ydro
Alexandra Basin Redevelopment Project
Strategic Infrastructure Application
Review of Coastal Processes Related
Issues
Environmental Ltd
No. 4 Caiseal Riada,
Clarinbridge,
Galway, Ireland.
tel +353 91 796734
email: [email protected]
For An Bord Pleanála
Ref:-29N.PA0007.
Report No HEL 085101 v1.1
10th December 2014
1.
INTRODUCTION
1.1.1 Anthony Cawley senior hydrologist and director of Hydro Environmental Ltd. was
appointed by An Bord Pleanála to examine and advise the Bord in respect to the
marine hydrology and flooding issues arising from the proposed Alexandra Basin
Redevelopment Project (ref PL29N.PA0034).
1.1.2 The brief given by An Bord Pleanála in respect to this review is summarised as
follows:
1.
2.
3.
4.
5.
6.
The effectiveness of the Coastal Process Models used by the applicant and the
adequacy of the simulations for the
 Tidal Regime,
 Wave Climate,
 Sediment transport, including patterns of erosion and deposition,
 Sediment dispersal from the Burford Bank, and
 Dredging Plumes.
The accuracy of the collected data and site surveys
The accuracy of the model simulations for storm events
The capacity of the Burford Bank to accommodate the dredged material
The potential for changes to the tidal regime, wave climate and sediment transport
(erosion and deposition) to the North and South of the section of channel that runs
from the East of the port to the North Bull and Poolbeg lighthouses.
The potential for changes to the tidal regime, wave climate and sediment transport,
and in particular the potential for sediment deposition on the south and east coast of
Howth and the Coast of Dalkey Island and the Muglins (in the vicinity of the Annex 1
Reefs which are indicated on the NPWS maps for the Rockabill to Dalkey Island c.
SAC).
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7. The adequacy of the sampling methodology and the sampling depths for the
detection of contaminated material in Alexandra Basin, the Liffey Channel and the
approach channel and the accuracy of the results.
8. The accuracy of the simulations and predictions contained in the flood risk
Assessment for Dublin Port and Clontarf.
9. The issue of Climate Change (rising sea levels) and its implications (if any) for the
proposed development.
2.
REVIEW METHODOLOGY
2.1.1 The assessment involved a full review in respect to the coastal processes and marine
hydrology issues of the planning application and supporting EIS and modelling
studies, the Flood Risk Assessment and NIS submitted as part of the Planning
Application and the response to further information request from An Bord Pleanála
(August 2014) and the various submissions and observations from third parties,
Statutory and Non-Statutory bodies and the Dublin City Council Planning Authority.
2.1.2 The attendance at the oral hearing proceeding that commenced on the 8th October
2014.
2.1.3 A final review of all documentation including the various briefs of evidence and
submissions to the oral Hearing and the oral hearing transcripts.
2.1.4 The Coastal Process and Flood Risk Assessment published in the EIS and the
response to further information was carried out by Mr Adrian Bell of RPS on behalf of
the Applicant.
3.
PROPOSED DEVELOPMENT
3.1.1 The application is for the redevelopment of Alexandra Basin and Berths 52 and 53
with associated works in Dublin Port and Dublin Bay. The works will include the
dredging and reconfiguring of Alexandra Basin to Accommodate larger vessels,
including Cruise Ships).
3.1.2
A summary of the proposed development works are:

The works will include the dredging and reconfiguring of Alexandra Basin to
Accommodate larger vessels, including Cruise Ships) including construction of
new quays and jetties, remediation of contamination on the bed of the basin,
capital dredging to deepen the basin and to achieve the specified depths of 10m Chart Datum (CD) at the new berths.
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
the infilling of the basin at Berths 52 and 53 and the Graving Dock No. 2 to
provide additional storage space with new quayside berths with a double tiered
Ro-Ro ramp.

Deepening of the fairway and approach channel to Dublin Port to increase the
ruling depth from -7.8m CD to -10.0m CD.

It is also proposed to provide a 220m long surge protection/retaining wall at
Poolbeg Marina with a new floating pontoon along the edge of navigation
channel at Poolbeg Marina. This structure will also serve to protect vessels
moored in the marina from the wash produced by vessels manoeuvring in the
vicinity of North Wall Quay Extension.
3.1.3 The works will include extensive capital dredging of the basins, the Liffey channel
and approach channel to Dublin Port over a distance of 10.3km producing an
estimated overall capital dredging volume of 6.37 million m3. This estimate is broken
down into 3.2million m3 of mainly fine sand from the outer channel east of the Bulls,
2.7million m3 of mainly silty material from the inner harbour channel and 0.47m3 of
mainly silty material from the Alexandra Basin West and the adjoining channel. This
indicates that approximately 5.9 million m3 of capital dredge will be disposed of in the
Burford Bank disposal site over a construction period of 6 to 10years.
3.1.4 The sediments within the Alexandra Basin were found to be unsuitable for disposal at
sea, and for direct use as fill material, they were found not to meet the criteria for
inert waste. The proposal for these contaminated dredged sediments is to stabilise
on site and reuse as fill material within the Berth 52/53 and the Graving Dock No. 2.
The proposed treatment of the contaminated sediment will be through a process of
stabilisation / solidification involving the addition of a stabilisation mix that physically
reduces the mobility and chemically binds contaminants to the produced solid matrix.
The actual stabilisation mix will be designed specifically for the sediment in the
Alexandra Basin and will involve lab and field tests to arrive at the most suitable
binder mix for stabilisation.
4.
Potential Hydrological Impacts of Development
4.1.1 The principal sources of hydrological and potentially ecological impact from the
proposed Alexandra Basin Redevelopment Project are summarised as follows:

Dredging works within the Alexandra Basin and within the Navigation Channel
and the potential generation of suspended sediments and sediment deposition
within estuarine and coastal waters of Dublin Bay and the potential release of
contaminated sediments as a result of the disturbance.

