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limits
limits
695000N
695500N
733500E
734000E
734500E
695469.521N733368.480E
695497.517N734375.344E
694755.868N734396.043E
694727.870N733389.023E
CM1CM2
Sheet Title:
Bathymetric Survey of the
JOB NUMBER:
PH12024
DRAWING NUMBER:
HS 64-1/11
DRAWN BY:
Gary Curtin
20/08/2011
DATE:
SCALE:
1:2000 ON A1
APPROVED:
REVISION:
D01
J.B.J.
WICKLOW PORTDUMP SITEon I.T.M.
SITE:
SURVEYED BY:
HYDROGRAPHIC SURVEYS LTD
CLIENT:
Wicklow Port CompanyNorth QuayWicklow
The Cobbles,Crosshaven,Co. Cork
tel: +353 21 4831184
Dump Site
Memoir
1. Constructed on I.T.M.2. Horizontal control by D.G.P.S.
3. Soundings in metres and decimetres reduced toChart Datum.
PRODUCED BY:
HYDROGRAPHIC SURVEYS LTDThe Cobbles,Crosshaven,Co. Cork
tel: +353 21 4831184
6°8°10°
52°
54°
Site Location
Hydrographic Surveys Site
CONSULTING ENGINEERS:
Gerard Higgins & AssociatesTwomilewaterWicklow
Memoir
CMCurrent Meter locations shown thus
333500E
195000N
195500N
334000E
334500E
limits
limits
333500E
334000E
334500E
195434.975N333446.518E
195462.982N334453.599E
194721.173N334474.306E
194693.164N733467.069E
CM1
CM1CM2
Sheet Title:
Bathymetric Survey of the
JOB NUMBER:
PH12024
DRAWING NUMBER:
HS 64-2/11
DRAWN BY:
Gary Curtin
20/08/2011
DATE:
SCALE:
1:2000 ON A1
APPROVED:
REVISION:
D01
J.B.J.
WICKLOW PORTDUMP SITEon I.N.G.
SITE:
SURVEYED BY:
HYDROGRAPHIC SURVEYS LTD
CLIENT:
Wicklow Port CompanyNorth QuayWicklow
The Cobbles,Crosshaven,Co. Cork
tel: +353 21 4831184
Dump Site
Memoir
1. Constructed on Irish National Grid.2. Horizontal control by D.G.P.S.
3. Soundings in metres and decimetres reduced toChart Datum.
PRODUCED BY:
HYDROGRAPHIC SURVEYS LTDThe Cobbles,Crosshaven,Co. Cork
tel: +353 21 4831184
6°8°10°
52°
54°
Site Location
Hydrographic Surveys Site
CONSULTING ENGINEERS:
Gerard Higgins & AssociatesTwomilewaterWicklow
Memoir
CMCurrent Meter locations shown thus
Wicklow Port Current Metering 21st August 2012
Easting (ING) Northing (ING) Easting (ITM) Northing (ITM)
Site 1 West side of disposal site 333506 195101 733427 695136
Site 2 East side of disposal site 334383 195111 734304 695146
Site 1 West side of disposal site
1m below Surface
1m below
Surface Mid-depth Mid-depth
1m above
Bottom
1m above
Bottom
Tide Time Total depth Current speed m/s Dir Current speed m/s Dir Current speed m/sDir Wind Comment
LW 8:00 11.2 0.565 330 0.497 322 0.4 316 S-SW Force 3-4
HW-6.25 8:32 11.1 0.68 317 0.577 328 0.53 305
HW-5.75 9:04 11.1 0.615 313 0.567 309 0.518 302
HW-4.5 9:40 11.4 0.488 309 0.55 297 0.468 294
HW-4 10:10 11.7 0.611 304 0.565 298 0.376 299
HW-3.5 10:49 11.9 0.549 267 0.499 255 0.432 253
HW-3 11:38 12.3 0.361 228 0.337 235 0.22 228 Tide Slackening
HW-2.75 12:10 12.5 0.179 204 0.32 203 0.267 182 S Force 2
HW-2 12:45 12.7 0.377 202 0.284 168 0.169 154 S-SE Force 4
HW-1 13:45 13.2 0.624 148 0.665 159 0.526 152 SW Force 4-5
HW-0.5 14:14 13.3 0.771 140 0.772 150 0.583 159 SW Force 5-6
HW 14:56 13.2 0.797 150 0.825 152 0.748 152
HW+1 15:43 13.2 0.947 150 0.991 159 0.995 168
HW+1.5 16:16 12.8 1.029 163 1.007 150 0.935 161 W Force 2-3
HW+2.25 17:10 12.5 0.819 160 0.718 151 0.772 148 W Force 3
HW+3 17:40 12.4 0.659 163 0.667 155 0.539 150
HW+3.5 18:15 12 0.521 155 0.533 154 0.296 153 W Force 2
HW+4 18:48 11.6 0.44 139 0.327 162 0.215 125
HW+4.5 19:29 11.3 0.119 344 0.166 173 0.257 324
LW 7:52
HW 14:52
21-Aug-12
Site 2 East Side of disposal site 21-Aug-12
1m below Surface
1m below
Surface Mid-depth Mid-depth 1m above Bottom
1m above
Bottom
Tide Time Total depth Current speed m/s Dir Current speed m/s Dir Current speed m/s Dir Wind
HW-6.25 8:30 13.6 0.748 323 0.688 337 0.642 328 S Force 3-4
HW-6 8:49 13.6 0.788 335 0.881 326 0.762 321
HW-5.5 9:25 13.9 0.894 319 0.867 330 0.737 319
HW-5 9:55 14.1 0.83 324 0.96 322 0.515 317
HW-4.25 10:32 14.3 0.811 322 0.766 315 0.674 330
HW-3.5 11:25 14.7 0.733 322 0.616 320 0.588 310 S Force 1-2
HW-3.5 11:55 14.9 0.531 315 0.482 311 0.428 291 S Force 2
HW-2.5 12:24 15.2 0.246 315 0.344 325 0.099 279 S Force 3
HW-1.75 13:06 15.1 0.253 199 0.451 190 0.341 181
HW-0.75 13:59 15.4 0.519 161 0.759 163 0.621 158 SW Force 4-5
HW-0.25 14:36 15.6 0.937 161 0.871 152 0.807 156 Force 5-6
HW+0.25 15:12 15.4 1.071 156 1.074 158 1.025 150 W Force 4
HW+1 15:59 15.3 1.199 155 1.13 147 1.082 160
HW+1.5 16:30 15.1 1.073 145 1.116 161 1.102 161
HW+2.5 17:25 14.6 0.959 155 0.889 159 0.896 159 W Force 2
HW+3 17:56 14.2 0.79 144 0.776 156 0.738 149 W Force 3
HW+3.5 18:29 14 0.479 158 0.569 157 0.543 159
HW+4.25 19:04 13.8 0.309 170 0.279 147 0.237 155
LW 7:52
HW 14:52
Wicklow Port Tidal Curve 21st August 2012
0.00
0.50
1.00
1.50
2.00
2.50
3.00
6:00 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00
Time
Tid
e m
C.D
.
