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The Murrough Coastal Protection Study

Coastal Processes

4

COASTAL PROCESSES

4.1

COASTAL PROCESS MODELS

The coastal processes along the frontage have been analysed using advanced computational modelling techniques.

The simulations have been undertaken using the

Mike21/Litpack suite of coastal process software developed by the Danish Hydraulics Institute. The bathymetry for the models has been taken from the hydrograhic surveys along the study area frontage together with data from the Irish National seabed survey, hydrograhic surveys of the Coding and Arklow banks and UK Admiralty data as digitally supplied by C-Map of Norway. Figure 4.1 shows the model bathymetry of the section of the Irish Sea around Wicklow and the Murrough.

Figure 4.1

Model bathymetry of Irish Sea around Wicklow and the Murrough

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4.2

TIDES AND WATER LEVELS

4.2.1

Water levels

The tidal levels at Wicklow to chart datum and to OD Malin are as follows: Chart Datum

OD Malin

Mean High Water Springs

2.70 m

0.89 m

Mean High Water Neaps

2.20 m

0.39 m

Mean Water Level

1.69 m

-0.12 m

Mean Low Water Neaps

1.10 m

-0.71 m

Mean Low Water Springs

0.70 m

-1.11 m

The extreme water levels at the frontage have been determined by statistical analysis of the top 50 storm surge events from the period 1961 to 2006. These events were simulated using RPS storm surge model of the Western Atlantic and Irish Coastal Waters developed for the Irish Coastal Protection Strategy Study currently being undertaken by RPS for the DCMNR. The extreme water levels at Wicklow derived from this analysis were as follows. Return Period Event

4.2.2

Water level to OD Malin

1 in 1 year

1.558 m

1 in 2 year

1.650 m

1 in 5 year

1.772 m

1 in 10 year

1.864 m

1 in 20 year

1.956 m

1 in 50 year

2.077 m

1 in 100 year

2.169 m

1 in 200 year

2.261 m

Tidal Flows

The tidal flows around the study area were derived from RPS’s flexible mesh tidal model of the Irish coastal waters. This model, the extent of which is shown in Figure 4.2 below, was also used to simulate the storm surges and extreme water levels for the study.

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Figure 4.2

Coastal Processes

Extent of RPS Tidal model of Irish Coastal Waters

Figure 4.3 shows the typical north going flood tide flow pattern and surface elevations around the study area for mid tide conditions. It will be seen that while the tidal currents around Wicklow Head are strong, the currents are much weaker in Wicklow Bay and the current direction reverses along the shoreline in the southern part of the bay. Figure 4.4 shows the surface elevations and flow patterns during the typical south going ebb tide. As with the flood tide the currents are strong around Wicklow Head but relatively weak within Wicklow Bay.

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Figure 4.3

Typical flood tide flow patterns and surface elevation at mid tide

Figure 4.4

Typical ebb tide flow patterns and surface elevation at mid tide

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The net movement of water over the complete tidal cycle, i.e. the residual flow regime, is shown in Figure 4.4. This diagram shows the net direction and velocity of movement of a particle in the water over a complete tidal cycle. This residual tidal drift is important in ascertaining the long term movement of suspended sediment or other matter in the water as a result of tidal action. It will be seen that there is a pronounced net south easterly drift in the southern part of the study area.

Figure 4.5

Residual tidal flow pattern In Wicklow Bay over a complete tidal cycle

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4.3

WIND AND WAVES

4.3.1

Offshore Wind and Wave

Coastal Processes

The offshore wind and wave data for the study was derived from the UK Met Office’s western European waters wave model for a point at 5.66oW, 53.00oN for the 12 year period 1990 to 2001. This wind and wave data consist of a 3 hourly time series of wind speed and direction as well as spectral significant wave heights, mean wave periods and mean wave directions. The wind rose for the offshore wind in excess of 20 knots is shown in Figure 4.6. It will be seen that while the majority of strong winds come from the west to southwest directions, strong winds do occur from the southeast sector for about 2% of the time in an average year.

