limits - environmental protection agency, · pdf filelimits limits 695000n 695500n 733500e...

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.


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


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).


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


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


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.


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


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


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


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.


30/11/2012

Figure 10 Spring Tide Hydrodynamics on 18th Sep 2012 (1.5hrs before HW (Wicklow))


30/11/2012

Figure 11 Spring Tide Hydrodynamics on 18th Sep 2012 (1.5hrs after HW (Wicklow))


30/11/2012

Figure 12 Spring Tide Hydrodynamics on 18th Sep 2012 (4hrs After HW (Wicklow))


30/11/2012

Figure 13 Spring Tide Hydrodynamics on 18th Sep 2012 (1hrs After LW (Wicklow))


30/11/2012

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


30/11/2012

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


30/11/2012

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)


30/11/2012

Figure 18 Mean Shear Stress Magnitude over 40day tidal simulation

Figure 19 Maximum Shear Stress Magnitude over 40day tidal simulation


30/11/2012

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


30/11/2012

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


30/11/2012

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)



30/11/2012

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)


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)



30/11/2012

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


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(vii) Low water

(viii) Low + 1hr


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(ix) Low Water + 2hrs

(x) Low Water + 3hrs


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(xi) Highwater – 2hrs

(xii) Highwater – 1 hrs


30/11/2012

(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

[email protected]

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

1

Natura Impact Statement for the Proposed Disposal of Dredged Spoil

at Wicklow Port Authority’s Dredge Disposal Site


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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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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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• 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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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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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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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

8

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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

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