APPENDIX AI

Model Addendum

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MARCON COMPUTATIONS INTERNATIONAL Advanced Englneertnp and Envirnnmentsl SoIutIons

ADDENDUM TO FINAL REPORT

For

Arklow Harbour Dredge Modelling Study

Prepared for

Arup Consulting Engineers on behalf of Arklow Harbour Commissioners

2 1 st July 2009

MarCon Computations International Ltd. No. 10, NUIG Business Innovation Centre,

Upper Newcastle, Galway, Jrehd

Tel I Fax: +353 91 583543 Email: Web www.marcon.ie

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MARCON COMPIITATIONS INTFRNATIDNAL

Table of Contents

Modelling Methodology ..................................................................................................................... 3

Model Input Parameters .................................................................................................................... 4 Analysis of Potential Sediment Transport ....................................................................................... 4 Long term stability of post-disposal cap .......................................................................................... 5

Model Outputs .................................................................................................................................... 7

Discussion and Conclusions ............................................................................................................... 8 Analysis of Potential Sediment Transport ....................................................................................... 8 Long term stability of post-disposal cap ......................................................... .... . ... . ... . ... ... . .. . .. . .... ... 9

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MARCON COMPUTATIONS INTERNATIONA- Mvmced Englneeriw and Envimnrnental Solutions

,

Introduction

MarCon Computations International Ltd., (MarCon), were commissioned by Arup Consulting Engineers, on behalf of Arklow Harbour Commissioners to undertake a mathematical modelling study of the Arklow Port dredge spoil disposal operations.

The objective was to determine the fate of dredged material disposed at open sea sites in response to local environmental forces of tides, waves and currents. The site for the disposal of the dredged material was investigated for the proposed operation. The geographic location of Disposal Site A is presented in Figure 1.

1 Figure 1: Location of Disposal Site A.

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MARCON COMPUTATIONS INTERNATIONA, AdvnCed Engmneadng am Enwonmental Sdutlonn a.- a

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Modelling Methodology

The Multiple Dump FATE (MDFATE) model is maintained and distributed by the US Army Corps of Engineers, Engineer Research and Development Center in Vicksburg, MS. The model predicts post-disposal bathymetry for dredge material disposed of in open water disposal sites. MDFATE simulates the convective descent, dynamic collapse and long-term passive diffusion of the dredge material. The MDFATE model output consists of a bathymetric grid defining the dredge material on the seafloor after simulation of the three processes listed above.

The MDFATE model was used to simulate the stability of the post-disposal cap at the proposed Disposal Site A, off the Arklow coastline. The MDFATE model was also used study to undertake a sensitivity analysis of potential rates of sediment transport for the dredge material at the proposed Disposal Site A, off the Arklow coastline.

The MDFATE model input parameters are a mixture of imperial and metric units. Model parameters detailed in this report will be quoted in metric units and, wh the imperial unit specified to MDFATE will be detailed in brackets.

The results are plotted as grids displaying the thickness of the material of the post-disposal cap and associated footprint of the post-disposal cap one year after the disposal operation commences.

C\MarCon\Contracts\MCO6O~ArklowDredgeU)oments\Outgoing\MCO6O~docOOZ.pdf 3

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Advanced Englneerinp and Environmental Soluti

ModelInputP ~ n

Analysis of Potential Sediment Transport

MDFATE facilitates qualitative sensitivity testing for the calculation of transport rates to determine the potential range of sediment transport rates for selected disposal sites. Results of the sediment transport sensitivity test are presented below for Disposal Site A. The MDFATE calculations are only applicable to median particle size diamebs (D50) ranging from 0.04 - 4.00 mm

Data used in the sediment transport rate predictions are:

water depth (D): 9.5 m (31 3) wave height (H): 0.0 m (03)

current speed (S): 0.0 - 0.9 ds (0.0 - 3.Oflsec)

Results of the sediment transport sensitivity test for varying D50, without wave effects, are shown below in Figure 2.

D50 0.041 -0.091 mm

wave period 0: 1%

0.0. I --tt--fi- 0 53 la, 150 z50 MO 350 400 450

Figure 2: D50 sensitivity of sediment transport rates @50=0.041mm - 0.091mm)

It is predicted that depth averaged velocities upto 0.45ds (I.Sj/s) will induce volumetric transport rates ranging fiom 0.0 to approx 850 m3/s/m (lOO,OOOfl/s@) at D5O = 0.041 mm.

fD501 wave e .. From the sensitivity analysis contained in the Main Report, it has been shown that long period waves (1 lsec) of significant height (1.8m) have the greatest effect on sediment transport rates for particles with a D50 of 0.lmm. To determine the effect of such waves on a range of sediment sizes the following analysis was undertaken. Data used in the transport rate prediction utility are:

wave period (T): 11 sec wave height (H):

D50 0.041 mm - 0.091

1.8 m (6j)

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MARCON COMPUTATIONS INTFWATIONA Advanced Enplneennp and Envimnmenlal SoILtI

water depth @): current speed (S):

9.5 m (313) 0.0 - 0.9 m/s (0.0 - 3.Oft/sec)

Results of the sediment transport sensitivity test for wave height = 1.8m (6f, and wave period = 11 sec are shown below in Figure 3.

It is predicted that depth averaged velocities upto 0.45m/s (1.5jVs) will induce volumetrk transport rates ranging ffom 0.0 to approx 17,500 m3/s/m (2,000,000$/s&) at D50 = 0.041 mm, 0.0 to approx 3,500 m3/s/m (W,Wfl/s&) at D50 = 0.051 mm, and less than 1,750 m3/s/m for D5O size classes greater than 0.051mm

Long term stability of post-dlpposd cap

Disposal Site A, as presented in Figure 1 above, was schematised to the model as a regular grid of width 90Om (2,9503) and length 60Om (1,95@). The grid resolution was specified as 15.25m (5oft).

