4. WATER QUALITY

4.1 Introduction

This section outlines the potential impacts associated with the proposed cable laying activities between Chi Ma Wan Peninsula and Cheung Chau via Pui O on receiving water quality. The potential water quality impacts associated with the Project have been considered. These include impacts from cable laying, general construction works and runoff., and tThe environmental acceptability of the impacts on receiving water quality wereas determined and mitigation measures recommended where necessary..

In order to define the nature and quantify the extent of the potential impacts associated with the marine based works, sediment plume modelling was undertaken using an approved and calibrated mathematical model which simulated various likely scenarios in accordance with the requirements of the Study Brief. Other water quality assessments have been undertaken using qualitative assessments and are based on practical experience of similar projects in Hong Kong (such as “Focussed Environmental Impact Assessment Study : Laying a Second 132kV Submarine Cable Transmission link from Lau Fau Shan to Shekou”, ERM 1996)..

The potential water quality impacts associated with the Project have been considered. These include impacts from dredging, general construction works and runoff, and the environmental acceptability of the impacts on receiving water quality were determined and mitigation measures recommended.

The water quality assessment has followed the guidelines given in Annexes 6 and 14 of the EIA-TM and has focussed on assessing the construction impacts associated with implementing the project and provides details of any mitigation measures and monitoring requirements which may be necessary to ensure residual impacts are acceptable and comply with current standards and guidelines.

4.2 Relevant Legislation, Regulations, Guidelines and Evaluation Criteria

The Water Pollution Control Ordinance (WPCO) (Cap. 358) enacted in 1980 is the principal legislation controlling water quality in Hong Kong along with Annexes 6 and 14 of the Technical Memorandum on Environmental Impact Assessment Process (EIA-TM). Under the WPCO, Hong Kong waters are classified into 10 Water Control Zones (WCZs) and statutory Water Quality Objectives (WQOs) are specified for each WCZ.

This cable laying project will take place in the Southern Water Control Zone which was gazetted in L.N. 204 of 1988. The WQOs for this WCZ are the evaluation criteria for assessing the water quality impacts during dredging or jet ploughing activities. The WQOs also apply to the protection of water quality with respect to the potential off-site discharges from construction sites, and disposal of sewage from the construction work force.

WQOs have been established in terms of the physical, chemical and biological water quality in the specific Water Control Zone to achieve the level of protection required for each beneficial use. For the current situation beneficial uses are:

· human health; and

· aquatic life

Relevant WQOs for this assignment are included in Table 4-1 which indicate the ambient value and the compliance level.

Table 4-1 Water Quality Objectives (WQOs) for Southern Water Control Zone

Water Quality Objective	Part or Parts of Zone
A. AESTHETIC APPEARANCE (a) Waste discharges shall cause no objectionable odours or discolouration of the water. (b) Tarry residues, floating wood, articles made of glass, plastic, rubber or of any other substance should be absent. (c) Mineral oil should not be visible on the surface. Surfactants should not give rise to a lasting foam. (d) There should be no recognisable sewage-derived debris. (e) Floating, submerged and semi-submerged object at a size likely to interfere with the free movement of vessels, or cause damage to vessels, should be absent. Waste discharges shall not cause the water to contain substance which settle to form objectionable deposits.	Whole Zone Whole Zone Whole Zone Whole Zone Whole Zone Whole Zone
B. BACTERIA (a) The level of Escherichia coli should not exceed 610 per 100 ml, calculated as the geometric mean of all samples collected in one calendar year. (b) The level of Escherichia coli should not exceed 180 per 100 ml, calculated as the geometric mean of all samples collected from March to October inclusive in one calendar year. Samples should be taken 3 times in a calendar month at intervals of between 3 and 14 days.	Secondary Contact Recreation Subzones and Fish Culture Subzones Bathing Beach Subzones
C. DISSOLVED OXYGEN (a) Waste discharges shall not cause the level of dissolved oxygen to fall below 4 milligrams per litre for 90% of the sampling occasions during the year, values should be calculated as the water column average (arithmetic mean of at least 3 measurements at 1 metre below surface, mid-depth, and 1 metre above seabed). In addition, the concentration of dissolved oxygen should not be less than 2 milligrams per litre within 2 metres of the seabed for 90% of the sampling occasions during the year. (b) The dissolved oxygen level should not be less than 5 milligrams per litre for 90% of the sampling occasions during the year; values should be calculated as water column average (arithmetic mean of at least 3 measurements at 1 metre below surface, mid-depth and 1 metre above seabed). In addition, the concentration of dissolved oxygen should not be less than 2 milligrams per litre within 2 metres of the seabed for 90% of the sampling occasions during the year. (c) Waste discharges shall not cause the level of dissolved oxygen to be less than 4 milligrams per litre.	Marine waters excepting Fish Culture Subzones Fish Culture Subzones Inland waters of the Zone
D. PHpH (a) The pH of the water should be within the range of 6.5-8.5 units. In addition, waste discharges shall not cause the natural pH range to be extended by more than 0.2 units. (b) The pH of the water should be within the range of 6.0-9.0 units. (c) The pH of the water should be within the range of 6.0-9.0 units for 95% of samples. In addition, waste discharges shall not cause the natural pH range to be extended by more than 0.5 units.	Marine waters excepting Bathing Beach Subzones; Mui Wo (A), Mui Wo (B), Mui Wo (C), Mui Wo (E) and Mui Wo (F) Subzones. Mui Wo (D) Subzones and other inland waters. Bathing Beach Subzones
E. TEMPERATURE (a) Waste discharges shall not cause the natural daily temperature range to change by more than 2.0 degree Celsius.	Whole Zone
F. SALINITY (a) Waste discharges shall not cause the natural ambient salinity level to change by more than 10%.	Whole Zone
G. SUSPENDED SOLIDS (a) Waste discharges shall neither cause the natural ambient level to be raised by 30% nor give rise to accumulation of suspended solids which may adversely affect aquatic communities. (b) Waste discharges shall not cause the annual median of suspended solids to exceed 20 milligrams per litre. (c) Waste discharges shall not cause the annual median of suspended solids to exceed 25 milligrams per litre.	Marine waters Mui Wo (A), Mui Wo (B), Mui Wo (C), Mui Wo (E) and Mui Wo (F) Subzones. Mui Wo (D) Subzones and other inland waters.
H. AMMONIA (a) The ammonia nitrogen level should not be more than 0.021 milligram per litre, calculated as the annual average (arithmetic mean), as unionised form.	Whole Zone
I. NUTRIENTS (a) Nutrients shall not be present in quantities sufficient to cause excessive or nuisance growth of algae or other aquatic plants. (b) Without limiting the generality of objective (a) above, the level of inorganic nitrogen should not exceed 0.1 milligram per litre, expressed as annual water column average (arithmetic mean of at least 3 measurements at 1 metre below surface, mid-depth and 1 metre above seabed).	Marine waters
J. 5-DAY BIOCHEMICAL OXYGEN DEMAND (a) Waste discharges shall not cause the 5-day biochemical oxygen demand to exceed 5 milligrams per litre.	Inland waters of the Zone
K. CHEMICAL OXYGEN DEMAND (a) Waste discharges shall not cause the chemical oxygen demand to exceed 30 milligrams per litre.	Inland waters of the Zone
L. DANGEROUS SUBSTANCES (a) Waste discharges shall not cause the concentration of dangerous substances in marine water to attain such levels as to produce significant toxic effects in humans, fish or any other aquatic organisms, with due regard to biologically cumulative effects in food chains and to toxicant interactions with each other. (b) Waste discharges of dangerous substances shall not put a risk to any beneficial uses o the aquatic environment.	Whole Zone Whole Zone

(Source: Adopted from CAP. 358, section 5, 1988 and L.N. 453 of 91)

The Technical Memorandum (TM), “Standards for Effluent Discharge into Drainage and Sewerage Systems, Inland and Coastal Waters”, issued under Section 21 of the WPCO defines acceptable effluent discharge limits to different types of receiving waters. With regard to inland waters, there is no distinction between different zones and the beneficial use of the inland waters is the only factor governing the quality and quantity of the effluent that should be met. Under the TM, inland waters are classified into four groups. These are given below in Table 4-2.

