Table of Contents

 

1.    INTRODUCTION

1.1      Description of the Project

1.2      Study Objectives

1.3      Report Structure

2.    WATER QUALITY IMPACT ASSESSMENT

2.1      Introduction

2.2      Assessment Methodology

2.3      Legislation and Standards

2.4      Sensitive Receivers

2.5      Baseline Conditions

2.6      Water Quality Criteria

2.7      Sediment Sampling, Elutriation Test and Particle Size Analysis

2.8      Assessment of Potential Impacts during Dredging

2.9      Concurrent Dredging/Disposal Activities

2.10    Assessment of Potential Impacts after Dredging

2.11    Recommended Maximum Dredging Rates and Proposed Dredging Schedule

2.12    Mitigation Measures and Good Site Practice

2.13    Summary and Conclusions

3.    ECOLOGY

3.1      Introduction

3.2      Legislation and Assessment Criteria

3.3      Overview of Habitats in the Study Area

3.4      Cetaceans

3.5      Benthic Infauna

3.6      Intertidal Flora and Fauna

3.7      Subtidal Ecology

3.8      Key Sensitive Receivers

3.9      Impact Identification and Prediction

3.10    Cumulative Impacts

3.11    Mitigation Measures

3.12    Summary and Conclusions

4.    FISHERIES IMPACT ASSESSMENT

4.1      Introduction

4.2      Legislation and Standards

4.3      Baseline Conditions

4.4      Culture Fisheries

4.5      Impact Assessment

4.6      Mitigation Measures

4.7      Summary and Conclusions

5.    Construction Noise

5.1      Introduction

5.2      Legislation and Standards

5.3      Noise Sensitive Receivers

5.4      Con-Current Projects

5.5      Assessment Methodology

5.6      Assessment Results

5.7      Mitigation Measures

5.8      Monitoring and Audit Requirement

5.9      Summary and Conclusions

6.    WASTE MANAGEMENT

6.1      Introduction

6.2      Legislation and Standards

6.3      Potential Sources of Waste

6.4      Sediment Quality

6.5      Sediment Disposal

6.6      Potential Impact of Sediment Disposal

6.7      Mitigation Measures

7.    Environmental Monitoring and Audit

7.1      Introduction

7.2      Water Quality

7.3      Marine Ecology

7.4      Fisheries

7.5      Noise

7.6      Waste Management

7.7      Real-Time Reporting of Monitoring Data

8.    summary and conclusions

8.1      Maximum Dredging Rates

8.2      Water Quality

8.3      Marine Ecology

8.4      Fisheries

8.5      Noise

8.6      Waste Management

8.7      Operational Impact

8.8      Mitigation Measures

8.9      Good Site Practice

8.10    Overall Conclusions

REFERENCES

 

List of APPENDICES

Appendix 1      Elutriation Test

Appendix 2      Review of Sediment Quality

Appendix 3      Water Quality Modelling Output

Appendix 4      Implementation Schedule

Appendix 5      Noise Calculations for Grab Dredger Option between 23:00 and 07:00

Appendix 6      EM&A Manual (Construction Phase) [under a separate cover]

 

List of Tables

Table 2.1         Water Quality at EPD Routine Monitoring Stations – Dry Season 2000 (January - March & October – December)

Table 2.2         Water Quality at EPD Routine Monitoring Stations – Wet Season 2000 (April – September)

Table 2.3         Relevant Water Quality Objectives at the Relevant Water Control Zones

Table 2.4         Background SS Concentrations and Tolerance Elevation at Sensitive Receivers

Table 2.4a       Elutriation Test Results

Table 2.5         Estimated Maximum Dredging Rates and Critical Sensitive Receivers

Table 2.6         Predicted Maximum Elevation of SS Levels at WQSRs, Grab Dredger with Silt Curtains at the Estimated Maximum Dredging Rates

Table 2.7         Predicted Maximum Elevation of SS Levels at WQSRs, TSHD at the Estimated Maximum Dredging Rates

Table 2.8         Predicted DO Levels (Baseline DO Less the Maximum Oxygen Depletion) at WQSRs, Grab Dredger with Silt Curtains at the Estimated Maximum Dredging Rates

Table 2.9         Predicted DO Levels (Baseline DO Less the Maximum Oxygen Depletion) at WQSRs, TSHD Option at the Estimated Maximum Dredging Rates

Table 2.10       International Standards for Heavy Metals and Micro-Pollutants in Marine Environment

Table 2.11       Standards for Heavy Metals and Micro-Pollutants Recommended for This EIA

Table 2.12       Predicted Maximum Cumulative SS Elevation at WQSRs, Grab Dredger with Silt Curtains, Recommended Maximum Dredging Rates

Table 2.13       Predicted Maximum Cumulative SS Elevation at WQSRs, TSHD, Recommended Maximum Dredging Rates

Table 2.14       Estimated Maximum Dredging Rates for Worst-Case Scenarios

Table 2.15       Recommended Maximum Dredging Rates for Worst-Case Scenarios

Table 2.16       Recommended Maximum Dredging Rates for Working Zones

Table 2.17       Proposed Working Schedule - Grab Dredger Option

Table 2.18       Proposed Working Schedule - TSHD Option

Table 3.1         Biomass of Benthic Assemblages from the Lamma Island Area

Table 3.2         Density (m-2) of Intertidal Flora and Fauna Recorded at the West Coast Sites During the Wet Season

Table 3.3         Results of Dive Surveys Conducted along West Lamma Coast

Table 3.4         Dive Survey Conditions, 10 December 2001

Table 3.5         Area and Percentage Mortality of Soft and Hard Coral Recorded in the Three Survey Sites

Table 3.6         A Comparison of Subtidal Conditions Found during SCUBA Surveys in 1998 and 2001

Table 3.7         Summary of Ecological Impact Assessment and Evaluation

Table 5.1         Sound Power Level of Plant Likely to Be Deployed

Table 5.2         Working Schedule and the Allocation of Dredgers for Grab Dredger Option

Table 5.3         Shortest Distances between the Dredging Area and the NSRs

Table 5.4         Construction Noise Level during Normal Working Hours

Table 5.5         Construction Noise Level during Day Time on Holidays

Table 5.6         Construction Noise Level during Evenings

Table 5.7         Construction Noise Level during Night Time

Table 7.1         Action and Limit Levels for Water Quality

 

LIST OF FIGURES

Figure 1.1        Project Area

Figure 1.2        Provisional Dumping Sites and Vessel Movement Restricted Area

Figure 2.1        Environmental Sensitive Receivers

Figure 2.2        Working Zones for Lamma Channel Improvement

Figure 2.3        Illustration of Cage Type Silt Curtain

Figure 3.1        Major Habitat Types around South Lamma

Figure 3.2        OPCF Study Area Showing Survey Sub-Areas and Transect Lines in Hong Kong

Figure 3.3        Finless Porpoise Distribution in a) Winter and Spring and b) Summer and Autumn. Taken from OPCF, 2000

Figure 3.4        Green Algae Abundant in the Intertidal Zone in the Winter Months (January 2002 Hung Shing Ye) and Hard Shore Habitats (Western Lamma)

Figure 3.5        Dive Survey Sites used by ERM 1998 and for this Study

Figure 3.6        Location of Dive Surveys Conducted by HKIED along the West Lamma Coast

Figure 3.7        Percentage Cover for Substrate or Sessile Organism Recorded in Each of the Three Transects and Mean for Each Site in December 2001 at the West Coastal Sites T2, T4 and T6 at Lamma Island

Figure 3.8        Location of Key Porpoise Habitat around the Project Area

Figure 4.1        Fishing Areas around Lamma Island

Figure 7.1        Environmental Monitoring Stations

 


ABBREVIATIONS

AFCD

- Agricultural, Food and Conservation Department

CCC

- Criteria Continuous Concentration

CITES

- The Convention on the International Trade in Endangered Species of Wild Fauna and Flora

CMC

- Criteria Maximum Concentration

DO

- Dissolved Oxygen

EIA

- Environmental Impact Assessment

EIAO

- Environmental Impact Assessment Ordinance

EIA-TM

- Technical Memorandum on Environmental Impact Assessment Process

EM&A

- Environmental Monitoring and Audit

EPD

- Environmental Protection Department

HEC

- Hongkong Electric Company Limited

HKIEd

- Hong Kong Institute of Education

HKPSG

- Hong Kong Planning Standards and Guidelines

IUCN

- International Union for the Conservation of Nature

LMX

- Lamma Power Station Extension

NSR

- Noise Sensitive Receiver

OPCF

- Ocean Park Conservation Foundation

PD

- Principal Datum

SOD

- Sediment Oxygen Demand

SR

- Sensitive Receiver

SS

- Suspended Solids

TIN

- Total Inorganic Nitrogen

TKN

- Total Kjedahl Nitrogen

TOC

- Total Organic Carbon

TLC EM&A Manual

- Environmental Monitoring and Audit Manual for Backfilling of Marine Borrow Areas at East Tung Lung Chau

TSHD

- Trailer Suction Hopper Dredger

USACE

- United States Army Corporate Engineers

USEPA

- United States Environmental Protection Agency

WCZ

- Water Control Zone

WPCO

- Water Pollution Control Ordinance

WQ

- Water Quality

WQO

- Water Quality Objective

WQSR

- Water Quality Sensitive Receiver

 


1.                  INTRODUCTION

1.1              Description of the Project

1.1.1        The Hongkong Electric Company Limited (HEC) is responsible for the generation and supply of electricity to Hong Kong and Lamma Island.  At present, all electric power is generated from Lamma Power Station (LPS) which is located on Lamma Island.  LPS, with a total installed capacity of 3,420MW, comprises 2,500MW coal-fired units for base load operation and 920MW gas turbine units for peak lopping  operation.  The annual coal consumption in 2001 is about four million tons.

