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Enterprise Energy Ireland Ltd Corrib Field Development Project

Dispersion Modelling Study Broadhaven Bay

Addendum Report

May 2002

KkK McCLUiE MORTON CONSULTING ENGLNEERS

ELMWOOD HOUSE 74 BOUCHER ROAD BELFAST BT12 6RZ Telephone i44 28 9066 7914 Facsimile +44 28 9066 8286

#

KIRK MCCLURE MORTON

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KIRK MCCLURE MORTON

/

ENTERPRISE ENERGY IRELAND LTD

CORRIB FIELD DEVELOPMENT PROJECT

DISPERSION MODELLING STUDY - BROADHAVEN BAY

ADDENDUM REPORT

Report Status: Final Report

Date: May 2002

Reference: Kirk McClure Morton, 2002. Corrib Field Development Project,

Dispersion Modelling Study - Broadhaven Bay. A report for

Enterprise Energy Ireland Ltd.

Report number: RW1212a

,o Contract number: KMM 5222.00

Commissioned by: Enterprise Energy Ireland Ltd., Corrib Project Tear-n, Park House,

Frascati Road, Blackrock, Co. Dublin, Ireland.

; . Enquiries relating to this report should be directed to:

Kirk McClure Morton, Ehnwood House, 74 Boucher Road, Belfast, BT12 6R2, N Ireland

Telephone: +44 28 9066 7914

Facsimile: +44 28 9066 8286

l E-mail: [email protected]

. . 5222.00/MB/RWl212a

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Dispersion Modelling Report - Addendum Contents

CONTENTS

Page

1.0

2.0

3.0

a ,,‘. _ 4.0

5.0

INTRODUCTION

HYDRODYNAMIC MODELLING

WATER QUALITY MODELLING

3.1 General 3.2 Dispersion Model Simulations

CONCLUSIONS OF THE STUDY

REFERENCES 26

1

4

4 4

22

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0 Corrib Field Development Project Enterprise Energy Ireland Ltd. Dispersion Modelling Report Addendum Introduction

1.0 INTRODUCTION

Enterprise Energy Ireland Ltd. has commissioned a series of studies with the

ultimate aim of developing the Corrib natural gas field, which lies off the western

seaboard of County Mayo, Ireland. Due to the exposed location of the Corrib gas

field, on-site pre-processing of the raw gas is not considered to be feasible.

Consequently, all processing to meet sales gas specifications will be carried out at a

new onshore reception terminal to be built at Bellanaboy Bridge in County Mayo.

Within the proposed reception terminal any liquids contained within the gas stream

will be extracted, produced water separated out, and passed through a

comprehensive treatment process. The aim of the proposed treatment process is to

reduce concentrations of metallic elements in the effluent to or below the EQS level

prior to discharge. Before discharge, the treated produced water will be combined

with treated rainwater and any fnewater runoff. This second wastewater stream will

also have been treated to EQS levels in a separate treatment process. The preferred

method for the disposal of the resulting treated effluent is by direct discharge to the

receiving waters of Broadhaven Bay and the western seaboard of Ireland.

The onshore development is presently the subject of a planning application and

Foreshore Licence application for which a study of the dispersion of metallic

elements contained in the effluent discharge was required to determine the likely

environmental impact on Broadhaven Bay. Kirk McClure Morton was therefore

commissioned by Enterprise Energy Ireland Ltd to study dispersion within

Broadhaven Bay using computational modelling techniques and prepare a report as

supporting information for the aforementioned applications.

This study indicated that an outfall extending to the 40m contour in Broadhaven Bay

would be sufficient to prevent a significant increase in background concentrations of

metallic elements occurring as detailed in Reference 1. Subsequent to the

submission of the various application and discussions with various interested parties

it was decided to investigate the impact of discharging the effluent at a point outside

5222.OO/MB/RWI212a 1 KlRKMcClURE

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C&rib Field Development Project Enterprise Energy Ireland Ltd. Dispersion Modelling Report Addendum Introduction

the boundary of the Broadhaven Bay Special Area of Conservation. It is this latter

element of the dispersion study that is the subject of this addendum report.