Figure 4.6

Offshore wind rose for winds in excess of 20 knots – 1990 to 2001

The offshore wave rose for waves of significant height greater than 2 metres is shown in Figure 4.7 below.

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Figure 4.7

Coastal Processes

Offshore wave rose for waves greater than 2m in height – 1990 to 2001

It will be seen that while the greatest percentage of the larger waves come from the south, the greatest percentage of the biggest waves in the sea state come from the south southeast to southeast direction. It will also be seen that wave approach the area from the northeast direction much more frequently than they do from the east. The offshore wave rose gives the distribution of wave heights with direction in the Irish Sea to the east of the banks that lie offshore along the east coast of Ireland. These banks and other nearshore features will significantly alter the size and direction of the waves that approach the study area shoreline.

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4.3.2

Coastal Processes

Nearshore Wave Climate

The nearshore wave climate was established by transforming the offshore wave data to the nearshore area using the DHI Nearshore Spectral Wind-wave model, Mike21 NSW. This model takes account of refraction, shoaling, wave breaking and bed friction losses due to the effects of changes in the seabed bathymetry as the waves travel from offshore to inshore. The model also takes account of wind wave generation and directional spreading so that all the relevant wave transformation processes are included in the model simulations. The wave transformation modelling was undertaken using three wave model bathymetries, southeast, east and northeast, as shown in Figure 4.8 so that the full 12 years of 3 hourly data could be accurately transformed inshore.

Figure 4.8

Orientation of wave transformation model bathymetries

The wave climate at the 10m contour (to Chart Datum) was established for a series of points along the frontage by transforming some 35,064 wave events from offshore to inshore. Figure 4.9 below shows the inshore wave roses for all the waves for the 12 year period 1990 to 2001 inclusive.

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Figure 4.9

Coastal Processes

Inshore wave rose for all waves – 12 year period 1990 to 2001 inclusive

It will be seen from Figure 4.9 that there is a noticeable variation in wave climate along the frontage. This is particularly true for the larger waves in the wave climate as can be seen in Figure 4.10 below.

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Figure 4.10

Coastal Processes

Inshore wave roses for waves greater than 1.5m significant wave height

Generally the northern end of the frontage is more exposed than the southern end of the study area. The largest waves approach the northern end of the frontage from the southeast while the largest waves approach the southern end of the study area from the northeast.

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4.3.3

Coastal Processes

Extreme Wave Conditions and Joint Probability of Waves and Water Levels

Whilst the long term wave climate is most relevant in relation to the ongoing sediment transport, extreme wave and water level data is required for the design of any future coastal protection works. An extreme value analysis was therefore undertaken for the waves and winds as part of the study. Extreme Offshore Wave and Wind While extreme water levels are important in coastal flooding, it is the combination of extreme waves and water levels that results in high rates of coastal retreat at the Murrough. An extreme value analysis was therefore undertaken of the offshore waves and winds of the Wicklow coast to allow a joint probability study of extreme wave and water levels to be included in this study. The extreme value analysis of the 3 hourly offshore wind and wave data for the 12 year period January 1990 to December 2001 was undertaken using the MIKE21 EVA toolbox. Work undertaken in connection with the Irish Coastal Protection Strategy study showed that there was a difference in the correlation between extreme waves and water levels for storms from the southeast, east and northeast sectors. Thus, the offshore wave climate was first divided into these direction sectors and an extreme value analysis undertaken for each sector separately.

A similar analysis was undertaken for the offshore wind speeds

associated with these wave events. An example of the EVA analysis for the offshore wave heights for storms from the southeast sector is shown in Figure 4.11. It will be seen that offshore wave events with a return period of 1 in 200 years have a significant wave height in excess of 7.75 m. The EVA analysis for the offshore wave heights for storms from the northeast sector is shown in Figure 4.12. It will be seen that offshore wave events with a return period of 1 in 200 years from this direction have a significant wave height of about 6.5 m.

