ANNEX 4A Water Quality Method Statement - epd.gov.hk · treatment and disposal of the sewage arising from ... This Method Statement presents information on ... conservative estimates

ANNEX 4A

Water Quality Method Statement

CONTENTS

4A1 INTRODUCTION 1

4A1.1 BACKGROUND 1

4A1.2 PURPOSE OF THE METHOD STATEMENT 3

4A1.3 INTERPRETATION OF THE REQUIREMENTS: KEY ISSUES AND CONSTRAINTS 3

4A1.4 MODEL SELECTION 3

4A1.5 GRID REFINEMENT 4

4A1.6 COASTLINE & BATHYMETRY 5

4A1.7 BOUNDARY CONDITIONS 7

4A1.8 AMBIENT ENVIRONMENTAL CONDITIONS - WATER TEMPERATURE, SOLAR

RADIATION AND WIND 7

4A1.9 VECTOR INFORMATION 7

4A1.10 UNCERTAINTIES IN ASSESSMENT METHODOLOGIES 8

4A2 WATER SENSITIVE RECEIVERS 9

4A3 CONSTRUCTION PHASE 11

4A3.1 CALCULATION METHOD 11

4A3.2 WORKING TIME 13

4A3.3 OVERVIEW OF DREDGING PLANT - GRAB DREDGERS 13

4A3.4 CONSTRUCTION SCENARIO – GRAB DREDGING FOR SUBMARINE OUTFALL 15

4A3.5 SEDIMENT INPUT PARAMETERS 16

4A4 OPERATION PHASE 17

4A4.1 OPERATION SCENARIO 17

4A5 EMERGENCY DISCHARGE 20

4A5.1 EMERGENCY SITUATIONS 22

4A5.2 MITIGATION MEASURES 22

4A5.3 MODEL EMERGENCY SCENARIOS 25

4A6 CUMULATIVE IMPACTS 29

4A7 MODEL SCENARIOS 30

ENVIRONMENTAL RESOURCES MANAGEMENT B&V

0227541_ANNEX 4A_WQMS_20160616.DOC 09 AUGUST 2016

ANNEX 4A - 1

4A1 INTRODUCTION

4A1.1 BACKGROUND

The Drainage Services Department (DSD) is undertaking a project named

“Outlying Islands Sewerage Stage 2 – South Lantau Sewerage Works” (PWP Item

331DS) (hereinafter referred to as the “Project”). This Project is to provide a

sewerage system that includes trunk and village sewerage, sewage pumping

stations, a sewage treatment works (STW) and associated effluent facilities,

and submarine outfall. This project is intended to provide proper collection,

treatment and disposal of the sewage arising from South Lantau, including

areas in Shui Hau, Tong Fuk, Cheung Sha, San Shek Wan, Pui O and Ham Tin.

The proposed sewerage works is located in South Lantau. The treatment

level is proposed to be secondary (biological) treatment level with

disinfection. Membrane Bio-Reactor (MBR) technique is the recommended

treatment option at the San Shek Wan STW. The extent of the sewerage

works is indicated on Figure 4A.1. The key elements of the Project are:

a) Provision of village sewerage to unsewered areas of Shui Hau, Tong Fuk,

Cheung Sha, San Shek Wan, Pui O and Ham Tin in South Lantau.

b) Construction of trunk sewer, sewage pumping stations and the associated

rising mains along South Lantau Road for collection and conveyance of

sewage from unsewered areas mentioned in item a) above to the

proposed San Shek Wan STW; and

c) Construction of a STW at San Shek Wan, associated effluent pumping

facilities and a submarine outfall.

The Project will require an Environmental Permit from the Hong Kong SAR

Government. In relation to this, DSD has prepared a Project Profile for

Application for an Environmental Impact Assessment (EIA) Study Brief which

was submitted to EPD on 6 August 2009. The EIA Study Brief (No. ESB-

209/2009) was issued by EPD on 15 September 2009.

Following the issuance of the EIA Study Brief, DSD commissioned the study

“Outlying Islands Sewerage Stage 2 – South Lantau Sewerage Works –

Investigation” (Agreement No. CE 55/2009 (DS)) for EIA and preliminary

design studies of the Project. As part of the EIA, computational

hydrodynamic and water quality modelling will be undertaken to quantify

and evaluate potential water quality impacts associated with the construction

and operation of this Project.



ANNEX 4A - 2

Figure 4A.1 Proposed Construction Works for Outlying Islands Sewerage Stage 2 – South Lantau Sewerage Works

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ANNEX 4A - 3

4A1.2 PURPOSE OF THE METHOD STATEMENT

This Method Statement presents information on the approach for the water

quality modelling and assessment works for the EIA. It is important to note

that at the time of writing this Method Statement the detailed engineering

information for both construction and operation activities is yet to be

confirmed and therefore a general approach as to how the modelling works

would be carried out is presented herein, with relevant assumptions provided

as appropriate.

The methodology has been based on the following three focus areas, as

follows:

• Model Selection;

• Input Data; and

• Scenarios.

4A1.3 INTERPRETATION OF THE REQUIREMENTS: KEY ISSUES AND CONSTRAINTS

The objectives of the modelling exercise are to assess:

• Effects of construction, which comprises the study of the dispersion of

sediments released during the construction/ installation of one submarine

outfall connected to the San Shek Wan STW;

• Effects of STW operation on water quality due to the effluent discharge

from STW via the proposed submarine outfall; and

• Any cumulative impacts due to other projects or activities within the

study area.

The construction and operational effects will be studied by means of computer

modelling.

4A1.4 MODEL SELECTION

The existing Western Harbour Model (referred to as “original model”) (1) of the

Delft3D hydrodynamic (Delft3D-FLOW) and water quality (Delft3D-WAQ)

suite of models will be used to simulate potential impacts on water quality

during construction and operation of the Project.

The model will have the required spatial extent for this study, i.e. waters

extending up to the south boundary of Southern Water Control Zone

(SWCZ) – about 9.5 km away from the Project site (Figure 4A.2).

(1) ERM - Hong Kong, Ltd (2006) EIA Study for Liquefied Natural Gas (LNG) Receiving Terminal and Associated

Facilities. For CAPCO.



ANNEX 4A - 4

In order to assess the immediate dilution of the effluent discharge into Pui O

Wan at point of discharge, near field simulation of dilution in the mixing zone

will be modelled. The near-field simulation will be conducted using

CORMIX model.

