Wicklow County Council Study Report The Murrough Coastal Protection Study Coastal Processes IBE0055/AKB/SG/Aug 07 9 t 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 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 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
4.2.2 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 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 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 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 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 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 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 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
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Figure 4.16 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
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Figure 4.18 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 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 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 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