Table 4-2 Different Groups of Inland Water Specified in the TM

Inland Water Grouping	Beneficial use
Group A	Abstraction for potable water supply
Group B	Irrigation
Group C	Pond fish culture
Group D	General amenity and secondary contact recreation

(Source: Adopted from Technical Memorandum of Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters)

For this Project Group D waters prevail for the inland waters the :-.

The WQOs which prevail for this Project with discharge standards for Group D waters are given in Table 4-3.

Table 4-3 Standards for Effluents Discharged into Group D Inland Waters

(All units in mg/L unless otherwise stated; all figures are upper limits unless otherwise indicated)

Flow rate (m^3/day) Determinand	£200	>200 and £400	>400 and £600	>600 and £800	>800 and £1000	>1000 and £1500	>1500 and £2000	>2000 and £3000
pH (pH units)	6-10	6-10	6-10	6-10	6-10	6-10	6-10	6-10
Temperature (^oC)	30	30	30	30	30	30	30	30
Colour (lovibond units) (25mm cell length)	1	1	1	1	1	1	1	1
Suspended solids	30	30	30	30	30	30	30	30
BOD	20	20	20	20	20	20	20	20
COD	80	80	80	80	80	80	80	80
Oil & Grease	10	10	10	10	10	10	10	10
Iron	10	8	7	5	4	2.7	2	1.3
Boron	5	4	3.5	2.5	2	1.5	1	0.7
Barium	5	4	3.5	2.5	2	1.5	1	0.7
Mercury	0.1	0.05	0.001	0.001	0.001	0.001	0.001	0.001
Cadmium	0.1	0.05	0.001	0.001	0.001	0.001	0.001	0.001
Other toxic metals individually	1	1	0.8	0.8	0.5	0.5	0.2	0.2
Total toxic metals	2	2	1.6	1.6	1	1	0.5	0.4
Cyanide	0.4	0.4	0.3	0.3	0.21	0.1	0.1	0.05
Phenols	0.4	0.3	0.2	0.1	0.1	0.1	0.1	0.1
Sulphide	1	1	1	1	1	1	1	1
Sulphate	800	600	600	600	600	400	400	400
Chloride	1000	800	800	800	600	600	400	400
Fluoride	10	8	8	8	5	5	3	3
Total phosphorus	10	10	10	8	8	8	5	5
Ammonia nitrogen	20	20	20	20	20	20	20	10
Nitrate + nitrite nitrogen	50	50	50	30	30	30	30	20
Surfactants (total)	15	15	15	15	15	15	15	15
E. coli (count/100ml)	1000	1000	1000	1000	1000	1000	1000	1000

(Source: Adopted from Technical Memorandum of Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters)

4.3 Potential Sources of Impact

4.3.1 Sources Inventory

The potential sources of water quality impacts are from:As identified in Section 2 there are various activities taking place in the marine environment which could adversely affect water quality and impact on the marine sensitive receivers. These are primarily :

The main aspects of the project which have the potential to impact water quality and thus potentially sensitive receivers these are:

· Cable laying which is described in section 2 detail in section ___ and may take place using dredging or direct burying methods although it is recognised that the method favoured method (by the the Client) is theis direct burying method as it is shorter, has fewer environmental impacts to address and is generally more in keeping with the approach of minimising at source the environmental impacts of projects (through choice of method or plant);

· Formation of the temporary workings platform at the tunnel portal at Pui O; and

· Laying the cable in the typhoon shelter at Cheung Chau.

The methods considered for cable laying have been subjected to detailed modelling using a sediment plume model. For the pther aspects of the work, qualitative assessments have ben adopted. Justifications for this are given in the form of the construction method as described below.

The temporary working platforms area at Pui O will be formed by placing a seawall in the location shown in Figure 3.3. Material excavated from the tunnel will be used to form the works platform and by adopting a strategy of filling behind the forward formed seawall, no “fines” will be released to the receiving marine waters.. The method of construction which is designed for pollution prevention, will be to place concrete blocks as an outer seawall (within no gaps) and fill behind the seawall with excavated material from the tunnel. Tunnel spoil will largely be large fraction rock and rubble. Notwithstanding this there will be no opportunity for any small fraction materials (“fines”) to be released to the receiving waters as there will be no gaps permitted in the seawall. Once the tunnelling works have been completed the fill material will be excavated, disposed of off-site and the seawall dismantled, thus reinstating the beach to its original state. No detailed modelling was undertaken as the impacts are temporary and very minor. and transient.

When laying the cables in the typhoon shelter, an opening will need to be made by dismantling the area shown in Figure 4.21. The cast concrete sleeve will then be placed in the seawall, which shall be reconstructed and divers deployed to “lay” the cables through the sleeve to rest in the cable trough. The “U” shaped sleeve is 4.9m (long) x 0.75m (high) for each circuit which passes through the breakwater. Only two circuits are placed through the breakwater according to the current design proposed.

It is estimated that the diver would need around 10 days to lay each circuit in the typhoon shelter (by hand). This work will have minimal impact on the sediments in the bottom of the typhoon shelter and no detailed assessment has been carried out. As a precaution the Contractor should be prepared to provide a silt screen for the diver to work within should this be required. Reference should be made to section 4.6 which details mitigation measures.

Other potential impacts include the off-site runoff during land based construction activities and dewatering of slurry generated during the driving of the tunnel, and .

Other issues include the liquid waste (sewage and greywater) generated by the construction workers. which is discussed in Section 9 under the solid and liquid wastes section.

For the assessment of off-site spillage the focus was on qualitative assessments and definition of mitigation measures to be included in the Contract Documents. Guidelines following ProPECC Note PN1/924 “Construction”, “Site Drainage” would be used as a reference guide. Mitigation measures are also described in Section 4.6.

For the treatment of wastewater generated by the tunnel construction, it is likely that this would need to focus on removal of sediment. This could be by sedimentation tank or by the use of a mobile microfiltration plant, should the required discharge standard not be met by conventional sedimentation techniques. The standards to achieve are set out in the Technical Memorandum on “Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters”. Further discussion of this waste is described in Section 9.

Dredging for and/or laying of cables;

·Reclamation works at Pui O Beach;

·Construction runoff; and

·Sewage generated by the workforce.

The water quality assessment has followed the guidelines given in Annexes 6 and 14 of the EIA-TM. The water quality assessment h and has focussed on assessing the construction and operational impacts associated with implementing the project and provides details of any mitigation measures and monitoring requirements which may be necessary to ensure residual impacts are acceptable and comply with current standards and guidelines.

It should be noted that as the working platform at Pui O is temporary there will be no potential impediment to tidal flows following completion of the works, therefore no post construction assessment is required.

For the remaining assessments, attention has focussed on the cable laying works.

In accordance with the requirements of the EIA-TM, the assessments given in the following sections are, as far as practical, quantitative..