The suitability of the proposed disposal site west of the Burford Band and the
long term fate of the sediments deposited there.

The effect of the deepened navigation channels on coastal processes –
hydrodynamics, wave climate, sediment transport and dispersion.

The stability of the navigation channel and its potential future maintenance
dredging requirements.
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
5.
Flood Risk Assessment and compliance with the Flood Risk Management
Planning Guidelines (DoEHLG,2009).
Characterisation of Dredge Material and Disposal
5.1.1 Sediment samples were taken in August 2013 by Hydrographic Syrveys Ltd. Cork
from 16 locations within the Alexandra Basin, 7 locations within the navigation
channel inside the Bulls and a further three in the navigation channel east of the
Bulls. These samples were taken generally at 2 to 3 depths (surface, 1m and 2m).
These were analysed by the National laboratory Services, Environment Agency for
England and Wales for a suite of contaminants specified by the Marine Institute. The
sediment results were compared against Marine Institute Guidelines for the
assessment of dredge material for disposal in Irish Waters. This review concluded
that the sediments within the Alexandra Basin West to be unsuitable for disposal at
sea. These sediments were also assessed for their suitability for use for fill material
and were found not to meet the criteria for inert waste.
5.1.2 The proposal for these contaminated dredged sediments from the Alexandra Basin
West is to treat on site and reuse as fill material within the Berth 52/53 and the
Graving Dock No. 2. The proposed treatment of the contaminated sediment is
through a process of stabilisation / solidification involving the addition of a
stabilisation mix that physically reduces the mobility and chemically binds
contaminants to the produced solid matrix. This treatment process is a well used
method for remediation and reuse of contaminated sediments.
5.1.3 The actual stabilisation mix will be designed specifically for the sediment in the
Alexandra Basin and will involve lab and field tests to arrive at the most suitable
binder mix for stabilisation.
5.1.4 The proposed dredge sediments from the upper section of the Liffey Channel
adjacent to Alexandra Basin West are classified by the Applicant as slightly to
moderately contaminated material suitable for sea disposal with conditions and the
remainder of the Navigation channel Seaward as material suitable for sea disposal.
5.1.5 It is considered that the number of sediments taken in the Navigation channel
between Alexandra Basin West to Bull Wall are too sparse to allow a determination of
the dredge area sediments that can be classified as moderately, slightly and
uncontaminated.
5.1.6 Within the channel sampling at surface, 1m and 2m depths is considered sufficient to
establish the vertical profile of contamination. Where sampling is located at higher
ground (i.e. on channel banks) then the depth of sampling needs to increase so as to
provide a profile down to the proposed channel base of -10m Chart datum.
6.
COASTAL PROCESSES MODELLING
6.1
Hydrodynamic Modelling
6.1.1 The impact of the Dublin Port proposal development on Coastal Processes in Dublin
Bay and the Liffey Channel was assessed by the Applicant using a coupled MIKE 21
hydrodynamic, wave and sediment transport model. MIKE 21 is internationally
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recognised as one of the leading computational hydraulic software programmes for
such applications. MIKE21 provides the option of a flexible mesh finite volume code
which allows the mesh resolution to vary in size within the study area for
computational efficiency. This allows for increased resolution in the immediate area
of interest or where complicated geometry dictates a higher resolution. The model
was set up for the Irish Sea with high refinement in the Dublin Bay and Dublin Port
Area having maximum mesh size equivalent of 500 by 500m and scaling down to a
minimum of 15m by 15m within the River Liffey channel and port area. High
resolution of the grid was demonstrated in the vicinity of the Bull Walls and within the
Liffey channel.
6.1.2 The model resolution within the area of interest Dublin Bay and Dublin Port, Liffey
and Tolka Estuaries is considered sufficiently refined to assess accurately the effect
of the proposed development on the tidal regime.
6.1.3 This hydrodynamic model is 2-dimensional in the horizontal plane (easting and
northing) and depth averaged in the vertical. Within Dublin Bay, outside of the
stratification influence from the Liffey and its tributeries it is generally accepted and
demonstrated by various studies and measurements that the water colume is well
mixed vertically and can be reasonably represented by a 2D hydrodynamic model.
6.1.4 To support the hydrodynamic model the following surveys were carried out during
June and July 2013.
 Bathymetric and geophysical survey of basin areas and Harbour channel area
and approaches by Hydrographic Survey’s Ltd
 Current metering surveys at two sites in Dublin Bay using Acoustic Doppler
Current Profilers deployed for a lunar 1month period.
 Sediment sampling, including particle size analyses (PSA) at 36 sites covering
the harbour channel and approaches
 It is understood that tidal data from Dublin Port and offshore gauges was also
available
6.1.5 The data input to the model as described include a recent bathymetric survey of the
shipping channel basin and approaches carried out in Hydrographic Surveys Lts. In
2013 and supplemented by the Irish National Seabed Survey, INFOMAR and other
unnamed local bathymetric surveys. The open sea tidal boundaries to the Dublin
Bay model were taken from the ICPSS tidal Surge Model (Irish Coastal Protection
Strategic Study for the OPW). The ICPSS east coast wave model was used to
gather wave boundary data for the Dublin Bay Model. The Dublin Bay model extends
south of Bray to north of Portmarnock and 5km east of Howth Head. River flow
boundaries for the Liffey, Dodder and the Tolka were specified as flow hydrographs
in the model.
6.1.6 The Dublin Bay Model was calibrated against the Accustic Doppler Current Profilers
(ADCP’s) which recorded tidal velocities, directions and water depths over a lunar