1.1.2        A coal jetty designed for 100,000 DWT coal vessels and a dedicated Navigation Channel (the Channel) have been in operation since the early 1980's to facilitate delivery of coal from overseas to LPS by ocean going vessels.  To ensure an adequate underkeel clearance for safe operation of coal vessels involved, maintenance dredging was carried out at the Channel in 1989/90 to bring the seabed level to - 16.5mPD.   Again due to natural siltation, the water depth at the Channel has now been reduced to about - 14mPD which is close to the limit for safe navigation of coal vessels of Panamax size (about 65,000 DWT) which is the smallest class of ocean going coal vessel in the market.

1.1.3        Even with the first new 300MW gas-fired unit in service at Lamma Power Station Extension, the coal-fired units of LPS still have to operate to meet the majority of electricity demand.  The annual coal consumption is around four million tons.

1.1.4        As the siltation is building up and in view of the importance to have in place a safe navigation channel with adequate water depth for ocean going coal vessels, maintenance dredging must be carried out in 2003.  Otherwise, coal supply to the LPS will be seriously hampered and the reliability of electricity supply will be jeopardized.  The proposed maintenance dredging work involves deepening the existing Channel to - 16mPD with an estimated total dredging volume of 2.98 million m3.

1.1.5        The Project Area is illustrated in Figure 1.1.  The grey area shows the limit of the Channel where dredging will be required under this Project. According to the latest bathymetric survey of the Channel, there is already sufficient water depth and no dredging will be required in the remaining section of the Channel in the south (beyond the grey area in Figure 1.1).

1.1.6        The dredging is scheduled to take place between May 2003 and December 2003.  In order to minimise the dredging period thus minimising the period of potential disturbance to environment and to suit the normal operating mode of dredgers, it is planned that the dredgers will be operating on a 24-hour basis during the dredging period.  Marine vessels will transport the dredged sediment to the Government approved disposal sites at East Ninepin, East Tung Lung Chau and East Sha Chau as shown in Figure 1.2. 

1.1.7        Coal ships currently deliver about six shipments of coal per month to the Lamma Power Station via the Channel.  All dredging works will be temporarily suspended when coal ships are navigating the Channel.

1.1.8        As the total dredging volume exceeds 500,000m3, the dredging operation of the Channel improvement is a designated project according to the Environmental Impact Assessment (EIA) Ordinance, Schedule 2, Part I, C.12.  A Project Profile was submitted by HEC to EPD on 11 June 2001 for application of an EIA Study Brief, which (Brief No. ESB-078/2001) was issued by EPD on 23 July 2001.  Hyder Consulting Ltd was commissioned by HEC to undertake the EIA Study in accordance with the EIA Study Brief.

1.2              Study Objectives

1.2.1        The purpose of this EIA is to provide information on the nature and extent of potential environmental impacts arising from the proposed Project and other projects taking place concurrently, with specific objectives as follows:

(a)        To describe the proposed Project and associated works together with the requirements for carrying out the proposed Project;

(b)        To consider alternative method(s) for the dredging work and design to ensure safe delivery of fuel to the Power station; and to compare the environmental benefits and dis-benefits of each of the method(s) and design in selecting a preferred one;

(c)        To identify and describe elements of the community and environment likely to be affected by the proposed Project and/or likely to cause adverse impacts to the proposed Project, including natural and man-made environment;

(d)        To propose the provision of infrastructure or mitigation measures so as to minimize pollution, environmental disturbance and nuisance during construction and operation of the proposed Project;

(e)        To identify, predict and evaluate the residual (i.e. after practicable mitigation) environmental impacts and the cumulative effects expected to arise during the construction and operation phases of the proposed Project in relation to the sensitive receivers (SRs) and potential affected uses;

(f)         To identify, assess and specify methods, measures and standards, to be included in the detailed design, construction and operation of the proposed Project which are necessary to mitigate these environmental impacts and reducing them to acceptable levels;

(g)        To investigate the extent of the secondary environmental impacts that may arise from the proposed mitigation measures, and to identify the constraints associated with the mitigation measures recommended in the EIA study as well as the provision of any necessary modification;

(h)        To identify, within the study area, any individual project(s) that fall under Schedule 2 of the EIA Ordinance (EIAO); to ascertain whether the findings of this EIA study have adequately addressed the environmental impacts of those projects; and where necessary to identify the outstanding issues that need to be addressed in any further detailed EIA study; and

(i)         To design and specify environmental monitoring and audit (EM&A) requirements, if required, to ensure the implementation and the effectiveness of the environmental protection and pollution control measures adopted.

1.3              Report Structure

1.3.1        Following this introductory section, Section 2 presents an assessment of the potential impacts on water quality (WQ).  Potential WQ sensitive receivers (WQSRs) are identified.  The baseline WQ conditions at those SRs are collated and the potential impacts of the Project on the SRs assessed.

1.3.2        Based on the potential WQ impacts, an assessment of the potential impacts on the marine ecology and fisheries has been carried out and the findings are presented in Sections 3 and 4.

1.3.3        The potential noise impact generated from the dredgers and barges to be deployed for the works is assessed in Section 5, taking into account the other ongoing noise generating activities around the area.

1.3.4        The management and disposal of the dredged marine sediment is presented in Section 6.  Section 7 outlines the Environmental Monitoring and Audit (EM&A) requirements recommended from this EIA and the main conclusions, recommendations and environmental outcomes of the EIA Study are summarised in Section 8.

1.3.5        Detailed technical information generated from this EIA Study is provided in the Appendices.

1.3.6        Appendix 1 presents the methodology, sampling location and detailed testing results of the Elutriation Test specifically conducted for this EIA Study, which aims to quantify the potential release of various contaminants from the seabed sediment into the water column in the vicinity of dredging.

1.3.7        The sediment quality data collected in and around the Channel is presented in Appendix 2.

1.3.8        Detailed graphical outputs from the water quality modelling are provided in Appendix 3.  An Implementation Schedule for the mitigation measures and good site practice recommended from this EIA Study is presented in Appendix 4. 

1.3.9        Detailed Noise Calculations are presented in Appendix 5 and an EM&A Manual (Construction Phase) is presented in Appendix 6 (under a separate cover).

 


2.                  WATER QUALITY IMPACT ASSESSMENT

2.1         Introduction

2.1.1        The Lamma Power Station Navigation Channel Improvement Work involves dredging of marine sediment within the existing Navigation Channel and disposal of the dredged sediment.

2.1.2        During the dredging operation, marine sediment leaking through the dredger will be released into the water column, thus leading to the elevation of the suspended solids (SS) level in the marine water. The coarse material, such as sand and gravel, of the released sediment will settle quickly back to the seabed close to the dredging site. However fine particles, such as silt and clay, together with any contaminants they may carry, will be transported away from the site by oceanic currents and tides, which may then affect WQSRs.

2.1.3        The potential WQSRs have been identified. The potential impact of the dredging operation on the SRs in terms of both pollution elevation and sediment deposition have been quantified, taking into account con-current dredging and disposal activities during the same period.  Necessary mitigation measures have been recommended.

2.2         Assessment Methodology

2.2.1        The WQSRs have been chosen based on the EIA study of the Lamma Power Station Extension completed in 1998 and the unique features of this dredging operation. Marine sediment sampling and elutriation test (following the USACE recommended methods) of the samples collected have been performed to determine the degree of mobilization of any likely contaminants.  WQ modelling has then been carried out to ascertain the potential elevation of water pollution in terms of various WQ parameters at the WQSRs. As the WQ model developed from the earlier EIA study of the Lamma Power Station Extension is intended for the same Project Area, and has been fully calibrated and verified, it has also been adopted for this EIA study.  Based on the results of the modelling, the elutriation tests and the baseline WQ conditions, the maximum dredging rates which would ensure no unacceptable environmental impacts are estimated.