4.3.2 Baseline Conditions

Marine Water Quality

The Study Area is located within Southern Waters in a water body where oceanic and estuarine waters interchange. The Southern Water Control Zone which is the second largest water control zone (WCZ) with a marine mud disposal area to the south of Cheung Chau. The WCZ also covers a statutory marine reserve in Cape D’Aguilar which is of great scientific interest and conservationversation significance but which is remote from the subject site. The major land areas in the zone include the southern part of Lantau Island, all outlaying islands in the southern territorial waters of Hong Kong Island. and the southern part of Hong Kong Island.

There are fFour maricultural zones are located within this WCZ at Cheung Sha Wan, Po Toi Island, Cheung Sha Wan (southeast coast of Lantau Island), Sok Kwu Wan and Lo Tik Wan of Lamma Island. Apart from Chi Ma WanCheung Sha Wan all other fish culture zones are remote from the influence of the proposed cable layingmarine works. Moreover, a navigation fairway is running through The Adamasta Channel is a designated navigation fairway which is used by high speed vessels plying to and from Macau, inter-island ferries and fishing vessels . Consequently water movements are complex with seasonal variations observed with estuarine or oceanic influences dominating at different times of the year.

Based on the data obtained from Marine Water Quality in Hong Kong in 1999 published by the Environmental Protection Department (EPD), the closest monitoring stations to the project are at SM11, SM12 and SM13 within the WCZ to the north and the west of Cheung Chau and to the southeast of Lantau Island. Locations of the indicating representative water sampling stations in the Southern WCZ (in the current context) are shown in Figure 4.12. Water quality statistics for the representative stations are presented in Table 4-4.

Table 4-4 Marine Water Quality in the Southern Water Control Zone at Selected Stations

Determinand	SM11	SM12	SM13
Temperature (^oC)	23.5 (17.5 – 27.2)	23.7 (18.5 – 27.5)	23.8 (18.6 – 27.3)
Salinity (psu)	30.3 (25.4 – 32.7)	30.5 (25.8 – 33.4)	30.4 (24.7 – 33.4)
Dissolved Oxygen (mg/L)	6.5 (5.0 – 9.3)	6.6 (5.3 – 9.6)	6.3 (4.7 – 8.0)
Dissolved Oxygen Bottom (mg/L)	6.5 (4.9 – 8.9)	6.5 (5.3 – 9.4)	6.4 (5.2 – 8.2)
Dissolved Oxygen (% Saturation)	91 (70 – 118)	92 (75 – 125)	88 (67 – 104)
Dissolved Oxygen Bottom (% Saturation)	91 (70 – 112)	91 (76 – 123)	89 (74 – 107)
PpH	8.0 (7.8 – 8.3)	8.0 (7.8 – 8.4)	8.0 (7.9 – 8.4)
Secchi Disc Depth (m)	2.0 (1.0 – 3.5)	1.8 (1.0 – 3.0)	1.8 (1.0 – 3.5)
Turbidity (NTU)	8.2 (3.7 – 10.7)	9.2 (4.0 – 13.6)	12.7 (3.8 – 22.4)
Suspended Solids (mg/L)	6.4 (2.5 – 11.3)	8.0 (2.8 – 12.7)	9.7 (2.0 – 20.7)
5-day Biochemical Oxygen Demand (mg/L)	0.7 (0.3 – 1.2)	0.8 (0.3 – 1.3)	0.7 (0.3 – 1.2)
Ammonia Nitrogen (mg/L)	0.06 (0.04 – 0.11)	0.04 (0.01 – 0.05)	0.03 (0.01 – 0.05)
Unionized Ammonia (mg/L)	0.003 (0.001 – 0.006)	0.002 (0.001 – 0.003)	0.002 (<0.001 – 0.004)
Nitrite Nitrogen (mg/L)	0.03 (0.01 – 0.05)	0.02 (<0.01 – 0.04)	0.02 (<0.01 – 0.04)
Nitrate Nitrogen (mg/L)	0.14 (0.02 – 0.29)	0.14 (0.01 – 0.30)	0.13 (<0.01 – 0.32)
Total Inorganic Nitrogen (mg/L)	0.23 (0.08 – 0.38)	0.19 (0.04 – 0.35)	0.19 (0.02 – 0.37)
Total Kjeldahl Nitrogen (mg/L)	0.2 (0.12 – 0.36)	0.22 (0.13 – 0.35)	0.21 (0.08 – 0.33)
Total Nitrogen (mg/L)	0.41 (0.28 – 0.51)	0.38 (0.24 – 0.55)	0.36 (0.18 – 0.52)
Ortho-phosphate (mg/L)	0.02 (0.01 – 0.04)	0.02 (0.01 – 0.03)	0.02 (0.01 – 0.03)
Total-Phosphorus (mg/L)	0.04 (0.02 – 0.05)	0.03 (0.02 – 0.04)	0.03 (0.02 – 0.04)
Silica (as SiO2) (mg/L)	0.9 (0.1 – 3.0)	1.0 (0.1 – 2.8)	1.1 (0.1 – 3.0)
Chlorophyll-a (mg/L)	3.3 (1.0 – 6.6)	3.2 (1.2 – 6.0)	2.4 (0.6 – 5.4)
Phaeo-pigment (mg/L)	1.0 (0.4 – 1.6)	1.0 (0.5 – 2.0)	0.8 (0.2 – 1.9)
E.coli (cfu/100mL)	4 (1 – 120)	12 (2 – 200)	2 (1 – 15)
Faecal Coliforms (cfu/100mL)	9 (1 – 350)	28 (4 – 510)	5 (1 – 90)

Note: 1. Except as specified, data presented are depth-averaged results.

2. Depth-averaged results at each station are calculated as arithmetic means of measurements at all available depths (i.e. S, M, B) except for E.coli and faecal coliforms which are geometric means.

3. Data presented are annual arithmetic means except for E.coli and faecal coliforms which are annual geometric means.

4. Data enclosed in brackets indicate the ranges.

5. Shaded cells indicate non-compliance with the WQOs.

(Source: Adopted from EPD Marine Water Quality for 1999)

The wWater quality at the representative stations was generally good with a low level of sewage bacteria (indicative of faecal organismsations) being present and with . Aall the parameters, of water quality except for total inorganic nitrogen (TIN), complying with the relevant WQOs. As these representative monitoring stations are shielded by Lantau Island, the influence of the Pearl River flow is insignificant, which is . This can be reflected by the relatively small range of in the salinity (25 to 33 psu).

With reference to the report entitled of the project “Focussed Environmental Impact Assessment (EIA) Study : Laying a second 132kV Submarine Cable Transmission Link from Lau Fau Shan to Shekou” it was noted that the . The ambient level of suspended solids wais assumed to be the 90^th percentile of the reported concentrations. Using this precedent case the , and the predicted values of 90^th percentiles at SM11, SM12 and SM13 were calculated to be are 9.1, 12.2 and 17.8 mg/L respectively. The mean ambient value is calculated to be 13.0 mg/L.

As the WQO is defined as 130% of the ambient, the WQO compliance threshold is set (for this assessment) at 16.9 mg/L. Therefore, the maximum allowable elevated level for suspended sediment is due to the proposed works is 3.9 mg/L. It should be stressed that this compliance level acceptability level should be confirmed through the baseline monitoring to be carreid carried out by the Contractor prior to commencing marine works. This baseline monitoring will provide athe more accurate level of suspended solids which can be released into the water column while still complying with the Water Quality Objectives – as it will represent the situation at the time of the works.