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Figure 4.11

Extreme Value Analysis of Offshore Wave Heights - SE sector

Figure 4.12

Extreme Value Analysis of Offshore Wave Heights - NE sector

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Joint Probability Analysis As noted above, it is the combination of high waves with high water levels that is particularly influential in potential coastal retreat at the Murrough. A joint probability analysis of waves and water levels was undertaken for storms approaching the Murrough from the southeast, east and northeast sectors.

The joint probability analysis was undertaken using the

techniques and methods recommended by the UK’s Environment Agency and DEFRA in their “Use of Joint Probability Methods for Flood and Coastal Defence” Document FD2308. Previous wave and water level data studies for the south east coast of Ireland undertaken by RPS for the Wicklow Coast show that the correlation between wave height and water levels has a coefficient of 0.6 for storms from the south-southeast and east-southeast sectors while the coefficient for storms from the northeast is only 0.1. These coefficient values were used in the joint probability analysis for the Murrough. Figures 4.13, 4.14 and 4.15 show the results of the joint probability analysis of offshore wave heights and water levels for the south-southeast, east-southeast and northeast storms respectively. The appropriate values of the wave height and water level combinations are shown in tables 4.1, 4.2 and 4.3.

Offshore Wave Height [Hs m]

Joint exceedence curves SSE waves & WL 9.00 8.00

Return period (years) 1 2 5 10 20 50 100 200

7.00 6.00 5.00 4.00 3.00 2.00 1.00 0.00 0.00

0.50

1.00

1.50

2.00

2.50

3.00

Sea Levels at Wicklow [mOD] Figure 4.13

Joint Exceedence Curves – Wave Height & Water Level – SSE sector

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Offshore Wave Height [m]

Joint exceedence curves ESE waves & WL 7.00

Return period (years) 1 2 5 10 20 50 100 200

6.00 5.00 4.00 3.00 2.00 1.00 0.00 0.00

0.50

1.00

1.50

2.00

2.50

3.00

Sea Levels at Wicklow [mOD] Figure 4.14

Joint Exceedence Curves – Wave Height & Water Level – ESE sector

Offshore Wave Height [Hs m]

Joint exceedence curves NE waves & WL 7.00

Return period (years) 1 2 5 10 20 50 100 200

6.00 5.00 4.00 3.00 2.00 1.00 0.00 0.00

0.50

1.00

1.50

2.00

2.50

3.00

Sea Levels at Wicklow [mOD]

Figure 4.15

Joint Exceedence Curves – Wave Height & Water Level – NE sector

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Joint exceedence return period (years) 1 2 5 10 Sea Level Wicklow mOD 1.038 1.160 1.252 1.344 1.466 1.558 1.650 1.772 1.864 1.956 2.077 2.169 2.261 2.475

20

50

100

200

7.66 7.66 7.66 7.66 7.55 7.38 7.19 6.87 6.62 6.34 5.81 5.36 #N/A #N/A

7.78 7.78 7.78 7.78 7.74 7.61 7.46 7.22 6.99 6.73 6.38 6.01 5.56 #N/A

Offshore wave height off Wicklow (m) 6.15 5.98 5.53 5.07 4.47 4.02 #N/A #N/A #N/A #N/A #N/A #N/A #N/A #N/A

6.48 6.48 6.17 5.73 5.13 4.67 4.22 #N/A #N/A #N/A #N/A #N/A #N/A #N/A

6.80 6.80 6.72 6.47 5.99 5.54 5.09 4.49 #N/A #N/A #N/A #N/A #N/A #N/A

7.07 7.07 7.07 6.83 6.50 6.18 5.74 5.14 4.69 #N/A #N/A #N/A #N/A #N/A

7.28 7.28 7.28 7.18 6.86 6.61 6.33 5.80 5.34 4.89 #N/A #N/A #N/A #N/A

7.51 7.51 7.51 7.51 7.30 7.10 6.83 6.51 6.19 5.76 5.16 #N/A #N/A #N/A

Table 4.1 Joint exceedence return period values for wave heights and water levels SSE sector