Figure 4A.2 Geographical Extent of the Model and Area of Interest

4A1.5 GRID REFINEMENT

The grid size of the original model near South Lantau is in the order of about

150 m. Local grid refinement of the water quality and hydrodynamic model

grids would be carried out to provide improved resolution (less than 75 m) in

the key area of interest. The refinement of the model grids has been

implemented by means of nesting to the Western Harbour Model in this Study

(Figure 4A.3). The refined model domain extends from the southern coast of

the Lantau Island to the boundary of the Southern Water Control Zone

(WCZ). The grid size ranges from about 40 m x 40 m in the immediate

vicinity of the South Lantau coastline to about 140 m x 140 m near the

Southern WCZ open boundary. The refined model grid is hereinafter called

the “refined model”. The hydrodynamic model grid will subsequently be

adopted without further horizontal aggregation in the water quality models.

Since parts of the new model used in this study will be refined to compute

flow behaviour with a higher resolution in the vicinity of the South Lantau

coastline, we will show that the refined model is consistent with the original

model, computed water levels, depth-averaged current speed and directions

will be compared at two selected locations in the refined model.



ANNEX 4A - 5

Comparisons will be made between the two models for the wet and dry

seasons for model calibration and validation.

Figure 4A.3 Model Grid of the Refined Model and the Original Model

4A1.6 COASTLINE & BATHYMETRY

Bathymetry and coastline information will be updated to represent any

constructed structures in the water as indicated in Figure 4A.4. It may be

noted with reference to the construction method and scale of works (refer

Section 4A3), the proposed submarine outfall will not be represented as a

structure above the seabed in the model.

Original Model Grid

Refined Model Grid



ANNEX 4A - 6

Figure 4A.4 Bathymetry and Coastline to be used in the Refined Model (1)

Source: (1) Hydrographic Office, Hong Kong Electronic Navigational Chart (ENC), 2011

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ANNEX 4A - 7

4A1.7 BOUNDARY CONDITIONS

The refined model was linked to the original model. Water levels and three-

dimensional velocities generated by the WHM were defined at the open

boundaries of the detailed model. For the interested parameters, depth-

averaged values were derived and the averages of the whole 15-days spring-

neap cycle were calculated. These averages were then used as the boundary

conditions for the new model.

4A1.8 AMBIENT ENVIRONMENTAL CONDITIONS - WATER TEMPERATURE, SOLAR

RADIATION AND WIND

The ambient environmental conditions are closely linked to the processes of

water quality changes. Meteorological forcing including wind speed, solar

surface radiation and water temperature for the dry and wet seasons need to

be defined in the hydrodynamic and water quality computations of the

detailed model. The data for meteorological forcing were based on the past

records from Hong Kong Observatory.

The wind conditions applied in the water quality simulation were 5 m/s NE

for dry season and 5m/s SW for the wet season, which was identical to the

hydrodynamic model. The same average wind speed and direction were

adopted in the original model. On the other hand, monthly averaged values

of solar surface radiation and water temperature were used in the model.

The monthly averaged solar radiation was calculated based on the hourly data

recorded at this station. The average values of solar radiation adopted were

132 W/m2 in the dry season and 237 W/m2 in the wet season.

The ambient water temperature was determined based on the EPD routine

monitoring data collected within the Southern WCZ. The average water

temperature values used in the water quality model were 16 °C in the dry

season and 29 °C in the wet season.

It is assumed that wind speed, solar radiation and water temperature are

constant over the entire domain of the model.

4A1.9 VECTOR INFORMATION

The current patterns under the baseline situation will be generated as an

output of the baseline hydrodynamic modelling. They will be presented as

vectors showing the current direction and velocities.



ANNEX 4A - 8

4A1.10 UNCERTAINTIES IN ASSESSMENT METHODOLOGIES

4A1.10.1 Uncertainties in Sediment Transport Assessment

Uncertainties in the assessment of the impacts from suspended sediment

plumes will be considered when drawing conclusions from the assessment.

In carrying out the assessment, the worst case assumptions have been made in

order to provide a conservative assessment of environmental impacts. These

assumptions are as follows:

• The assessment is based on the peak dredging rates. In reality, these will

only occur for short period of time;

• The calculations of loss rates of sediment to suspension are based on

conservative estimates for the types of plant and methods of working; and

• The assumptions of the dredger forward speed are made only for the

purposes of modelling. The actual dredging rates may not be the same

and will be subject to the weather constraints, site conditions and

continued operational progress. In reality, the dredger moving speed

would equate to the result of dividing the total volume of dredged

materials (m3) by the duration of the dredging works (day).

The conservative assumptions presented above allow a prudent approach to

be applied to the water quality assessment.

4A1.10.2 Uncertainties arising from Operations

The following uncertainties in the operations have not been included in the

modelling assessment:

• Ad hoc navigation of marine traffic;

• Propeller scour of seabed sediments from vessels;

• Near shore scouring of bottom sediment; and

• Access of marine barges back and forth the site.

4A1.10.3 Limitation in Water Quality Models

CORMIX has two key limitations. It assumes steady-state conditions and

unidirectional, uniform flow in the receiving waterbody. Secondly, CORMIX

has simplified geometric capabilities. It assumes an idealized waterbody

with straight sides and a uniform bottom.



ANNEX 4A - 9

4A2 WATER SENSITIVE RECEIVERS

The water quality sensitive receivers (WSRs) have been identified in

accordance with Annex 14 of the Technical Memorandum on EIA Process (EIAO,

Cap.499, S.16). These WSRs are illustrated in Figure 4A.5 and listed in

Table 4A.1. The representative WSRs are included as discrete model output

points in Figures 4A.6.