month survey period. The hydrodynamic model was further verified against previous
hydrodynamic studies including ADCP data from the Ringsend Waste Water
Treatment Plant outfall study (2010) by the Danish Hydraulics Institute (DHI) which
deployed an ADCP recorder 500m west of the Burford Bank. The three ADCP’s site
locations are reasonably representative of the study area near the navigation
channels and the proposed disposal site. For practical reasons the ADCP’s could not
be deployed directly in the navigation channels due to potential for damage and
disturbance by ships and appled adjacent to the channels.
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6.1.7 The Dublin Bay Model was calibrated against the two ADCP sites (1 to 3) which
recorded tidal velocities, directions and water depths over a lunar month survey
period and further model verification was performed against previous hydrodynamic
studies including ADCP data from the Ringsend Waste Water Treatment Plant DHI
outfall study (2010) which deployed an ADCP recorder 500m west of the Burford
Bank.
6.1.8 The calibration and verification results presented in the EIS and further clarified in the
RFI response demonstrate that the Dublin Bay hydrodynamic model is fit for purpose
in respect to simulating tidal levels and tidal circulations within Dublin Bay and Liffey
channel.
6.1.9 A 3-dimensional MIKE 3 hydrodynamic model was developed for the immediate and
inner Dublin Bay area in order to simulate the presence of salinity gradients within the
water column (salt wedge) from the Liffey and its tributaries. This model was set up
using the same bathymetry and boundary condition sources as the 2D depth
averaged model. The model was run for 5 vertical layers whose layer thickness
directly varies with depth (one fifth of the depth, a 10m depth will be simulated by 5
no. 2m layer depths).
6.1.10 In the EIS the 3D modelling calibration is presented in Appendix 7 using the ADCP
recorders at location 2 and 3 and shows comparisons of velocity and direction
(modelled and measured) for three vertical locations (i.e. near bed, mid-column and
near surface). These comparisons show reasonable agreement at each of the three
vertical depths. The model was not calibrated against salinity profiles within the
study area and therefore the study does not demonstrate the ability of the model to
reproduce accurately the density variation under different river flows, tides and
meteorological (wind, temperature) conditions in terms of horizontal and vertical
water column variations in salinity within the Liffey Channel.
6.1.11 In respect to modelling the complex density stratification effect on the tidal flows
produced by baroclinic forces, the use of only five vertical layers is unlikely to be
sufficiently refined to accurately represent the salinity wedge in the vertical,
particularly under varying freshwater inflows and is likely to result in high numerical
dispersion between layers resulting in almost barotrophic, well mixed density
conditions. A justification for the selection of a 5 layer model was given in the
response to the RFI quoting examples by DHI (2010 and 2006) and Bedri et al (2011)
which also used a 5 layer 3-D set-up to model Dublin Bay. It has not been
demonstrated in the EIS or in the Further information submitted that the 3D
hydrodynamic modelling carried out is capable of representing reasonably accurately
the salinity (density) gradients within the water column of the Liffey Channel.
6.1.12 Notwithstanding the limitations of the 3D hydrodynamic model in terms of calibration
and vertical refinement, the model is considered sufficiently calibrated and refined to
model the potential impact of the proposed changes in bed bathymetry within the
basin and navigation channel on the tidal regime and tidal flows and levels. The
hydrodynamic modelling is also considered sufficiently accurate for use in the
sediment transport simulations of suspended sediment concentrations and in
predicting the resultant deposition from the proposed capital dredging operations.
6.1.13 The simulation results show clearly that the proposed capital dredge scheme does
not affect the tidal curve of extreme flood levels within the Bay and in particular within
Dublin Port and the Tolka Estuary.
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6.1.14 The simulations show a reduction in velocities within the deepened Liffey channel
with the largest change predicted in the channel just landward of the Bull Wall
entrance as a result of the increased channel flow area.
6.1.15 The flood risk simulations show, for a combined extreme tide and 100year river flood
flow, no perceptible impact on the tide/flood levels as a result of the proposed capital
dredge scheme and thus it is concluded that the proposed development will not
exacerbate tidal flooding within Dublin Bay.
6.2
Wave Climate Modelling
6.2.1 The wave climate simulations use the MIKE 21 Spectral Wave (SW) Model. This
model was built using the same computational mesh as the 2D hydrodynamic model
for Dublin Bay using same sea bed bathymetry and mesh structure and topology. It
is stated that this model allows for wave-breaking, wave current interaction, wave
diffraction, wave refraction, wind-wave generation wave energy reflection along
Boundaries.
6.2.2 The model open sea boundary is driven by the ICPSS East Coast Wave Model. The
model is used to assess the effect that the proposed channel dredge works has on
the wave climate within Dublin Bay and whether it has a potential to impact on flood
risk. The wave model was also coupled with the hydrodynamic model to drive the
sediment transport and bed evolution simulations for assessing morphological
change.
6.2.2 Its wave definition at the offshore open sea boundary is well founded being
generated from a large scale calibrated and validated East Coast wave model
developed by RPS as part of the Irish Coastal Protection Strategic Study. For this
study the Dublin Port wave model was not calibrated against any inshore wave data
(no deployment of a Wave Ryder buoy) and therefore its accuracy has not been
demonstrated in respect to the inshore predictions. The predictions from this model