Joint exceedence return period (years) 1 2 5 10 Sea Level Wicklow mOD 1.038 1.160 1.252 1.344 1.466 1.558 1.650 1.772 1.864 1.956 2.077 2.169 2.261 2.475

20

50

100

200

6.06 6.06 6.06 6.06 5.71 5.23 4.74 4.13 3.66 3.17 2.54 2.07 #N/A #N/A

6.53 6.53 6.53 6.53 6.39 5.92 5.44 4.80 4.32 3.87 3.23 2.75 2.28 #N/A

Offshore wave height off Wicklow (m) 2.90 2.72 2.24 1.77 1.15 0.68 #N/A #N/A #N/A #N/A #N/A #N/A #N/A #N/A

3.38 3.38 2.93 2.45 1.83 1.36 0.89 #N/A #N/A #N/A #N/A #N/A #N/A #N/A

4.02 4.02 3.85 3.37 2.73 2.26 1.79 1.17 #N/A #N/A #N/A #N/A #N/A #N/A

4.46 4.46 4.46 4.06 3.43 2.95 2.47 1.84 1.38 #N/A #N/A #N/A #N/A #N/A

4.95 4.95 4.95 4.72 4.12 3.64 3.16 2.52 2.05 1.58 #N/A #N/A #N/A #N/A

5.59 5.59 5.59 5.59 5.01 4.52 4.08 3.44 2.96 2.48 1.86 #N/A #N/A #N/A

Table 4.2 Joint exceedence return period values for wave heights and water levels ESE sector

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Joint exceedence return period (years) 1 2 5 10 Sea Level Wicklow (mOD) 1.038 1.160 1.252 1.344 1.466 1.558 1.650 1.772 1.864 1.956 2.077 2.169 2.261 2.475

20

50

100

200

5.38 4.88 4.49 4.09 3.49 2.99 2.48 1.80 1.28 0.77 0.09 #N/A #N/A #N/A

5.74 5.27 4.88 4.50 3.96 3.50 3.00 2.32 1.81 1.29 0.61 0.10 #N/A #N/A

Offshore wave height off Wicklow (m) 2.42 1.74 1.22 0.71 0.03 #N/A #N/A #N/A #N/A #N/A #N/A #N/A #N/A #N/A

2.94 2.26 1.75 1.23 0.55 0.04 #N/A #N/A #N/A #N/A #N/A #N/A #N/A #N/A

3.60 2.95 2.44 1.93 1.25 0.73 0.22 #N/A #N/A #N/A #N/A #N/A #N/A #N/A

4.07 3.46 2.96 2.45 1.77 1.25 0.74 0.06 #N/A #N/A #N/A #N/A #N/A #N/A

4.47 3.93 3.47 2.97 2.29 1.78 1.26 0.58 0.07 #N/A #N/A #N/A #N/A #N/A

5.00 4.48 4.09 3.63 2.98 2.47 1.95 1.27 0.76 0.25 #N/A #N/A #N/A #N/A

Table 4.3 Joint exceedence return period values for wave heights and water levels NE sector

Extreme Wave Height and Water Levels Inshore At present, a joint return period of 1 in 200 years is specified as the design standard for new coastal works in Ireland. Thus the range of 1 in 200 year event combinations shown in tables 4.1, 4.2 and 4.3 above were transformed inshore using computational modelling techniques. Due to the presence of the Codling Bank, the most arduous inshore conditions do not necessarily come from largest offshore wave conditions. Therefore, every wave and water level combination was considered in the analysis. The waves were first transformed to the nearshore area using the MIKE21 NSW wave model. The wave transformation through the surf zone to the beach was undertaken using the LITPACK wave profile model as this included the simulation of variations in water level due to wave set-up. Figures 4.16, 4.17 and 4.18 show examples of the wave transformation to the nearshore area. The figures show the wave heights and mean wave directions for the most arduous 1 in 200 year return period event from the south-southeast, east-southeast and northeast respectively. The effect of the Codling Bank on the height of the waves that approach the Murrough can be clearly seen in these diagrams.