Table 4A.1 Water Quality Sensitive Receivers (WSRs) in the Vicinity of the Project Site

Description Location Approximate

Shortest Distance

from Proposed

Outfall Diffuser

(km)a

Model

Output Location

Fisheries Sensitive Receivers

Recognised Spawning/

Nursery Grounds

Fisheries Spawning/ Nursery

Grounds in South Lantau

Within outfall

footprint

SR14

Fish Culture Zones Cheung Sha Wan 4 km

SR28

Ecological Sensitive Receivers

Mangroves Shui Hau Wan 4.38 km

SR 12

Pui O 1.09 km

SR1-2

Chi Ma Wan 2.60 km

SR29

Intertidal Mudflat Shui Hau Wan 4.38 km

SR12

Pui O 1.16 km

SR1

Horseshoe Crab Habitat/

Nursery Site

Shui Hau Wan

Shek Kwu Chau

4.38 km

4.20 km

SR12

SR20

Marine Mammal Habitats Finless Porpoise --

SR21 -27

Chinese White Dolphin --

SR21 -27

Marine Park Potential Southwest Lantau

Marine Park

6.65 km SR21-SR23

Special Site of Scientific Interest

(SSSI)

Proposed Shui Hau Wan Site 4.38 km SR12

Ecologically Important Streams Pui O Within project

boundary

SR1

Tong Fuk Within project

boundary

SR9



ANNEX 4A - 10

Description Location Approximate

Shortest Distance

from Proposed

Outfall Diffuser

(km)a

Model

Output Location

Coral Communities b Pui O Wan

Siu A Chau,

Shek Kwu Chau

Cheung Chau

Hei Ling Chau

Chi Ma Wan

1.00 km

0.74 km

0.43 km

4.00 km

5.4 km

6.5 km

6.30 km

2.85 km

SR3

SR4

SR5

SR21

SR30

SR32 & SR34

SR35

SR31

Water Quality Sensitive Receivers

Gazetted Bathing Beaches Tong Fuk 3.22 km SR8

Upper Cheung Sha 2.63 km SR7

Lower Cheung Sha 0.97 km SR6

Pui O 1.02 km SR2

Secondary Contact Recreational

Zones

Southern Lantau and Chi Ma

Wan Coastlines

Within outfall

footprint

SR10, SR13, SR15-SR17

Coastal Protection Areas Tong Fuk Miu Wan 4.95 km SR 11

Shui Hau Wan 4.38 km SR 12

Tong Fuk 2.75 km SR8

Upper Cheung Sha 1.46 km SR7

Lower Cheung Sha 0.97 km SR6

Pui O 0.99 km SR1-2

Seawater Intakes Pumping Station at

Tai Kwai Wan

6.45 km SR19

Note:

a. Measured “as the crow flies”, i.e. directly without taking into account land mass or other structures. The

presence of such masses would naturally affect any direct / indirect impact to these receivers; however, for

conservatism they have been removed

b. Source: AECOM (2010), Engineering Investigation and Environmental Studies For Integrated Waste Management

Facilities Phase I – Feasibility Study, for EPD,

http://www.epd.gov.hk/eia/register/report/eiareport/eia_1932011/EIA/EIA_PDF/Figures/FIGURE%205

b.1.pdf

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Prepared

Date

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OUTLYING ISLANDS SEWERAGE STAGE 2SOUTH LANTAU SEWERAGE WORKS -

INVESTIGATION

THE GOVERNMENT OF THEHONG KONG

SPECIAL ADMINISTRATIVE REGIONDRAINAGE SERVICES DEPARTMENT

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Water Quality Sensitive Receivers in the Vicinity of the Project Site

Figure 4A.5

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# EPD Routine Marine Water Monitoring Station

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k Proposed SSSI

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Proposed Sewer Alignment

Submarine Outfall

Gazetted Bathing Beaches

SSSI

Freshwater and Brackish Wetland

Ecologically Important Streams

Location of Major Stream

Mangroves

Project Boundary

Coastal Protection Area

Intertidal Mudflat

Fish Culture Zone

Recognised Spawning/ Nursery Grounds

Potential Marine Park

Secondary Contact Recreational Zones

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Kilometres

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Figure 4A.6



ANNEX 4A - 11

4A3 CONSTRUCTION PHASE

For the construction phase the WAQ model will be used to directly simulate

the following parameters:

• suspended sediments (SS); and

• sediment deposition.

It is assumed that the worst-case construction phase impacts will be at the

commencement of dredging, when there is no depression formed to trap

sediments disturbed during dredging works.

4A3.1 CALCULATION METHOD

The depletion of DO and the elevation in nutrient levels associated with the

release of SS will be calculated using the modelled maximum SS

concentrations. This method has been adopted in recently approved EIAs

(1) (2).

Dissolved Oxygen Depletion

The degree of DO depletion exerted by a sediment plume is a function of the

sediment oxygen demand of the sediment, its concentration in the water

column and the rate of oxygen replenishment. The impact of the sediment

oxygen demand on DO concentrations will be calculated based on the

following equation (3):

DO (gO2/m3) = SS (gDW/m3) × fraction of organic matter in sediment

(gC/gDW) × 2.67 (gO2/gC)

The assumption behind this equation is that all the released organic matter is

eventually re-mineralized within the water column. This leads to an

estimated depletion with respect to the background DO concentrations. This

DO depletion depends on the quality of the released sediments, i.e. on the

percentage of organic matter in the sediment. The fraction of organic matter

in sediment will be taken as 0.008 gC/gDW based on averaged data from EPD

Sediment Monitoring Stations SS6 (2004-2013) located near the Project Site

(Figure 4A.5).

This is a conservative prediction of DO depletion since oxygen depletion is not

instantaneous and will depend on tidally averaged suspended sediment

(1) ERM - Hong Kong, Ltd (2006) EIA Study for Liquefied Natural Gas (LNG) Receiving Terminal and Associated Facilities. For CAPCO. Register No.: AEIAR-106/2007, http://www.epd.gov.hk/eia/register/report/eiareport/eia_1252006/html/index.htm

(2) ERM - Hong Kong, Ltd (2010) EIA Study for Black Point Gas Supply Project. For CAPCO. Register No. AEIAR-150/2010, http://www.epd.gov.hk/eia/register/report/eiareport/eia_1782009/index.html

(3) ERM - HK Ltd (2010). Development of an Offshore Wind Farm in Hong Kong. Final Environmental Impact

Assessment. For the Hong Kong Electric Company



ANNEX 4A - 12

concentrations. It is worth noting that the above equation does not account

for re-aeration which would tend to reduce impacts of the SS on DO

concentrations in the water column. The proposed analysis, which is on the

conservative side, will not, therefore, underestimate the DO depletion.

Further, it should be noted that, for sediment in suspension to exert any

oxygen demand in the water column will take time and, in the meantime, the

sediment will be transported and mixed or dispersed with oxygenated water.

As a result, the oxygen demand and the impact on DO concentrations will

diminish as the suspended sediment concentrations decrease.