2.3         Legislation and Standards

2.3.1        The following legislation is applicable to the evaluation of WQ impacts associated with the dredging and disposal of marine sediment.

·            Environmental Impact Assessment Ordinance and Technical Memorandum on Environmental Impact Assessment Process (EIA-TM)

·         Water Pollution Control Ordinance (WPCO)

·         Work Branch Technical Circular No. 3/2000, Management of Dredged/Excavated Sediment (WBTC No. 3/2000)

2.3.2        The WPCO is the primary legislation for the control of water pollution and WQ in Hong Kong. Under WPCO, Hong Kong waters are divided into 10 Water Control Zones (WCZs) each zone has a designated set of statutory Water Quality Objectives (WQOs).  The Lamma Power Station Navigation Channel and most of the potential WQSRs lie within the Southern WCZ.  The WQOs for these WCZs are the evaluation criteria for assessing the WQ impacts during dredging activities.

2.4         Sensitive Receivers

2.4.1        The dredging activities of the Navigation Channel will have the potential to directly affect WQ in the water along the western coast of Lamma Island. Beneficial uses have been defined in accordance with the requirements of the Hong Kong Planning Standards and Guidelines (HKPSG), which have been transposed into the EIA Technical Memorandum (TM).  WQSRs have been identified in these potentially affected areas under the broad designations of gazetted and non-gazetted bathing beaches, water intakes, fish culture zones and sites of ecological value.  The following list of WQSRs are identified in each of the categories:

Gazetted Bathing Beaches: Hung Shing Ye Beach and Lo So Shing Beach.

Water Intakes: Kennedy Town WSD Intake, Sha Wan Drive Intake, Queen Mary Hospital Intake, Wah Fu Estate Intake and HEC Lamma Power Station Intake. A new seawater intake at Cyberport is expected to start operating towards late 2002.  The water intake is close to the seawater intake at Wah Fu Estate and is between the seawater intakes at Wah Fu Estate and at Queen Mary Hospital. The impact of this project on the seawater intake at Cyberport is considered to be acceptable as long as the impact on those at Wah Fu Estate and Queen Mary Hospital is acceptable.

Fish Culture Zones: Lo Tik Wan Fish Culture Zone and Sok Kwu Wan Fish Culture Zone.

Sites of Ecological Interest: Kau Yi Chau (fish fry nursery area), Pak Kok (corals present), Shek Kok Tsui (corals present), Luk Chau (corals present) and  south Lamma water (finless porpoise habitat). Due to its ecological importance, the south Lamma water could be potentially designated as a marine park.  Two points, South Lamma 1 and South Lamma 2, which are situated in the closest part of the south Lamma water to the works area, have been chosen to facilitate the quantification of the potential WQ impact of this Project on the south Lamma water.  As long as the impact on this part of the water is acceptable, the impact on the other parts of the south Lamma water will be less and therefore also acceptable.

2.4.2        There are a number of EPD routine water monitoring stations around the Study Area.  The WQ data monitored from those stations has been used in this EIA to derive the baseline WQ conditions at the identified WQSRs. The locations of the above WQSRs and the EPD WQ monitoring stations are shown in Figure 2.1.

2.5         Baseline Conditions

Hydrodynamic Conditions

2.5.1        The Channel, located at the western side of the Lamma Power Station and extending along the outskirt of Ha Mei Wan, is within the West Lamma Channel.  The residual flow through the West Lamma Channel is towards the south. The average residual flow through a cross-section from Cheung Chau Island in the west to Shek Kok Tsui (just to the north of Lamma Power Station) in the east is 700 m3/s and 2800 m3/s approximately during the dry and the wet season respectively [1].  The average tidal flow through the cross-section in the dry season is 26,000 m3/s at the flood tide and 33,800 m3/s at the ebb tide. In the wet season, the average flow is increased to 27,700 m3/s at the flood tide and 40,700 m3/s at the ebb tide.

2.5.2        The tidal currents in the Channel with a peak speed of just over 0.5 m/s, are predominantly from the southeast to the northwest at the flood tide and in the opposite direction during the ebb tide, i.e. from the northwest towards the southeast.

Water Quality Conditions

2.5.3        The Channel lies within the Southern WCZ.  EPD carries out routine monitoring of marine WQ at the nearby stations, namely SM4, SM5, SM6, SM7, WM1 and WM2 as shown in Figure 2.1.

2.5.4        A number of EPD routine monitoring stations are in the vicinity of the dredging site, of which the closest is less than 1 km to the east of the Channel.  Relevant WQ data for this site, SM5, and other nearby stations around Lamma Island and within the Southern and Western Buffer WCZs are summarised in Tables 2.1 and 2.2 for the dry (October to March) and wet (April to September) season respectively.  This data was monitored in 2000.  It is the most up to date information from EPD and is therefore used in this EIA to represent the baseline WQ conditions. All monitoring stations are within 8 km of the Project Area.

Table 2.1        Water Quality at EPD Routine Monitoring Stations – Dry Season 2000 (January - March & October – December)

WQ Parameter

SM3

SM4

SM5

SM6

SM7

WM1

WM2

DO (mg/L)

6.99 (A)

6.69 (S)

7.17(B)

(5.58-7.77)

6.80 (A)

6.57 (S)

6.95 (B)

(5.50-7.61)

6.75 (A)

6.51 (S)

6.93 (B)

(6.08-7.33)

7.09 (A)

7.08 (S)

7.13 (B)

(5.74-7.63)

6.92 (A)

6.83 (S)

7.00 (B)

(5.33-8.63)

6.72 (A)

6.54 (S)

6.81 (B)

(4.48-7.80)

6.35 (A)

6.19 (S)

6.45 (B)

(4.19-7.88)

DO (% Sat.)

92.08

(83.63-97.77)

89.81

(82.46-94.51)

89.53

(83.82-98.24)

93.42

(86.31-98.33)

91.08

(79.72-109.8)

88.88

(66.65-100.2)

83.93

(62.51-99.94)

BOD5 (mg/L)

0.49

(0.10-1.37)

0.78

(0.35-1.47)

0.93

(0.34-2.17)

0.91

(0.31-2.13)

1.08

(0.41-2.90)

0.75

(0.15-2.00)

0.85

(0.33-1.87)

NH4 (mg/L)

0.03

(0.01-0.07)

0.05

(0.01-0.10)

0.03

(0.01-0.05)

0.03

(0.01-0.05)

0.05

(0.02-0.11)

0.05

(0.03-0.06)

0.08

(0.02-0.13)

NH3 (mg/L)

0.001

(0.000-0.002)

0.001

(0.000-0.002)

0.001

(0.000-0.001)

0.001

(0.000-0.002)

0.002

(0.001-0.002)

0.001

(0.000-0.002)

0.002

(0.000-0.004)

NO2 (mg/L)

0.02

(0.01-0.08)

0.02

(0.01-0.07)

0.02

(0.01-0.04)

0.03

(0.01-0.10)

0.04

(0.01-0.16)

0.04

(0.01-0.13)

0.05

(0.01-0.19)

NO3 (mg/L)

0.10

(0.05-0.14)

0.10

(0.05-0.16)

0.09

(0.04-0.13)

0.10

(0.03-0.15)

0.12

(0.03-0.26)

0.09

(0.04-0.15)

0.12

(0.06-0.17)

SS (mg/L)

6.06

(1.13-10.70)

7.11

(0.87-26.53)

5.62

(1.30-10.87)

8.46

(1.67-18.37)

5.37

(2.13-12.83)

6.29

(1.80-13.00)

8.38

(1.47-22.33)

TIN (mg/L)

0.15

(0.10-0.20)

0.17

(0.11-0.21)

0.14

(0.09-0.16)

0.15

(0.10-0.28)

0.21

(0.08-0.45)

0.18

(0.10-0.27)

0.25

(0.17-0.36)

TN (mg/L)

0.27

(0.19-0.34)

0.29

(0.21-0.35)

0.23

(0.18-0.27)

0.27

(0.20-0.42)

0.33

(0.18-0.57)

0.30

(0.26-0.39)

0.40

(0.30-0.49)

Note:       1.     Data presented are depth-averaged, except as specified.

2.        Data presented are Dry Season arithmetic mean.

3.        Data enclosed in brackets indicate the ranges.

4.        Shaded cells indicate non-compliance with WQOs.

5.        (A), (S) and (B) represented the value of depth-averaged, surface layer and bottom layer respectively.

 

2.5.5        The data in Tables 2.1~2.3 shows that the baseline WQ in the vicinity of the Channel was generally good, with compliance of WQOs for dissolved oxygen and unionised ammonia being achieved in both seasons at all monitoring stations.  The total inorganic nitrogen (TIN) exceeded the WQO limits at all five stations in Southern WCZ. It is worth noting that the WQO for TIN is not exceeded at Stations WM1 and WM2, even though the average concentration was higher than those in Southern WCZ. This is because the TIN level in the WQOs for Western Buffer WCZ was higher than that for the Southern WCZ. 