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Figure 4.16

Coastal Processes

Significant Wave Height and Mean Wave Direction 1 in 200 year return

period event from the SSE – Offshore Wave Height 6.987m – Water Level +1.86m OD

Figure 4.17

Significant Wave Height and Mean Wave Direction 1 in 200 year return

period event from the ESE – Offshore Wave Height 5.916m – Water Level +1.68m OD IBE0055/AKB/SG/Aug 07

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Figure 4.18

Coastal Processes

Significant Wave Height and Mean Wave Direction 1 in 200 year return

period event from the NE – Offshore Wave Height 5.270m – Water Level +1.28m OD While the largest waves offshore come from the south-southeast, the effect of the Codling Bank is such that the largest waves that approach the majority of the frontage along the Murrough come from east-southeast direction. Figures 4.19, 4.20 and 4.21 shows the variation in the wave heights and water levels for the most arduous combined 1 in 200 year return period event as the waves run in through the surf zone and approach the southern middle and northern sections of the frontage (points C, F and H in Figure 4.10). It will be seen from these diagrams that the maximum significant wave height at the shoreline for the 1 in 200 year event occurs in the middle of the frontage with a significant wave height value of 3.927 metres with mean wave periods of 9.38 seconds.

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Figure 4.19

Coastal Processes

Variation in significant wave height and mean water level across the

profile C during a combined 1 in 200 year return period event

Figure 4.20

Variation in significant wave height and mean water level across the

profile F during a combined 1 in 200 year return period event

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Figure 4.21

Coastal Processes

Variation in significant wave height and mean water level across the

profile H during a combined 1 in 200 year return period event Impact of Sea Level Rise The impact of sea level rise due to global warming by 2100 was simulated by re running the extreme wave analysis with the water levels increased by 0.5m. The results of the analysis showed that for a 1 in 200 year return period joint probability of waves and water levels, the maximum wave height at the middle of the frontage would increase from 3.927m significant height to 4.137m significant height.

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4.3.4

Coastal Processes

Sediment Transport

The sediment drift along the frontage was modelled by simulating the transport across eight coastal profiles A – H as shown in Figure 4.22. The simulations were run for the full 12 years of 3 hourly wave data that had previously been transformed to the nearshore area. The model simulations were undertaken using the LITDRIFT programme which uses the wave data together with the coastal profile and distribution of sediment to calculate the movement of the sediments at each 3 hourly time step. The output of the model gives the amount of sediment moving both north and south along the frontage as well the distribution of the sediment transport across the coastal profile. Figure 4.22

Coastal Profiles

The waves that break along the shoreline lift the bed sediments, which are then transported along the shoreline by the wave driven currents.

Thus the area of greatest sediment

transport due to wave action tends to be relatively close to the shoreline. Figures 4.23 to 4.26 show the distribution and magnitude of the average annual sediment transport at profiles A, C, F and H. (Figure 4.22). In Figures 4.23 to 4.26, the red line represents the amount and distribution of sediment moving from south to north while the blue line represents the amount and distribution of sediment moving from north to south. The purple line in these diagrams shows the resulting net sediment drift. It will be seen from the figures that only a relatively small amount of sediment by-passes Wicklow Harbour and that the general net drift is from south to north.

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Figure 4.23

Sediment Transport – Profile A (Wicklow Harbour)

Figure 4.24

Sediment Transport – Profile C (southern part of frontage)

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Figure 4.25

Sediment Transport – Profile F (central part of frontage)

Figure 4.26

Sediment Transport – Profile H (northern part of frontage)

It will be seen from these figures that the main area for longshore sediment transport is in the coarse sands that lie in the lower part of the beach with the net drift being from south to north. There is only a small amount of longshore drift of the shingle with the sediment transport being mainly cross-shore draw down during storm events. The tides alone are not sufficiently strong along the frontage to move the coarse sediment along the shoreline. However during storms the tides will move sand in the offshore area. As already noted in Section 4.2.2, Tidal Flows, there is a back eddy in the flood tide caused by Wicklow Head thus sand will tend to move in a southeasterly direction in the southern part of the study area.

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