Nutrients

An assessment of nutrient release during dredging will be carried out based

on the predicted SS elevation and the testing results of EPD sediment

monitoring station. In the calculation it is assumed that all TIN and

unionised ammonia (UIA) concentrations in the sediments are released to the

water. This is a highly conservative assumption and will result in the

overestimation of the potential impacts.

Ammonia nitrogen is the sum of ionised ammonia (NH4-N) and unionised

ammonia (UIA). Under normal conditions of Hong Kong waters, more than

90% of the ammonia nitrogen would be in the ionised form. For the purpose

of assessment, a correction (as a function of temperature, pH, and salinity) has

been applied based on the EPD monitoring data, i.e. temperature of 24°C,

salinity of 28 ppt and pH of 8 which represent the typical conditions of Hong

Kong waters. From this it is derived that UIA constitutes 5% of ammonia

nitrogen. In view that the mineralisation of the organic nitrogen will also

contribute to ammonia, the calculation of NH3-N is based on average TKN

(Total Kjeldahl Nitrogen, which is a measurement of both ammonia nitrogen

and organic nitrogen) concentrations (mg/kg) in the sediment at EPD station

SS6 (2004-2013). Note that it is a highly conservative approach since it is

assumed that 100% of organic nitrogen will be mineralised to ammonia but

this is unlikely to occur in reality.

The maximum SS concentration at each WSR is multiplied by 5% of TKN concentration in the sediment to predict the maximum UIA elevations.

UIA = Max SS × TKN × 5%

Where (1) it is assumed that 100% of organic nitrogen in TKN will be mineralised to ammonia;

(2) TKN concentration in the sediment is 321×10-6 kg/kg.

The maximum predicted SS concentration at each WSR is multiplied by the average concentration of TIN in sediment (mg/kg) in the SWCZ based on EPD Sediment Monitoring Stations to give the maximum TIN increase (mg/L) in water column. TIN is composed of ammonia nitrogen, nitrate nitrogen and nitrite nitrogen. In accordance with sediment test results of TKN (100% of organic nitrogen in TKN is assumed to be mineralised to ammonia as conservative method) from the nearest EPD station SS6 (2004-2013) and the



ANNEX 4A - 13

sediment survey results (nitrate and nitrite nitrogen) from the other approved

EIA (1), the calculations of TIN elevation are shown below.

TIN = Max SS × (TKN + nitrate nitrogen + nitrite nitrogen)

Where (1) it is assumed 100% of organic nitrogen in TKN will be mineralised to ammonia and TKN concentration in the sediment is 321×10-6 kg/kg;

(2) total nitrate and nitrite nitrogen in the sediment (1) is 1×10-6 kg/kg.

4A3.2 WORKING TIME

It is understood that the entire effluent outfall pipe from the proposed San

Shek Wan STW will be installed via an underground cavity by directional

drilling. For the outfall portion extending from the coastline, it will be

installed by directional drilling under the seabed (at a depth of approximately

-65 mPD). As such, installation of this portion of the outfall pipe is not

anticipated to affect water quality. Dredging is limited to an area of the

seabed at the discharge point of the outfall (about 39m x 33m at the end of the

outfall pipe) for the installation of the above-seabed portion (i.e. diffusers) of

the outfall.

The works programme for dredging activities for the proposed submarine

outfall in Pui O Wan is based on the assumption of a 12 working hour day

with 7 working days per week, using grab dredgers.

4A3.3 OVERVIEW OF DREDGING PLANT - GRAB DREDGERS

Grab dredgers may release sediment into suspension by the following

mechanisms:

• Impact of the grab on the seabed as it is lowered;

• Washing of sediment off the outside of the grab as it is raised through the

water column and when it is lowered again after being emptied;

• Leakage of water from the grab as it is hauled above the water surface;

• Spillage of sediment from over-full grabs;

• Loss from grabs which cannot be fully closed due to the presence of

debris;

• Release by splashing when loading barges by careless, inaccurate

methods; and

(1) ERM (2009). Development of a 100MW Offshore Wind Farm in Hong Kong. The Hong Kong Electric Co. Ltd.



ANNEX 4A - 14

• Disturbance of the seabed as the closed grab is removed.

In the transport of dredging materials, sediment may be lost through leakage

from barges. However, dumping permits in Hong Kong include

requirements that barges used for the transport of dredging materials have

bottom-doors that are properly maintained and have tight-fitting seals in

order to prevent leakage. Given this requirement, sediment release during

transport is not proposed for modelling and its impact on water quality will

not be addressed under this Study.

Sediment is also lost to the water column when discharging material at

disposal sites. The amount that is lost depends on a large number of factors

including material characteristics, the speed and manner in which it is

discharged from the vessel, and the characteristics of the disposal sites. It is

considered that potential water quality issues associated with disposal at the

intended government disposal site(s) have already been assessed by CEDD

and permitted by EPD, hence and the environmental acceptability of such

disposal operations is demonstrated. Therefore modelling of impacts at

disposal sites does not need to be addressed and reference to relevant studies

will be provided in the EIA for this Project where appropriate.

Loss rates have been taken from previously accepted EIAs in Hong Kong (1)(2)(3)

and have been based on a review of worldwide data on loss rates from

dredging operations undertaken as part of assessing the impacts of dredging

areas of Kellett Bank for mooring buoys(4). The assessment concluded that

for 8 m3 (minimum) grab dredgers working in areas with significant amounts

of debris on the seabed (such as in the vicinity of existing mooring buoys) that

the loss rates would be 25 kg m-3 dredged, while the grab dredger bucket size

in areas where debris is less likely to hinder operations could be 12 or 16 m3,

with a loss rate of 17 kg m-3. It is assumed there is little debris based on the

fact the area is away from marine works and heavy marine traffic / industry.

The value of 17 kg m-3, for dredgers with grab size of 12 or 16 m3, will

therefore be used for this Study.

Generally, a split-bottom barge could have a capacity of 900 m³. A bulk

factor of 1.3 would normally be applied, giving a dredging rate of about 700

m³ per barge. The hopper dry density for an 800 to 1,000 m3 capacity barge is

around 0.75 to 1.24 ton m-3. Assuming 12 working hours per day for the

proposed construction activities, with allowance on the demobilisation of

(1) ERM - Hong Kong, Ltd (2006) EIA Study for Liquefied Natural Gas (LNG) Receiving Terminal and Associated Facilities.

