Climatic and marine dynamic controls in the study area

A good knowledge of current and past storm patterns, and their effects, is desirable for coastal assessments (Stone & Orford, 2004).

Ireland is positioned on the path of major North Atlantic storms. This greatly influences wind directions and wave heights in Irish coastal waters which are exposed to strong wave energy and regular low-pressure systems (Füssel 2007; Devoy, 2008;

Sweeney et al., 2008;. Consequently, storm surges in the Irish Sea are associated with major Atlantic depressions, usually from a westerly direction (Sweeney, 2000).

Surge strength depends on the speed, intensity and size of the depression as it approaches Ireland (Orford, 1989). The effect of wind on surge levels largely depends on topography, particularly in shallow waters where tide and wave heights get amplified.

Gentler gradients, like those in the study area, will influence the impact of future sea-level rise in surges heights (Devoy, 2008; Wang et al., 2008). In the shallow-wide Irish continental shelf, wind speed and direction rather than atmospheric pressure, influences storm surge height (McFadden et al., 2007a). Hence, in the south Irish Sea, surge height is dominated by the low-pressure effect whereas in the North Irish Sea the wind effect adds ~72% to the height of the surge (Lowe et al., 2001). Extreme surge heights are expected in both the North and South Irish Sea (Flather and Smith, 1998; Lowe et al., 2001; Woodworth et al., 2005) accompanied by changes in 10-50 year return periods (Lowe and Gregory, 2005; Wang et al., 2008;). Winter and spring storm wave heights might also increase (Gleeson et al., 2013).

In recent years, exceptionally frequent and intense winter cyclone activity with associated extreme wind speeds, tidal surges and low pressure have caused serious damage on Irish coasts (E-surge, 2014; Met Éireann, 2015; Matthews et al,. 2016). The east coast of Ireland was affected by important surge flooding in February 2002 and October 2004. In January 2014 a surge of about one metre coincided with one of the highest spring tides of the year causing intense flooding (Plate 3. 3).

Track of storms and tide conditions could influence flooding potential. In December 1989 the biggest storm surge on record (0.937m) caused by a low pressure system traversing Ireland did not cause much flooding when it hit Dublin fully but at

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low tide; whereas the 2002 event was a deeper low-system tracking further north and yet it brought intense coastal winds that derived on extreme surge.

Plate 3. 3. Extreme water-levels at the Liffey rivers mouth in Docklands (Co. Dublin) produced by storm-surge that hit Ireland in early January 2014. (Source: Silvia Caloca).

A tide ranging up to 2.2m OD would not cause flooding in Dublin, unless it is combined with a storm surge. This will generate water levels of over 2.5m OD Malin at which the flood warning is activated. This happened in February 2002 when the highest spring tides of the year (1.95m OD) coincided with a surge 0.91m resulting in an extreme water level of ~2.9m OD (the highest on record). See Figure 3.3.

Interestingly enough, sea-level rise on the top of spring tides will not always produce inundation. However, it will have an impact on frequency of extreme events.

Thus, 40cm of extra water level from sea-level added to the 2002 event, would convert the 1-100 year event into 1-5year event (Dollard, 2003; IAE, 2007). Sea-level rise will also impact on the 1 in 200-year return period, increasing significant wave heights and raising water levels by at least 0.5m by 2100 (RPS, 2007). Assuming a medium RCP scenario of 0.48m in the future, the 100-year extreme weather occurrence will happen every 1-2 years in Ireland by 2100.

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Figure 3. 3. Surge record of 1 in 70yr extreme event that hit Dublin City and Fingal in February 2002. (Met éireann. (2002); Available at http://www.met.ie/climate-ireland/weather-events).

3.2.1. Tidal regimes

Tides are one of the dynamic controls that significantly vary in scale around Ireland. Tidal regimes in the study area range from meso (spring tidal range 2-4 m) but also include microtidal areas in the southern part (<2 m (Carter, 1991a) (Figure 3.4).

Most of the tidal motion at the Irish Sea comes from oscillations of the Atlantic Ocean tidal regime. The structure of co-tidal elevation is virtually the same for both M2 and S2 constituents (lunar and solar forces), supporting the major influence of the amphidromic point (zero tide) on the Irish Sea (Figure 3 5).

Tidal heights refer to Chart Datum (CD) and this datum varies from port to port.

However, it is usually set to-or near to-the Lowest Astronomical Tide (LAT) at the nearest port. Mean sea-level at Dublin is 2.46m CD whereas it is 1.30 m CD at Arklow.

This small variation in mean sea-level between the two ports is as a result of the influence of land masses and friction inertia, among other factors, and has an effect on tidal movements (ECOPRO, 1996).

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Figure 3. 4. Variation of spring tidal ranges around the Irish coastline (Carter, 1991a).

Maximum tidal range on the east coast is associated with the shelf areas that underlie the potential amplification of shallow waters. Hence, there is a spring tide elevation gradient on the Irish coast from 0.6m at Arklow to 4.5m in Dundalk Bay.

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Figure 3. 5. Left. Influence of the amphidromic point on spring tidal range in the Irish Sea. Right. Tidal currents maximum depth averaged flow (m/s). Source ECOPRO (1996).

3.2.2. Exposure to wave climate

Irish Sea coasts only receive about 20% of the wave energy occurring on open Atlantic coasts. While locally-generated sea waves dominate, swell waves entering the Irish Sea through St George's Channel and the North Channel, have an important role (Carter, 1983).

Waves from 90-180 degrees, following the strong south to west air flows are predominant in this area. In the relatively low-energy coasts of the east of Ireland, deep-water Hs waves decrease northwards, and rarely exceed 8-10m during storms. The median (Hs modal) in the Irish Sea region is 1.6-2 m with extreme (1 in 1000) wave heights of 1.9-2m (Orford, 1989; Carter et al., 1993; Gallagher et al., 2014). See Figure 3.6. Future projections indicate that significant wave heights in this area are due to increase (McGrath et al., 2012).

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Figure 3. 6. Annual Significant wave Height (Hs) for the period 1979 to 2012 (Gallagher et al., 2014).

Figure 3. 7. Significant wave heights (Hs) and mean direction from a northerly storm at high tide approaching North Dublin (OPW, 2010).

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Storm’s trajectory and bathymetric controls, such as offshore banks, affect wave trajectories as they approach the coast (Lozano et al,. 2004; Regnauld et al., 2004).

Wave heights and directions from northerly storms can overexpose vulnerable areas such as e.g. Sutton (OPW, 2010) (Figure 3. 7).