Date Time Tide Wicklow Port Tide
CD
8/21/2012 7:17 0.54
8/21/2012 7:22 0.49
8/21/2012 7:27 0.49
8/21/2012 7:32 0.48
8/21/2012 7:37 0.45
8/21/2012 7:42 0.44
8/21/2012 7:47 0.45
8/21/2012 7:52 0.43 LW
8/21/2012 7:57 0.45
8/21/2012 8:02 0.45
8/21/2012 8:07 0.44
8/21/2012 8:12 0.46
8/21/2012 8:17 0.47
8/21/2012 8:22 0.46
8/21/2012 8:27 0.50
8/21/2012 8:32 0.50
8/21/2012 8:37 0.50
8/21/2012 8:42 0.54
8/21/2012 8:47 0.55
8/21/2012 8:52 0.59
8/21/2012 8:57 0.62
8/21/2012 9:02 0.64
8/21/2012 9:07 0.67
8/21/2012 9:12 0.70
8/21/2012 9:17 0.74
8/21/2012 9:22 0.79
8/21/2012 9:27 0.81
8/21/2012 9:32 0.86
8/21/2012 9:37 0.89
8/21/2012 9:42 0.95
8/21/2012 9:47 0.98
8/21/2012 9:52 1.02
8/21/2012 9:57 1.06
8/21/2012 10:02 1.10
8/21/2012 10:07 1.14
8/21/2012 10:12 1.19
8/21/2012 10:17 1.25
8/21/2012 10:22 1.28
8/21/2012 10:27 1.33
8/21/2012 10:32 1.37
8/21/2012 10:37 1.40
8/21/2012 10:42 1.46
8/21/2012 10:47 1.50
8/21/2012 10:52 1.55
8/21/2012 10:57 1.58
8/21/2012 11:02 1.63
8/21/2012 11:07 1.67
8/21/2012 11:12 1.72
8/21/2012 11:17 1.76
8/21/2012 11:22 1.80
8/21/2012 11:27 1.85
8/21/2012 11:32 1.88
8/21/2012 11:37 1.91
8/21/2012 11:42 1.95
8/21/2012 11:47 1.97
8/21/2012 11:52 2.00
8/21/2012 11:57 2.04
8/21/2012 12:02 2.09
8/21/2012 12:07 2.11
8/21/2012 12:12 2.13
8/21/2012 12:17 2.19
8/21/2012 12:22 2.20
8/21/2012 12:27 2.23
8/21/2012 12:32 2.25
8/21/2012 12:37 2.28
8/21/2012 12:42 2.30
8/21/2012 12:47 2.33
8/21/2012 12:52 2.36
8/21/2012 12:57 2.39
8/21/2012 13:02 2.41
8/21/2012 13:07 2.43
8/21/2012 13:12 2.44
8/21/2012 13:17 2.49
8/21/2012 13:22 2.51
8/21/2012 13:27 2.54
8/21/2012 13:32 2.54
8/21/2012 13:37 2.57
8/21/2012 13:42 2.60
8/21/2012 13:47 2.60
8/21/2012 13:52 2.65
8/21/2012 13:57 2.67
8/21/2012 14:02 2.69
8/21/2012 14:07 2.69
8/21/2012 14:12 2.71
8/21/2012 14:17 2.72
8/21/2012 14:22 2.73
8/21/2012 14:27 2.75
8/21/2012 14:32 2.76
8/21/2012 14:37 2.76
8/21/2012 14:42 2.77
8/21/2012 14:47 2.78
8/21/2012 14:52 2.79 HW
8/21/2012 14:57 2.78
8/21/2012 15:02 2.76
8/21/2012 15:07 2.73
8/21/2012 15:12 2.73
8/21/2012 15:17 2.71
8/21/2012 15:22 2.71
8/21/2012 15:27 2.69
8/21/2012 15:32 2.67
8/21/2012 15:37 2.67
8/21/2012 15:42 2.64
8/21/2012 15:47 2.61
8/21/2012 15:52 2.59
8/21/2012 15:57 2.56
8/21/2012 16:02 2.53
8/21/2012 16:07 2.50
8/21/2012 16:12 2.46
8/21/2012 16:17 2.43
8/21/2012 16:22 2.40
8/21/2012 16:27 2.36
8/21/2012 16:32 2.31
8/21/2012 16:37 2.28
8/21/2012 16:42 2.23
8/21/2012 16:47 2.19
8/21/2012 16:52 2.13
8/21/2012 16:57 2.10
8/21/2012 17:02 2.03
8/21/2012 17:07 1.98
8/21/2012 17:12 1.93
8/21/2012 17:17 1.86
8/21/2012 17:22 1.80
8/21/2012 17:27 1.75
8/21/2012 17:32 1.70
8/21/2012 17:37 1.65
8/21/2012 17:42 1.60
8/21/2012 17:47 1.54
8/21/2012 17:52 1.45
8/21/2012 17:57 1.42
8/21/2012 18:02 1.35
8/21/2012 18:07 1.33
8/21/2012 18:12 1.27
8/21/2012 18:17 1.22
8/21/2012 18:22 1.16
8/21/2012 18:27 1.12
8/21/2012 18:32 1.08
8/21/2012 18:37 1.03
8/21/2012 18:42 0.98
8/21/2012 18:47 0.95
8/21/2012 18:52 0.92
8/21/2012 18:57 0.86
8/21/2012 19:02 0.84
8/21/2012 19:07 0.80
8/21/2012 19:12 0.79
8/21/2012 19:17 0.77
8/21/2012 19:22 0.76
8/21/2012 19:27 0.74
8/21/2012 19:32 0.73
8/21/2012 19:37 0.71
8/21/2012 19:42 0.71
8/21/2012 19:47 0.70
8/21/2012 19:52 0.70
Report No. HEL095501 v1.1
Hydrodynamic Modelling Assessment
of the Wicklow Harbour Dredge Disposal Site
Prepared for
Wicklow Port Authority.
November 2012
ydro
Environmental Ltd
No. 4 Caiseal Riada,
Clarinbridge,
Co. Galway.
Tel / Fax: 091 796734
email: [email protected]
Hydrodynamic Modelling of the Wicklow Harbour Dredge Disposal Site Report HEL095101 v1.1
30/11/2012
Hydrodynamic Modelling Assessment
of the Wicklow Harbour Dredge Disposal Site
Job No.: 095101
Report No.: HEL095101 v1.1 Prepared by: Anthony Cawley BE, MEngSc, CEng MIEI Date: 22
nd November 2012
Issue Final
© 2012 Hydro Environmental Ltd
DISCLAIMER
This report has been prepared for Wicklow Port Authority for a dredge disposal license from
the EPA. Hydro Environmental Ltd. accept no responsibility or liability for any use that is
made of this document other than by the Client for the purposes for which it was originally
commissioned.
Hydrodynamic Modelling of the Wicklow Harbour Dredge Disposal Site Report HEL095101 v1.1
30/11/2012
CONTENTS 1. Introduction ............................................................................................................... 1
1.1 Background ........................................................................................................................................... 1
1.2 Dredge Operation .................................................................................................................................. 1
1.3 TELEMAC Hydraulic Software System .................................................................................................. 2
1.4 CHC PSed Sediment Transport Model .................................................................................................. 3
2. Hydrodynamic Modelling ........................................................................................... 4
2.1 Finite Element Model Structure ............................................................................................................. 4
2.2 Boundary and Initial conditions Specification ........................................................................................ 5
2.3 Model Verification .................................................................................................................................. 6
3. Hydrodynamic Simulations ...................................................................................... 10
3.1 Tidal Circulation ................................................................................................................................... 10
3.2 Bed Shear Analysis and discussion..................................................................................................... 17
3.3 Sediment Transport Simulation and Discussion .................................................................................. 24
Hydrodynamic Modelling of the Wicklow Harbour Dredge Disposal Site Page 1 Report HEL095101 v1.1
30/11/2012
1. Introduction
1.1 Background
Hydro Environmental Ltd was appointed by Gerard Higgins and Associates Consulting Engineers on
behalf of Wicklow Port Authority to perform a hydrodynamic sediment transport assessment of
dredge material from a proposed dredge disposal site in coastal waters 1.5km northeast of Wicklow
Harbour. This study is in support to an application from the Port Company to the EPA for a dredge
disposal permit under the Dumping At Sea Acts 1996 and 2004.
The proposed dredge disposal site is 74.7ha in area and located in water depth increasing west to
east from 12 to 15m below msl. This study attempts to quantify the sediment impact of disposing of
dredge material at the proposed site with particular attention to the potential impact on the nearby
Wicklow Reef SAC site which is only 500m to the east and southeast (refer to Figure 1). This disposal
site has been used and licensed previously (1998) to dispose dredge material excavated from the
Harbour.