The principal objectives of Kirk McClure Morton’s additional study were therefore:

to utilise the hydrodynamic model of Broadhaven Bay developed during the

original study to provide simulated tidal flow conditions for input to the

dispersion models;

to establish a series of dispersion models to simulate the advection, dispersion

and fate of a range of constituents in the effluent discharged from a potential

outfall position outside Broadhaven Bay;

to establish the impact of the discharge on existing background concentrations

of metallic elements within Broadhaven Bay and the SAC.

The additional model simulations have been completed and the potential impact for

receiving water quality of extending the outfall to a point on the 60m contour line is

discussed in subsequent sections of this document. Following completion of this

modelling study, Enterprise Energy Ireland advised that the diffuser is to be located

at KP 71.0, approximately on the 64m contour, outside Broadhaven Bay. This

adjustment is not expected to significantly alter the predictions for the 60m depth, as

discussed within.

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: ., ; 0 Corrib Field Developmtint Project Enterprise Energy Ireland Ltd.

Dispersion Modelling Report Addendum Hydrodynamic Modelling

2.0 HYDRODYNAMIC MODELLING

Kirk McClure Morton’s hydrodynamic model of Broadhaven Bay and a portion of

the adjoining Atlantic seaboard of County Mayo was developed using the

TELEMAC-2D suite of computational modelling software developed by LNH in

Paris and distributed in the UK and Ireland by H.R. Wallingford Ltd. This model

solves the two-dimensional depth-integrated shallow water equations, which

represent the flow in rivers, estuaries and seas using a finite element technique,

permitting the use of very flexible unstructured triangular grids to represent the

model domain. Full details of the model implementation and the correlation with

field observations are presented in Section 2 of the main dispersion modelling

report, Reference 1.

The tidal flow regime predicted by the hydrodynamic model was compared with

observations and notes contained in sailing directions, the Admiralty coast pilot and

current and level information collected specifically for this purpose. Generally the

predicted tidal current regime was found to be broadly in line with the observations

presented in the Irish Coast Pilot, Reference 2.

Similarly the correlation between the’ model predictions and field observations of

tidal levels and currents was judged sufficient to give confidence that the model was

predicting the correct tidal exchange between the various inlets and outer

Broadhaven Bay. Hence the model predictions were considered to be representative

of tidal movements within the Bay.

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Corrib Field Development Project Enterprise Energy Ireland Ltd. Dispersion Modelling Report Addendum Water Quality Modelling

3.0 WATER QUALITY MODELLING

3.1 General

0 ‘_

‘. 0. .. 3.2 .’

Computational models for the assessments of effluent dispersion in the marine

environment comprise a suite of post processing packages associated with the

hydrodynamic model. These models are designed to examine the consequences of

both existing and proposed discharges to the water environment. For this study the

effluent dispersal model, PLUME-RW, was utilised to model the release of treated

effluent from the proposed gas reception terminal at Bellanaboy Bridge into the

receiving waters off the County Mayo coastline.

Details of the dispersion model implementation and the assumptions made

regarding dispersion characteristics are presented in Section 3 of Reference 1. The

accuracy of the various assumptions was confirmed by simulating a selection of the

dye release experiments undertaken as part of the main study and the results

obtained were sufficient to conclude that the dispersion model was capable of

accurately predicting the mean advection of the dye patches. Thus confirming the

applicability of the diffusion coefficients etc. used in the dispersion model

simulations to the receiving waters of Broadhaven Bay.