Analysis of dDissolved oxygen is defined both in terms of concentration (mg/L) and saturation (%). According to the WQO compliance evaluation, only DO concentration is concerned and the ambient level of dissolved oxygen is assumed to be the 10^th percentile of the reported concentrations. The ranges and mean values at the representative sampling stations are presented in Table 4-4. The 10^th percentiles at SM11, SM12 and SM13 are predicted to be 5.4, 5.4 and 5.1 mg/L respectively. The mean ambient value is estimated to be 5.3 mg/L. For the DO saturation, the 10^th percentiles at SM11, SM12 and SM13 are predicted to be 78.2%, 77.9% and 74.1% respectively and the mean ambient level is estimated to be 76.7%. These ambient values indicate a well-oxygenated baseline environment.

Sediment Quality

Marine sediment quality data is collected by EPD in the vicinity of the sStudy aArea. has revealed that the sediments are classified as The “Classification of Dredged Sediment for Marine Disposal” under Work Bureau Technical Circular (WBTC) No. 3/2000 is summarised in. The sediment quality criteria for the classification of sediment is summarised as shown in Table 4-5. This defines the level of further testing (if any) which needs to be carried out before disposal routes can be defined assuming that disposal of material is required. It should be noted that only if dredging is undertaken will disposal of marine deposits be requiriedd. The direct burying method does not generate waste for off-site disposal which is another reason why it is a favoured construction method..

Table 4-5 Sediment Quality Criteria for the Classification of Sediment

Contaminants	Lower Chemical Exceedance Level (LCEL)	Upper Chemical Exceedance Level (UCEL)
Metals (mg/kg dry wt.)
Cadmium (Cd)	1.5	4
Chromium (Cr)	80	160
Copper (Cu)	65	110
Lead (Pb)	75	100
Mercury (Hg)	0.5	1
Nickel (Ni)*	40	40
Zinc (Zn)	200	270
Metalloid (mg/kg dry wt.)
Arsenic (As)	12	42
Organic-non-PAHs (mg/kg dry wt.)
PCBs	23	180
Organic-PAHs (mg/kg dry wt.)
Low Molecular Weight PAHs	550	3160
High Molecular Weight PAHs	1700	9600

* The contaminant level is considered to have exceeded the UCEL if it is greater than the value shown.

SS5 and SS6 are the two closest sediment monitoring stations to the proposed submarine cable across the Adamasta Channel. In addition, the proposed submarine cable will pass through Cheung Chau typhoon shelter albeit using divers to lay the cables in this final section rather than direct burying or dredging techniques. Therefore, the quality of typhoon shelter sediments at SS7 iswas also taken into account. The routine sediment quality monitoring data at SS5, SS6 and SS7 collected by EPD are summarised in Table 4-6.

Table 4-6 Monitoring Results of Sediment Quality between 1995 and 1999

Parameter	Average Concentrations			Unit SS7
	SS5	SS6	SS7SS6
Cadmium	0 - 0.9	0 - 0.9	0 - 0.90 - 0.9	(mg/kg dry wt.)0 - 0.9
Chromium	0 – 49	0 - 49	0 - 490 - 49	0 - 49
Copper	0 - 54	0 - 54	³650 - 54	³65
Lead	0 - 64	0 - 64	65 - 740 - 64	65 - 74
Mercury	0 - 0.7	0 - 0.7	0 - 0.70 - 0.7	0 - 0.7
Nickel	0 - 34	0 - 34	0 - 340 - 34	0 - 34
Zinc	0 - 140	0 - 140	150 - 1900 - 140	150 - 190
Arsenic	0 - 14	0 - 14	0 - 140 - 14	0 - 14
PCBs	6 - 10	0 - 5	11 - 200 - 5	(mg/kg dry wt.)11 - 20
PAHs	51 - 100	0 - 50	101 - 200

Note: 1. Data presented are in annual medians of monthly samples

2. Shaded cells indicate non-compliance with exceedance of the LCEL.

(Source: Adopted from EPD Marine Water Quality in Hong Kong for 1999)

Based on the above monitoring results, the qualities of the sediment quality is relatively similar for the three locations except for Zn in SS7.samples collected at the marine sediment monitoring locations are similar. The average concentrations of the heavy metals in the marine sediments at SS5 and SS6 fall within the same range while the typhoon shelter sediments at SS7 hasve higher average concentrations of copper, lead and zinc. The results show that the contents of Mercury and Arsenic exceed the Lower Chemical Exceedance Level (LCEL) but complies with the Upper Chemical Exceedance Level (UCEL). Therefore no contamination of sediment is considered.

The average concentrations of polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) are different at the three monitoring stations. The tendency of increasing concentrations is in the sequence of SS6, SS5 and SS7. The high PAHs at Cheung Chau typhoon shelter may be related to the combustion use of of petrol eum ion the fishing boats.

4.4 Sensitive Receivers

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-TM. As required under the Study Brief all water bodies, water and stream courses, groundwater systems and other sensitive or beneficial uses have been identified within a 6km radius from the project sites. The study area of the water quality impact assessment is presented in Figure 4.3. In some cases however the effects of the works cannot be discerned so far from the site and thus these sensitive receivers may be discounted. Potentially sensitive receivers include :include: These include Adamasta Channel, Cheung Chau Typhoon Shelter, Hei Ling Chau Typhoon Shelter, Silver Mine Bay, Cheung Sha Lower Beach, Pui O Beach and other gazetted and non-gazetted beaches within the Study.. y Area.

Monitoring locations within the Study Area are illustrated on Figure 4.23. Monitoring locations for beneficial uses have been identified in accordance with the requirements of the EIA-TM. These include:

· Marine life;

· Mariculture;

· Beaches and other recreational areas;

· Fish spawning grounds and fish culture zones; and

· Areas for navigation/shipping including typhoon shelters, marinas and boat parks.

The project essentially requires cable laying across the Adamasta Channel and the potential impacts relate to elevation of suspended solids in the water column. Taking these factors into account only two of the potentially sensitive receivers within the 6km radius have the potential to be adversely affected during the course of the cable laying works. These are the Cheung Sha Wan Fish Culture Zone (shown as station R1 on Figure 4.4) and Tai Kwai Wan beach on Cheung Chau (shown as R13 on Figure 4.4).

Other potentially sensitive receivers such as Pui O Wan are too far away from the works area and the effects of tidal currents would disperse any sediment plumes before reaching this area and for this reason Pui O Wan has been discounted from any further assessment relating to sediment plume modelling. Similarly the beach at Tung Wan (R7) has been discounted as it is on the east of Cheung Chau whereas the works will take place on the west of the island.

In order to assess the extent of the impacts of sediment release to the water column a series of assessment stations were selected which include the two sensitive receivers as indicated above. Although the results from all other assessment points shown on Figure 4.4 have been included to assist in the interpretation of the data, attention is focussed upon the impacts of the works on the sensitive receivers R1 and R13.

