After a winter of extreme weather events, Dr Jimmy Murphy outlines coastal protection techniques and describes how new strategies for dealing with large-scale coastal erosion can help future-proof coastal structures
Civil

 

Author: Dr Jimmy Murphy, coastal engineering manager, Hydraulics & Maritime Research Centre, Department of Civil and Environmental Engineering, University College Cork

Repairing damaged infrastructure or protecting coastal assets, as funding allows, works as long as the severe storms are relatively infrequent. If the recent severe events do not recur for another 10-20 years, as has been the case in the past, then repair may be the most feasible option.

The winter storms of 2013-2014 brought the fragility of Ireland’s coastline to the public’s attention making erosion and flooding very topical issues and prompting much media and political discussion. Already, and like the storms themselves, the attention has subsided and whilst the Government has resolved to repair much of the damage, there is a continuing failure to tackle the underlying issue of how on a national basis do we manage our coastline and deal with coastal erosion.

However, if they become more frequent (every 5-10 years) as climate analysts would suggest, then a tipping point will be reached where we need to strategise and manage for the long term. Ireland is already doing this in terms of flood protection schemes and considerable resources have been committed, but coastal erosion still remains a low national priority.

THREATS TO IRELAND’S COASTLINE

Ireland has about 6000km of coastline, half of which is categorised as soft coastline (non rocky) and although there are no consistent figures, perhaps up to 500km of this is actively eroding and is considered at risk. Soft coastlines are naturally dynamic whose position will depend on storm events and sediment supply, although sea-level rise and climate change are often now being attributed to affecting erosion rates.

Coastal erosion at Rossbeigh, Co. Kerry (top pic: summer 2000; bottom pic: November 2006)

Storms not only bring high waves to the shoreline, but also elevated water levels through storm surges, which allow waves to directly attack the dunes and cliffs. Storm surges are influenced by meteorological conditions, primarily low atmospheric pressure and onshore winds and can contribute significantly to the water levels.

An analysis of the Irish tide gauges from November 2013 to March 2014 showed that water levels were frequently more than 1m higher than the predicted astronomical tide. On one occasion, the Galway Bay tide gauge showed the water levels to be 1.5m higher than predicted. As tide gauges are usually located in sheltered locations, the full extent of water level rise during storm events may not be detected.

Higher water levels due to waves breaking on the shoreline (wave setup) are significant on exposed coastlines. For a particular hurricane event on the US eastern coastline, a water level rise of 5.4m was recorded with 3m of this attributed to wave setup.

The impacts of sea level rise (SLR) on coastal erosion are not easy to estimate and even though figures for SLR of the order of 1.8mm/year are often quoted, the actual change at a site will depend on land movements. Some coastlines are sinking whilst others are uplifting so, overall, it is incorrect to apply a universal policy. Irrespective of this, a sea-level rise of 0.5m is specified as a design requirement in many Irish coastal projects. It is likely that better design would be achieved by quantifying storm surge and wave setup, rather than estimating SLR.

Climate change as it relates to coastal erosion normally means changes to the storm patterns giving rise to more frequent severe events. The recent storms represented a cluster of severe events and for a particular analysis carried out by the author, it was found that the one-in-one-year wave condition was exceeded seven times in a period of just greater than one month.

The storms that occurred ranged in severity between one-in-three-year to one-in-20-year events. These storms could be driven by climate change or could be a consequence of a rare climatic event, only time will tell which is the case.

MANAGEMENT STRATEGIES AND PROTECTION TECHNIQUES

In ideal circumstances, the coastline should be allowed to evolve naturally and humans should adapt to this change. This is not a feasible option and intervention is often required to provide protection to important infrastructure. Given sufficient resources, it is possible to ‘engineer’ a solution for any coastal erosion scenario that arises but this also is not a feasible option.

In many countries, there is an overall coastal erosion management strategy which dictates on a national, regional or local level what approach will be adopted should erosion occur. Such strategies are normally based on detailed knowledge of coastal processes and environmental forcings. In Ireland, the Irish Coastal Protection Strategy Study (ICPSS) has been ongoing for a number of years and whilst it has not yet developed a strategy, it has produced erosion risk maps based on detailed site surveys and numerical modelling.

In general, there are four main coastal erosion strategy options:

  • Hold the line – this involves maintaining the position of the shoreline primarily through engineering intervention. This strategy has been applied to the full length of the Dutch coastline since the 1990s. In implementing this strategy, the Dutch have taken some coastal protection measures to a whole new level and can do so based on a thorough understanding of coastal processes.
  • An example would be the ‘Sand Engine’ project, whereby 21 million m3 of sand was pumped into one location to create a sand stockpile that is currently being moved naturally by wave action to nourish the coastline. A ‘hold the line’ policy is expensive and is normally applied when valuable infrastructure is at risk i.e. roads, towns etc. This strategy is relevant to Irish coastal towns such as Kilkee and Lahinch, which are under threat as it inconceivable to think that they will be allowed to be taken by the sea. Therefore, at some stage, detailed plans will be required and funding will have to be committed to secure their longterm future.
  • Retreat the line/managed realignment – this strategy allow for coastal recession but on a managed basis with the view that it may, in the long term, be the only financially justifiable option. This can be a hard strategy to justify, as often property and land need to be sacrificed. But on the plus side, the creation of saltmarsh can have positive environmental and eco-tourism benefits. The £28 million Medmerry scheme in the UK is the most significant implementation of such a policy to date.
  • A potential Irish application of ‘managed realignment’ would be in Rossbeigh in Co. Kerry. The dune system of this barrier beach breached in 2008 and has since widened, exposing a land and properties on the lee side to increased flooding. Restoring or protecting Rossbeigh beach is not really a viable option, but lower-cost measures, such as dykes, at the affected locations may be feasible.
  • Advance the line – this pushes the coastal position seaward and is usually undertaken on a local scale. Beach nourishment or land reclamation could be taken as examples of advancing the line.
  • Do nothing – allows minimal intervention and best defines Ireland’s approach if we were to have a strategy. Implementing a ‘do nothing’ approach and allowing the coastline to evolve naturally requires planning restrictions or minimum development setback distances from the shoreline. In some cases, it can be specified that no development can take place on the seaward side of the road closest to the coastline in others a fixed distance is specified i.e. 100m.
  • It is important that this strategy is well communicated and discussions are held with affected property owners. The best example of this is in the UK, where part of the town of Happisburgh in Norfolk was given up due to the prohibitive costs of coastal protection. In this case, the local council implemented a policy of compensating the affected homeowners.