Dispersion Model Simulations

0

A series of dispersion model simulations were carried out in order to identify the

potential impact of discharging the proposed dry weather loading from the

Bellanaboy Bridge Terminal on receiving water quality in Broadhaven Bay during

this additional study. The predicted dry weather discharge was selected for

investigation since this had previously been shown to be the most onerous loading

condition. A possible outfall position on the route of the proposed gas transmission

pipeline but outside the boundary of the SAC in approximately 60m of water was

selected for investigation during this study. The influence of wind generated surface

currents was included in the analysis of this outfall position to illustrate any

potential variation in impact during different climatic conditions. This analysis has

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“ Corrib Field Development Project Dispersion Modelling Report Addendum

Enterprise Energy Ireland Ltd. Water Quality Modelling

been restricted to reviewing the impact during neap tidal conditions as the model

results presented in the main report indicated this to be the most adverse tidal state.

The following plume simulation runs were carried out in order to assess the impact

of the proposed effluent discharge and determine the likely impact on receiving

water quality in Broadhaven Bay.

Run15

1’ 0 ,. i

Run16

Run 17

Run18

Run19

Dry weather discharge from an outfall extending to the 60m contour off

Broadhaven Bay during neap tidal cycles and calm weather.


Broadhaven Bay during neap tidal cycles and northerly winds.


Broadhaven Bay during neap tidal cycles and northwesterly winds.


Broadhaven Bay during neap tidal cycles and northeasterly winds.


Broadhaven Bay during neap tidal cycles and westerly winds.

An envelope of the maximum concentration of each constituent of the fmal effluent

attained at every point in the model grid has been plotted for each model run. These

envelopes represent the highest concentration reached in every cell in the model

although this may have been present for only a short period during the tidal cycle.

The envelope therefore marks the outer limit of the plume’s influence and the

excursion of the effluent plume at any given time during the tidal cycle will lie

entirely within the area covered by the plume envelope..

Contours equivalent to an increase of 0.5, 1.0 and 1.5 percent over the existing

background concentrations of each of the constituents within Broadhaven Bay have

5222.OO/MB/R W1212a 5 KIRK MCCLURE

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Corrib Field Devklolhent Project Enterprise Energy Ireland Ltd. Dispersion Modelling Report Addendum Water Quality Modelling

been drawn to illustrate the potential

quality.

impact of the effluent discharge on water

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5. The effluent was discharged over a series of neap tides from an outfall extending to

the 60m contour off Broadhaven Bay in order to allow a stable distribution of effluent

concentrations to develop.


60m OUTFALL OPTION

Run 15 Dry weather discharge from an outfall extending to the 60m contour off Broadhaven

Bay during neap tidal cycles and calm weather conditions.

This series of model runs was intended to determine the impact on water quality in

Broadhaven Bay of discharging treated effluent from the Gas Reception Terminal outside the

designated SAC during dry weather conditions.

The following assumptions were made for these computer runs:

1. The effluent was discharged at a rate of 82.3 m3/hour for one hour commencing at

one hour after each high water i.e. 2 hours per day.

2. Treated produced water in the effluent stream was discharged at a concentration

equivalent to the EQS for each constituent of the effluent.

3. No temporal decay was applied i.e. concentrations are only reduced by the processes

of natural dispersion.

4. Calm weather conditions prevailed during each of the model runs,

6. The effluent mixed vertically throughout the water column, i.e. no stratification

occurred.

The furthest excursions of the resulting effluent plumes in Broadhaven Bay are shown in

Runs 15a and b.

. . . . ,a 1 :-.: ,,: .)

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m .- Corrib Field Development Project Enterprise Energy Ireland Ltd. Dispersion Modelling Report Addendum Water Quality Modelling

60m OUTFALL OPTION


Bay during neap tidal cycles and northerly winds.

This series of model runs was intended to determine the impact of northerly winds on the

dispersion of effluent from the proposed Gas Reception Terminal and the resulting impact on

water quality within Broadhaven Bay during dry weather conditions.








4. A 5.5m.k northerly wind prevailed during each of the model runs.





occurred.


Runs 16a and b.

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Corrib PiePd Development Project Enterprise Energy Ireland Ltd. ,Dispsrsion M,odelling Report Addendum Water Quality Modelling

50m OUTl?ALL OPTION

Rnn 17 Dry weather discharge from an outfall extending to the 60m contour off Broadhaven

Bay during neap tidal cycles and north westerly winds.