Table 2.2        Water Quality at EPD Routine Monitoring Stations – Wet Season 2000 (April – September)

WQ Parameter

SM3

SM4

SM5

SM6

SM7

WM1

WM2

DO (mg/L)

5.95 (A)

6.75 (S)

5.40 (B)

(4.91-7.14)

6.24 (A)

6.41 (S)

5.82 (B)

(4.62-8.18)

7.28 (A)

7.59 (S)

6.56 (B)

(4.82-9.24)

6.47 (A)

7.61 (S)

5.31 (B)

(5.04-8.24)

7.03 (A)

7.11 (S)

6.59 (B)

(4.77-10.31)

5.54 (A)

6.22 (S)

4.93 (B)

(3.89-6.81)

5.74 (A)

6.06 (S)

5.43 (B)

(4.15-8.06)

DO (% Sat.)

86.95

(72.35-107.92)

91.94

(68.10-123.65)

107.58

(71.50-139.19)

95.30

(74.69-124.47)

103.18

(69.95-154.10)

80.31

(55.38-99.83)

83.12

(59.50-120.78)

BOD5 (mg/L)

0.66

(0.37-0.81)

0.77

(0.61-1.21)

1.31

(0.78-2.63)

1.11

(0.70-2.31)

1.33

(0.58-3.07)

0.82

(0.22-1.33)

0.94

(0.35-1.90)

NH4 (mg/L)

0.04

(0.02-0.10)

0.06

(0.03-0.10)

0.03

(0.01-0.06)

0.04

(0.02-0.08)

0.07

(0.03-0.13)

0.04

(0.02-0.06)

0.07

(0.02-0.13)

NH3 (mg/L)

0.003

(0.001-0.008)

0.003

(0.002-0.008)

0.002

(0.000-0.006)

0.003

(0.001-0.007)

0.004

(0.002-0.011)

0.002

(0.001-0.003)

0.003

(0.001-0.006)

NO2 (mg/L)

0.02

(0.002-0.032)

0.02

(0.01-0.04)

0.02

(0.00-0.04)

0.02

(0.00-0.04)

0.03

(0.01-0.04)

0.02

(0.00-0.05)

0.04

(0.01-0.08)

NO3 (mg/L)

0.07

(0.01-0.15)

0.09

(0.02-0.22)

0.12

(0.00-0.31)

0.01

(0.01-0.26)

0.17

(0.04-0.30)

0.12

(0.03-0.23)

0.19

(0.11-0.26)

SS (mg/L)

5.48

(3.03-8.83)

3.55

(1.47-6.17)

3.71

(2.63-6.10)

3.83

(2.80-5.37)

4.89

(2.57-6.53)

4.81

(1.83-7.00)

3.49

(1.37-5.73)

TIN (mg/L)

0.13

(0.05-0.22)

0.16

(0.10-0.30)

0.17

(0.02-0.39)

0.18

(0.03-0.34)

0.26

(0.19-0.36)

0.19

(0.07-0.32)

0.30

(0.19-0.43)

TN (mg/L)

0.24

(0.13-0.34)

0.29

(0.17-0.49)

0.33

(0.10-0.63)

0.32

(0.11-0.58)

0.47

(0.27-0.86)

0.29

(0.17-0.42)

0.42

(0.29-0.55)

Note:       1.     Data presented are depth-averaged, except as specified.

                2.     Data presented are Wet Season arithmetic mean.

                3.     Data enclosed in brackets indicate the ranges.

                4.     Shaded cells indicate non-compliance with WQOs.

5      (A), (S) and (B) represented the value of depth-averaged, surface layer and bottom layer respectively.

 

Table 2.3        Relevant Water Quality Objectives at the Relevant Water Control Zones

Locations of WQSR

Southern WCZ

Western Buffer WCZ

Victoria Harbour (Phase 3) WCZ

WQSR ID (see Fig 2.1)

1, 7 - 15

3-6

2

PH

6.5 – 8.5

Dissolved Oxygen (depth average, 90% of the sampling occasion during the year)

4 mg/L

Dissolved Oxygen (within 2 m from seabed, 90% of the sampling occasion during the year)

2 mg/L

Un-Ionised Ammonia Nitrogen (annual average)

0.021 mg/L

Total Inorganic Nitrogen (annual depth average)

0.1 mg/L

0.4 mg/L

0.4 mg/L

Suspended Solids

< 30% increase over the ambient level

2.6         Water Quality Criteria

2.6.1        Impacts from SS may be caused by sediment plumes being transported to SRs, such as water intakes, bathing beaches and areas of high ecological value, leading to the elevation of the SS levels at the SRs.  The level of elevation will determine whether the impact is acceptable.  The WQOs in terms of SS for the Southern and Western Buffer WCZs are defined as being an allowable maximum elevation of 30% above the background for bathing beaches and sites of ecological interest.  Following recent research [29] by City University for AFCD, a maximum SS level of 50mg/l is recommended for the marine fish culture zones. The maximum SS levels allowed for the water intakes are 100mg/l for HEC’s Lamma Power Station cooling water intake, 20mg/l for the remaining water intakes.  Based on the allowable maximum SS levels and the baseline SS levels, the SS tolerant elevation can be determined as in Table 2.4.  As the seawater intakes are normally located in the mid-depth or above, the assessment criteria for the seawater intakes has been based on the pollution levels at the surface layer and the depth-averaged concentrations.

2.6.2        The WQOs in terms of other WQ parameters for the WQSRs have been taken as the same as those for the respective WCZs.

2.6.3        Silt and clay, also called cohesive sediment, will form large particles by the process of flocculation after being released into the water column, which will then settle back to the seabed, resulting in a smothering effect.  This smothering effect can be detrimental to the corals found near the western coast of Lamma Island.  Following the EIA for Sand Dredging at the West Po Toi Marine Borrow Area [30], a limit on the sedimentation rate of 0.1kg/m2/day was applied to this EIA in view of the ecological interests found in and around the area.

2.6.4        With reference to a previous EIA study for a 1,800MW Gas-Fired Power station at Lamma Extension [3], the background SS value for this EIA has been represented by the 90th percentile of the reported concentrations.

2.6.5        EPD routine WQ monitoring data has been used as the source of reported concentrations, with the monitoring station nearest to each SR being defined as representative of that SR.  The predicted maximum SS elevation resulting from the dredging has been compared to the SS tolerance values in Table 2.4 and the predicted maximum concentration elevation of other pollutants plus the baseline level has been compared to Table 2.3, to determine the acceptability of the WQ impacts.


Table 2.4        Background SS Concentrations and Tolerance Elevation at Sensitive Receivers

SRs (Relevant Monitoring Station)

Dry Season

Wet Season

90th Percentile
(mg/l)

Tolerance Elevation (mg/l)

90th Percentile
(mg/l)

Tolerance Elevation (mg/l)

Luk Cha

(SM3)

Aa

9.18

2.75

8.07

2.42

Sb

6.85

2.06

4.30

1.29

Bc

14.50

4.35

21.00

6.30

Lo Tik Wan Fish Culture Zoned

(SM3)

A

9.18

40.82

8.07

41.93

S

6.85

43.15

4.30

45.7

B

14.50

35.5

21.00

29.00

Sok Kwu Wan Fish Culture Zoned

(SM4)

A

15.92

34.08

5.18

44.82

S

5.30

44.7

4.80

45.20

B

25.85

24.15

7.75

42.25

Hung Shing Ye Beach

Lo So Shing Beach

S. Lamma 1

S. Lamma 2

(SM5)

A

8.65

2.60

5.42

1.63

S

6.00

1.80

4.35

1.31

B

12.30

3.69

6.10

1.83

Sha Wan Drive Intakee

Queen Mary Hospital Intakee

Wah Fu Estate Intakee

(WM1)

A

11.67

8.33

6.95

13.05

S

7.00

13.00

3.60

16.40

Pak Kok

Shek Kok Tsui

(WM1)

A

11.67

3.50

6.95

2.09

S

7.00

2.10

3.60

1.08

B

16.50

4.95

14.00

4.20

Kennedy Town WSD Intakee

(WM2)

A

19.00

1.00

5.10

14.90

S

15.50

4.50

4.55

15.45

Kau Yi Chau

(WM2)

A

19.00

5.70

5.10

1.53

S

15.50

4.65

4.55

1.37

B

25.50

7.65

5.85

1.76

HEC Power Station Intake f

(SM5)

A

9.18

90.82

8.07

91.93

S

 

6.85

 

93.15

 

4.30

 

95.70

 

Note:       a. A represents depth-averaged

                b. S represents surface layer

                c. B represents bottom layer

                d. Fish Culture Zones can tolerate a SS level of up to 50mg/l according to AFCD

                e. Flushing seawater intakes can tolerate a SS level of up to 20mg/l

                f. HEC Power Station cooling water intake can tolerate a SS level of up to 100mg/l according to HEC.