COASTAL PROTECTION TECHNIQUES

The first three strategies are usually implemented by means of engineering intervention through the application of various coastal protection techniques. The table below lists the main coastal protection methods that are applied worldwide. The ‘High Cost’ measures require engineering design whilst the ‘Low Cost’ measures are often undertaken by communities in co-operation with local authorities.

High Cost Low Cost
Offshore breakwatersSand by-passingGroynesSubmerged groynesBeach nourishment

Artificial headlands

Mudflat restoration

Silt redistribution

Seawalls & revetments

Buried dune toe protection

Cliff stabilisation

Beach ridge restructuringWave barrier fencingDune recontouringSand stabilisationSand trap fencing

Artificial dune ridge building

Marram grass planting

Dune fertilisation

Walkways

 

The most common protection measure in Ireland is revetment structures. They represent a ‘Hold the Line’ approach, whereby the large rocks/gabions/concrete armour units placed against a cliff or dune face form a barrier to resist further erosion.

Revetment at Waterville, Co. Kerry

In the past, very ad hoc methods of design and construction were employed that have resulted in various problems with these structures. If proper design methods are followed, then revetments can not only protect but also enhance a beach. What has been found to be particularly important is to design for shallow slopes (1:2.5 – 1:4), as this leads to better sand retention on the beach.

Beach nourishment is very common worldwide and involves pumping dredged sand onto the beach to create a buffer against erosion. Over time, this sand is moved away by nearshore processes and the nourishment need to be repeated. This recurrent cost is one of the reasons there are only a few significant applications in Ireland (Rosslare and Bray).

Any solution to a coastal erosion problem must be specifically tailored to the site. In the past, there have been a lot of failures due to blindly applying protection measures that were seen to work on other locations. There is currently no method that is guaranteed to work and even if a structure is designed and constructed to best practice, it may still fail or not perform as expected. The reason is that there is a not a full understanding of nearshore coastal processes and how beaches react to particular events.

There are a large number of variables that influence nearshore sediment processes, i.e. wave exposure, near shore bathymetry, tidal currents and elevations, wind conditions, sediment properties and supply, and understanding the interaction of all of these is very challenging. Numerical models that combine waves, tidal currents and sediment transport help bridge the knowledge gap and are often used to design coastal structures.

SHORELINE MONITORING

On soft coastlines, sediment is constantly being moved by waves, currents and winds. Before designing a coastal protection scheme, it is important to have some knowledge of the variability of the topography and bathymetry. This can be done by regular surveys and the availably of past information greatly increases confidence levels in a design.

In Ireland, it is not feasible to survey our entire soft coastline but any location that is under particular threat and is an identified priority area would benefit from regular surveys. Topographic and bathymetric surveys can be carried out quite economically and the information that they provide would be invaluable to the understanding of beach processes and the design of coastal protection measures. It is known that some local authorities were looking for this information in the aftermath of recent storms.

Shallow water bathymetric surveying

Ideally, surveys should be carried out twice yearly – in the spring and autumn. At the Hydraulics and Maritime Research Centre (UCC), topographic surveys are carried out using a GPS often fixed to a jeep. For shallow-water bathymetric surveys, a jetski has been fitted with an echo sounder and GPS – this can work in areas not practical for normal vessels.

Ireland is moving to a stage where we will need to develop a strategy regarding coastal erosion and, whilst progress is being made, we are still lagging far behind our European neighbours. It is likely that the severity and frequency of Atlantic storms will be the main driving force in determining when we will start to act proactively rather than just reacting as storm events occur.

Acknowledgements

The author would like to thank the Marine Institute for supplying tide data from the gauges on the Irish Tide Network.

http://www.engineersjournal.ie/wp-content/uploads/2014/05/www.engineersir-012-1024x714.jpghttp://www.engineersjournal.ie/wp-content/uploads/2014/05/www.engineersir-012-300x300.jpgDavid O'RiordanCivilclimate change,infrastructure,United Kingdom
  Author: Dr Jimmy Murphy, coastal engineering manager, Hydraulics & Maritime Research Centre, Department of Civil and Environmental Engineering, University College Cork Repairing damaged infrastructure or protecting coastal assets, as funding allows, works as long as the severe storms are relatively infrequent. If the recent severe events do not recur for...