This series of model runs was intended to determine the impact of north westerly winds on

the dispersion of effluent from the proposed Gas Reception Terminal and the resulting impact

on water quality within Broadhaven Bay during dry weather conditions.






3, No temporal decay was applied i.e. concentrations are only reduced by the processes


4. A T.Sm/s north westerly wind prevailed during each of the model runs.





occurred.

The fhrthest excursions of the resulting effluent plumes in Broadhaven Bay are shown in

Runs 17a and b.

5222.OO/MB/R Wi212a 13

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60m OUTFALL OPTION

Run 1X Dry weather discharge from an outfall extending to the 60m contour off Broadhaven

Bay during neap tidal cycles and north easterly winds.

This series of model runs was intended to determine the impact of north easterly winds on the




1. The eMuent was discharged at a rate of 82.3 m3ihour for one hour commencing at






4. A 5Srnfs north easterly wind prevailed during each of the model runs.




6. The effluent mixed vertically throughout the water colur~, i.e. no stratification

occurred.


Runs 18a and b.

0 ‘k. / a,

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Corrib Field Development Project Enterprise Energy Ireland Ltd. Dispersion Modelling Report Addendum Water Quality Modelling,

60m OUTFALL OPTION


Bay during neap tidal cycles and westerly winds.

This series of model runs was intended to determine the impact of westerly winds on the




1.

2.

3.

4.

5.

6.

The effluent was discharged at a rate of 82.3 m3/hour for one hour commencing at


Treated produced water in the effluent stream was discharged at a concentration


No temporal decay was applied i.e. concentrations are only reduced by the processes


A 5.5mls westerly wind prevailed during each of the model runs.

The effluent was discharged over a series of neap tides from an outfall extending to



The effluent mixed vertically throughout the water column, i.e. no stratification

occurred.


Runs 19a and b.

.I .’ 0 -,., ;. .‘Zi w

19 KIRK MCCLURE

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: ..) :, ._” .,.

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Corrib Field Development Project Enterprise Energy Ireland Ltd. Dispersion Modelling Report Addendum Conclusions of the Study

4.0 CONCLUSIONS OF THE STUDY

The implications for receiving water quality in Broadhaven Bay of discharging

effluent from the proposed onshore gas reception terminal at Bellanaboy Bridge via a

new outfall outside the bay have been assessed using computational modelling

techniques. The following conclusions have been drawn with the aid of the

computerised dispersion model simulation runs:

Hydrodynamic and water quality models of Broadhaven Bay and its approaches have

been set-up and verified against data recorded specifically for this project and other

published data. The hydrodynamic models have been proven to be capable of

simulating the tidal flow regime within the Bay and the various passages leading to

the inner parts of Broadhaven Bay and the Sruwaddacon.

A series of effluent dispersion simulations have been completed for an outfall position

outside the Bay using neap tidal predictions and a range of weather conditions. The

results of the dispersion modelling have been compared to recorded background

concentrations of various metallic elements within Broadhaven Bay in order to assess

the significance of the predicted impact.

Dispersion modelling was completed for an outfall extending to the 60m contour

outside Broadhaven Bay at IGR 070969,343698. Subsequent to the completion of the

dispersion modelling Enterprise Energy Ireland Ltd advised that the diffuser is to be

located at KP 71.0, approximately on the 64m contour outside the Bay. However, the

slight increase in water depth and movement of the outfall are not expected to

significantly alter the predictions of the model from those presented for the 60m water

depth. Only the dry weather discharge was modelled using neap tidal conditions as

these were known to be the worst case combination as a result of the extensive

modelling undertaken during the main study.

The maximum percentage increase in background concentration of the various

metallic elements during calm weather conditions is presented in Table 1.

