series: County Series and National Grid
• Edition: All revisions and editions from 19th century to present day
• Scale: 1:10,560 & 1:10,000, but 1:2500 also used
• Names of other data used:
  1. Land-Line.Plus
  2. Aerial photographs
  3. DGPS (Differential Global Positioning System) survey data of cliff top position
• Source:
  1. Digimap
  2. East Riding of Yorkshire Council.
  3. East Riding of Yorkshire Council.

Aims and Objectives

Aim

To investigate the terminal groyne effect and ascertain if coastal defences have led to increased cliff erosion down drift.

Objectives

1. Using historical maps (1854 to present day), aerial photographs and other literature, undertake a detailed survey of coastal defences, including type and dates of defence construction and subsequent extensions.
2. To create a GIS of historical cliff top positions at Holderness.
3. To calculate cliff top retreat and retreat rates before and after defence construction.
4. To ascertain if retreat rates have increased down drift of defences, and if so, calculate by how much and the longshore distance of coast affected.

Methodology

1. Land-line data was downloaded from Digimap to create a base map on GIS (ArcView 3.2) for each study site.
2. Digimap was used to download historical maps (1:10,560 or 1:10,000, and 1:2500) from 1854 to the present day. To document a history of coastal defence for each study site, the historical maps were added to the GIS, together with aerial photographs. Additions and changes to the defences were documented, including the type of defence, location and length of defences. The cliff top position on each map or photograph was traced onto the GIS, together with DGPS surveys undertaken.
3. To calculate cliff top retreat, Thieler’s et al (2003) Digital Shoreline Analysis System (DSAS) was used to cast transects from a baseline across the historical cliff lines. The retreat was measured and referenced to the oldest cliff line.
4. Data was transferred from ArcView 3.2 to Excel and retreat rates before and after coastal defence construction were calculated and compared.

Results/Outcomes

The use of historical maps was essential, providing the main source of information about coastal defence construction prior to aerial photography. With the maps easy to download, invaluable time, travel to local records offices and subsequent costs were saved. The Digimap site provided a clear explanation into how the resources were digitised and their limitations. Overall, Digimap provided good quality trustworthy data.

The maps provided an excellent data source for documenting coastal defences and establishing the approximate date of construction. By mapping cliff top position, retreat and retreat rates were recorded before and after defence construction. The results indicated that the down drift coast always became setback after defence construction. However given map uncertainties, retreat rate only increased down drift after defence construction in five out of the nine cases tested, some over tens of metres, others over tens of kilometres. Extending defences down drift did not always produce an increase in cliff top erosion. This is important as it questions the theory of how and why the terminal groyne effect occurs and how different parts of the coastal system are affected. The results indicated that the effect defences have on the coast varies and is influenced by many factors including the type of defence, geomorphology and longshore drift rate. If only short stretches of the coast are defended in the future, adjacent to longer unprotected sections, the protected areas will form artificial headlands as the adjacent coastline continues to retreat. This research has helped understand how artificial headlands form and how scientists and engineers can better predict future shoreline positions, thus assisting with medium to long term shoreline planning.

The maps also provided an unexpected insight into social history. For example, the results indicated that very high retreat (>2.5m/yr) occurred near Withernsea in the middle of the 19th century. Journal articles referencing the reasons for the high retreat are scarce, but one article attributed it to the mining of sand and gravel on beaches that were subsequently used to build the railways line. Subsequent inspection of the maps supported this theory, and hence the town twice benefited from its beaches, - firstly due to the sale of sand and shingle to the railway company, and secondly through the railway bringing Victorian tourists to the town.

Additional Information

Click on images for larger versions.

Historic cliff top positions at Hornsea, Holderness. The coastline continues to retreat after defence construction, with retreat rates increasing. The star indicates the location where the photograph below was taken.

Cliff retreat down drift of defences at Hornsea. Photograph taken on 9th August 2006.

References and/or acknowledgements

References

Thieler, E. R., Martin, D., and Ergul, A., 2003. The Digital Shoreline Analysis System, version 2.0: Shoreline change measurement software extension for ArcView. USGS Open-File Report 03-076. Accessed on 2nd October 2006.

Acknowledgements

School of Civil Engineering and the Environment, University of Southampton – funded via an EPSRC studentship supervised by Professor Robert Nicholls and Dr Max Barton.

Additional data and support was provided by East Riding of Yorkshire Council.