do not appear to have been compared with other previous studies or measurements.
6.2.3 The inshore wave climate in Dublin Bay and within the Liffey channel and Tolka
Estuary has been derived by transforming wave data from past storm events inshore.
It is acknowledged that this approach has been extensively used in many other
comparative studies without the availability of inshore wave measurements. Such
approach is currently being used in the OPW cFRAMs / ICPSS studies for the Irish
Coastline inundation studies.
6.2.4 In the EIS the wave-current interaction was not modelled and therefore the effect of
potentially increased or reduced velocities or change in velocity direction arising from
the deepened and straightened navigation channel were not assessed. It was
considered by the consultants that such an effect on the wave climate would not be
significant, based on their experience. To support this claim wave simulations that
included the wave-current interaction with and without capital dredge were submitted
to the oral hearing. The wave plots showed little or no discernible change in wave
climate magnitudes with and without the wave-current interaction.
6.2.5 This model is generally not very applicable to complex harbour applications in which
wave reflection and diffraction processes are important. The suitability of the spectral
wave model is questionable for the inner Liffey and Tolka area given the importance
of wave diffraction and wave reflection.
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6.2.6 Given that the proposed development involves the deepening of the navigational
channel as opposed to a new infill structure, that would directly reflect and diffract the
wave field, and the fact that the assessment is primarily for comparative purposes it
is considered that the MIKE 21 Spectral Wave Model is sufficiently accurate to
quantify the potential wave climate impact and flood risk implications.
6.2.7 Wave Climate simulations were carried out using offshore wave data from the UK
Met Office European Wave model for a 15year record period (1989 to 2004) and
used to select the largest event for each of the northeast, east and southeast
directions. These events had off shore waves with wave significant heights of 4.6,
4.1 and 5.4m and peak wave periods of 8.9, 8.8 and 10.4seconds respectively.
6.2.8 Simulations with and without the proposed capital dredge scheme were carried out
for these events and difference plots produced to compare the impact on wave
heights of the pre and post Capital Dredge scenarios. The difference plots indicate
that up the Liffey channel adjacent to the Alexandra Basin storm wave heights will
potentially be increased by between 0.1 and 0.2m. Within the Lifffey channel closer
to the Bull Wall channel entrance wave height increases of 0.4 to 0.5m are predicted
under easterly waves and also wave height increases up the Tolka estuary towards
Clontarf road of 0.1 to 0.2m under southeasterly. These increases may potentially
have ramifications for flood Risk along the Liffey Channel shoreline and at Clontarf.
This is discussed later in Section 6.5.
6.2.9 Additional simulations were carried out as part of the response to the RFI to
investigate the effect of tidal current refraction on the imposing wave field. The model
simulations which investigated a southeasterly storm event at mid flood and mid-ebb
showed some small differences in wave height as a result of current refraction but
these effects did not impact the comparative difference between the pre and post
dredge cases presented in the EIS report and the conclusions reached.
6.3
Sediment Transport Modelling
6.3.1
Dredging Operation
6.3.1.1 Two different sediment transport models were used to assess the proposed
development, A MIKE 3D model was used to assess the capital dredge activities
within the Liffey channel area and a MIKE 2D sediment transport model was used to
assess the fate of the dredge material to be disposed at the proposed offshore spoil
disposal site near the Burford Bank. These models were coupled to their respective
3D and 2D hydrodynamic models. The calibration of these models is based on the
hydrodynamic model calibration discussed earlier. There was no specific calibration
exercise carried out for the sediment transport module in terms of settling velocities,
dispersion coefficients, critical shear stresses for resuspension etc. This is generally
the case with the application of sediment transport models, with selected coefficients
being based on theoretical and peer reviewed empirical formula and coefficients.
6.3.1.2 Winter flows in the Liffey, Tolka and Dodder were specified in the model along with
the Power Station inflows and discharge flows and cooling water temperatures from
the North Wall, Synergen and Poolbeg plants.
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6.3.1.3 A total of 36 seabed grab samples distributed over the proposed navigation dredging
area were collected for sediment size distribution analysis. This sediment distribution
data was used to define the fraction size to be modelled for the dredging operation
and disposal site simulations.
6.3.1.4 Within the Liffey channel section a silt was modelled having three equal fractions of
3µm, 20µm and 200µm at a loss rate of 1% at in the near bed layer and 1% as an
overspill at the surface.
6.3.1.5 The proposal is to confine dredging operations within the Liffey channel to winter
months only (October to March) in order to negate any potential impact on salmonid
migration (particularly smolts) and summer bird feeding, notably terns, in the vicinity
of the dredging operations.
6.3.1.6 Separate six month winter simulations were carried out using MIKE 3 model to
represent the winter dredge program for the inner, middle and Outer Liffey Channel
sections. Model output of suspended sediment were presented at the four principal
stages of the tidal cycle during the dredging operation when suspended sediments
were at their highest concentration in the Tolka Estuary. Silt deposition plots were
presented at the end of the six month dredging simulation for the inner, middle and
outer Liffey channel dredging operations. Peak and mean suspended sediment
concentrations at the Cooling intakes to the three power stations were also presented