 

2.7         Sediment Sampling, Elutriation Test and Particle Size Analysis

2.7.1        An elutriation test has been carried out as part of this EIA study to assess the elutriation potential of various contaminants in the sediment during dredging. 

2.7.2        Under normal conditions, the contaminants would bind firmly with the bottom sediment and remain in the seabed. However, it is possible that these contaminants might be released into marine water if mechanical actions such as dredging activities take place, thus affecting the ambient marine WQ.

2.7.3        The objective of carrying out elutriate testing was to investigate the potential for the sediment-bonded pollutants being released into the ambient marine water (in the immediate vicinity of dredging) during dredging activities for the Project. ALS Environmental, a laboratory accredited by HOKLAS, was appointed to conduct sample collection and elutriation analysis.

2.7.4        Sediment samples were collected in December 2001 from the Project Area at three locations (Sample A - 828240E, 808540N; Sample B - 828540E, 807270N and Sample C - 828780E, 806000N) as shown in Figure 2.1 using grab samplers. Representative blank marine water samples were also collected for the test. The three sediment samples were homogenized in the laboratory and extracted with the collected marine water in accordance with the elutriation method as recommended by USACE.

2.7.5        The sampling locations were chosen so that they are representative of the Channel and are close to the three dredging locations (representing the worst-case scenarios) to be modelled for WQ assessment.

The following WQ parameters were analyzed:

·         Nitrogen Nutrients

Ammonia nitrogen (NH3-N and NH4-N), Nitrite (NO2-N), Nitrate (NO3-N), Organic Nitrogen, Total Kjeldahl Nitrogen (TKN) and Total Inorganic Nitrogen (TIN).

·         Heavy Metals

Silver, Arsenic, Cadmium, Chromium, Copper, Nickel, Lead, Zinc and Mercury.

·         Micro-Pollutants

PCBs, PAHs, TBT and chlorinated pesticides.

·         Five-Day Sediment Oxygen Demand (SOD5)

2.7.6        Unlike the above parameters for which the analysis was carried out on the mixture of sediment and water, sediment samples were analysed for SOD5.

2.7.7        The detection limits for the various WQ parameters were established based on the review of the international marine WQ standards which will be discussed later in this Report. A summary of the elutriation test results is given in Table 2.4a.

2.7.8        All heavy metals were found to be below the detection limits for all samples.

2.7.9        TBT, all PAHs, PCBs and chlorinated pesticides were also below the detection limit for all samples.

2.7.10    SOD5 were found to be 453, 706 and 380 mgO2/kg (dry weight sediment) for the samples collected at the locations A, B and C respectively.

Table 2.4a      Elutriation Test Results

Parameters

Unit

Sample A

Sample B

Sample C

Blank Water

pH Value @ 25'C

 

7.6

7.6

7.8

8.1

Redox Potential

mV

266

263

262

260

Silver

ug/L

<1

<1

<1

<1

 

 

 

 

 

 

Heavey Metals

 

 

 

 

 

Arsenic

ug/L

<10

<10

<10

<10

Cadmium

ug/L

<0.5

<0.5

<0.5

<0.5

Chromium

ug/L

<5

<5

<5

<5

Copper

ug/L

<1

<1

<1

<1

Nickel

ug/L

<1

<1

<1

<1

Lead

ug/L

<1

<1

<1

<1

Zinc

ug/L

<10

<10

<10

<10

Mercury

ug/L

<0.05

<0.05

<0.05

<0.05

 

 

 

 

 

 

Nutrients

 

 

 

 

 

Ammonia as N

mg/L

1.76

2.74

1.23

0.01

Nitrite + Nitrate as N

mg/L

0.02

0.02

0.02

0.11

Organic Nitrogen as N

mg/L

0.4

0.3

0.7

0.4

Total Kjeldahl Nitrogen as N

mg/L

2.2

3

1.9

0.4

Total Inorganic Nitrogen

mg/L

1.78

2.76

1.25

0.12

 

 

 

 

 

 

PCBs

 

 

 

 

 

PCB8

ug/L

<1

<1

<1

<1

PCB18

ug/L

<1

<1

<1

<1

PCB28

ug/L

<1

<1

<1

<1

PCB44

ug/L

<1

<1

<1

<1

PCB52

ug/L

<1

<1

<1

<1

PCB66

ug/L

<1

<1

<1

<1

PCB77

ug/L

<1

<1

<1

<1

PCB101

ug/L

<1

<1

<1

<1

PCB105

ug/L

<1

<1

<1

<1

PCB118

ug/L

<1

<1

<1

<1

PCB126

ug/L

<1

<1

<1

<1

PCB128

ug/L

<1

<1

<1

<1

PCB138

ug/L

<1

<1

<1

<1

PCB153

ug/L

<1

<1

<1

<1

PCB169

ug/L

<1

<1

<1

<1

PCB170

ug/L

<1

<1

<1

<1

PCB180

ug/L

<1

<1

<1

<1

PCB187

ug/L

<1

<1

<1

<1

PCB195

ug/L

<1

<1

<1

<1

PCB206

ug/L

<1

<1

<1

<1

PCB209

ug/L

<1

<1

<1

<1

 

 

 

 

 

 

ORGANOCHLORINE PESTICIDES

 

 

 

 

 

alpha-BHC

ug/L

<0.5

<0.5

<0.5

<0.5

beta- & gamma-BHC

ug/L

<1

<1

<1

<1

delta-BHC

ug/L

<0.5

<0.5

<0.5

<0.5

Heptachlor

ug/L

<0.5

<0.5

<0.5

<0.5

Aldrin

ug/L

<0.5

<0.5

<0.5

<0.5

Heptachlor epoxide

ug/L

<0.5

<0.5

<0.5

<0.5

Endosulfan 1

ug/L

<0.5

<0.5

<0.5

<0.5

Dieldrin

ug/L

<0.5

<0.5

<0.5

<0.5

4,4'-DDE

ug/L

<0.5

<0.5

<0.5

<0.5

Endrin

ug/L

<0.5

<0.5

<0.5

<0.5

Endosulfan 2

ug/L

<0.5

<0.5

<0.5

<0.5

4,4'-DDD

ug/L

<0.5

<0.5

<0.5

<0.5

Endrin aldehyde

ug/L

<0.5

<0.5

<0.5

<0.5

Endosulfan sulfate

ug/L

<0.5

<0.5

<0.5

<0.5

4,4'-DDT

ug/L

<2

<2

<2

<2

Endrin ketone

ug/L

<0.5

<0.5

<0.5

<0.5

Methoxychlor

ug/L

<2

<2

<2

<2

Cypermethrins

ug/L

<0.5

<0.5

<0.5

<0.5

 

 

 

 

 

 

POLYNUCLEAR AROMATICS

 

 

 

 

 

Naphthalene

ug/L

<2

<2

<2

<2

Acenaphthylene

ug/L

<2

<2

<2

<2

Acenaphthene

ug/L

<2

<2

<2

<2

Fluorene

ug/L

<2

<2

<2

<2

Phenanthrene

ug/L

<2

<2

<2

<2

Anthracene

ug/L

<2

<2

<2

<2

Fluoranthene

ug/L

<2

<2

<2

<2

Pyrene

ug/L

<2

<2

<2

<2

Benz(a)anthracene

ug/L

<2

<2

<2

<2

Chrysene

ug/L

<2

<2

<2

<2

Benzo(b) & (k)fluoranthene

ug/L

<4

<4

<4

<4

Benzo(a)pyrene

ug/L

<2

<2

<2

<2

Indeno(1.2.3-cd)pyrene

ug/L

<2

<2

<2

<2

Dibenz(a.h)anthracene

ug/L

<2

<2

<2

<2

Benzo(g.h.i)perylene

ug/L

<2

<2

<2

<2

 

 

 

 

 

 

ORGANOTIN COMPOUNDS

 

 

 

 

 

Tributyltin - Soluble

ng Sn/L

<5

<5

<5

<5

 

2.7.11    The majority of the TIN (in excess of 98%) consists of ammoniacal nitrogen.  There is significant elevation of all nitrogen components over the background levels except for NO2-N and NO3-N.

2.7.12    The level of NO2-N and NO3-N in the elutriation samples was found to be lower than the background levels, which is believed to be due to the sediment, on mixing with the seawater, acting as an oxygen absorption media in the mixing process which converts part of NO2-N and NO3-N into NH3-N.

2.7.13    In addition, the sediment samples were analysed for particle size distributions.  All samples showed that 95% or more of the sediment was silt and clay.