For CAPCO. Register No.: AEIAR-106/2007,

http://www.epd.gov.hk/eia/register/report/eiareport/eia_1252006/html/index.htm

(2) ERM (2005). Detailed Site Selection Study for a Contaminated Mud Disposal Facility within the Airport East/East of Sha

Chau Area. EIA and Final Site Selection Report. For CEDD. Approved on 1 September 2005. Register No.: AEIAR-

089/2005, http://www.epd.gov.hk/eia/register/report/eiareport/eia_1062005/index.htm

(3) ERM (2000). Construction of an International Theme Park in Penny’s Bay of North Lantau together with its Essential

Associated Infrastructures – Final EIA Report. For CEDD. Approved on 28 April 2000. Register No.: AEIAR-032/2000

http://www.epd.gov.hk/eia/register/report/eiareport/eia_0412000/index.html

(4) ERM (1997). EIA: Dredging an Area of Kellett Bank for Reprovisioning of Six Government Mooring Bays. Working Paper on

Design Scenarios. For CEDD.



ANNEX 4A - 15

filled barge and remobilisation of empty barges, approximately 5 barges could

be filled per day, giving a daily dredging rate of approximately 3,500 m3.

4A3.4 CONSTRUCTION SCENARIO – GRAB DREDGING FOR SUBMARINE OUTFALL

The dredging operations for the outfall will be carried out by one closed grab

dredger. The estimated dredged volume is approximately 4,700 m3 in total.

Working hours are assumed to be 12 hours per day with a maximum

dredging rate of 3,500 m3 day-1 (i.e. 0.081 m3 s-1) per dredger, giving a rate of

release (in kg s-1) of sediment for one dredger as follows:

Loss Rate (kg s-1)

= Dredging Rate (m3 s-1) * Loss Rate (kg m-3)

= 0.081 m3 s-1 * 17 kg m-3

= 1.3773 kg s-1

Therefore a continuous release rate of 1.3773 kg s-1 for one dredger will be

adopted in the model for release throughout the whole water column. Given

the small extent of marine dredging area, one stationary source at the outfall

discharge point is assumed in the model to represent the grab dredger.

Table 4A.2 summarises the inputs defined in the sediment dispersion

simulation for construction phase modelling scenario.



ANNEX 4A - 16

Table 4A.2 Summary of Model Inputs for Construction Phase Modelling

Emission Point Dredging for Submarine Outfall

No. of Working Plant 1 Grab Dredger with a grab size of 12 or

16m3

Dredging Rate m3/day/plant 3,500

Operation Duration hours 12

Loss Type Continuous

Loss Rate kg m-3 17

Loss Rate kg s-1 1.3773

Input Layer Whole Column

4A3.5 SEDIMENT INPUT PARAMETERS

For simulating sediment impacts the following general parameters will be

assumed:

• Settling velocity – 0.5 mm s-1

• Critical shear stress for deposition – 0.2 N m-2

• Critical shear stress for erosion – 0.3 N m-2

• Minimum depth where deposition allowed – 0.1 m

• Resuspension rate – 30 g m-2 d-1

The above parameters have been used to simulate the impacts from sediment

plumes in Hong Kong associated with uncontaminated mud disposal into the

Brothers MBA (1) and dredging for the Permanent Aviation Fuel Facility at Sha

Chau (2). The critical shear stress values for erosion and deposition were

determined by laboratory testing of a large sample of marine mud from Hong

Kong as part of the original Water Quality and Hydraulic Mathematical

Model (WAHMO) studies associated with the new airport at Chek Lap Kok.

(1) Mouchel (2002a). Environmental Assessment Study for Backfilling of Marine Borrow Pits at North of the Brothers.

Environmental Assessment Report.

(2) Mouchel (2002b). Permanent Aviation Fuel Facility. EIA Report. Environmental Permit EP-139/20



ANNEX 4A - 17

4A4 OPERATION PHASE

4A4.1 OPERATION SCENARIO

For the study of operational effects, the approach requires several steps:

1) Running near-field models (i.e. CORMIX) for the operational discharges

to characterise the initial mixing of the effluent discharge. The results

from the CORMIX analysis will also provide information of the near field

dispersion and dilution of the effluent plumes.

2) Adapting the hydrodynamic model for the new conditions, i.e. to include

the discharges from San Shek Wan STW.

3) Running the hydrodynamic model for the specified conditions (wet/dry

season) covering a spring-neap cycle.

4) Running the water quality model. A “full water quality model”

approach is proposed in which nutrients will be simulated explicitly.

The objective of the water quality modelling is to quantitatively assess the

concentrations of key water quality variables (e.g. SS, E. coli, NH3-N, etc.)

as a result of effluent discharge from the submarine outfall in the far field.

The general water quality is the result of transport phenomena and

transformation and retention processes. The operation of the project may

locally affect the transport patterns. Transformation and retention processes

are not affected. Consequently, validation of the Delft3D-WAQ model is not

required.

4A4.1.1 Effluent Discharge

The latest project information suggests that the treatment capacity, i.e.

Average Dry Weather Flow (ADWF) of the proposed San Shek Wan STW is

about 5,800 m3/day. The Peak Wet Weather Flow (PWWF) is 14,500 m3/day.

The ADWF and PWWF will be simulated in the operation phase modelling

scenarios using CORMIX and Delft3D-WAQ models.

The proposed effluent standards upon commissioning of the STW are

summarised in Table 4A.3. The effluent standards proposed for the STW are

30 mg/L, 30 mg/L, 20 mg/L and 1000 cfu/100mL for TN, TSS, BOD, and

E. coli respectively.

Table 4A.3 Proposed Effluent Discharge Standards of STW upon Commissioning and

Effluent Discharge Standards Adopted in Water Quality Modelling

Parameters (1) Proposed Effluent Discharge Standards for STW (2)



ANNEX 4A - 18

Parameters (1) Proposed Effluent Discharge Standards for STW (2)

Flow rate (m3/day) 5,800 (ADWF); 14,500 (PWWF)

Total Nitrogen (TN) 30

BOD 20

TSS 30

E. coli (cfu/100ml) 1,000

Note:

(1) All units in mg/L unless otherwise stated.

(2) Proposed water discharge standards are upper limits of effluent concentration for the STW design.