Sensitive receivers Those which could be potentially affected include:

·Adamasta Channel

·Cheung Chau Typhoon Shelter

·Pui O Beach

·Tai Kwai Wan

·Tai Long Wan

·Yi Long Wan

·Sai Wan

·Po Yue Wan

·Pak Tso Wan

·Tung Wan Tsai

Those which arwe outsidewith the area of influence of the works are not considered further and include:

·Cheung Sha Wan Fish Culture Zone

·Fish Fry Nursery Area

·Hei Ling Chau Typhoon Shelter

·Silver Mine Bay

·Cheung Sha Lower Beach

·Tung Wan Chai

·Sai Wan Typhoon Shelter

·Kwun Yam Wan

·Mong Tung Wan

Of the foregoing identified sensitive receivers within the six km radius of the works, some have been identified as being outside the area of influence of the work. Some of the sensitive receivers have not been ascribed specific assessment points. These include the Cheung Sha Wan Fish Culture Zone is outwith the main tidal flows and sediment plumes are expected to disperse before reaching this area. Hei Ling Chau and Sai Wan are typhoon shelters both have impediments to flow and ingress of pollutants due to the breakwaters and Pui O beach was not included as the landmass of the Chi Ma Wan Peninsula and the tidal regime in the area were considered to be impediments to the migration of sediments from the main stream of the Adamasta Channel.

For the water quality impact assessment, 38 monitoring locations for beneficial uses have been selected as part of the assessment process. 27 monitoring locations represent the water sensitive receivers of beneficial uses and the rest of 11 are used for assess ing purposeThe assessment points indicated on Figur. Locations of these WSRs and assessment points are presented in Figure 4.42., with aare included with a brief description in Table 4-.7. A brief description of the WSRs is summarised given in Table 4-7 and 4-8 respectively.

Table 4-7 Description and Coordinates of the Water Quality AssSenstiveessment PointReceivers Monitoring Locations

LocationDescription	Beneficial UseType	Assessment Point	Easting	Northing
Cheung Sha Wan Fish Culture Zone	Sensitive ReceiverFish Cultural Zone	R1*	810 687.330	818 743.944
Tai Kwai Wan	Sensitive Receiver (Non-gazetted Beach)	R13*	808 727.202	820 715.976
Tung Wan	Sensitive Receiver Gazetted Beach	R7*	808 103.527	821 798.480
Nam Tam Wan	Rugged Coast	R8*	806 894.147	821 408.058
Po Yue Wan	Rugged Coast	R24**	806 725.004	819 775.129
Pak Tso Wan	Non-gazetted Beach	R25*	806 385.343	820 077.863
Tai Long Wan	Non-gazetted Beach	R26**	808 794.506	817 867.812
Tai Long Wan	Non-gazetted Beach	R27**	808 789.923	817 679.750
Yi Long Wan	Non-gazetted Beach	R28**	808 374.320	817 346.308
Yi Long Wan	Non-gazetted Beach	R29**	808 478.223	817 170.606
Yi Long Pai	Non-gazetted Beach	R30*	808 262.784	817 069.695
Mong Tung Wan	Non-gazetted Beach	R38*	809 440.312	815 880.115
Cheung Chau Typhoon Shelter	Typhoon Shelter	R18**	808 108.111	820 693.041
		R19**	807 906.423	820 885.690
		R20**	807 584.012	820 871.393
		R21**	807 474.000	820 605.354
		R22**	807 498.292	820 453.979
		R23**	807 194.387	820 265.925
Adamasta Channel	Navigation Fairway	R2*	810 458.139	820 129.182
		R3*	810 052.929	821 064.905
		R4*	809 792.935	821 551.115
		R5*	809 791.652	820 500.719
		R6*	809 559.160	821 055.732
		R9*	809 515.618	820 009.596
		R10*	809 272.676	820 560.022
		R11**	809 067.820	819 542.974
		R12*	808 887.635	820 165.550
		R14**	808 466.217	820 569.500
		R15**	808 648.422	819 138.386
		R16**	808 442.957	819 774.439
		R31**	808 131.778	818 010.005
		R32**	808 035.518	817 505.448
		R33*	807 966.761	817 005.478
		R34*	807 499.947	816 504.704
		R35**	807 687.148	818 339.435
		R36**	807 572.552	817 784.422
		R37*	807 471.708	817 253.170

Note: * Monitoring locationsWater sensitive receivers lie outside the area of 100m away from the submarine cables.

** Monitoring locationsWater sensitive receivers fall within the area100m in the vicinity of the submarine cables.

Table 4.8 Description and Coordinates of the Supplementary Assessment Points

Note: * Assessment Points lie outside the area of 100m away from the submarine cables.

** Assessment Points fall within the area100m in the vicinity of the submarine cables.

The identified sensitive receivers include a fish culturale zone and a fish fery nursery area, nine non-gazetted and on gazetted beaches and the navigation fairway. The existing water quality at these sensitive receivers can be represented by the closest EPD’s monitoring station, SM12, as discussed in the previous section 4.3.2.

Cheung Sha Wan Fish Cultural Zone and the fish fry nursery area located toon the east side of Chi Ma Wan Peninsula and a. They are not directly affected by the proposed installation of submarine cables.

Tai Long Wan, Yi Long Wan, Yi Long Pai and Mong Tung Wan are locateds along the south-west coast of Chi Ma Wan Peninsula while Tai Kwai Wan, Po Yue Wan, Pak Tso Wan, Nam Tam Wan and Tung Wan are on Cheung Chau. Only Tung Wan is a gazetted beach.

The proposed submarine cables are running extend across the Adamasta Channel. As ships and ferries travel through this fairway regularly and transverse the alignment of the submarine cables, the dispersion of the water pollutants may be complicated. It should be noted that the assessment has not taken account of propeller wash as this was deemed to be minor compared to other natural hydrodynamic forces.

4.5 Construction Phase Impacts

4.5.1 Impact Sources

Construction activities include land-based works for the installation of the underground cable. Construction activities may cause adverse impacts on the water quality of the receiving waters due to silt laden runoff, and direct contamination of waters during construction works.

Potential impacts associated with the cable laying activities include the physico-chemical changes to the water column due to the release of suspended solids and the hitherto bound contaminant. s.

The nature and extent of the impacts depend on the quality of the sediments as well as:

· Dispersion characteristics within the receiving waters

· Method of cable laying

· Rate of cable laying

· Proximity and nature of the sensitive receivers

Environmental impacts associated with dredging can be subdivided as follows:

·physical effects : resuspension and redeposition of particles during the dredging operations and changes in physical habitat due to smothering of benthos.

·chemical effects : oxidation of released sediments, potential proliferation of bacteria which feed on resuspended organic matter.

·biological effects : short term alteration in phytoplankton productivity as a result of decreased light penetration or proliferation of some species at the expense of others due to overabundance of nutrients, and adverse impacts of silt and clay particles on the branchiae of fish, and abrasive effects on crustaceans.

·social implications : visual impacts of dredging in terms of the recreational use of waters, damage to commercial fishing grounds or high levels of suspended solids at seawater intake points.

Key issues which were addressed through this assessment include:

(i) the definition of the extent of the potential sediment contaminationplume; and

(ii) determination of the response of the marine environment to the potential release into the water column of trace metals and organic micropollutants from the material being dredged; and

(iii) definition of the mitigation measures which will minimise the impacts of dredging release of sediments to the lowest acceptable level.

The geochemical form of trace metals within sediment samples plays a significant role in determining the potential impacts on the marine environment. Although bulk chemical analyses provides an indication of the total contaminant levels within a sediment sample, the mere presence of a contaminant does not necessarily infer that it will either have an adverse impact on water quality or be available for uptake by aquatic organisms.

Metals may be :

(i) bound tightly within the crystalline lattice structure of the minerals within sediments and are not thus release except through the weathering process. Such metals, including aluminium and magnesium, are usually inert and are thus biologically unavailable;

(ii) bound through a variety of ionic interactions, involving negatively charged surfaces of minerals or large organic molecules and positively charged cations including trace metals. Substances in this form, often termed exchangeable cations, are relatively easily mobilised particularly under acidic conditions; or

(iii) dissolved in interstitial waters (pores) and although this fraction is generally relatively small these contaminants are comparatively easily mobilised and are often available to biota. Contaminants in interstitial waters exist as free ions, in various organic and inorganic complexes, and their concentration is independent of the total contaminant level.