in the EIS. It was concluded that the sediment impact on the intake flow was not
significant.
6.3.1.7 No sediment transport model simulation results were presented in the EIS for the
proposed capital dredging works within the approach navigation channel east of the
Bull Walls. The nature of the sediment to the east of the Bull Walls being primarily a
fine sand suggest a significantly lesser impact than the dredging operations within the
Liffey Channel inside the Bull Walls as the sand will settle rapidly at or near the
dredging source with limited ability to disperse away or remain
suspended/resuspended.
6.3.1.8 The sediment transport model simulation results presented in the EIS of the dredge
plume are considered to be representative of the likely impact that the proposed
dredging operation will have on suspended and deposited sediment concentrations
within the Liffey Channel and adjoining Tolka Estuary (i.e. inside the Bull Walls). The
simulations indicate that outside of the immediate dredge operation area the
maximum suspended sediment concentration are less than 25, 30 and 50mg/l for the
outer, middle and inner dredge simulations respectively. The simulations show the
deposition rates at the end of the six month dredging campaign are almost
imperceptible at between 0.05 and 0.2µm sediment depth (i.e. 0.00005mm and
0.0002mm).
6.3.1.9 The simulations for dredging operations within the inner Liffey Channel indicate the
potential for high sustained suspended solids spreading across the entire liffey
channel width and based on the EIS output plots having sustained concentrations in
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excess of 50mg/l. Dredging in the middle and outer channel section are shown to
produce lower sustained concentrations of less than 20mg/l.
6.3.1.10
To lessen the suspended solids concentration in the inner Liffey It is
recommended that overspilling at the surface of the dredger is avoided for all
dredging activities within the inner Liffey channel section. This will significantly
reduce significantly the potential for elevated suspended sediment concentration from
the dredging operation. It is recommended that outside of the immediate dredging
zone that the increase in suspended sediments over the ambient does not exceed a
maximum concentration of 50mg/l.
6.3.2
Capital Dredge Disposal Sediment transport modelling
6.3.2.1 The 2-D MIKE 21 hydrodynamic and sediment transport coupled model was used to
simulate the fate of the dredge sediment to be deposited at the proposed licensed
disposal site located to the West of the Burford Bank. It is noted that disposal of
dredge material at this site will be subject to a Dumping at Sea Permit by the EPA
involving its own consent process and approval by the EPA.
6.3.2.2 Tidal velocities and wave climate measurements were not available for this disposal
site and calibration of the hydrodynamic model was based on ADCP data results
from a DHI(2010) study for a location 500m away to the southwest. The dredge
material to be deposited there is made up of a fine sand seaward of the Bulls and a
fine silt from within the Liffey Channel landward of the Bull walls. The simulation
assumed that the volume of dredge material to be disposed of at the site was evenly
distributed over the 6year period representing 0.177million m3 per month and is
deposited on a 24/7 basis over the 6month winter dredging campaign. The
simulation assumes that the 6year disposal campaign deposits evenly over the full
licensed area.
6.3.2.3 The silt simulation results show that a small portion of the silt remains within the
disposal site with the majority of it re-suspended and dispersed widely by the
relatively strong north-south ambient currents. What remains on the site is the
heavier 200µm sand fraction of the silty sediment dredged from the Liffey channel
producing a maximum depth at the disposal site of 30mm. Deposition plot of the fine
silt at the end a 6month campaign indicates that potential deposition rates are less
than 0.15g/m2 (represents a sediment depth of approx 0.0001mm per m2) within the
majority of the Bay and slightly higher silt values shown near the shorelines on the
south Bull with deposition depths of < 0.3 g/m2 (<0.0002mm per m2) of fine silt.
These levels are imperceptible. The suspended sediment plots show high dispersive
movement with the ambient current both in north and south directions indicating that
a high proportion of the silt will disperse up and down along the Irish Coastline.
Impact to the intertidal and sub-tidal reef habitat (1170) s within the Rockabill to
Dalkey Island SAC based on the model output is shown to be less than 0.0001mm
per m2, which is imperceptible in terms of natural siltation in coastal areas.
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6.3.2.4 The fine sand simulation modelled 3 fractions of 60 100 and 200 µm at a deposit rate
of 108kg/s which is equivalent to 0.177 million m3 of spoil per month. The modelling
simulations show that virtually all of the sand fractions remain at the disposal site
under normal tidal flow conditions with the velocity magnitude unable to re-suspend
the fine sand fractions. Sediment transport simulations under storm wave conditions
from the southeast were examined using coupling of the sediment transport the
hydrodynamic and wave climate models. A ten day south easterly storm event was
simulated and the sediment deposition results at the end of Storm shows that
approximately 20% of the sand was removed from the disposal site and generally
deposited in the direction of the Wind towards the Howth Headland and the North
Bull with some deposition shown within the mapped reef habitat area at Howth Head
of 30 to 50mm based on Figure 9.51 in the EIS. Such sand deposition rates are not
considered significant given the natural sediment budget and the exposed nature to
wave Climate of Howth Headland area, with such deposition being transitory.
6.3.2.5 The sediment transport simulations show that the silt fraction deposited at the
disposal site will be dispersed widely by tidal flows whereas and a large portion of the
fine sand fraction will remain insitu under tidal flows and is only removed by storm
wave conditions with the dominant wave direction from the south and southeast.