2.7.14    The details of the sampling methodology and the elutriation testing results are provided in Appendix 1. 

2.7.15    During the period from 1994 to 1998 three sets of sediment quality data were collected in and around the Channel. A summary of the sediment quality data is provided in Appendix 2.  All measured contaminant levels of the samples are below the Lower Chemical Exceedance Level (LCEL) as defined in the Works Bureau Technical Circular (WBTC) No. 3/2000.  Data from these three sediment quality studies indicated there have been no significant changes in sediment quality during the 4-year period and the sediment in and around the Channel belongs to Category L.  An earlier study [3] also confirmed that the marine sediment at the site of the Lamma Power Station Extension, which is close to the northern section of the Channel, is uncontaminated.

2.8         Assessment of Potential Impacts during Dredging

The Water Quality Model

2.8.1        The WQ effect has been assessed using WQ modelling. The hydrodynamic and WQ model constructed using Delft3D for a previous EIA study, Environmental Impact Assessment of a 1,800MW Gas-Fired Power Station at Lamma Extension [3], has been adopted for the current study.  The coverage and the computational grid system of the model are shown in Figure A3.1 in Appendix 3.

2.8.2        Due to lack of site specific data, a sediment settling velocity of 2m/d, derived from the calibration and verification of the Upgraded Model for Hong Kong waters, has been adopted for this EIA.

2.8.3        The dredging period is scheduled for between May 2003 and December 2003, covering both the dry and wet seasons. As the tidal circulation patterns in the vicinity of the Channel differs significantly between the two seasons, the model has been run for a typical 15-day spring to neap tidal cycle each for the dry and wet season for each dredging scenario. As in the previous EIA study for the Lamma Power Station Extension, the dry season simulation period has been from 6th to 20th January 1993 with a 5-day ‘spin-up’ period from 1st to 5th January 1993, and for the wet season the simulation period has been from 1st to 15th August 1992 with a ‘spin-up’ period from 28th to 31st July 1992.  Those periods have been chosen since the existing model was already setup for those periods and they also represent a typical dry season period and wet season period in Hong Kong.

Dredging Scenarios

2.8.4        As different dredging contractors may be equipped with different dredgers, HEC would like to have the flexibility of appointing the most suitable contractors for carrying out the proposed dredging works. To this end, it would be most appropriate to estimate the maximum dredging rates which would ensure no unacceptable environmental impacts rather than fixing a detailed plant inventory at this early stage.  In order to minimise the dredging period and therefore the associated costs, it is planned that the dredgers will be operating on a 24-hour basis during the dredging period.

2.8.5        The Channel to be dredged is divided equally in terms of the Northing Coordinates into four working zones: ABn, ABs, BCn and BCs as shown in Figure 2.2.  While the location of dredging will be moving all the time within the Channel, the most adverse impact of the whole dredging operation can be represented by dredging at either Dredging Location A, B or C as shown in Figure 2.2.  Without dividing the project area into smaller working zones, the only environmentally acceptable uniform dredging rate would have to be based on the worst case scenario, i.e. dredging at Location C.  In view of the relatively large area to be dredged, this would significantly and unnecessarily lengthen the dredging programme.

2.8.6        The impact of dredging at Dredging Location A would represent the worst-case scenario for the WQSRs to the north of the Channel such as those in the Western Buffer WCZ.

2.8.7        Dredging at Dredging Location C would represent the worst-case scenario for the WQSRs to the south of the Channel such as those in the South Lamma water. Dredging Location C represents the most southern point where the dredging will take place. According to the latest bathymetric survey of the Channel, there is already sufficient water depth and no dredging will be required in the southern part of the Channel beyond Dredging Location C.

2.8.8        The other adverse scenarios which may not be covered by dredging at either Dredging Location A or C can be accounted for by  dredging at Dredging Location B.

2.8.9        Based on the maximum dredging rates for dredging at Dredging Locations A, B or C, the maximum dredging rates for dredging at the individual working zones can be conservatively estimated as follows:

2.8.10    The maximum dredging rate at the mid point between A and B can be linearly interpolated from those at A and B. Likewise, the maximum dredging rate at the mid point between B and C can be linearly interpolated from those at B and C.  When a working zone has two different estimated dredging rates at the northern and southern boundaries, the lesser is recommended for the working zone. 

2.8.11    It should be noted that the maximum dredging rates thus estimated assume that all dredging activities take place at one single location (i.e. A or B or C) or at one single working zone (i.e. ABn or ABs or BCn or BCs).  The maximum dredging rates for dredging activities taking place at more than one working zones concurrently will be addressed at the end of this section.

2.8.12    Two types of dredgers, i.e. grab dredgers and trailer suction hopper dredgers (TSHD), are likely to be deployed for the works based on HEC’s current and previous projects involving dredging in this area.  In view of the relatively large quantity of the sediment to be dredged, medium to large size of grab with a grab capacity of no less than 8 m3 will be deployed for the grab dredger option.  The maximum dredging rates have therefore been estimated using WQ modelling for those two types of dredgers. 

2.8.13    For the TSHD option, it was assumed that there would be only one TSHD to be deployed at any one time for the dredging work, operating intermittently at a dredging cycle of 30 minute dredging and 2 hour travelling between the dredging site and the allocated dumping sites.

2.8.14    For grab dredgers, cage-type silt curtains as illustrated in Figure 2.3 will be deployed surrounding the grab and dredging will only take place within the water enclosed by the silt curtains which are mounted to the dredging barge.  The silt curtains can move along with the dredging barge. According to HEC’s measurements for the dredging practice in the area, the reduction rate in SS concentration of the silt curtains is typically between 76% and 81%. For the purpose of this assessment, a reduction rate of 75% is adopted. It was assumed that the dredging would be carried out continuously with the dumping of the dredged material being handled by barges and tugboats. A maximum of five grab dredgers will be operating at any one time and the actual number of dredgers used will depend on the maximum dredging rates allowed.

2.8.15    The sediment loss rates, or the S-Factors, of 4.25 kg/m3 has been adopted in this EIA for the grab dredger option with cage-type silt curtain and grab capacity ³ 8 m3.  This follows the EIA for International Theme Park and Associated Development [6] in which a S-Factor of 17kg/m3 was adopted for area of low debris.  The Channel is low in debris and boulders as it is relatively remote from shore, has been dredged twice since 1981 and has been mainly used by the ships delivering coal fuel to Lamma Power Station.  Applying a 75% reduction rate to the S-Factor of 17kg/m3, due to use of cage-type silt curtain, a S-Factor of 4.25kg/m3 is obtained.   The S-factors for a TSHD (no overflow or Lean Mixture Overboard) typically ranges from 3 to 4 kg/m3 [5] and a S-factor of 4 kg/m3 has been adopted in this EIA, following a previous EIA study conducted for the same area [3]. 

2.8.16    The leakage from grab dredgers can be throughout the water column as the dredger is lifted from the seabed to the barge.  However, any leakage from TSHD would be close to the seabed.

Suspended Sediment (SS)

2.8.17    For each dredging scenario, a unit sediment discharge rate was modelled.  Based on the predicted SS elevation and the baseline SS level at each vertical layer and at each SRs, the most critical SRs (where the percentage SS elevation over the baseline level is the largest among all WQSRs) were identified as shown in Table 2.5.  The maximum dredging rates were then estimated by scaling the unit sediment discharge rate by the ratio between the allowed elevation above the baseline SS level in Table 2.4 and the predicted SS elevation at the most critical SRs under the unit discharge rate.  The maximum dredging rates estimated for various dredging scenarios are also provided in Table 2.5.  Based on the resulting SS elevation and the baseline SS level, Shek Kok Tsui is the most critical WQSR when dredging at Dredging Location A. The south Lamma water generally represents the most critical WQSR when dredging at Dredging Locations B or C for both types of dredgers.  However the WQSR at Hung Shing Ye becomes most critical when dredging at Dredging Location B using TSHD during the dry season. The contours of the SS elevations predicted under the estimated maximum dredging rates for various scenarios are provided in Figures A3.7 to A3.12 and A3.25 to A3.30 in Appendix 3.

2.8.18    It should be noted that the maximum dredging rates estimated in Table 2.5 are the total dredging rates allowed in the entire Project Area at any one time.  Take the grab dredger with silt curtains option during the wet season for example.  The total dredging rate should be no greater than 40,100 m3/day if all dredging activities take place at Dredging Location A, no greater than 34,916 m3/day if all dredging activities take place at Dredging Location B and no greater than 23,521 m3/day if all dredging activities take place at Dredging Location C.

2.8.19    It can be seen that the location of the critical SR differs between dredging locations, but remain largely unchanged for different seasons and for both grab dredgers and TSHD.  As expected, the most critical SRs for dredging at Dredging Locations A and C are at Shek Kok Tsui and the South Lamma Water respectively.  When dredging at Dredging Location B, the south Lamma water is also the most critical SR for both grab dredgers and TSHD except for the TSHD option during the dry season when Hung Shing Ye becomes the most critical SR.