Tentative outfall design and modelling parameters for CORMIX as adopted

from Review of Sewerage Scheme for South Lantau (Review Study) in 2008

are tabulated in Table 4A.4 and Table 4A.5 at this stage of assessment. It is

worth noting that the diffuser is located approximately 500 m from the nearest

coastline and more than 100 m from the boundaries of the most adjacent

gazetted beaches (Figure 4A.7).

Table 4A.4 Tentative Outfall Design

Parameter Information

No. of discharge ports

in the diffuser

4

Diameter of discharge

port

0.3m

Configuration of

discharge port

All discharge ports are located at two ends of the diffuser, 2 ports at

each end. The two ends are connected with a pile to form a diffuser

line. All the discharge ports are pointing vertically upward

Location of diffuser

from the nearest

coastline

About 500m from the nearest coastline; > 100 m from the boundaries

of the most adjacent gazetted beaches

Discharge Depth 5m below water surface (Discharge point is 1m above seabed; seabed

is -4.8 mPD)

")")

")

!(

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OUTLYING ISLANDS SEWERAGE STAGE 2SOUTH LANTAU SEWERAGE WORKS -

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Measured Distance from the Outfall Diffuser to Pui O & Lower Cheung Sha Beach Boundaries

Figure 4A.7

Prepared

Date Scale

0 0.5 10.25Kilometers´

Key

Project Boundary

Cheung Sha Lower Beach

Pui O Beach

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!( sewage treatment works

Proposed Sewer

Proposed Submarine Outfall

Proposed Rising Main

Proposed Emergency Bypass

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ANNEX 4A - 19

Table 4A.5 CORMIX Modelling Scenarios and Corresponding Parameters

Parameter

Scenarios

D10 / D50 / D90 W10 / W50 / W90

Dry season Wet season

Discharge

Parameters

Discharge Type Submerged vertical discharge at sea bottom with 4 x 0.3 m

diameter pipe

Total discharge

flowrate

ADWF = 5,800 m3/day = 0.067 m3/s;

PWWF = 14,500m3/day = 0.168 m3/s

Upper limit

Concentration of

Effluent (at ADWF

and PWWF under

normal operation)

TSS = 30 mg/L

BOD = 20 mg/L

TN = 30 mg/L

E. coli = 1,000 cfu/100ml

Ambient

Conditions

Ambient Velocity 0.01m/s| 0.03m/s| 0.05m/s 0.01m/s| 0.05m/s| 0.09m/s

Water Depth at

discharge outfall 5 m

Average Surface

Water Density 1023.6 kg/m3 1013 kg/m3

Average Bottom

Water Density 1023.6 kg/m3 1016.7 kg/ m3

Ambient Wind

Speed

2 m/s

(CORMIX’s recommended value for conservative design

condition)



ANNEX 4A - 20

4A5 EMERGENCY DISCHARGE

The latest scheme indicates there are six trunk sewage pumping stations

(SPSs) in Shui Hau, Tong Fuk, Cheung Fu Street, Cheung Sha, San Shek Wan

and Pui O; and one STW in San Shek Wan. The work flow for the operation

of South Lantau sewerage system is shown in Figure 4A.8.



ANNEX 4A - 21

Figure 4A.8 Flow Chart of the Operation for South Lantau Sewerage System

Note:

(1) According to DSD/ST2, there will be a maintenance team station in San Shek Wan STW for regular maintenance and inspection for all SPSs and STW during

office hour.

(2) According to DSD/ST2, there will be existing mobile team from Ma Wan and emergency team from Siu Ho Wan to back-up any emergency scenasrio in South

Lantau Sewerage Works during office and non-office hours.

(3) In general, normally peak flow would only last for a short period (about 2 hours). Therefore, ADWF is more appropiate to estimate the volume required for

emergency storage.

(4) As requested by DSD/ST2, a safety outlet would be provided in all SPSs and STW to prevent sewage flooding at the SPSs, STW, equipment damage and

flooding/backflow to upstream sewerage network (and perhaps flooding on road, underground facilities or at houses).



ANNEX 4A - 22

4A5.1 EMERGENCY SITUATIONS

There are two types of emergency situations for sewerage system: 1)

equipment failure and 2) power failure. The SPSs and STW are all designed

to have one standby unit (e.g. standby pumps and standby treatment units) in

case of equipment failure. Dual power supplies will be in place in case of

power breakdown. In case of failure of dual power supply, emergency

storage for SPSs and STW would be in place to prevent emergency discharge.

A 6-hour (ADWF) emergency storage will be provided to the STW and each

SPS respectively. In addition, the equalization (EQ) tank in STW can provide

additional 6-hour ADWF emergency storage.

For Pumping Stations (SPSs)

When equipment in use fails the standby unit will cut in. If dual power

supplies are down at the same time, the 6-hour emergency storage would be

provided. Raw sewage discharge would overflow from SPSs when

emergency storage is used up.

For Sewage Treatment Works (STW)

The standby unit will cut in if equipment in use fails. In the case of dual

power supply failure, 12-hour emergency storage (6-hour emergency storage

tank and 6-hour equalization tank) will store the sewage temporarily until the

storage capacity is exceeded. If emergency storage is used up, sewage will

overflow through the safety pipelines and outlets and will drain to nearby

coastal waters, i.e. Pui O Bay. As no treated effluent will be generated when

power is down, overflow passage or extra temporary storage for treated

effluent is not necessary.

4A5.2 MITIGATION MEASURES

Standby units, emergency storage units and dual power supply will be

provided for the sewage pumping stations and STW to minimize the potential

of unplanned events and emergency discharge of untreated sewage. The

emergency designs for sewage pumping stations and STW are summarized in

Table 4A.6.