Metals, nutrients and organic materials which are bound into the sediment interstitial waters or adsorbed to the cation exchange complex, are the most mobile and potentially available contaminants in dredged material. Anoxic sediments will frequently contain trace elements which are readily mobilised.

Contaminants, such as PCBs and chlorinated hydrocarbons are more complicated than organo-metals as they are not bound within mineral lattices, they do not occupy positions in clays or form sulphides or other insoluble compounds, nor are they part of the exchangeable fraction. Instead, synthetic organics are almost exclusively found in the adsorbed form, usually associated with dissolved and particulate organic pollutants and carbon in sediments, connected to various organic molecules by Van der Waals forces.

Only a fraction of the PCBs and chlorinated hydrocarbons which are bound to the sediments are available in the interstitial water of sediments, and therefore it may be concluded that only a very small percentage of the overall contaminant load will be bioavailable.

Redox potential (Eh) is one of the most important factors influencing the remobilization of metals from sediments. Anoxic sediments which are characterised by an Eh of -100 mV or less, while well oxygenated waters will, in contrast, have an Eh of >400mV. If anoxic sediments are dredged from or disposed of in well oxygenated waters (i.e. dissolved oxygen levels greater than 4 mg/L), the physio-chemical state of metals within the sediments may be affected and some metals will become more mobile.

In addition to redox potential, the pH of both the sediments and the receiving waters can also affect the availability of some metals with a consequential increase in the amount of metal released initially to the water column.

In addition to the increase in suspended solids to the water column potential impacts also relate to the possible smothering of benthic biota and marine organisms through irritation of the gills or other membranes. A change in the deposition layer may result in the disturbance to the benthic infauna although recent survey data has shown that the rate of reworking of the seabed is much more rapid than previously considered and this should not be such a problem as formerly considered.

Another aspect which has often been considered in assessments of dredging impacts is the change in dissolved oxygen levels due to the release of suspended solids into the water column (during dredging). This is a particular issue if the sediments are heavily contaminated especially with organic matter. In this regard however cognisance should be given to the actual field survey data collected by CLP on another project which involved monitoring of the dissolved oxygen levels during the dredging works.

During construction there is the potential for erosion and sediments to be washed into receiving waters. The potential sources of these impacts and the potential effects have also been assessed.

4.5.2 4.5.2 Dissolved Oxygen Depletion

Another aspect which has often been considered in assessments of dredging impacts is the change in dissolved oxygen levels due to the release of suspended solids into the water column (especially during dredging).

The effects of the resuspension of sediments in terms of dissolved oxygen depletion has been may be estimated using athe standard relationship relating the sediment oxygen demand and daily oxygen uptake rate with concentration of sediment release during dredging. : However such

DO_dep = C x SOD x K x 0.001

where DO_dep = reduction in dissolved oxygen level in mg/l

C = tidal average suspended solids concentration in kg/m³

SOD = sediment oxygen demand in mg/kg sediment

K = daily oxygen uptake rate, 0.23

This Such equations provides very conservative results which need to be subsequently verified during onsite monitoringby collecting sediment samples from the dredging location and conducting laboratory analyses for SOD. It should be stressed that the sediments in the area to be disturbed are not generally dredged would not be expected to be highly polluted and coupled with a relatively low dredging rate, it would be sediment release, it is expected that DO depletion would be negligible and a very transient, situationshort term impact. This has not in this regard however cognisance should be given to the actual field survey data collected by CLP on another project which involved monitoring of the dissolved oxygen levels during the works and confirmed that the level of dissolved oxygen depletion was negligible.

4.5.3 Modelling Assumptions

Hydrodynamic Regime

As tThe reclamation area on formation of the temporary working platforms area to the east side of Pui O beach is small (18055180 m²) and temporary, no and will be formed behind a seawall with no dredging work will be involved. Thus As noted previously, the water quality impact caused by construction of this temporary working platformreclamation area is expected to be minimal and negligible when comparing with the laying of submarine cables. The temporary working platform was not refore, the construction of this reclamation was not included in the modelsin the hydrodynamic models as this facility will be dismantled following excavation of the tunnel.. The tidal flow models used for this assessment were therefore not modified to take account of any potential impediment to flow associated with this project. The model used was that set up under EPD contract No WP00-84 using the local fine grid model with a coastline representing the coastline .for the year 2000.

Sediment Plume Model Assumptions

The dredging modelling scenarios focussed on disturbance due to laying the submarine cables and used have been modelled using DELFT3D-PART with the following parameters values:

· settling velocity = 1.28 x 10^-4 m/s (based on the fact that sandy marine deposits prevail in the Study Area)

· critical shear stress for sedimentation = 0.05 Pa

· critical shear stress for erosion = 0.1 Pa

· vertical dispersion coefficient in wet season = 10^-6 m²/s

· vertical dispersion coefficient in dry season = 10^-3 m²/s

·assuming a release of 400,000 particles to the model

A total of 400,000 particles were The amount quantity of sediment released to the water column by a dredger is commonly represented by the ‘S’ factor, which is expressed in terms of the total quantity of sediment released by the dredger per cubic metre dredged, can be used as a reasonable estimate of the to simulate the quantity of sediment being released resuspended withininto the water column. According to the “Contaminated Spoil Management Study, October 1991, undertaken by MCL (then known as Mott MacDonald Hong Kong Limited), EPD”, the upper limit of the S factor among the various dredge types a dredger, 30% of sediment loss rate of 30% is the upper level of losses to the water column as a result of dredging. This was used in the model to is assumed. This represents a conservative worst case scenarioeee.vv

For the simulation of the direct burying technique it was assumed that 100 % of the sediment would be released to the lowest layer in the water column (as a worst case assumption). This assumption was made is because the direct burying technique essentially ploughs through the seabed creating a trench and laying a cable at the same time. It should be stressed that the material being ploughed is essentially fluidised mud which is not available for dispersion in the water column. However in order to simulate the worst case scenario the 100% release rate was used and the assumption made that the effects would be confined to the bottom layer of the model (ie the lowest 20% of the water columniAlso, it is assumed that 100% of sediment will be resuspended for direct burying. This allows aIt demonstrates the worst scenario to be simulate).

According to the “Contaminated Spoil Management Study, October 1991, undertaken by MCL (then known as Mott MacDonald Hong Kong Limited), EPD”, a sediment loss rate of 30% is the upper level of losses to the water column as a result of dredging. This was used in the model to represent a conservative worst case scenario and does not allow for any mitigation measures such as good operational control or control of the rate of lifting of material or even the effects of using sealed or at least closed grabs. The modelling assumption made was that the sediment would be released over the extent of the water column for the dredging scenario to simulate the worst case effects and also to represent the spillage from grabs during the lifting process.d..

During dredging or other disturbance to the bottom sediments, materials will be resuspended, and in accordance with the foregoing the loss rate this has been assumed to be 30%.