2.8.20    According to the WQ modelling results, the WQ impact on South Lamma 1 and South Lamma 2 are among the highest in the south Lamma water due to its close vicinity to the southern section of the Project Area.  The WQ impact on the other parts of the south Lamma water is considerably less and is negligible in the water around Sham Wan. 

2.8.21    Table 2.6 summarises the maximum (over the 15-day spring to neap tidal cycle) SS elevation at the bottom layer (B), surface layer (S) and averaged over the water depth (A) at the SRs if grab dredgers with silt curtain are used at the maximum dredging rates.  The similar information for TSHD at the maximum dredging rates is provided in Table 2.7.  It is worth noting that the SS concentration elevation presented in Tables 2.6 and 2.7 and in Appendix 3 is the maximum concentration over the 15-day spring to neap tidal cycle.  As such, the resulting SS concentration elevation will be lower most of the time.

 


Table 2.5        Estimated Maximum Dredging Rates and Critical Sensitive Receivers

 

 

 

 

 

 

 

S-Factor (kg/m3, Grab with silt curtain)

4.25

 

4.00

 

 

 

S-Factor (kg/m3, TSHD, no overflow)

4

 

 

 

 

 

 

Wet Season (April to September)

Dry Season (October to March)

Dredging Location

A

B

C

A

B

C

Grab with Silt Curtain

 

 

 

 

 

 

Critical Sensitive Receiver

SKTd (Sb)

SL1e (Bc)

SL1e (S)

SL1e (S)

SL1e (S)

SL2e (S)

Max Loss Rate  (kg/s)

1.972

1.717

1.157

2.967

1.812

1.926

Predicted Max Dredging Rate (m3/s)

0.4641

0.4041

0.2722

0.6982

0.4264

0.4532

Predicted Max Dredging Rate (m3/day)

40,100

34,916

23,521

60,328

36,837

39,160

TSHD

 

 

 

 

 

 

Critical Sensitive Receiver

SKTd (B)

SL1e (B)

SL1e (B)

SKTd (Aa)

HSYf (A)

SL1e (B)

Max Loss Rate (kg/s)

3.494

2.912

1.356

11.479

8.827

5.077

Predicted Max Dredging Rate (m3/s)

0.8735

0.7280

0.3390

2.8698

2.2068

1.2693

Predicted Max Dredging Rate (m3/day)

15,094

12,580

5,858

49,589

38,133

21,933

Note:      a. A represents depth-averaged                                       b. S represents surface layer

                c. B represents bottom layer                                             d. SKT - Shek Kok Tsui

                e. SL1 and SL2 - South Lamma 1 and 2 respectively  f. HSY - Hung Shing Ye Beach

 

Sediment Deposition

2.8.22    Under the maximum dredging rates estimated, the predicted deposition rates are less than 0.01 kg/m2/day along the west coast of Lamma Island and less than 0.001 kg/m2/day in the south Lamma water for all dredging scenarios.  The contours of the predicted sediment deposition rates for the dredging scenarios are provided in Figures A3.13 to A3.18 and A3.31 to A3.36.


Table 2.6          Predicted Maximum Elevation of SS Levels at WQSRs, Grab Dredger with Silt Curtains at the Estimated Maximum Dredging Rates

Sensitive Receiver

Depth

Dredging Location A

(mg/l)

Dredging Location B (mg/l)

Dredging Location C (mg/l)

Dry

Wet

Dry

Wet

Dry

Wet

Kau Yi Chau (fish fry nursery area)

A

0.168

0.161

0.089

0.106

0.051

0.054

S

0.167

0.074

0.089

0.047

0.051

0.025

B

0.168

0.252

0.088

0.169

0.051

0.088

Kennedy Town WSD Intake

A

0.287

0.123

0.130

0.100

0.087

0.053

S

0.274

0.098

0.124

0.066

0.074

0.034

B

0.294

0.138

0.133

0.120

0.093

0.065

Sha Wan Drive Intake

A

0.298

0.135

0.135

0.107

0.091

0.053

S

0.271

0.151

0.118

0.108

0.078

0.036

B

0.298

0.129

0.140

0.110

0.097

0.060

Queen Mary Hospital Intake

A

0.297

0.142

0.136

0.111

0.092

0.054

S

0.272

0.168

0.119

0.123

0.080

0.040

B

0.297

0.129

0.141

0.110

0.097

0.061

Wah Fu Estate Intake

A

0.297

0.172

0.138

0.124

0.097

0.056

S

0.290

0.301

0.136

0.221

0.086

0.066

B

0.284

0.107

0.135

0.081

0.100

0.047

Pak Kok

(corals present)

A

2.131

1.313

1.212

0.906

0.669

0.360

S

1.703

0.551

0.796

0.453

0.401

0.174

B

2.501

1.720

1.456

1.345

0.840

0.531

Shek Kok Tsui

(corals present)

A

2.588

1.966

1.497

1.081

0.884

0.421

S

1.820

1.080

0.844

0.687

0.505

0.303

B

3.202

3.052

1.860

1.599

1.118

0.607

Luk Chau

(corals present)

A

1.187

0.433

0.612

0.334

0.373

0.146

S

1.129

0.455

0.528

0.349

0.275

0.123

B

1.197

0.263

0.665

0.239

0.417

0.118

Lo Tik Wan Fish Culture Zone

A

0.614

0.272

0.305

0.193

0.195

0.083

S

0.612

0.336

0.304

0.253

0.194

0.090

B

0.610

0.191

0.304

0.161

0.195

0.081

HEC Power Station Intake

A

0.562

0.524

1.319

1.092

1.231

0.671

S

0.347

0.297

0.671

0.580

0.631

0.341

B

0.663

0.628

1.663

1.340

1.516

0.918

Hung Shing Ye Beach

A

0.561

0.343

1.438

0.614

1.383

0.364

S

0.380

0.258

0.962

0.375

0.808

0.166

B

0.680

0.482

1.764

0.873

1.680

0.501

Lo So Shing Beach

A

0.957

0.731

1.461

1.064

1.913

0.671

S

0.905

0.925

1.271

0.974

1.276

0.467

B

0.956

0.672

1.628

1.362

2.261

0.971

Sok Kwu Wan Fish Culture Zone

A

0.316

0.120

0.150

0.083

0.097

0.048

S

0.310

0.133

0.149

0.091

0.095

0.051

B

0.323

0.094

0.153

0.069

0.100

0.045

South Lamma  1 (finless porpoise habitat)

A

1.683

0.996

1.860

1.497

2.059

1.176

S

1.800

1.047

1.800

1.193

1.604

1.305

B

1.460

0.821

1.865

1.830

2.488

1.563

South Lamma  2 (finless porpoise habitat)

A

1.216

0.542

1.135

0.957

1.857

0.914

S

1.255

0.653

1.313

0.574

1.800

0.599

B

1.187

0.295

1.040

0.918

1.844

0.781

Note: A – Depth-averaged, B – Bottom Layer, S – Surface Layer


Table 2.7        Predicted Maximum Elevation of SS Levels at WQSRs, TSHD at the Estimated Maximum Dredging Rates

Sensitive Receiver

Depth

Dredging Location A (mg/l)

Dredging Location B (mg/l)

Dredging Location C (mg/l)

Dry

Wet

Dry

Wet

Dry

Wet

Kau Yi Chau (fish fry nursery area)

A

0.144

0.080

0.087

0.051

0.028

0.016

S

0.144

0.034

0.087

0.024

0.028

0.007

B

0.144

0.149

0.087

0.084

0.277

0.030

Kennedy Town WSD Intake

A

0.251

0.046

0.135

0.040

0.050

0.013

S

0.238

0.034

0.130

0.027

0.044

0.009

B

0.257

0.055

0.135

0.048

0.053

0.016

Sha Wan Drive Intake

A

0.263

0.048

0.140

0.038

0.050

0.013

S

0.238

0.038

0.125

0.029

0.044

0.010

B

0.263

0.050

0.140

0.043

0.055

0.015

Queen Mary Hospital Intake

A

0.263

0.050

0.140

0.040

0.053

0.013

S

0.238

0.040

0.125

0.029

0.044

0.010

B

0.257

0.052

0.140

0.045

0.053

0.015

Wah Fu Estate Intake

A

0.257

0.044

0.140

0.037

0.053

0.012

S

0.270

0.048

0.145

0.037

0.047

0.010

B

0.238

0.038

0.130

0.032

0.055

0.011

Pak Kok (corals present)

A

2.126

0.853

1.322

0.473

0.388

0.102

S

0.878

0.109

0.603

0.076

0.216

0.017

B

2.954

1.787

1.770

0.883

0.494

0.174

Shek Kok Tsui (corals present)