Under such a transport mechanism the sand is likely to remain in Dublin Bay and will
rejoin the natural sediment Budget of the Bay. The simulations indicate no
depositional hot spots that will end up receiving significant amounts of sediment from
the disposal site. The depositional impact on the reef habitat within the Rockabill to
Dalkey Island SAC and the Lamby Island SAC based on modelling results is
considered to be a slight to imperceptible impact and transitory in nature given the
exposure of the reef areas to wave climate and tidal currents.
6.3.2.6 The following mitigation from the EIS is noted and recommended for the disposal at
sea of contaminated sediments: Sediments immediately adjacent to Alexandra Basin
West where low levels of contamination have been found will be disposed of at sea
only at slack tide and immediately covered by sand or gravel, if required by the EPA.
All dumping operations will be undertaken according to industry standards and will be
conducted in accordance with any restrictions imposed by the Dumping at Sea
Permit which is issued by the EPA. Furthermore, all dredgers will have GPS track
plotters fitted to ensure that they remain on course throughout the entire
dredging/dumping operation.
6.3.2.7 The ability for storm wave events to gradually remove the sand fractions from the
disposal site has implications for the grading of a capping layer that is proposed
should it be deemed necessary for mitigation of the sea disposal of the slight to
moderate contaminated sediment from the Liffey Channel section. It is proposed in
the EIS that the source of the capping layer is from the outer approach channel
dredging operations. The grading of this capping layer needs to be designed taking
cognisance of the ambient hydrodynamics and wave climate at the disposal site.
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6.3.2.8 It is recommended that a hydrographic monitoring station at the disposal site be
established for the duration of the six year capital dredge operation. Monitoring of
the hydrodynamics and wave climate (ADCP and Wave Rider buoy), turbidity, salinity
and temperature at the disposal site should be carried out.
6.4
Channel Morphology
6.4.1 A review of previous maintenance dredging records and channel bathymetry surveys
(annual surveys 2007 to 2012) is presented in the EIS. The review indicates a
consistent trend of deposition along the north side of the approach channel from the
end of the North Bull Wall towards Buoy No. 3. The sediment source for this is most
likely to be predominantly the littoral drift and wave erosion from the south side of the
channel.
6.4.2 The records also indicated the width of the approach channel landward of the Bull
walls had a tendency to narrow. The source of this sediment was identified as being
most likely fluvial sediments (primarily silt) which settle in the channel and are
displaced from the centre to the sides by manoeuvring ships.
6.4.3 Sediment transport morphological modelling was carried out to assess the impact of
the capital dredge scheme on the sediment transport regime and the stability of the
existing and proposed approach channel taking account of tides, waves, and seabed
evolution changes. To account for the morphological changes on hydrodynamics the
predicted changes to the sea bed level was updated in the hydrodynamic and wave
climate models automatically throughout the simulation period.
6.4.4 Similar to the wave climate modelling the largest offshore storm events from the UK
15 year record for wind directions north east, east and south East directions were run
in combination with high spring tides to ensure the most arduous conditions for
sediment movement in the Bay.
6.4.5 To assess the impact of the proposed scheme a comparison between pre and post
capital dredge bed levels was presented as a difference plot for three wave storm
directions.
6.4.6 This modelling approach is considered to be accepted practice in assessing
morphological impacts and includes storm information for the Irish Sea over a 15
year observations period (1989 to 2004). This has the advantage of providing actual
realistic storm wave profiles that reflect the variation and duration of a storm event,
making the design inputs realistic.
6.4.7 Under northeast storm the main evolution changes are to the south Bull bank and
along the north bull island shoreline area with largest change in the vicinity of the
north and south Bull Walls. It is evident from the difference plot that similar evolution
patterns occur under the pre and post capital dredge scenarios. The northeast storm
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direction is shown to have little impact on the approach channel in terms of sea bed
change.
6.4.8 The pre and post capital dredge simulations for the easterly storm event show similar
patterns of bed evolution with the main areas of change occurring in the vicinity of the
North and South Bull walls and some changes to the south and north of the approach
channel near the Liffey Bull wall entrance. Near the entrance local deposition on the
north side of the approach channel and local erosion on the south side is predicted.
The difference plot indicates that the post capital dredge case will have higher
deposition in the approach channel near the Bull entrance than the existing (pre
capital dredge case) under storm events.
6.4.9 The southeast storm event represents the most severe wave climate of the three
directions considered. Similar to the other directions it showed no significant
difference in evolution change to sea bed levels between the pre and post Capital
dredge scenarios outside of the channel itself. In both cases the main evolution
change occurs in the vicinity of the Bull walls, in the vicinity of the Liffey entrance, to
the south Bull sand bank and to the north Bull island shoreline. The simulations
show deposition to the northside of the channel and erosion to the southside of the
channel near the Bull Wall entrance. The difference plot indicates that the post
capital dredge case has higher deposition in the approach channel near the Bull
entrance than the existing (pre capital dredge case).
6.4.10 The main conclusions that can be reached from these simulation results are:

The proposed new channel will not result in any significant change in the
morphological response of the sea bed in Dublin Bay outside of the channel area.

Similar to the existing case there will be a tendency for deposition towards the
northern side of the channel and erosion to the southern side.

The simulation output for the east and southeast storm directions indicate an
increase in deposition within the approach channel near the Bull wall entrance under
the proposed capital dredge case. Given the localised nature of this increase it is
unlikely that magnitude of future maintenance dredging will be significantly increased.
6.4.11 The effect of the Liffey channel capital dredge on deposition rates of river silt does
not appear to have been evaluated in the EIS. The hydrodynamic impact of the
capital dredge scheme is to lower the tidal velocities by 5 to 15% on both ebbing and
flooding tides based on the time series plots provided in the response to the RFI.
This lowering of the velocity is likely to allow for slightly more favourable conditions
for silt deposition within the channel. It is anticipated that increased ship activity is
likely to mobilise this silt.
6.4.12 Based on the above it is expected that the new channel will require maintenance
dredging of a slightly higher magnitude to that required by the existing channel.
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6.5
Flood Risk Assessment
6.5.1 A flood risk assessment was carried out for the development and presented in
Chapter 10 of the EIS. The objective of the study was to assess the flood risk at the
proposed development and assess the potential change in flood risk to the
neighbouring areas as a result of the development.
6.5.2 The study examined the various sources of flood risk to the development and to
adjacent neighbouring areas and concluded that the flood risk to the site comes from
coastal flooding and that other sources of flood risk, fluvial (Liffey and tributaries),
pluvial flooding, storm water flooding, groundwater flooding and foul drainage does
not pose a significant flood risk. This finding applies also to the neighbouring
shoreline areas that are vulnerable to flooding.
6.5.3 This assessment examined the combination of extreme tides, extreme waves and
extreme fluvial floods as part of the flood risk assessment study. The fluvial flood
flows used to describe the 100year and 1000year return period events are
considered to be appropriate and represent current best available estimates. The
study also concluded that in the vicinity of the Port area the impact of fluvial floods
both individually and combined with tides is not significant and does not influence the
flood risk for either the development and the surrounding areas including the existing
Port lands and Clontarf Sea Frontage.
6.5.4 Combination of these other sources of flood risk with coastal flooding does not
generate significant flood risk at the key return periods of 200year (threshold for
Flood Risk Zone A – High Probability of Flooding) and 1000year (threshold for Flood
Risk Zone B – Moderate Probability of Flooding).
6.5.5 Coastal Flooding results from a combination of high tide levels and storm surges
generated during extreme storm events. A 2D MIKE21 flexible mesh hydrodynamic
model developed by the Consultants RPS and used to inform the Irish National
Coastal Protection Strategy for the OPW was used to predict extreme tidal levels
adjacent to Dublin Port and within the Tolka Estuary. This model was calibrated
against a wide range of tidal measurements from various locations around Ireland
including gauged data from the Main Ports such as Dublin, Belfast, Cork, Galway etc.
and along the relevant UK coastline. This model predicted the 200year tide to be
3.07 and the 1000year tide to be 3.24m O.D. Malin.
6.5.6 A long record of tide levels is available for Dublin Port (continuous from 1924 to date)
on which to statistically estimate the 200year (0.5% APR) and the 1000year (0.1%
APR) return period tide levels at Dublin Port gauge. This record included the highest
storm surge event of 2.95m OD occurring on the 1st February 2002. Such estimates
are relevant for the entire sea frontage area along the Liffey, Dodder and Tolka
Estuaries. Statistical analysis of the is relatively long high tide level record suggest
200 and 1000year surge tide levels of 3.13m OD and 3.31m OD. This compares
reasonably well with the RPS model estimates of 3.07 and 3.28m OD.
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Inclusion of Climate Change
6.5.7 The current guidance in Ireland for the application of climate change in respect to
future sea level rise is to consider a Mid Range Future Scenario (MRFS) and a High
End Future Scenario (HEFS) with recommendations that for the MRFS +500mm sea
level rise allowance is considered and for the HEFS +1000mm is considered. In
considering future flood risk to development the current Irish Guideline is to use the
MRFS of 500mm. This gives a potential future 200year tide level of 3.57mO.D.
6.5.8 The influence of wave climate from offshore and local wind waves on the Flood Risk
at the Port Development has not been addressed in the Flood Risk Assessment for
the development (i.e. the potential for wind waves combined with tidal surge
propagating up the Liffey Channel and into the Alexandra Dock. The omission of this
is possibly due to the location of the Alexandra basin and Berth 52/53 off the Liffey
channel and being somewhat sheltered from direct impact of these storm waves. It is
considered given the height of the proposed Quay walls that sufficient freeboard is
available to protect against such waves taking account of joint probability with the
tidal surge highwater flooding.
6.5.9 The effect of the proposed development on storm waves was taken into account in
assessing the impact on flood risk to neighbouring area of Clontarf. In this respect
results from a joint probability of wave and tide return period from the Eastern cFRAM
study was used.
The analysis shows for the worst combined event that the
proposed capital dredge scheme increases the wave height approaching the sea
defences by 3cm at Clontarf road.
6.5.10 The applicant concludes based on the topography of the defences and sea frontage
that this predicted small magnitude increase in wave height will have no perceptible
impact on flood risk, in terms of the volume of water breaching the defences from
overtopping waves. No overtopping calculations were included with the FRA to
support this conclusion but given the very marginal increase in wave heights it
considered probable that the impact at Clontarf will be imperceptible.
6.5.11 No assessment has been made on the potential for increase wave height
propagating up the Liffey on the South Quays area which has been shown in the
wave climate difference plots suggesting 0.1 to 0.2m increase in the main channel
and less than 0.1m along the south bank. Allowing for joint probability this impact is
likely to be less than 5cm and retained within the channel.
Flood Risk
6.5.12 The proposed design quay levels at Alexandra Basin West vary between 3.9 and
4.2m O.D. whereas the existing quay levels are at 3.2m and 4m OD Malin. The
proposed new Quay levels provide a freeboard of between 800 and 1100mm over
the estimated 200year tide level for the present day scenario and 300 to 600mm for
the Mid Range Future climate change scenario.
6.5.13 The redevelopment will infill existing berth 52/53 raising it to 4.6m OD Malin which
takes it well out of the high and moderate flood risk zones even under future sea level
rise MRFS scenarios.
6.5.14 The flood mapping shows the Application site to be primarily in Flood C with the
areas that are in Flood Zone A and B comprising the hard standing areas associated
with the quay, cargo handling and temporary storage areas. At Berth 52/53 the
application site is primarily in Flood zone B.
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6.5.15 Flood Mapping presenting present day 200year and 1000year coastal flood
inundation for the entire Dublin Port complex was submitted to the oral hearing. This
mapping represents the Dublin Port lands that are located in Flood Zones A, B and
C. The mapping indicates that for the 200year flood event, some local flooding to the
Alexandra Dock Storage area and to a small section of the Alexandra Road near the
entrance to the Dock occurs. Under 1000year flood event extensive flooding of
Alexandra Road and the Alexandra Dock storage area is shown. Importantly the
mapping shows that access to the Dock off the East Wall Road is in Flood Zone C.
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7.
CONCLUSIONS AND RECOMMENDATIONS
A comprehensive suite of computational models have been applied to evaluate the
flood risk, the baseline coastal processes and to quantify hydrological and
environmental impacts. These models use the Danish Hydraulics Institute (DHI)
MIKE hydrodynamic computational software system which is internationally
recognised as one of the leading hydraulic software packages for coastal process
applications and assessment of Harbours and Ports. The model
resolution/refinement, modelling approach, and in general the supporting
hydrographic survey data (bathymetric survey, sediment distribution measurements
and ADCP tidal flow and depth measurements are considered to be sufficiently
detailed to support the hydrodynamic model.
The tidal modelling using MIKE 2D and 3D variable mesh hydrodynamic models is fit
for purpose and suitably calibrated against hydrographic survey data for assessing
the impact of the Capital Dredge on the tidal regime. The model set-up and results
have been shown to compare well with other previous studies and measurement
data.
The Wave climate modelling using a spectral Wave model is not considered to be
the most accurate model available for the Dublin Port and Clontarf shoreline area
due to the potential significance of diffraction and reflection processes which are not
well represented in the Spectral Wave model. However given that the potential
impact will arise from the deepening of the navigational channel as opposed to
reflection off a structure and the fact that it is a comparative study it is concluded that
the spectral wave model is sufficiently accurate to quantify the potential impact on
wave climate by the proposed Capital Dredge Scheme.
The sediment transport models and modelling approach used to evaluate the
dredging operation within the Liffey Channel area, the sea disposal at the proposed
licensed disposal site near the Burford Bank and the morphological modelling of the
sea bed evolution within Dublin Bay are considered fit for purpose and meet normal
Irish and International practice for such assessments.
The conclusions that can be reached from the Flood Risk Assessment are as
follows:

The proposed development is appropriate development for the flood risk
zones identified.
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
The flood risk to the development is primarily tidal but potentially
compounded by wave climate

The proposed Quay levels are located above the 200year with sufficient
freeboard to account for climate change and potential local wind and sea
waves propagating up the Liffey in combination with storm tides.

The site is accessible under 200 and 1000year flood conditions.

The development is unlikely to materially impact flood risk in the
neighbouring areas of Clontarf and the south Quays.

The proposed development will significantly replace lands in Flood
zones A and B with raised grounds having elevations above the
estimated 1000year flood level.

The flood risk assessment includes the recommended current Irish
practice for Climate Change allowance

The flood risk Assessment meets the requirements of the Flood Risk
Management Planning Guidelines in terms of suitability, impact and
residual flood risk.
The simulation results for dredging operations within the inner Liffey Channel
indicate the potential for high sustained suspended solids spreading across the
entire Liffey channel width with concentrations in excess of 50mg/l. Dredging in the
middle and outer channel section are shown to produce lower sustained
concentrations of less than 20mg/l.
The simulations show that deposition rates outside of the channel dredge area as a
result of the capital dredging operation will be insignificant.
To lessen the suspended solids concentration in the inner Liffey It is recommended
that overspilling at the surface of the dredger is avoided for all dredging activities
within the inner Liffey channel section. This will significantly reduce the potential
suspended sediment from the dredging operation. It is recommended that outside of
the immediate dredging zone that increase in suspended sediment concentration
over the ambient does not exceed a maximum of 50mg/l.
The sediment transport simulations of the Burford Bank disposal site show that the
silt fraction deposited at the site will be dispersed widely by tidal flows whereas and a
large portion of the fine sand fraction will remain insitu under tidal flows and is only
removed by storm waves conditions with the dominant wave direction from the south
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and southeast. Under such a transport mechanism the sand is likely to remain in
Dublin Bay and likely to rejoin the natural sediment budget within the bay.
The simulations indicate no depositional hot spots that will end up receiving
significant amounts of sediment from the disposal site. The depositional impact on
the reef habitat within the Rockabill to Dalkey Island cSAC and the Lamby Island
cSAC based on modelling results is considered to represent a slight to imperceptible
impact and transitory in nature given the exposure of the reef areas to wave climate
and tidal currents.
It is recommended that a continuous hydrographic monitoring station at the disposal
site be established for the duration of the six year capital dredge operation.
Monitoring of the hydrodynamics and wave climate (ADCP and Wave Rider buoy),
turbidity, salinity and temperature at the disposal site should be carried out by this
station. The grading of the proposed capping layer for the contaminated sediments
needs to be designed taking cognisance to the ambient hydrodynamics and wave
climate so that it remains in place and does not erode under adverse tide and storm
conditions.
The main conclusions reached from reviewing the morphological assessment of the
navigation channel are:

The proposed new channel will not result in any significant change in the
morphological response of the sea bed in Dublin Bay outside of the channel
area.

Similar to the existing case there will be a tendency for deposition towards the
northern side of the channel and erosion to the southern side.

The simulation output for the east and southeast storm directions indicate an
increase in deposition within the approach channel near the Bull wall entrance
under the proposed capital dredge case. Given the localised nature of this
increase it is unlikely that magnitude of future maintenance dredging will be
significantly increased.

It is expected that the new channel will require maintenance dredging of a
slightly higher magnitude to that required with the existing channel.
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