Sediment Release Rates

Sediment release rates were calculated using the following assumptions:

for dredging

· ‘s’ factors taken from the contaminated Spoil Management Study, October 1991 assumed for a bucket type dredger s = 15-30

· assumed release of sediment was over extent of water column to represent worst case

· assumed dredging rate was 547m³/day

· 12 hours working day was assumed

· sandy marine deposits would be dredged and not clay

· dry density of sediment was 488kg/m³

for direct buying

· thewo fastest advancing rates wasere assumed to be 150 m/hr and represented the worst case scenario for the model scenarios to simulate different rates of workassumed in the model although note however an assessment to be made a assumption to be made assumed S=100 (reference to previous report)thereafter the two rates are referred to as ‘slow’ and ‘fast’ direct buryi ieng; 150m/hr and 400m/hr. Although for the purposes of the assessment the two rates were used, in practice the rate of work on-site is closer to 150m/hr and thus the focus of attention for the assessment has been placed on the results of scenarios simulating 150m/hr;

· the “trench” was assumed to be 3m deep and 0.3m wide;

· release of sediment was all assumed to be to the bottom layer of the model (ie 20% of water column).

The direct burying method will cover the distance between the two shorelines in a shorter period of time than the dredging works. (referred to as the ‘fast’ rate). tTmeans infers that multitriple A period of 13 weeks in total has been identified in the progamme given in Section 2 for the direct burying method. The direct burying simulations have been carried out with releases during a spring and a neap tide assuming one cable (or multi-core) laid at one time over a seven day period. The model also assumes releases of sediment at various points across the channel. This approximates the time varying releases into the water column as the DELFT3D-PART was not designed to deal with continuously moving discharge locations.

The progress for dredging is considerably slower with an advance rate of approximately 4 m/h. It will take therefore take about 13-18 weeks, assuming 12 hour working day, which is considerably more than a spring-neap cycle. The dredging scenarios have therefore been carried out for a spring neap cycle.

4.5.4 Assessment Methodology

Once in suspension, fine sediment will be carried by the tidal currents and dispersedd, possibly over a large area depending on tidal conditions and the point of release in the tidal cycles. During transport by the tidal currents, the fine sediment will tend to flocculate forming larger particles which will settle under gravity on the seabed. The rate of settling for cohesive sediments will depend on the concentration and on the local tidal currents. Once the tidal currents become sufficiently weak, the sediment will settle to the seabed and begin to consolidate. If the tidal currents become large enough, the settled material will be eroded and put back into suspension for further transport by the tidal currents where the rate of erosion will depend on the tidal currents and the degree of consolidation which may have taken place.

Sediment release rates were calculated using the following assumptions:

for dredging

·‘s’ factors taken from the contaminated spoil management study, October 1991 assumed for a bucket type dredger s = 15-30

·assumed release of sediment was to upper layer of water column to represent worst case

·assumed dredging rate was 547m³/day

·12 hours working day was assumed

·dry density of sediment was 488kg/m³

for direct buying

·advancing rates of 150 or 400m/hr were used on advise from Contractors as being achievable rates for undertaking this type of work using available equipment

·assumed S=100 (reference to previous report)

·trench assumed to be 3m deep, 0.3m wide

·release of sediment

The direct burying methods cover the distance between the shorelines in a short period. It will take approximately 15 hours to cover the distance of 2300m when using the 150 m/h. Therefore, the direct burying simulations have been carried out with releases during a spring and a neap tide.

The progress for dredging is considerably slower with an advance rate of approximately 4 m/h. It will take therefore about 13-18 weeks, assuming 8 hour working days, which is considerably more than a spring-neap cycle. The dredging scenarios have therefore been carried out for a spring neap cycle.The DELFT3D-PART model is a particle tracking model which releases discrete particles that represent the released sediments. Each released particle in the model represents a certain mass of sediments. Settling and resuspension are included in the model, although its main purpose is to simulate the extent of the sediment plumes. The DELFT3D-PART model is driven by the hydrodynamic model results from a previous study carried out by Delft Hydraulics on behalf of the EPD (Local Fine Grid Model of the North-Western Waters and Western Harbour - WP00-084). The results of the particle tracking model are based on a grid with cells of 50*50m. All runs were carried out for both the wet as well as the dry season periods.

Assessments have been carried out by mathematical models. These are described in details in Section 4.7.

Assessments of the potential impacts associated with the off-site discharges have been addressed using a qualitative approach. Domestic sewage arisings have been considered in terms of potential waste arisings, location and disposal arrangements.

The maximum number of construction workers on-site is estimated to be 50 employees. According to the Sewerage Manual (Part 1), Drainage Services Department, 1995, the flow and loads caused by the construction workforce can be estimated and the results are presented in Table 4-8.

Table 4-8 Estimated Flow and Loads Caused by the Construction Workforce

This estimate of liquid waste arisings has not been included in the model input file as it is not expected to represent a direct discharge to receiving waters. Rather the wastes will be treated prior to disposal.

4.5.5 Flow Regime

The DELFT3D-PART model was set up to represent the coastline which was present at the time of the flow measurements in 20001996. The model validation was carried out for the two representative tide types – spring and neap during the wet season by using the Fine Grid Model (FGM). Graphical presentations of the computational grid and depth schematisation of the FGM are provided in Appendix D. The simulated results of water level, depth-averaged current magnitude, salinity and temperature are validated with update 1997 model results. The graphical plots are also presented in Appendix D.

4.5.6 Scenarios Simulated

For the study of the effects of cable laying between the Chi Ma Wan Peninsula and Cheung Chautowards Cheung Chau, a number of (DELFT3D-PART) simulations of the sediment plumes plumes that will be generated were carried out.

The sediment plume model has been used to simulate the following scenarios which are based on the two previously definedfollowing cable laying methods. Sediment and associated release rates for each scenario have been calculated as follows:

· Direct burying at 150m/hr, with a slow speed of 150 m/h, sediment release rate 24.4 kg/s

Iie 150 m per hour x 0.4 m wide x 3 m deep trench = 180 m³ per hour

180/3600 m³/s x 488 kg/m³ = 24.4kg/s (removal)

·Direct burying at 400m/hrwith a fast speed of 400 m/h, sediment release rate 65.1 kg/s

Ie 400 m per hour x 0.4 m wide x 3 m deep trench = 480 m³ per hour

480/3600 m³/s x 488 kg/m³ = 65.1kg/s (removal)

· Dredging with a single trench, sediment release rate 2.8 kg/s

Iie Volume of sediment being disturbed during dredging = 17.1m² x 4m per hour

= 68.4 m³ per hour

Spill rate = 68.4/3600 m³/s x 488kg/m³ x 0.3 = 2.8 kg/s

· Dredging with three trenches, sediment release rate 8.34 kg/s

Iie Volume of sediment being disturbed during dredging = 17.1m² x 4m per hour x 3 = 205.2 m³ per hour

Spill rate = 205.2/3600 m³/s x 488kg/m³ x 0.3 = 8.3 kg/s

Sediment release rates were calculated using the following assumptions:

for dredging

·‘s’ factors taken from the contaminated spoil management study, October 1991 assumed for a bucket type dredger s = 15-30

·assumed release of sediment was to upper layer of water column to represent worst case

·assumed dredging rate was 547m³/day

·12 hours working day was assumed

·dry density of sediment was 488kg/m³

for direct buying

·advancing rates of 150 or 400m/hr were used on advise from Contractors as being achievable rates

·assumed S=100 (reference to previous report)

·trench assumed to be 3m deep, 0.3m wide

·release of sediment

The direct burying methods cover the distance between the shorelines in a short period. It will take approximately 15 hours to cover the distance of 2300m when using the 150 m/h. Therefore, the direct burying simulations have been carried out with releases during a spring and a neap tide.

The progress for dredging is considerably slower with an advance rate of approximately 4 m/h. It will take therefore about 13-18 weeks, assuming 8 hour working days, which is considerably more than a spring-neap cycle. The dredging scenarios have therefore been carried out for a spring neap cycle.