1.2 Dredge Operation
The dredge material for sea disposal will be excavated from within the Wicklow Harbour area itself
with harbour bed levels being excavated down to 5.5m below OS Malin. It is proposed that the
dredging be carried out using a barge mounted excavator and that disposal of dredged material shall
be by means of a bottom opening barge at the proposed dump site. The total amount to be
disposed is estimated to be 54,260 tonnes with the dredge-disposal operation period being
completed over two months. The proposed monthly and weekly loadings are 27,000 and 6,000
tonnes respectively. These rates represent a disposal rate of 10kg per second.
Laboratory analysis of the dredge material in Wicklow harbour was carried out to assess the physical
and chemical nature of the sediment. Sediment samples were taken from five representative
locations within the Harbour dredge area and a summary of the sediment characteristics
encountered are discussed latter in this chapter.
Table 1 Summary of Sediment Type from discrete sampling of dredge area
Sediment Type
Sampling Sites
0 1 2 3 4
Coarse Gravel 67 Medium Gravel 3 Fine Gravel 8 2.6
3.2
Coarse Sand 14.5 14.8 5 10.6 5.7
Medium Sand 4.5 32.4 43 55.9 86
Fine sand 2.5 28.6 29 17 5
Silt 0.5 21.6 23 13.3 3.3
Sample site 0 which is the most upstream location in the river channel presents sandy coarse gravel
whereas the remaining 4 samples representing the bulk of the Harbour dredge area consistently
presents a fine to medium Sand.
Hydrodynamic Modelling of the Wicklow Harbour Dredge Disposal Site Page 2 Report HEL095101 v1.1
30/11/2012
Figure 1 Proposed Dredge Disposal site and Wicklow Reef SAC.
1.3 TELEMAC Hydraulic Software System
The TELEMAC system and specifically TELEMAC 2D is the software of choice for modelling the
complicated hydrodynamics of the Irish Sea coastal waters off the Wicklow coastline. TELEMAC is a
software system designed to study environmental processes in free surface transient flows. It is
therefore applicable to seas and coastal domains, estuaries, rivers and lakes. Its main fields of
application are in hydrodynamics, water quality, sedimentology and water waves.
TELEMAC is an integrated, user friendly software system for free surface waters. TELEMAC was
originally developed by Laboratoire National d’Hydraulique of the French Electricity Board (EDF-
LNHE), Paris. It is now under the directorship of a consortium of organisations including EDF-LNHE,
HR Wallingford, SOGREAH, BAW and CETMEF. It is regarded as one of the leading software packages
for free surface water hydraulic applications and with more than 1000 Telemac Installations
Worldwide.
Wicklow Reef
SAC (002274)
Dredge Disposal Site
Dredge Area
Hydrodynamic Modelling of the Wicklow Harbour Dredge Disposal Site Page 3 Report HEL095101 v1.1
30/11/2012
The TELEMAC system is a powerful integrated modelling tool for use in the field of free-surface
flows. Having been used in the context of very many studies throughout the world (several
thousand to date), it has become one of the major standards in its field. The various simulation
modules use high-capacity algorithms based on the finite-element method. Space is discretised in
the form of an unstructured grid of triangular elements, which means that it can be refined
particularly in areas of special interest. This avoids the need for systematic use of embedded
models, as is the case with the finite-difference method. Telemac-2D is a two-dimensional
computational code describing the horizontal velocities, water depth and free surface over space
and time. In addition it solves the transport of several tracers which can be grouped into two
categories, active and passive, with salinity and temperature being the active tracers which alter
density and thus the hydrodynamics.
1.4 CHC PSed Sediment Transport Model
To simulate the sediment transport characteristics of the dredge site a Lagrangian Sediment Transport Model PSed (the Particle-based Sediment transport model) developed by the CHC Canadian Hydraulics Centre. The PSed model utilises the hydrodynamic output from the Telemac2d model requiring bathymetry, time varying water depths and velocities. For full details and background theory refer to the PSed4.3 Reference Manual (2006).
Hydrodynamic Modelling of the Wicklow Harbour Dredge Disposal Site Page 4 Report HEL095101 v1.1
30/11/2012
2. Hydrodynamic Modelling
2.1 Finite Element Model Structure
The total model domain covers the entire Irish Sea from ??? near to ???, refer to Figure 2 below. An
unstructured mesh of triangular elements was fitted to the domain with a total of 17,914 with 200m
refinement in the vicinity of the dredge disposal site and the Adjacent SAC and Shoreline Area and
increasing to 5000m elements remote from the shoreline. An element growth ratio of 1.15 from the
refined area growing to the larger elements was specified. The overall mesh is shown in Figure 2 and
an example of the mesh refinement in the vicinity of the study area is presented in Figure 3.
The bathymetry within the domain area was produced using the HSL local survey of the dredge
disposal site, relevant admiralty chart soundings and the BODC European Shelf 0.5minute
bathymetry and mapped onto the computation mesh nodes through linear triangulation. The
domain projection for model was set to UTM 29 North and this assists extrapolating boundary and
initial conditions from European shelf global solutions database. The vertical datum for all the data
sets inputted to the model was converted to mean Sea level.
Figure 2 Model Extent of Irish Sea model used to compute hydrodynamics and transport characteristics
North Open Boundary
South Open Boundary
Hydrodynamic Modelling of the Wicklow Harbour Dredge Disposal Site Page 5 Report HEL095101 v1.1
30/11/2012
Figure 3 Refinement of Mesh near Wick Head and the disposal Site
2.2 Boundary and Initial conditions Specification
Boundary conditions driving the Irish Sea Hydrodynamic Model were generated from regional/local tidal solutions by OSU (Oregon State University). The regional solution was derived from the European Shelf structured grid Barotrophic tidal model that covers the North-East Atlantic Ocean with 11 harmonic constituents (M2, S2, N2, K2, K1, O1, P1, Q1, M4, MS4 and MN4), refer to Figure 9 for extent of the European Shelf Model coverage. The solution gives amplitudes and phases for tidal elevation and transport from which two horizontal components of the current can be deduced (by dividing transport by water depth). The resolution of the local European Shelf solution is 1/30 degree and is referenced to mean sea level. The harmonic constants from the ES model were interpolated on to the relevant boundary nodes of the local model. These harmonic constants were also used to reconstruct the initial conditions within the domain at simulation commencement and to define the time varying boundary conditions at each boundary node and at each time step.
For this study the south and north open boundaries were specified as imposed U, V and H (velocity and depth boundaries) and the east and west boundary (British and Irish Shorelines) as a land boundary (zero normal Flux boundary). Given the imposed / clamped nature of the open sea boundaries, a radiation condition at these boundaries using the Thompson Method was necessary to allow numerical noise to propagate freely out of the computational domain rather than becoming trapped and oscillating unrealistically within the domain.
Wicklow Reef
SAC
Dredge
Disposal Site
SPA
Hydrodynamic Modelling of the Wicklow Harbour Dredge Disposal Site Page 6 Report HEL095101 v1.1
30/11/2012
Figure 4 NOA European Shelf Model
2.3 Model Verification
The hydrodynamic model was verified against the tidal velocity and depth measurements performed
by Hydrographic Surveys Ltd (21st August 2012). Two current metering locations S1 and S2 in the
disposal area (refer to figure 5), provided a 12hour velocity measurements at near surface, mid-
depth and near-bottom for a spring tide condition. The hydrodynamic model was run for a
simulation period commencing the 20th August 2012 and finishing on the 28th September 2012
(40day simulation) for a computational time step of 30seconds and simulation results were output
every 3min’s for the complete model domain and stored in a binary results database. Time series of
water depths and depth averaged velocities were generated for each of the measurement points
from this results database. A final calibrated manning’s roughness of 0.018 was used with a full k-
turbulence model to simulate eddy viscosity / turbulence and accurately produce the observed
hydrodynamics.
