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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
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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
a, KIRK M’XXURE
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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
5222.OO/MB/RW1212a
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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.
Dry weather discharge from an outfall extending to the 60m contour off
Broadhaven Bay during neap tidal cycles and northerly winds.
Dry weather discharge from an outfall extending to the 60m contour off
Broadhaven Bay during neap tidal cycles and northwesterly winds.
Dry weather discharge from an outfall extending to the 60m contour off
Broadhaven Bay during neap tidal cycles and northeasterly winds.
Dry weather discharge from an outfall extending to the 60m contour off
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.
Corrib Field Development Project Enterprise Energy Ireland Ltd. Dispersion Modelling Report Addendum Water Quality Modelling
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 :-.: ,,: .)
5222.OO/MB/R WI 212a
a> 7
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m .- Corrib Field Development Project Enterprise Energy Ireland Ltd. Dispersion Modelling Report Addendum Water Quality Modelling
60m OUTFALL OPTION
Run 16 Dry weather discharge from an outfall extending to the 60m contour off Broadhaven
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.
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. A 5.5m.k northerly wind prevailed during each of the model runs.
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.
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 16a and b.
5222.OO/MB/RWI212a 10 KIRK MCCLURE MORTON
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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.
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. A T.Sm/s north westerly wind prevailed during each of the model runs.
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.
6. The effluent mixed vertically throughout the water column, i.e. no stratification
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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Corrib Field Development Project Enterprise Energy Ireland Ltd. Dispersion Modelling Report Addendum Water Quality Modelling
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
dispersion of effluent from the proposed Gas Reception Terminal and the resulting impact on
water quality within Broadhaven Bay during dry weather conditions.
The following assumptions were made for these computer runs:
1. The eMuent was discharged at a rate of 82.3 m3ihour 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. A 5Srnfs north easterly wind prevailed during each of the model runs.
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.
6. The effluent mixed vertically throughout the water colur~, i.e. no stratification
occurred.
The furthest excursions of the resulting effluent plumes in Broadhaven Bay are shown in
Runs 18a and b.
0 ‘k. / a,
5222.OO/MB/R Wl212a 16 KJR$oM&C&l.RE
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Corrib Field Development Project Enterprise Energy Ireland Ltd. Dispersion Modelling Report Addendum Water Quality Modelling,
60m OUTFALL OPTION
Run 19 Dry weather discharge from an outfall extending to the 60m contour off Broadhaven
Bay during neap tidal cycles and westerly winds.
This series of model runs was intended to determine the impact of 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.
The following assumptions were made for these computer runs:
1.
2.
3.
4.
5.
6.
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.
Treated produced water in the effluent stream was discharged at a concentration
equivalent to the EQS for each constituent of the effluent.
No temporal decay was applied i.e. concentrations are only reduced by the processes
of natural dispersion.
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 60m contour off Broadhaven Bay in order to allow a stable distribution of effluent
concentrations to develop.
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 19a and b.
.I .’ 0 -,., ;. .‘Zi w
19 KIRK MCCLURE
MORTON
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: ..) :, ._” .,.
0
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.
5222.OO/MB/R WI 212a 22 KIRK MCCLURE MORTON
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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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Corrib Field Development Project Enterprise Energy Ireland Ltd. Dispersion Modelling Report Addendum Conclusions of the Study
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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Corrib Field Development Project Enterprise Energy Ireland Ltd. Dispersion Modelling Report Addendum Conclusions of the Study
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
KIRK MCCLURE MORTON
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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.
5222.OO/MB/RW1212a 26 KIRK MCCLURE MORTON
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