A

3.500

1.476

1.978

0.595

0.533

0.137

S

1.480

0.246

0.868

0.121

0.258

0.037

B

4.805

4.200

2.566

1.149

0.682

0.237

Luk Chau (corals present)

A

0.922

0.151

0.584

0.129

0.200

0.036

S

0.772

0.094

0.473

0.076

0.158

0.021

B

0.960

0.116

0.632

0.110

0.219

0.032

Lo Tik Wan Fish Culture Zone

A

0.470

0.073

0.289

0.068

0.105

0.020

S

0.496

0.078

0.294

0.078

0.105

0.021

B

0.452

0.073

0.289

0.067

0.105

0.021

HEC Power Station Intake

A

0.408

0.168

2.301

0.732

0.860

0.342

S

0.263

0.074

0.912

0.103

0.405

0.045

B

0.477

0.216

3.237

1.362

1.124

0.627

Hung Shing Ye Beach

A

0.433

0.086

2.595

0.191

1.035

0.093

S

0.276

0.044

1.669

0.041

0.580

0.019

B

0.533

0.139

3.256

0.461

1.293

0.218

Lo So Shing Beach

A

0.734

0.191

1.843

0.399

1.545

0.348

S

0.690

0.105

1.452

0.103

0.932

0.050

B

0.734

0.252

2.209

0.821

1.900

0.785

Sok Kwu Wan Fish Culture Zone

A

0.245

0.032

0.145

0.027

0.055

0.010

S

0.238

0.040

0.140

0.032

0.053

0.010

B

0.251

0.027

0.145

0.022

0.055

0.009

South Lamma  1 (finless porpoise habitat)

A

1.286

0.307

1.987

0.708

1.923

0.552

S

1.349

0.149

1.626

0.143

0.838

0.089

B

1.085

0.342

2.373

1.830

3.690

1.830

South Lamma  2 (finless porpoise habitat)

A

0.859

0.115

0.989

0.403

1.079

0.385

S

0.866

0.097

1.100

0.116

0.841

0.053

B

0.853

0.076

0.936

0.516

1.359

1.023

Note: A – Depth-averaged, B – Bottom Layer, S – Surface Layer


Ammoniacal Nitrogen

2.8.23    As mentioned in Section 2.7 above, the elutriation test revealed that more than 98% of the potential TIN elevation resulting from the proposed dredging works is contributed to by ammoniacal nitrogen.  The test showed that the potential elevation of the ammoniacal nitrogen is 1.75, 2.73 and 1.22 mg/l for the samples collected at Sampling Locations A, B and C respectively.  It should be noted that the test results only represent the potential pollution elevation in the very vicinity of the dredging locations.

2.8.24     In the marine environment, un-ionised ammonia exists in equilibrium with the ammonium ion.  The tested ammonia nitrogen concentrations are the sum of both forms of ammoniacal nitrogen.  Under a pH value of about 8 measured for the water samples collected from the site and at the likely temperature range of 10 to 280C in Hong Kong waters, more than 90% of the ammoniacal nitrogen would be in the ionised form.  This would lead to an elevation of un-ionised ammoniacal nitrogen of less than 0.175, 0.273 and 0.122 mg/l at Sampling Locations A, B and C respectively.  Compared to the threshold value of 0.021mg/l for un-ionised ammoniacal nitrogen as defined in the WQOs and taking into account the baseline ammoniacal nitrogen level, a dilution of just 9, 13 and 6 would be sufficient to enable the compliance with the WQOs for dredging at Locations A, B and C respectively.

2.8.25    Based on the Gaussian Dispersion Model [5] for a local velocity of 0.5m/s, a dispersion coefficient of 1m2/s and a local water depth of 15m, this dilution can be achieved within 50m of the dredging location in the direction normal to the main tidal stream (i.e. east-west direction).   However, in the direction along the main tidal stream of the Channel (i.e. north-south direction), the required distance will be 700m, 1700m and 400m approximately for dredging at Locations A, B and C respectively.  As most of the WQSRs (except for South Lamma 2) are in the inshore area to the east of the Channel, at least 1000m from the dredging area and outside the path of the tidal stream passing the Channel, the potential impact of the proposed Channel Improvement on those WQSRs in terms of the ammoniacal nitrogen concentration is therefore negligible.  South Lamma 2 is about 1200m to the nearest location of the Channel to be dredged (i.e. Location C) and the distance required along the main tidal stream to achieve the desired dilution is only 400m, the impact of the proposed dredging on South Lamma 2 is therefore also acceptable.

Dissolved Oxygen Depletion

2.8.26    The release of sediment contaminants into the water column consumes the dissolved oxygen in the ambient water.  The oxygen depletion resulting from the dredging under the maximum dredging rates has been modelled and the results are presented in Appendix 3 (Figures A3.19 to A3.24 and A3.37 to A3.42).  The sediment oxygen demand from the laboratory test (Appendix 1) and the estimated maximum dredging rates have been used to calculate the sediment depletion sources at Dredging Locations A, B and C.

2.8.27    Unlike the conventional definition of Sediment Oxygen Demand (SOD), the SOD tested from the laboratory for this EIA, having the unit of mgO2/kg sediment, is the total oxygen demand in 5 days due to the release of the sediment contaminants into the water.  Due to lack of sufficient information on the composition of the sediment pollutants, the SOD was modelled as a source of BOD5 to the receiving water through sediment loss during dredging.  The oxygen depletion in the WQSRs was calculated from the results of two water quality model runs.  One model run simulated the DO level at the WQSRs without the dredging operation.  The other simulated the DO level at the WQSRs during the dredging operation which releases SOD.  The difference in the DO level at the WQSRs between the two sets of the model results is the oxygen depletion resulting from the proposed dredging operation.  In both model runs, both the initial DO level and the DO level at the open boundaries were set at 7.6 mg/l.

2.8.28    As expected, the highest oxygen depletion is around the very vicinity of dredging.  However, the maximum oxygen depletion at the WQSRs is only 0.0022mg/l for the grab dredger option and is less than 0.001mg/l for the TSHD option.

2.8.29    As for SS, the oxygen depletion presented in Appendix 3 is the maximum oxygen depletion over the 15-day spring to neap tidal cycle.  The resulting oxygen depletion from the proposed dredging will be less than the maximum depletion most of the time.

2.8.30    From the baseline DO level (Table 2.2) and the predicted oxygen depletion resulting from the proposed dredging operation, the resulting DO levels at the WQSRs during the proposed dredging period are summarised in Tables 2.8 and 2.9.

2.8.31    It can be seen that for both options, the depth averaged DO levels at all the WQSRs are between 5.54 to 7.28mg/l and the bottom layer DO levels are between 4.93 to 7.16mg/l, well above the minimum requirement of 4 mg/l and 2 mg/l for the depth averaged and bottom layer DO respectively.  It is concluded that the proposed dredging operation has little effect on the DO levels at the WQSRs and the resulting DO level at the WQSRs will continue to comply with the DO level defined in the water quality criteria (Section 2.6) throughout the dredging period.

 


Table 2.8        Predicted DO Levels (Baseline DO Less the Maximum Oxygen Depletion) at WQSRs, Grab Dredger with Silt Curtains at the Estimated Maximum Dredging Rates

Sensitive Receiver

Depth

Dredging Location A (mg/l)

Dredging Location B (mg/l)

Dredging Location C (mg/l)

Dry

Wet

Dry

Wet

Dry

Wet

Kau Yi Chau (fish fry nursery area)

A

6.347

5.738

6.347

5.738

6.347

5.738

S

6.190

6.062

6.190

6.062

6.190

6.062

B

6.451

5.432

6.451

5.432

6.451

5.432

Kennedy Town WSD Intake

A

6.347

5.738

6.347

5.738

6.347

5.738

S

6.190

6.062

6.190

6.062

6.190

6.062

B

6.451

5.432

6.451

5.432

6.451

5.432

Sha Wan Drive Intake

A

6.724

5.544

6.724

5.544

6.724

5.544

S

6.539

6.221

6.539

6.221

6.539

6.221

B

6.813

4.928

6.813

4.928

6.813

4.928

Queen Mary Hospital Intake

A

6.724

5.544

6.724

5.544

6.724

5.544

S

6.539

6.221

6.539

6.221

6.539

6.221

B

6.813

4.928

6.813

4.928

6.813

4.928

Wah Fu Estate Intake

A

6.724

5.544

6.724

5.544

6.724

5.544

S

6.539

6.221

6.539

6.221

6.539

6.221

B

6.813

4.928

6.813

4.928

6.813

4.928

Pak Kok (corals present)

A

6.723

5.544

6.723

5.544

6.724

5.544

S

6.538

6.221

6.538

6.221

6.539

6.221

B

6.812

4.928

6.812

4.928

6.813

4.928

Shek Kok Tsui (corals present)

A

6.723

5.544

6.723

5.544

6.724

5.544

S

6.538

6.221

6.538

6.221