An option for scaling (increasing/decreasing) the magnitudes of the specified boundary tidal heights
and velocities from the European Shelf Tidal Solution was not required for this application with the
ES tidal solution generating realistic results without modification.
Hydrodynamic Modelling of the Wicklow Harbour Dredge Disposal Site Page 7 Report HEL095101 v1.1
30/11/2012
The time series comparisons between the measured and modelled hydrodynamics (velocity
magnitude, direction and tidal water depth) are presented in Figures 6 to 9 for the two
measurement sites.
Figure 5 Current Metering Locations within the Disposal Area for Model Verification
Hydrodynamic Modelling of the Wicklow Harbour Dredge Disposal Site Page 8 Report HEL095101 v1.1
30/11/2012
Figure 6 Computed and measured Tidal speeds at S1 for the 21st August 2012
Figure 7 Computed and measured tidal Direction at S1 for the 21st August 2012
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
06:00 07:00 08:00 09:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00
Vel
oci
ty (
m/s
)
Model Depth Averaged
1m below Surface
Mid-depth
1m above Bottom
0
50
100
150
200
250
300
350
400
06:00 07:00 08:00 09:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00
Vel
oci
ty D
irec
tio
n (
de
gree
s N
)
Model Depth Averaged
1m below Surface
Mid-depth
1m above Bottom
Hydrodynamic Modelling of the Wicklow Harbour Dredge Disposal Site Page 9 Report HEL095101 v1.1
30/11/2012
Figure 8 Computed and measured Tidal speeds at S2 for the 21st August 2012
Figure 9 Computed and measured tidal Direction at S2 for the 21st August 2012
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
06:00 07:00 08:00 09:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00
Vel
oci
ty (
m/s
)
Model - depth averaged1m below SurfaceMid-depth1m above Bottom
0
50
100
150
200
250
300
350
400
06:00 07:00 08:00 09:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00
Vel
oci
ty D
irec
tio
n (
de
gree
s N
)
Model - depth averaged1m below Surface1m above BottomMid-depth
Hydrodynamic Modelling of the Wicklow Harbour Dredge Disposal Site Page 10 Report HEL095101 v1.1
30/11/2012
3. Hydrodynamic Simulations
3.1 Tidal Circulation
A 40-day simulation period set from the 20th August to the 28th September 2012 representing the
varying tidal characteristics that include the predicted combination of spring, mean and neap tides
was used to assess the baseline tidal flow regime within the area of interest and to assess the
hydrodynamic impact of the proposed development.
Figures 10 to 14 present colour tonal vector plots of computed tidal velocities in the Irish sea over a
typical spring tide at approximately 3hour intervals from the simulation period for the 18th
September 2012 representing a spring tide. For clarity the velocity vectors are shown on a less
dense grid than the model mesh. These figures demonstrate the strong rectilinear type flow parallel
to the Irish shoreline in the vicinity of the Study Area and the overall circulation pattern within the
Irish Sea. Similar characteristics are evident on neaps tides with velocity magnitudes typically 40%
lower.
Based on the 40 day simulation period the residual (net current speed and direction) was calculated
and is presented in Figure 15 and 16. These figures shows a southward trending residual current off
Wicklow head which demonstrates that sediment transport from the Disposal site will dominantly
be in a southward direction through the SAC site.
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Figure 10 Spring Tide Hydrodynamics on 18th Sep 2012 (1.5hrs before HW (Wicklow))
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Figure 11 Spring Tide Hydrodynamics on 18th Sep 2012 (1.5hrs after HW (Wicklow))
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Figure 12 Spring Tide Hydrodynamics on 18th Sep 2012 (4hrs After HW (Wicklow))
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Figure 13 Spring Tide Hydrodynamics on 18th Sep 2012 (1hrs After LW (Wicklow))
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Figure 14 Spring Tide Hydrodynamics on 18th Sep 2012 (2hrs before HW (Wicklow))
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Figure 15 Computed Residual current in the Irish Sea over 40day simulation
Figure 16 Computed Residual current off Wicklow head over 40day simulation
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3.2 Bed Shear Analysis and Discussion
The sediment survey of the dredge material based on 5 samples indicates that the dominant
sediment classification for the disposal material from Wicklow Harbour to be equivalent to a medium
sand.
The movement of the sediment on the seabed is dependent on the tidal currents and the sediment
type (grain size). The tidal flow gives rise to generating shear stress along the seabed. When the
shear stress increases to a critical value, the sediment will move, refer to Table 3 below for critical
shear stress values for different non-cohesive sediment sizes. Shear stresses above 0.1 N/m2 (Pa) will
erode the silt fraction with the fine to medium sand requiring shear stresses of 0.18 N/m2 to 0.23
N/m2.
The hydrodynamic modelling provided velocity and depth information which was used to determine
the resulting bed shear stresses within the study area. Figures 18 and 19 present the average and
maximum shear stress magnitudes in the vicinity of the proposed disposal and SAC sites off Wicklow
Head. The simulation average shear stress (40days of varying tide simulation) exceeds 0.37N/m2
throughout the disposal site and the SAC reef site. This shows that the average shear stress exceeds
the critical shear value required to erode / mobilise coarse sand and the eastern section the Wicklow
reef SAC exceeds 0.83N/m2 which is the critical shear stress for a very coarse sand.
For the four Reference Locations presented in Figure 20 (A and B in the disposal site and C and D in
the SAC site) time series of bed shear and a resultant shear duration curve was developed from the
40-day simulation output. These results are presented in figures 21to 28 and clearly indicate that for
the majority of the time bed shear under tidal flows are sufficient to mobilise and erode medium and
coarse sand from the bed and that depositional conditions will only occur for a very limited period
over the tidal cycle. This information suggests that the proposed sandy sediments to be deposited
within the dredge disposal site will not remain and will be easily mobilised by the ambient tidal flows.
The computed Shear velocities also show that permanent deposition of this material within the SAC
Reef Area will not occur as the ambient tidal velocities are stronger there and will prevent permanent
settlement. Table 5 presents the duration of time various sediment classes from fine Gravel to Silt
will be mobilised by the ambient current.
Table 3 Standard Sediment grain size distributions (grain size in mm)
Desired Graduation Class
D average Porosity D10 D35 D50 D60 D90
Fine sand FS 0.08 0.4 0.07 0.09 0.1 0.11 0.15
Medium Sand MS 0.3 0.4 0.27 0.34 0.38 0.41 0.56
Coarse Sand CS 0.9 0.4 0.8 1.01 1.15 1.24 1.68
Fine Gravel FG 3 0.4 2.67 3.38 3.83 4.12 5.6
Medium Gravel MG 9 0.4 8 10.1 11.5 12.4 16.8
Coarse Gravel CG 30 0.4 26.7 33.8 38.3 41.2 56 D10 is 10% passing through sieve and D90 is diameter at 90%.passing through Sieve
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Figure 17 Critical Shear Stress versus Particle Diameter
Table 4 Sediment size classification and critical shear stress for erosion.