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Corrib IF’ieid Development Project Enterprise Energy Ireland Ltd. Dispersion Modelling Report Addendum Conclusions of the Study

Table 1 Predicted Impact of Effluent Discharge (60m Outfall)

The results of the modelling presented in Table 1 indicate that the effluent plume

resulting from the intermittent dry weather discharge is not predicted to increase

background concentrations by more than 1.2% during neap tides. Predicted increases

of greater than 0.5% of the existing background concentrations are restricted within

approximately 500m of the point of discharge.

The results of the dispersion model simulations for this longer outfall option are

compared with correspotiding results for the various outfall positions investigated in

the main study in Table 2 below.

Table 2 Comparison of the Impact of the Potential Effluent Discharges

5222.OO/MB/R W1212a 23 KIRK MCCLURE MORTON

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Examination of the data presented in Table 2 indicates that increasing the available

water depth at the point of discharge reduces the maximum concentration of each

constituent of the effluent predicted to occur within the receiving waters. However the

incremental difference reduces with increasing water depth to the point where moving

from a water depth of 30m to 40m does not materially alter the magnitude of the

predicted impact. Thus it was concluded in the original study that extending the

outfall into deeper water within the Bay i.e. beyond the 40m contour would not yield

any significant benefit in terms of water quality. This is borne out by the results of the

dispersion model simulations for the 60m water depth which generally do not indicate

a significant reduction in impact when compared to the equivalent data for the 40m

water depth.

The impact of wind derived currents on the dispersion of effluent from the deep water

outfall position outside Broadhaven Bay was also investigated using the dispersion

model. Again this exercise was restricted to an investigation of the impact on the

dispersion of the dry weather discharge during periods of neap tides as this had

previously been shown to be the worst case scenario. Model simulations for a light to

moderate breeze from four wind directions ranging from north east through north to

west were completed.

The dispersion modelling does not indicate any occasion when the wind induced

currents cause effluent to be advected into Broadhaven Bay in sufficient quantity to

increase the tioncentrations of metallic elements by more than 0.5% above

background. The effect of the wind induced currents is to reduce the magnitude of the

peak concentrations close to the outfall. This can be explained in terms of the

increased advection and dispersion of effluent within the Bay caused by wind induced

currents particularly during the periods when the tidal flow is slack. The proposed

60m outfall position is also sufficiently far offshore to prevent the impact at the

shoreline in Broadhaven Bay from exceeding 0.1% of the existing background levels.

The actual effect of the wind induced currents on the maximum concentrations of

metallic elements is summarised in Table 3 below.

5222.OO/MB/RWI212a 24 KJRK MCCLURE MORTON

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, L. I,;“,. :” _. .:; ‘,.‘i ; ;

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Table 3 Impact of Wind on Effluent Dispersion

l

The modelling results indicate that for effluent treated to at least the EQS standard

before discharge, the provision of an outfall extending to the 60m contour outside

Broadhaven Bay will not result in a significant adverse impact on water quality within

the Bay. In all cases investigated any impact in excess of 0.5% of the existing

background lies outside Broadhaven Bay with the worst case showing an increase of

1.2% in Chromium levels above the existing background. In many cases no increase

in excess of 0.5% of the existing background concentration is predicted to occur even

within the offshore waters.

Subsequent to the completion of this modelling study Enterprise Energy Ireland

advised that the diffiser is to be located at KP 71.0, approximately on the 64m

contour outside Broadhaven Bay. This revised position is approximately 220m further

offshore than the position investigated for this addendum report. Given the minimal

differences in impact associated with moving fie discharge from the 40m contour to

the 60m contour it is considered very unlikely that this &rther movement would have

any significant impact on the results of the dispersion modelling.

5222.OO/MB/RW1212a 25 w

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Corrib Field Development Project Enterprise Energy Ireland Ltd. Dispersion Modelling Report Addendum References

5.0 REFERENCES

1.

2.

Kirk McClure Morton, 200 1. Corrib Field Development Project, Dispersion Modelling

Study - Broadhaven Bay. A report for Enterprise Energy Ireland Ltd.

UK Hydrographic Office. Irish Coast Pilot.

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