Table 4A.6 Emergency designs for sewage pumping stations and STW

Item Pumping Station Time to

Restore the

Facilities

Mitigation Measures



ANNEX 4A - 23

Item Pumping Station Time to

Restore the

Facilities

Mitigation Measures

1 Cheung Sha

SPS

6 hour 1 stand-by

pump

Dual

power

supply

6-hour (ADWF)

emergency storage

2 San Shek Wan

SPS

6 hour 1 stand-by

pump

Dual

power

supply

6-hour (ADWF)

emergency storage

3 Cheung Fu

Street SPS

6 hour 1 stand-by

pump

Dual

power

supply

6-hour (ADWF)

emergency storage

4 Tong Fu SPS 6 hour 1 stand-by

pump

Dual

power

supply

6-hour (ADWF)

emergency storage

5 Shui Hau SPS 6 hour 1 stand-by

pump

Dual

power

supply

6-hour (ADWF)

emergency storage

6 Pui O SPS 6 hour 1 stand-by

pump

Dual

power

supply

6-hour (ADWF)

emergency storage

7 San Shek Wan

STW

12 hour 1 stand-by

treatment

units and 1

stand-by

pump

Dual

power

supply

12-hour emergency

storage

(6-hour emergency

storage tank and 6-

hour equalization tank)

Regular inspection and maintenance will be carried out by DSD’s maintenance

crew which will station in the San Shek Wan STW. During non-office hours,

existing mobile team from Ma Wan STW will be responsible to back up the

maintenance crew in San Shek Wan STW. An emergency team will assist the

existing mobile team from Ma Wan STW to restore the SPSs or STW in order

to reduce the maintenance time during non-office hour. The maintenance

sequence is illustrated in Figure 4A.9.



ANNEX 4A - 24

Figure 4A.9 Maintenance Sequence under Emergency Situation

Note:

(1) Regular Team/Mobile Team provides 24 hours maintenance service.

(2) The actual gathering time for Regular Team/Mobile Team should be less than the proposed gathering time (ie 1 hour). For more conservative estimation, 1 hour

gathering time is assumed for regular team or mobile team to prepare.

(3) Under the worst case scenario, the function of wet well and chamber inside SPSs can also serve as an emergency tank. The wet well will be designed to store

addition 2 hours ADWF. As a result, the total time for emergency storage is not less than 6 hours.



ANNEX 4A - 25

Given the designed emergency storage together with the standby unit and

dual power supplies, the likelihood of raw sewage discharge from SPSs and

STW is extremely low. Nevertheless, a Emergency Response Plan will be

developed to deal with the occurrence of emergency discharge from the STW

and SPSs. The Emergency Response Plan should include the following:

• Locations of the sensitive receivers in vicinity of the emergency discharge;

• A list of relevant governmental bodies to inform of and to ask for

assistance in the event of emergency discharge, including key contact

persons and telephone numbers;

• Reporting procedures are required in the event of emergency discharge;

and

• Responsibilities and procedures for clean-up of the affected water

body/sensitive receivers after the emergency discharge.

4A5.3 MODEL EMERGENCY SCENARIOS

The model will account for emergency situations where there is raw sewage

overflow and affect the potential sensitive receivers in the coastal areas.

There are two emergency discharge scenarios to be examined.

Emergency Scenario 1 - Emergency Discharge from All 6 SPSs

If all the mitigation measures such as standby equipment and dual power

supply fail together at the same time, raw sewage is designed to route to 6-

hour emergency storage facility to avoid direct discharge to the surrounding

environment.

Although the likelihood of raw sewage discharge from SPSs and STW during

emergency is rather low, EPD suggested including raw sewage overflow from

all 6 SPSs in the model runs as Emergency Scenario 1 to investigate the extent

of potential impacts, in particular the gazetted beaches in the study area. It is

assumed that raw sewage will overflow from the SPSs for 2 hours and

discharge into streams through safety pipelines and outlets as shown in

Appendix 4A.1. The streams would dilute and direct the discharged raw

sewage into coastal water. Average Dry Weather Flow at each SPS and

estimated loads shown in Table 4A.8 are adopted for modelling.

Emergency Scenario 2 - Emergency Discharge from STW and Pui O SPS

In the event of emergency situations during operation phase of the Project, the

12-hour and 6-hour emergency storage are designed for the STW and Pui O

Trunk SPS respectively. The emergency storage in Pui O SPS will store the

flow from eastern side of the STW (i.e. Pui O villages) and the emergency



ANNEX 4A - 26

storage in the STW will serve for the flow from western side of the Project

Catchment Area (i.e. San Shek Wan SPS) in case of emergency.

Consequently, when there is power failure at the STW while other pumping

stations are working well, the sewage from western side of the Project

Catchment Area would be expected to reach the STW and all of it would be

released untreated from the STW into nearby Pui O Bay (when emergency

storage capacity is exceeded) through the safety pipeline and outlet. The

sewage from the eastern side of the STW will overflow into Pui O stream from

Pui O SPS (when emergency storage capacity is exceeded) through safety

pipeline and outlet. The discharge locations from STW and Pui O SPS are

shown in Appendix 4A.1.

As per request of EPD, the flow of untreated effluent was considered in the

three-dimensional hydrodynamic and water quality model at the nearest

coastline as continuous pollutant loads as Emergency Scenario 2 although the

likelihood is rather low. Average Dry Weather Flow at the STW and

estimated loads as shown in Table 4A.8 are adopted for modelling. 2 hours of

raw sewage overflow were considered in the model.

If all the 6 SPSs are down altogether, no sewage is expected to be pumped to

STW. The emergency storage in the STW and Pui O SPS would be sufficient

for holding the untreated sewage already at the STW and overflow bypass

will not occur. Therefore the scenario of the emergency discharge from the

STW and all the 6 pumping stations altogether is not considered for

modelling.

In general, water quality modelling will be carried out for two emergency

scenarios shown in Table 4A.7 to simulate the impact due to the emergency

discharge of sewage from SPSs and STW.

Table 4A.7 Modelling Scenarios for Emergency Discharge of Raw Sewage

Scenario Discharge Point Discharge

Period

(hours)

Assumed Concentration

in Untreated Sewage

07 (wet) Emergency discharge at all 6 pumping

stations (release time: 09:00 – 11:00 of 7

Jul in model)

2 See Table 4A.8

08 (Dry) Emergency discharge at all 6 pumping

stations (release time: 17:00 – 19:00 of 22

Feb in model)

2 See Table 4A.8

09 (Wet) Emergency discharge at STW and Pui O

SPS (release time: 09:00 – 11:00 of 7 Jul

in model)

2 See Table 4A.8

10 (Dry) Emergency discharge at STW and Pui O

SPS (release time: 17:00 – 19:00 of 22 Feb

in model)

2 See Table 4A.8

Under the low ambient flow condition, the discharged pollutants would

accumulate within a more localized area and result in more localized yet



ANNEX 4A - 27

intense impacts on nearby waters than discharge during high ambient flow

condition. During the selected release time (Table 4A.7), the ambient is in

low flow condition. Hence emergency discharges during the low ambient

flow conditions (i.e. neap tide) would be considered as the worst case

scenarios.