Material Type Sediment Size (mm) Critical shear stress (N/m2)
Fine gravel 6 5.24
Very fine gravel 3 2.16
Very coarse sand 1.5 0.83
Coarse sand 0.75 0.37
Medium sand 0.38 0.23
Fine sand 0.19 0.18
Very fine sand 0.09 0.14
Coarse silt 0.047 0.11
Table 5 presents information from the Shear Duration curves to demonstrate the percentage of the
time that critical shear stresses for the various sediment classes are exceeded
Percentage of time exceeded
Sediment Class
Critical Shear
(N/m2) A B C D
Fine Gravel 5.24 0 0 0 0
Very Fine Gravel 2.16 0 2.35 4.7 10.3
Very Coarse Sand 0.83 31.3 40 37.5 48.7
Coarse Sand 0.37 61 65.3 65.1 68.7
Medium Sand 0.23 71.8 74.3 74.48 76.4
Fine Sand 0.18 75.7 77.7 78.1 79.3
Very Fine Sand 0.14 79 80.6 81.1 82.1
Coarse Silt 0.11 81.7 83.1 83.7 84.1
0.01
0.1
1
10
0.01 0.1 1 10
Cri
tica
l Sh
ear
Str
ess
(P
a)
Dia (mm)
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Figure 18 Mean Shear Stress Magnitude over 40day tidal simulation
Figure 19 Maximum Shear Stress Magnitude over 40day tidal simulation
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Figure 20 Computed Shear Stress Observation Points
Figure 21 Time series of computed Bed Shear Stress magnitudes at Site A
0
0.5
1
1.5
2
2.5
19/08/2012 00:00 24/08/2012 00:00 29/08/2012 00:00 03/09/2012 00:00 08/09/2012 00:00 13/09/2012 00:00 18/09/2012 00:00 23/09/2012 00:00 28/09/2012 00:00
She
ar S
tre
ss N
/m2
(P
a)
Time Series of computed Bed Shear at Site A
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Figure 22 Time series of computed Bed Shear Stress magnitudes at Site B
Figure 23 Time series of computed Bed Shear Stress magnitudes at Site C
Figure 24 Time series of computed Bed Shear Stress magnitudes at Site D
0
0.5
1
1.5
2
2.5
3
19/08/2012 00:00 24/08/2012 00:00 29/08/2012 00:00 03/09/2012 00:00 08/09/2012 00:00 13/09/2012 00:00 18/09/2012 00:00 23/09/2012 00:00 28/09/2012 00:00
She
ar S
tre
ss N
/m2
(P
a)Time Series of computed Bed Shear at Site B
0
0.5
1
1.5
2
2.5
3
19/08/2012 00:00 24/08/2012 00:00 29/08/2012 00:00 03/09/2012 00:00 08/09/2012 00:00 13/09/2012 00:00 18/09/2012 00:00 23/09/2012 00:00 28/09/2012 00:00
She
ar S
tre
ss N
/m2
(P
a)
Time Series of computed Bed Shear at Site C
0
0.5
1
1.5
2
2.5
3
3.5
19/08/2012 00:00 24/08/2012 00:00 29/08/2012 00:00 03/09/2012 00:00 08/09/2012 00:00 13/09/2012 00:00 18/09/2012 00:00 23/09/2012 00:00 28/09/2012 00:00
She
ar S
tre
ss N
/m2
(P
a)
Time Series of computed Bed Shear at Site D
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Figure 25 Computed Bed Shear Duration curve for Site A
Figure 26 Computed Bed Shear Duration curve for Site B
0.001
0.01
0.1
1
10
0 10 20 30 40 50 60 70 80 90 100
Shea
r St
ress
(P
a)
Percentage Exceedance
0.001
0.01
0.1
1
10
0 10 20 30 40 50 60 70 80 90 100
She
ar S
tre
ss (
Pa)
Percentage Exceedance
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Figure 27 Computed Bed Shear Duration curve for Site C
Figure 28 Computed Bed Shear Duration curve for Site D
0.001
0.01
0.1
1
10
0 10 20 30 40 50 60 70 80 90 100
She
ar S
tre
ss (
Pa)
Percentage Exceedance
0.001
0.01
0.1
1
10
0 10 20 30 40 50 60 70 80 90 100
Shea
r St
ress
(P
a)
Percentage Exceedance
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3.3 Sediment Transport Simulation and Discussion
Sediment from the disposal activity was modelled using PSed Model for a 40day simulation period
and an average loading rate of 10kg/sec of sediment at the disposal site. The disposal was inputted
as a line source (east to west) mid-way along the Disposal site and the material was released at 0 to
2m above the bed. The PSed Lagrangian tracking used the computed hydrodynamics from the
Telemac model presented earlier in this section. The sediment was set as medium sand reflecting
the general nature of the dredged material. Medium grained sand has the following characteristics:
density 2650kg/m3; grain size 0.0003m; and settling velocity 0.03857m/s.
The sediment simulations of a medium sand released continuously over a forty day simulation period
demonstrate that the sediment is mobilised as suspended and bed load for the majority of the time
due to the naturally high ambient shear velocities and only settles temporarily at slack periods of the
tidal cycle and slightly longer periods during low neap tides. The plume transport is dominantly
south and southeast from the disposal site and generally through the western section of the SAC
reflecting the hydrodynamics. Periods of deposition are very limited to the slack tides and are
immediately mobilised once velocities increase. The transported sediment will generally be as bed
load movement on the bed or in concentrated suspension towards the base of the water column.
Spring Tide Output after 30days is presented in Figure 29 (i to xiii). This demonstrates the path of the
sediment movement which is primarily southwards through the SAC site but with little opportunity
for deposition there and with any deposition being temporary and remobilised once velocities
increase. These plots also show that at certain stages of the tide a large portion of the SAC area is
free of the sediment. These plots represent the total sediment mass released (bed and suspended
loads) as concentration in mg/l averaged over the water depth. In reality these sediment
concentrations will be confined close to the sea bed either as a bed form or as a relatively shallow
bed layer of suspended sediment.
The simulation demonstrates that movement from the dredge site will result in sediment migrating
through the SAC area but with little opportunity for permanent settlement there.
Given that the dredging period will be for a 2month period the plots presented in Figure 29 would be
representative of the sediment activity during that period. Given the information from the
hydrodynamics and residual currents the mobilised sediment will completely migrate away initially
south and then east and north away from the coastline and the SAC and dredge site. It is expected
that within 2 to 4 months of completing the dredging activity the sediment load through the SAC will
have returned to ambient rates.
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(i) Highwater Sediment Concentration
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(ii) Highwater + 1hr
(iii) Highwater +2hr
(iv) Highwater +3hr
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(v) L0w Water -2hrs
(vi) Low Water -1hrs
Hydrodynamic Modelling of the Wicklow Harbour Dredge Disposal Site Page 28 Report HEL095101 v1.1
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(vii) Low water
(viii) Low + 1hr
Hydrodynamic Modelling of the Wicklow Harbour Dredge Disposal Site Page 29 Report HEL095101 v1.1
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(ix) Low Water + 2hrs
(x) Low Water + 3hrs
Hydrodynamic Modelling of the Wicklow Harbour Dredge Disposal Site Page 30 Report HEL095101 v1.1
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(xi) Highwater – 2hrs
(xii) Highwater – 1 hrs
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(xiii) Highwater
Figure 29 Sediment Concentration plots at Hourly intervals over Spring Tide after 30days continuous discharge of 10kg/sec at dredge disposal site.