The bottom topography of the disposal site was defined as a flat plane with a mean depth of 9.5m (313).

The post-disposal cap was defined as 18Om x 18Om. The top level of the post-disposal cap was specified as 1.4m (4.63) above bed level. The material in the post-disposal cap was specified as consisting of sand with a median grain size of 0.25- in accordance with grab samples taken at the site. The grid schematisation of Disposal Site A is presented as a colour ramp image in Figure 4 and as a wiremesh relief image in Figure 5.

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a a a a a a a a a a a a a a a a a a a a a ' a a a

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MARCON COMPUTATIONS INTERNATIONA

0

I

.*U - Z b . 7 ,

.13 ~ 27 .a

.m ~ 77 39

.39 ~ 27.71

.71 ~ 28.M

. w ~ 28.37

.51 - 20.70

.70 ~ 29.03

.03 - 29.36,

.3b ~ 29.69

.69 ~ 38.01

.01 . 30.34

.34 ~ 30.b7

.07 - 31.-

Figure 4: Schematic of post-disposal cap at Disposal Site A (colour ramp).

0.D.M.D.S A. 8 "OURS

Figure 5: Schematic of post-disposal cap at Disposal Site A (wire mesh)

MDFATE simulated tidally varying water depths derived f3om harmonic tidal constituents at Arklow Port obtained h m the United Kingdom Hydrographic Office publications, tidally varying current speeds analogous to the current speeds recorded during the hydrographic survey undertaken by Moore Group and a constant wave field throughout the disposal site with the following characteristics:

Wave height: 1.8m (6ft) Wave period: 11 sec Wave direction: westerly

MDFATE calculated the consolidation and bed load transport of the post-disposal cap in accordance with the above conditions. MDFATE output the thickness and shape of the of post- disposal cap one year after the disposal operation commenced and incorporating the physical effects of avalanching, consolidation and mound transport.

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MARC0 N COMPUTATIONS INTERN ATION A

rn --- Model Outputs

MDFATE calculated the height and extent of post-disposal cap one year after the disposal operation commenced. The calculation of the post-disposal cap height and shape after one year incorporated the physical effects of avalanching, consolidation and mound transport.

The model-predicted height and shape of the post-disposal cap is presented as a colour ramp image in Figure 6, as a colour contour image in Figure 7 and as a wire m e perspective view in Figure 8.

The predicted maximum height of the dredge material at Disposal Site B six weeks after the disposal operation commenced was approx. 0.12111 (0.423) with a total footprint of 18.2 hectares or 182,000 m2 (1.96~ I@@).

'.28 - 27.53 '.n - n.m '.87 - zB.21 1.21 - 28.54 1 .54 - 28.88 1.88 - 73.22 1.22 - n.56 1 . 5 6 ' - n.m

1.23 ~ 33.57 1.53 - m.30 1.90 - 31.2+ ..a - 31.58 .58 - 31.91

1.m - 30.23

Figure 6: Post-diiposal cap after one year (colour ramp

higure I : rosr-asposai cap a1rer one year (colour contou

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MARCON TOMPUTATIONS INTFRNATIONAL I

m

Discussion and Conclusions

Analysis of Potential Sediment Transport

A series of sensitivity analyses were performed for Disposal Site A to determine quantitatively the volumetric rates of sediment transport from the site for small median particle size classes. The various parameters examined included, median sediment grain size @50), mean wave height (H) and mean wave period (T) over a range of depth averaged current speeds. The main finding from the sensitivity analysis was that:

For current velocities below 0.45 ds, with no wave action, there is predicted to be, at most, a transport rate of 850 m3/s/m (lOO,OOOfr5ls/fr! for small particle sizes.

Current speeds were recorded at Disposal Site A as part of the current study from 23/12/08 to 06/01/09. Current Meter No.1 recorded no speeds in excess of 0.45 ds. Current Meter No.2 only recorded speeds in excess of 0.45 m/s for a total of 131 minutes during the course of the 14 day deployment.

No in-situ wave measurements have been undertaken as part of the current study. Access to online wave climate prediction models (http:/ /m .prism.ie) have indicated that as of 22/06/09 waves off the Arklow coastline were predicted to have a wave height of <0.25m with a period of >lOsec. Unfortunately, long term wave field statistics were not available h m this site for the current study.

Large waves with a long period have the ability to induce sediment transport at Disposal Site A but only if they have significant height and will only affect the smallest size classes of sediment material.

It is predicted that depth averaged velocities upto 0.45ds (1.5fls) will induce the following volumetric transport rates:

upto 17,500 m3/s/m (2,0OO,OOO~/s4) at D50 = 0.041 mm, upto 3,500 m3/s/m (400,OOOjP/s4) at D50 = 0.051 mm, less than 1,750 m3/s/m for D50 sue classes greater than 0.051mm

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MARC0 N COMPUTATION 5 INTERN AT1 ON A

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*At*-- --’ Long term stability of post-disposal cap

The MDFATE model predicted the shape and height of the post-disposal cap, subjected to continuously large wave conditions, one year after the dredging operation commenced.

The MDFATE model showed that the cap does have the potential to erode in the presence of large waves for a continuous period. However, the extreme conditions simulated would never occur continually for the duration of one year. It is likely that some erosion of the cap will occur during storm conditions but that over the long term the cap will remain in place, be subjected to consolidation and periodic erosion.

Based on the results of the MDFATE model, it is not predicted that the cap will erode over the period of one year, even subject to excessive wave climates.

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