ANNEX 4A - 28

Table 4A.8 Derivation of Raw Sewage Loads and Flow during Emergency Discharges

Proposed Sewage

Pumping Station/

Sewage Treatment

Works

Flow

(m3/day) a SS (kg/day)

BOD

(kg/day)

NH3N

(kg/day)

E. coli

(count/day) SS (mg/L)

BOD

(mg/L)

NH3N

(mg/L)

E. coli

(count/100

ml)

Shui Hau Trunk Sewage

Pumping Station

620 76.27 80.24 9.52 8.18E+13 123.02 129.42 15.35 1.32E+07

Tong Fuk Trunk Sewage

Pumping Station

1260 162.08 173.99 21.63 2.12E+14 128.63 138.09 17.16 1.68E+07

Cheung Fu Street Trunk

Sewage Pumping Station

590 80.24 91.22 9.31 7.84E+13 136.00 154.61 15.79 1.33E+07

Cheung Sha Trunk


920 106.12 115.81 14.51 1.30E+14 115.35 125.89 15.77 1.41E+07

San Shek Wan Trunk


450 48.39 52.11 5.91 5.05E+13 107.53 115.80 13.14 1.12E+07

Pui O Trunk Sewage

Pumping Station

1960 269.94 296.39 32.17 3.25E+14 137.73 151.22 16.41 1.66E+07

San Shek Wan Sewage

Treatment Works b

3840 473.10 513.37 60.88 5.52E+14 128.11 139.61 16.04 1.51E+07

Notes:

a. Expressed as Average Dry Weather Flow (ADWF);

b. A portion of raw sewage will be released from Pui O SPS during the failure of STW. Hence, the flow rate and pollutant release from STW during

emergency will be calculated as the designed flow rate/ pollution loading of STW minus flow rate/ pollution loading at Pui O SPS.



ANNEX 4A - 29

4A6 CUMULATIVE IMPACTS

According to publicly available sources, the identified potential concurrent

projects are the marine dumping activities near South Cheung Chau and the

proposed Integrated Waste Management Facilities (IWMF) potentially at Shek

Kwu Chau (2017 - 2022). These projects are at least 5 km from the proposed

submarine outfall and the water quality impacts from the construction of

IWMF were predicted to be localized near Shek Kwu Chau with reference to

EIA Study for Development of the Integrated Waste Management Facilities Phase 1.

Given the information above, these projects are not considered to be located in

sufficient proximity to cause cumulative effects.



ANNEX 4A - 30

4A7 MODEL SCENARIOS

The water quality modelling exercise will commence with the set-up of

hydrodynamic and water quality baseline models (covering a complete

spring/neap cycle for both the dry and wet seasons). It will be conducted

with regard to two main components, construction phase and operation phase

as detailed below.

• Construction Phase: the assessment will examine potential water quality

impacts arising from dredging for the installation of one submarine

outfall, with the extent of seabed dredging of about 4,700 m3 at 700 m

offshore;

• Operation Phase: the assessment will examine potential water quality

impacts due to the effluent discharge from the STW operation via the

outfall in Pui O Wan.

• Emergency Discharge: two scenarios of emergency discharge will be

examined for potential water quality impacts:

o Overflow of raw sewage from individual sewage pumping stations

through nearby streams into the nearby coastal water; and,

o Overflow of all collected raw sewage from the San Shek Wan STW and

Pui O SPS into the nearby coastal water through emergency safety

outlet and nearby stream.

Table 4A.9 summarizes the proposed near-field and far-field modelling

scenarios below:

Table 4A.9 Proposed Far-field and Near-field Model Scenarios

Scenario ID Project Phase Project Activity Period Seasons Inputs

Far-field Delft3D Model

01 Construction Dredging for

Submarine

Outfall Diffuser

2018 Wet Season

SS load as

presented in Table

4A.2

02 Construction Dredging for

Submarine

Outfall Diffuser

2018 Dry Season SS load as

presented in Table

4A.2

03 Operation Normal

Operation

(ADWF)

2036 Wet Season

Effluent Load as

presented in Table

4A.5 via STW

outfall

04 Operation Normal

Operation

(ADWF)

2036 Dry Season Effluent Load as

presented in Table

4A.5 via STW

outfall



ANNEX 4A - 31


05 Operation Normal

Operation

(PWWF)

2036 Wet Season

Effluent Load as

presented in Table

4A.5 via STW

outfall

06 Operation Normal

Operation

(PWWF)

2036 Dry Season Effluent Load as

presented in Table

4A.5 via STW

outfall

07 Operation Emergency

Scenario 1

2036 Wet Season Raw sewage

discharges as

presented in Table

4A.8 from all 6

pumping stations

via safety pipelines

and safety outlet


Scenario 1

2036 Dry Season Raw sewage

discharges as

presented in Table

4A.8 from all 6

pumping stations

via safety pipelines

and safety outlet


Scenario 2

2036 Wet Season Raw sewage

discharge as

presented in Table

4A.8 from STW and

Pui O SPS via safety

pipelines and safety

outlet


Scenario 2

2036 Dry Season Raw sewage

discharge as

presented in Table

4A.8 from STW and

Pui O SPS via safety

pipelines and safety

outlet

Near-field CORMIX Model

AD10 Operation Normal

Operation

(ADWF)

2036 Dry Season 10th Percentile of

Ambient Current

Velocity


Operation

(ADWF)


Ambient Current

Velocity


Operation

(ADWF)


Ambient Current

Velocity



ANNEX 4A - 32


AW10 Operation Normal

Operation

(ADWF)

2036 Wet Season 10th Percentile of

Ambient Current

Velocity


Operation

(ADWF)


Ambient Current

Velocity


Operation

(ADWF)


Ambient Current

Velocity

PD10 Operation Normal

Operation

(PWWF)


Ambient Current

Velocity


Operation

(PWWF)


Ambient Current

Velocity


Operation

(PWWF)


Ambient Current

Velocity

PW10 Operation Normal

Operation

(PWWF)


Ambient Current

Velocity


Operation

(PWWF)


Ambient Current

Velocity


Operation

(PWWF)


Ambient Current

Velocity

APPENDIX 4A.1

SAFETY ARRANGEMENT

Documents

ANNEX 4A Water Quality Method Statement - epd.gov.hk · treatment and disposal of the sewage arising from ... This Method Statement presents information on ... conservative estimates