Natura Impact Statement
for the
Proposed Disposal of Dredged Spoil
at Wicklow Port Authority’s
Dredge Disposal Site
Produced by
AQUAFACT International Services Ltd
On behalf of
Wicklow Port Authority
January 2013
AQUAFACT INTERNATIONAL SERVICES ltd
12 KILKERRIN park
TUAM rd
GALWAY city
www.aquafact.ie
tel +353 (0) 91 756812
fax +353 (0) 91 756888
Table of Contents
1. Introduction 1
1.1. Requirement for an Article 6 Assessment....................................................................................1
1.2. The Aim of this Report.................................................................................................................1
1.3. Background .................................................................................................................................2
2. Appropriate Assessment Process 2
2.1. Introduction ................................................................................................................................2
2.2. Stages..........................................................................................................................................4
2.2.1. Stage 1. Screening for Appropriate Assessment......................................................................................4
2.2.2. Stage 2. Appropriate Assessment...........................................................................................................5
2.2.3. Stage 3. Alternative Solutions ................................................................................................................5
2.2.4. Stage 4. Imperative Reasons of Overriding Public Interest (IROPI)/Derogation .......................................5
3. The Ecological Impact Assessment (EcIA) 6
3.1. Introduction ................................................................................................................................6
3.2. Description of the Project............................................................................................................6
3.3. Sediment Transport Model Results..............................................................................................7
3.4. Baseline Description of the Subtidal Environment .......................................................................8
3.4.1. Marine Benthos .....................................................................................................................................8
4. Natura 2000 Sites 8
4.1. Sites is the vicinity of the Proposed Development .......................................................................8
4.2. Characteristics of the Designated Site .........................................................................................9
4.3. Conservation Objectives............................................................................................................ 10
5. Assessment of the Likely Effects 11
5.1. Sediment Dispersion & Deposition ............................................................................................ 11
6. Mitigation Measures 13
7. Residual Impacts 13
8. Summary Impacts/Mitigation Table 13
9. References 14
List of Figures
Figure 2.1: Stages in the AA process (Source: DEHLG, 2009)............................................................................................4
Figure 4.1: Location of Wicklow Reef cSAC (Site Code: IE002274) in relation to the dredge disposal site (Source: Hydro
Environmental Ltd., 2012)....................................................................................................................................9
List of Tables
Table 4.1: Annex I habitats listed on the Natura 2000 Standard Data Form..................................................................10
Table 4.2: Other species of importance – from the Natura 2000 Standard Data Form ..................................................10
Table 3: Summary of Impacts and Mitigation ...............................................................................................................13
List of Appendices
Appendix 1 IE002274 Site Synopsis
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Natura Impact Statement for the Proposed Disposal of Dredged Spoil
at Wicklow Port Authority’s Dredge Disposal Site
Wicklow Port Authority
January 2013
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1. Introduction
1.1. Requirement for an Article 6 Assessment
Due to the fact that the proposed dredge spoil disposal operation site is located approximately 500m from
the Wicklow Reef cSAC (Site Code: IE002274), it is regarded as necessary that the proposal should have due
regard to Article 6 (3) of the EU Habitats Directive1 which states:
Article 6 (3): Any plan or project not directly connected with or necessary to the
management of the [Natura 2000] site but likely to have a significant effect thereon, either
individually or in combination with other plans or projects, shall be subject to appropriate
assessment of its implications for the [Natura 2000] site in view of the [Natura 2000] site’s
conservation objectives. This is transposed into national legislation by Regulation 31 of the
European Communities (Natural Habitats) Regulations 1997.
1.2. The Aim of this Report
This Natura Impact Statement (NIS) has been prepared in accordance with the current guidance (DEHLG,
2009, Revised February 2010) and provides an ecological impact assessment (EcIA) for the proposed disposal
operations.
The NIS provides the information required in order to establish whether or not the proposed development is
likely to have a significant impact on the Wicklow Reef cSAC (Site Code: IE002274) in the context of its
conservation objectives and specifically on the habitats and species for which the site has been designated.
By taking the ecological impact assessment in a step by step manner in relation to the habitats and species of
the Natura site, together with its conservation objectives, this report seeks to inform the screening process
required as the first stage of the process pursuant to Article 6.3 of the EU Habitats Directive and also to
provide full and detailed information as required for the second stage, that of Appropriate Assessment,
should the competent authority decide that such an assessment is required.
1 Council Directive 92/43/EEC on the conservation of natural habitats and of wild fauna and flora to beneficial consequences of primary importance
for the environment or, further to an opinion from the Commission, to other imperative reasons of overriding public interest.
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Natura Impact Statement for the Proposed Disposal of Dredged Spoil
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1.3. Background
This disposal site is located approximately 500m northwest of the Wicklow Reef cSAC. This disposal site has
been used and licensed previously (1998) to dispose dredge material excavated from Wicklow Harbour.
Wicklow Port Authority is applying to the Environmental Protection Agency (EPA) for a dredge disposal
permit under the Dumping At Sea Acts 1996 and 2004.
2. Appropriate Assessment Process
2.1. Introduction
There is a requirement, under Article 6(3) of the EU Habitats Directive (Directive 92/43/EEC), to carry out an
Appropriate Assessment. The first step of the Appropriate Assessment process is to establish whether, in
relation to a particular plan or project, Appropriate Assessment is required. Article 6(3) states:
‘Any plan or project not directly connected with or necessary to the management of the site but
likely to have a significant effect thereon, either individually or in combination with other plans or
projects, shall be subject to appropriate assessment of its implications for the site in view of the
site’s conservation objectives. In the light of the conclusions of the assessment of the implications
for the site and subject to the provisions of paragraph 4, the competent national authorities shall
agree to the plan or project only after having ascertained that it will not adversely affect the
integrity of the site concerned and, if appropriate, after having obtained the opinion of the general
public.’
If the Appropriate Assessment determines that a plan of project may adversely affect the integrity of a
Natura 2000 site, then Article 6 (4) may come into play. Article 6 (4) states that:
‘If, in spite of a negative assessment of the implications for the [Natura 2000] site and in the
absence of alternative solutions, a plan or project must nevertheless be carried out for imperative
reasons of overriding public interest, including those of a social or economic nature, Member States
shall take all compensatory measures necessary to ensure that the overall coherence of Natura
2000 is protected. It shall inform the Commission of the compensatory measures adopted’.
This NIS has been prepared in accordance with the following guidance documents:
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Natura Impact Statement for the Proposed Disposal of Dredged Spoil
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• Appropriate Assessment of Plans and Projects in Ireland - Guidance for Planning Authorities
(DEHLG 2009, Revised February 2010)
• EU Guidance document on Article 6(4) of the 'Habitats Directive' 92/43/EEC (EC, 2007);
• Assessment of plans and projects significantly affecting Natura 2000 sites. Methodological
guidance on the provisions of Article 6(3) and (4) of the Habitats Directive 92/43/EEC (EC, 2002);
and
• Managing Natura 2000 Sites: The provisions of Article 6 of the ‘Habitats’ Directive 92/43/EEC (EC,
2000).
Should a decision be reached to the effect that it cannot be said with sufficient certainty that the
development will not have any significant effect on the Natura 2000 sites, then, as is stated above, it is
necessary and appropriate to carry out an appropriate assessment of the implications of the development
for the sites in view of their conservation objectives.
The guidance for Appropriate Assessment (DEHLG, 2009, revised February 2010) states:
“AA is an impact assessment process that fits within the decision-making framework and tests of
Articles 6(3) and 6(4) and, for the purposes of this guidance, it comprises two main elements. Firstly
a Natura Impact Statement – i.e. a statement of the likely and possible impacts of the plan or
project on a Natura 2000 site (abbreviated in the following guidance to “NIS”) must be prepared.
This comprises a comprehensive ecological impact assessment of a plan or project; it examines the
direct and indirect impacts that the plan or project might have on its own or in combination with
other plans and projects, on one or more Natura 2000 sites in view of the sites’ conservation
objectives. Secondly, the competent authority carries out the AA, based on the NIS and any other
information it may consider necessary. The AA process encompasses all of the processes covered by
Article 6(3) of the Habitats Directive, i.e. the screening process, the NIS, the AA by the competent
authority, and the record of decisions made by the competent authority at each stage of the
process, up to the point at which Article 6(4) may come into play following a determination that a
plan or project may adversely affect the integrity of a Natura 2000 site”.
It is the responsibility of the competent authority, in this instance the EPA, to make a decision as to whether
or not the proposed disposal operation should be permitted, taking into consideration any potential impact
upon the Natura 2000 site in question.
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Natura Impact Statement for the Proposed Disposal of Dredged Spoil
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2.2. Stages
It is stated within the EU guidelines that “where, without any detailed assessment at the screening stage, it
can be assumed (because of the size or scale of the project or the characteristics of the Natura 2000 site)
that significant effects are likely, it will be sufficient to move directly to the appropriate assessment (Stage
Two) rather than complete the screening assessments explained below.”
The Commission’s methodological guidance (EC, 2002) promotes a four-stage process to complete the AA,
and outlines the issues and tests at each stage. An important aspect of the process is that the outcome at
each successive stage determines whether a further stage in the process is required.
The four stages are summarised diagrammatically in Figure 2.1 below.
Figure 2.1: Stages in the AA process (Source: DEHLG, 2009).
