MIDA - Marine Irish Digital Atlas

Coastal Erosion

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Trends and Causes

Soft cliffs, such as these in Myrtleville, Co. Cork, are particularly susceptible to erosion.

In Ireland, the rate of erosion along the most vulnerable parts of the coast is estimated at 0.2-1.6 metres per year (DELG, 2001). The most susceptible coastal systems are sand dunes and soft cliffs. In some cases, up to 10 m of soft cliffs can be eroded during a single storm event.

Erosion can be caused by a number of factors. Waves are the most important erosive agent along most coasts but their effect varies with wave energy and characteristics, and with the nature of the material exposed to wave attack (Summerfield, 1991). Where a coast is formed by steep cliffs which plunge straight into deep water, swell waves are not forced to break before they impact. As such, these waves are reflected with little loss of energy and rarely cause erosion. However, coastlines are more commonly subject to breaking waves that displace a considerable amount of kinetic energy as they break on a shoreline. Of the main types of breaking wave, 'plunging breakers' produce the highest pressures between the leading wave front and the land. The combined effect of air compression and the impact of a considerable mass of water is capable of dislodging fractured rock and other loose particles – this is known as quarrying. Breaking waves may also throw particles against the shore and this leads to the abrasion of shoreline materials – the effectiveness of this is highly dependent on wave energy and on the availability of suitable materials (e.g. pebbles) along the shore (Summerfield, 1991).

Dunes at the Raven Nature Reserve in Co. Wexford are subject to both erosion and accretion.

Once eroded, cliffed coastlines may stabilise, but the lost sediment can never be replaced. On the other hand, sandy beaches are dynamic sedimentary systems that naturally experience phases of erosion and accretion that operate over a range of time intervals (Thom and Hall, 1991). Frequent short-term changes are seasonal - erosion mostly occurs in seasons when storms that generate erosional wave regimes are more frequent. The amount of material removed depends on the strength and frequency of storm events. A 1 in a 100 year storm event may erode several meters of material, even part of a dune systems sitting several metres behind the normally active zone of accretion and erosion. Also, "several lower-magnitude storms that occur in quick succession can produce a similar degree of erosion because the intervening periods are too short for constructive swell waves to push a significant amount of sediment back to the shoreline" (Brooke, 2005).

Human activity can exacerbate erosion. Dredging, mining, land reclamation and wash from ships as well as coastal protection measures themselves can all contribute to the problem (DELG, 2001).

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Management implications

Rock revetments are used to halt erosion behind the beach at Rosscarbery, Co. Cork.

A number of so-called ‘hard’ and ‘soft’ coastal engineering tools can be used to reduce or prevent coastal erosion. ‘Hard’ techniques include protection structures such as sea walls and revetments. These are expensive to build and maintain and are often employed to protect hinterlands containing dense populations and industry and when the value of projected losses of land to flooding exceeds the cost of construction. In recent years it has been accepted that ‘hard’ engineering options work against the natural system and may lead to problems elsewhere. For example, the construction of a sea-wall removes an important source of sediment (the previously eroding shoreline), that could have accreted further along the coast. Accelerated erosion on this section of coast may therefore be a consequence.

Alternative 'soft' coastal management techniques aim to work with, and not against, natural processes. These techniques include managed realignment and beach nourishment (or recharge). Managed realignment involves the identification of a new line of defence and, where appropriate, constructing new defences landward of the original defences (DEFRA, 2001). It may be the preferred shoreline management option if the cost of maintaining the existing defences exceeds the value of the hinterland it protects, or if intertidal habitats need to be created to offset habitats lost to coastal developments. Beach nourishment involves pumping foreign sediment (often collected offshore) to raise the beach profile and to decrease the chance of coastal flooding.

The effects of beach recharge at Heacham Beach, Norfolk, UK. The photo on the top was taken in 1987, the one on the bottom in 1997 .

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Coastal Erosion in Ireland

Areas where erosion is greater than 0.5 m per annum are shown in yellow; where accretion is greater than 0.5 m per annum is shown in red (source: DELG, 2001).

Of the 7,800 kilometres of coast in Ireland, up to 3,500 km may be susceptible to erosion, because it is made up of soft sediments (Devoy, 2003). Much of this coastline is found on the East coast in particular in Co. Down; south of Dublin, between Killiney and Bray and in Co. Wexford. Some isolated bay areas on the South, West and North coasts are also made of soft materials where erosion can occur (DELG, 2001). Erosion of these soft materials averages 0.2-0.5m per year, although faster rates are found in some areas. Current sea-level rise around Ireland averages 1mm per year, but predictions suggest that this could reach rates of 4-6 mm per year by 2100. Such a rise will have implications for vulnerable coastal areas. Currently, of the over 1,700 km of coastline that is considered particularly sensitive to erosion, 350 km has defence structures of some kind (Devoy, 2003).

The need to manage coastal erosion has been recognised by the Irish Government. Over €44 million were allocated in the National Development Plan 2000-2006 to address coastal erosion issues. A range of techniques have been used in different areas across the country, from dune stabilisation in Brittas Bay Co. Wicklow to the use of rock armour in Rosscarbery Co. Cork.

Click here to view a map of Ireland's coastline.

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