2.2.1. Stage 1. Screening for Appropriate Assessment
Screening is the process that addresses and records the reasoning and conclusions in relation to the first two
tests of Article 6(3):
i. whether a plan or project is directly connected to or necessary for the management of the site, and
ii. whether a plan or project, alone or in combination with other plans and projects, is likely to have
significant effects on a Natura 2000 site in view of its conservation objectives.
If the effects are deemed to be significant, potentially significant, or uncertain, or if the screening process
becomes overly complicated, then the process must proceed to Stage 2 (AA). Screening should be
undertaken without the inclusion of mitigation, unless potential impacts clearly can be avoided through the
modification or redesign of the plan or project, in which case the screening process is repeated on the
altered plan. The greatest level of evidence and justification is needed in circumstances where the process
ends at the screening stage on grounds of no impact.
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2.2.2. Stage 2. Appropriate Assessment
This stage considers whether the plan or project, alone or in combination with other projects or plans, will
have an adverse effect on the integrity of a Natura 2000 site, and includes any mitigation measures
necessary to avoid, reduce or offset negative effects. The proponent of the plan or project will be required to
submit a Natura Impact Statement, i.e. the report of a targeted professional scientific examination of the
plan or project and the relevant Natura 2000 sites, to identify and characterise any possible implications for
the site in view of the site’s conservation objectives, taking account of in combination effects. This should
provide information to enable the competent authority to carry out the appropriate assessment. If the
assessment is negative, i.e. adverse effects on the integrity of a site cannot be excluded, then the process
must proceed to Stage 4, or the plan or project should be abandoned. The AA is carried out by the
competent authority, and is supported by the NIS.
2.2.3. Stage 3. Alternative Solutions
This stage examines any alternative solutions or options that could enable the plan or project to proceed
without adverse effects on the integrity of a Natura 2000 site. The process must return to Stage 2 as
alternatives will require appropriate assessment in order to proceed. Demonstrating that all reasonable
alternatives have been considered and assessed, and that the least damaging option has been selected, is
necessary to progress to Stage 4.
2.2.4. Stage 4. Imperative Reasons of Overriding Public Interest (IROPI)/Derogation
Stage 4 is the main derogation process of Article 6(4) which examines whether there are imperative reasons
of overriding public interest (IROPI) for allowing a plan or project that will have adverse effects on the
integrity of a Natura 2000 site to proceed in cases where it has been established that no less damaging
alternative solution exists.
The extra protection measures for Annex I priority habitats come into effect when making the IROPI case2.
Compensatory measures must be proposed and assessed. The Commission must be informed of the
compensatory measures. Compensatory measures must be practical, implementable, likely to succeed,
proportionate and enforceable, and they must be approved by the Minister.
2 IROPI reasons that may be raised for sites hosting priority habitats are those relating to human health, public safety or
beneficial consequences of primary importance to the environment. In the case of other IROPI, the opinion of the
Commission is necessary and should be included in the AA
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Natura Impact Statement for the Proposed Disposal of Dredged Spoil
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Wicklow Port Authority
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3. The Ecological Impact Assessment (EcIA)
3.1. Introduction
The approach and methodology used in the Ecological Impact Assessment for this Natura Impact Statement
has been undertaken with due regard to the EPA Advice Notes on Current Practice (2003); EPA ‘Guidelines on
the Information to be contained in Environmental Impact Statements’ (2002); and the Institute of Ecology
and Environmental Management’s Guidelines for Ecological Impact Assessment (IEEM, 2006).
The ecological characteristics of the Natura 2000 site are described in Section 4 of this statement, followed
by an Assessment of Likely Effects, Mitigation and Residual impacts in Sections 5, 6 and 7 respectively. The
conclusions are set out in section 8.
3.2. Description of the Project
The dredge material for sea disposal will be excavated from within the Wicklow Harbour area itself with
harbour bed levels being excavated down to 5.5m below OS Malin (Hydro Environmental Ltd., 2012). It is
proposed that the dredging be carried out using a barge mounted excavator and that disposal of dredged
material shall be by means of a bottom opening barge at the proposed dump site. The total amount to be
disposed is estimated to be 54,260 tonnes with the dredge-disposal operation period being completed over
two months. The proposed monthly and weekly loadings are 27,000 and 6,000 tonnes respectively. These
rates represent a disposal rate of 10kg per second.
Sediment analysis of the dredge material based on 5 samples indicates that the dominant sediment
classification is medium sand (Hydro Environmental Ltd., 2012). Medium sand has the following
characteristics: density 2650kg/m3; grain size 0.0003m and mean settling velocity 0.03857m/s. Fine to
medium sand requires a shear stress of between 0.18 to 0.23 N/m3 in order to be eroded (also known as
critical shear stress value).
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Natura Impact Statement for the Proposed Disposal of Dredged Spoil
at Wicklow Port Authority’s Dredge Disposal Site
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3.3. Sediment Transport Model Results
Hydro Environmental Ltd. (2012) modelled sediment transport and bed shear stress. Bed shear stress
analysis indicated that for the majority of the time (70 – 80%) bed shear under tidal flows are sufficient to
mobilise and erode medium and coarse sand from the bed and that depositional conditions will only occur
for a very limited period over the tidal cycle. This information suggests that the proposed sandy sediments to
be deposited within the dredge disposal site will not remain and will be easily mobilised by the ambient tidal
flows. The computed shear velocities also show that permanent deposition of this material within the SAC
Reef area will not occur as the ambient tidal velocities are stronger there and will prevent permanent
settlement.
The simulations of releasing medium sand continuously over a forty day period demonstrated that sediment
is mobilised for the majority of the time due to the naturally high ambient shear velocities and only settles
temporarily at slack periods of the tidal cycle. This temporary deposition occurs for longer periods during
neap tides.
The plume transport is dominantly south and southeast from the disposal site and generally through the
western section of the cSAC reflecting the hydrodynamics of the area. Periods of deposition are very limited
to the slack tides and are immediately mobilised once velocities increase. The transported sediment will
generally be as bed load movement on the bed or in concentrated suspension towards the base of the water
column.
The modeled outputs also show that at certain stages of the tide a large portion of the cSAC area is free of
the sediment. These plots represent the total sediment mass released (bed and suspended loads) as
concentration in mg/l averaged over the water depth. In reality these sediment concentrations will be
confined close to the sea bed either as a bed form or as a relatively shallow bed layer of suspended
sediment.
The simulation demonstrates that movement from the dredge site will result in sediment migrating through
the cSAC area but with little opportunity for permanent settlement there.
Given the information from the hydrodynamics and residual currents the mobilised sediment will completely
migrate away initially south and then east and north away from the coastline and the cSAC and dredge site. It
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is expected that within 2 to 4 months of completing the dredging activity the sediment load through the
cSAC will have returned to ambient rates.
3.4. Baseline Description of the Subtidal Environment
3.4.1. Marine Benthos
The substrate of the Wicklow Reef cSAC is a mixture of cobbles, bedrock and sand and is subject to strong
tidal streams (NPWS, 2001). Wicklow Reef is an example of a subtidal reef constructed by the honeycomb
worm Sabellaria alveolata. In Irish waters this worm normally constructs reefs on intertidal rocks, in areas
subject to some sand scour, and such reefs are widespread but uncommon. Sabellaria alveolata subtidal
reefs are known to occur in the Mediterranean but this example is an extremely unusual feature and may be
the first record for Britain and Ireland. The reef occurs at a depth of 12-30m and reaches a thickness of at
least 0.3-0.5m. It is composed of consolidated sand grains formed into a honeycomb structure by the
activities of the worm. There is a good diversity of species associated with the reef, including hydroids
(Hydrallmania falcata), a variety of polychaete worms, the snail Calliostoma zizyphinum, the bivalves
Musculus discors and Mytilus edulis, molluscs, bryzoans, barnacles, amphipods and crabs, starfish, brittlestars
and sea squirts. Three of the species associated with this biogenic reef are rare in Irish waters. The bryozoan
Phaeostachys spinifera is only known from five locations, with the majority on the west coast and no records
in the Irish sea south of Co. Antrim. The polychaete Eulalia ornata and the amphipod Unciola crenatipalma
are only known from one and two sites respectively in Ireland. Wicklow Reef is of high conservation value as
it is the only documented example in Ireland of a biogenic reef.
4. Natura 2000 Sites
4.1. Sites is the vicinity of the Proposed Development
The proposed development is located within 500m of the Wicklow Reef cSAC (Site Code: IE0002274) (See
Figure 4.1 for location map).
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Figure 4.1: Location of Wicklow Reef cSAC (Site Code: IE002274) in relation to the dredge disposal site (Source: Hydro
Environmental Ltd., 2012).
4.2. Characteristics of the Designated Site
Wicklow Reef cSAC is situated just to the north of Wicklow Head on the east coast of county Wicklow.
Wicklow Reef is an example of a subtidal reef constructed by the honeycomb worm Sabellaria alveolata.
Reefs are listed under Annex I of the EU Habitats Directive.
The ecological information in the Standard Data Form for the cSAC IE002262 is summarised in Tables 4.1 and
4.2. A site synopsis for this cSAC can be found in Appendix 1.
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Table 4.1: Annex I habitats listed on the Natura 2000 Standard Data Form
Qualifying Habitat Code % Cover
Reefs 1170 95
Table 4.2: Other species of importance – from the Natura 2000 Standard Data Form
Species Name Group
Sabellaria alveolata (Polychaete) Invertebrate
Phaeostachys spinifera (Bryozoan) Invertebrate
Eulalia ornata (Polychaete) Invertebrate
Unicola crenatipalma (Amphipod) Invertebrate
4.3. Conservation Objectives
The conservation objectives of the cSAC IE002274 are outlined below (NPWS, 2011).
The overall aim of the Habitats Directive is to maintain or restore the favourable conservation status of
habitats and species of community interest. These habitats and species are listed in the Habitats and Birds
Directives and Special Areas of Conservation and Special Protection Areas are designated to afford protection
to the most vulnerable of them. These two designations are collectively known as the Natura 2000 network.
European and national legislation places a collective obligation on Ireland and its citizens to maintain habitats
and species in the Natura 2000 network at favourable conservation condition. The Government and its
agencies are responsible for the implementation and enforcement of regulations that will ensure the
ecological integrity of these sites.
The maintenance of habitats and species within Natura 2000 sites at favourable conservation condition will
contribute to the overall maintenance of favourable conservation status of those habitats and species at a
national level.
Favourable conservation status of a habitat is achieved when:
• its natural range, and area it covers within that range, is stable or increasing, and
• the ecological factors that are necessary for its long-term maintenance exist and are likely to continue
to exist for the foreseeable future, and
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• the conservation status of its typical species is favourable.
The favourable conservation status of a species is achieved when:
• population data on the species concerned indicate that it is maintaining itself, and
• the natural range of the species is neither being reduced or likely to be reduced for the foreseeable
future, and
• there is, and will probably continue to be, a sufficiently large habitat to maintain its populations on a
long-term basis.
Objective: To maintain or restore the favourable conservation condition of the Annex I habitat(s) and /or the
Annex II species for which the SAC has been selected:
• Reefs [1170]
5. Assessment of the Likely Effects
5.1. Sediment Dispersion & Deposition
The area of the Irish Sea south of Dalkey down to the Celtic Sea experiences high current velocities
throughout the water column with speeds reaching in excess of 1 m/s. Such velocities cause sediment to be
mobilised on every tide and give rise to megaripples that stand several meters high. Silt and fine sand are
transported out of the area and fall to the sea bed in the quiescent areas of the northern Irish Sea and the
Celtic Sea. Species that occur in such conditions have evolved to be able to exist is such locations.
Sabellaria alveolata is found in exposed conditions as it relies on high velocities to mobilise sediments that
can be used for tube and ultimately reef construction. The model outputs indicate that the medium to fine
sand dispersed from the spoil ground will stay mobilised for approximately 75-80% of the time as it passes
through the cSAC. The transported sediment will generally be as bed load movement or in concentrated
suspension towards the base of the water column. Highest concentrations of 10-15 mg/l will occur in the
most western part of the cSAC and for the majority of the cSAC the concentrations are less than 5 mg/l.
An increase in suspended sediments may facilitate tube building and the fine fraction of the suspended
sediments will have higher levels of organic matter associated with them compared to coarser sediments and
may act as an additional food source for the reef-forming worms. Sabellaria alveolata has been classified as
having a very low sensitivity and a very high recoverability to an increase in suspended sediments (Jackson,
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2008). This classification would also apply to the other taxa that occur on the Wicklow. For this reason, it is
considered that the Wicklow Reef cSAC will not be impacted by the disposal of dredge spoil at the designated
site.
An increase in suspended sediments may however clog up the feeding apparatus of the worms. The model
predicts that the transported sediments will only settle temporarily at slack periods of the tidal cycle (and
slightly longer during neap tides). This sediment will be remobilised once velocities increase ca 1 hour after
slack water. Sabellaria alveolata has been classified as been not sensitive to and having an immediate
recoverability to smothering (Jackson, 2008), surviving burial under sand for up to several weeks.
The model also predicts that in the 2-4 months following the 2 month dredging/disposal campaign, all of the
mobilised sediment will have completely migrated away, initially south, then east and north away from the
coastline, the cSAC site and the dredge site. It is expected that within 2-4months of completing the dredging
activity, the sediment load passing through the cSAC will have returned to ambient rates.
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6. Mitigation Measures
No mitigation measures are required.
7. Residual Impacts
Two-four months following the cessation of dredging and disposal, the mobilised sediment will have
completely migrated away from the cSAC and therefore there will be no residual impacts.
8. Summary Impacts/Mitigation Table
Table 3: Summary of Impacts and Mitigation
Activity Impacting
On
Via Potential Level of
Impact
Proposed
Mitigation
Residual
Impact
Increased
Suspended
Sediments
Short Term
Negligible
None Negligible Spoil Disposal cSAC
Smothering Short Term
Negligible
None Negligible
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9. References
DEHLG. 2009. Appropriate Assessment of Plans and Projects in Ireland - Guidance for Planning Authorities
(Revised February 2010).
European Commission. 2000. Managing Natura 2000 Sites: The provisions of Article 6 of the ‘Habitats’
Directive 92/43/EEC. Office for Official Publications of the European Communities, Luxembourg.
European Commission. 2002. Assessment of plans and projects significantly affecting Natura 2000 sites.
Methodological guidance on the provisions of Article 6(3) and (4) of the Habitats Directive 92/43/EEC.
Office for Official Publications of the European Communities, Luxembourg.
European Commission. 2007. EU Guidance document on Article 6(4) of the 'Habitats Directive' 92/43/EEC.
Clarification of the concepts of: alternative solutions, imperative reasons of overriding public interest,
compensatory measures, overall coherence, opinion of the Commission.
Hydro Environmental Ltd. 2012. Hydrodynamic Modelling Assessment of the Wicklow Harbour Dredge
Disposal Site. Prepared for Wicklow Port Authority. November 2012.
Jackson, A. 2008. Sabellaria alveolata. Honeycomb worm. Marine Life Information Network: Biology and
Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the
United Kingdom. [cited 21/01/2013]. Available from:
<http://www.marlin.ac.uk/speciessensitivity.php?speciesID=4277>
NPWS. 2011. Conservation objectives for Wicklow Reef SAC [002274]. Generic Version 3.0. Department of
Arts, Heritage & the Gaeltacht.
NPWS. 2001. Wicklow Reef Site Synopsis Site Code: IE002274.
http://www.npws.ie/media/npwsie/content/images/protectedsites/sitesynopsis/SY002274.pdf
Appendix 1
IE002274 Site Synopsis