The PART model is a particle tracking model which releases discrete particles that represent the released sediments. Each released particle in the model represents a certain mass of sediments. Settling and resuspension are included in the model, although its main purpose is to simulate the extent of the sediment plumes. The PART model is driven by the hydrodynamic model results from a previous study carried out by Delft Hydraulics on behalf of the EPD (Local Fine Grid Model of the North-Western Waters and Western Harbour - WP00-084). The results of the particle tracking model are based on a grid with cells of 50*50m. All runs were carried out for both the wet as well as the dry season periods.

The following eighttwelve scenarios were investigated:

· direct burying, dry season, neap tide @ 150 m/hslow speed150m/hr

· direct burying, dry season, spring tide @ 150m/hrslow speed150 m/h

· direct burying, wet season, neap tide @ 150m/hrslow speed150 m/h

· direct burying, wet season, spring tide @ 150m/hrslow speed150 m/h

· direct burying, dry season, neap tide @ 400m/hr400 m/hfast speed

· direct burying, dry season, spring tide @ 400m/hrfast speed400 m/h

· direct burying, wet season, neap tide @ 400m/hrfast speed400 m/h

· direct burying, wet season, spring tide @ 400m/hr400 m/h

· dredging, single trench, dry season

· dredging, single trench, wet season

· dredging, three trenches, dry season

· dredging, three trenches, wet season

In each scenario, the operation is assumed to start at Cheung Chau and the location of the installation activities then moves during the simulation towards Tai Long Wan. Only one cable (or multitriple-core) is assumred to be laid at any given time. Results of the simulations are presented as numerical values, colour contour plots which are snapshots of concentrations are a given time and time histories which illustrate the concentration of suspended solids at a defined location over time. All these data are contained in Appendix E with and the results of the itme histories given in Appendix E indicate the length of time for the status quo to be restored following a peak concentration.a summary of the peak concentrations predicted at the sensitive receivers given in Appendix F. A summary of the peak concentrations is given in Tables 4-.8 and 4-.9 for direct burying and dredging respectively.

For the direct burying method, each cale (or triple-core ie 3 ‘strands’) being laid at 150 metres per hour day will take 15 hours days and 400 metres per hour will take 6 hours daysfor the operation to traverse the 2300 metres of the Adamasta Channel. The operators of the cable laying vessels indicate that the faster rate is around 450m/day. However the assumption made for the model was that if laying the cable of say 10x the usual speed was acceptable, then the slower speed would also be acce

ptable.

The specialists who built the model also confirm that the total amount of sediments released during the direct burying simulations is the same as derived during the study. This is because it directly related to the size of the trench, density of the sediments and loss rate of the method. If the advance rate is 24 times slower than used in the study, and the total of released sediments remains as it is, then the release rates for these runs would be 1/24th of what was prviouslypreviouslyused, over a period that is 24 times longer. These are considerable differences with what was used in the study. What will be clear is that the peak concentrations as indicated in the plots that were submitted at the time will overestimate the concentrations. They can probably be scaled by a factor of 24, although this will not be exact because there are some non-linear effects for which scaling would not be valid. Scaling the peaks of the time-histories will give you an indication of the increases in concentrationse . However, because of the essentially different nature of the conditions, and the much longer duration of the operation, the approach of the modelling should look very similar to the dredging scenarios, which also cover a longer period.

These spreading of these sediment releases plumes will bewill be affected by the time of ir timing in release into the tidal cycle. For the spring tide simulations, the cable laying has been assumed to start a few hours before low-low-water spring. For the neap tide simulations, the operation has been assumed to start a few hours after low-low-water neap. The simulations for direct burying last for 7 days.

For Using the traditional dredging techniques at a rate of 4 metres per hour, it will take a period of 575 hours will be required to traverse the Adamasta Channel. Since, the total dredging operation will extend well over several spring/neap tidal cycles (~ 15 days), it make little does not make much difference when the dredging starts in the tidal cycle. All of these simulations thus start at low-low-water spring and continue for 36 days (12 hours/day, 6 days of dredging followed by 1 day of rest).

Although direct burying is the preferred option for installing the submarine cables, the actual installation method employed will depend on the Contractor’s choice. In addition, a normal advancing rate in the range between 400 and 450 m/day will be applied for direct burying method. Therefore, the modelling results demonstrate the worst scenario for direct burying.

Due to the generic nature of the simulations and for the purposes of the presentation of the output time series, the starting date and time of the dredging has been made equivalent to 01 January, at 00:00:00h. The contour plot outputs (as shown in Appendix E) have been presented in terms of the number of hours or days that have elapsed since the start of the dredging.

The resulting sediment concentrations calculated by DELFT3D-PART represent only the increase in sediment concentrations above the background concentrations already present. DELFT3D-PART does not calculate the background concentrations. The background concentrations have been assumed to be average values using the data provided by EPD in their published routine water quality monitoring data as described in t ehe foregoing sections. This allows the acceptability or otherwise of the marine works to be assessed using the WQO as the criteria.

4.5.7 Evaluation of Impact

Graphical Presentations and Contour Plots

Graphical presentations and the contour plots of the depth average concentrations of sediments for comparing the results between extreme cases (such as dredging 1 trench and 3 trenches on the same plot, two speeds for direct burying ie “150 m/h slow” and “400 m/hfast on the same plot, etc.) are provided in Appendix E. General observations are that the direct burying method generates a more elongated and distinctive plume compared to the dredging .

The main features of the scenarios that will determine the elevated suspended solids concentration is the release rate and the release period. The direct burying methods produce significantly higher release rates, but for shorter periods than compared to the dredging method. The results of the dredging simulations also show that when dredging three trenches simultaneously the results are of the order of three times that for dredging one trench. The difference between the methods is directly visible in the results of the simulations. When examining the time-histories of the model results for the monitoring locations it can be noted that the maximum concentrations of the direct burying (at 150)) m/h) and the dredging scenario have the same order of magnitude. with the direct burying being more defined as a “plume”. The one trench approach shows concentrations at about one third of the three trench scenarios. whilst the slow150 m/h direct burying results in higher concentrations than the fast400 m/h rate of advance.

The highest concentrations tend to be observed within the typhoon shelter of Cheung Chau Wan, which is a semi-enclosed waterbody with relatively little exchange with surrounding waters. This over-exaggerates the potential impacts, because, as indicated in Section 2 the work carried out in the typhoon shelter will be by hand, by divers and thus the impacts will be significantly less than predicted by the model.

Here the difference between the direct burying and the dredging is immediately visible with significantly higher concentrations for the direct burying methods.

The sediment plumes from the direct

Although the sediment plumes for the direct burying method appear to be burying are well defined plumes, which extend for several kilometers, in particular for the spring tide, in which the concentrations are simulated to be between 10 and 50 mg/L for the fast rate of direct burying for a few hours after commencement of the dredging.and show high concentrations (10-50mg/l) at the centre of the plume, the plumes do not impinge on any of the defined sensitive receivers, and noNo exceedancess of the WQO’s are observed aAt the Cheung Sha Wan Fish Culture Zone (R1) and concentrations of around 1 mg/l were predicted on the spring tide during the wet season, while at R13 (Tai Kwai Wan (R13)were likely to be observedconcentrations of around 5mg/l were predicted on the spring tide of the dry season. The results need to be further examined to determine whether they exceed the WQO’s and reference is made to the peak values contained in Table 4.8. This peak concentration only lasts about four hours for condition simulated and affects in general the Adamasta Channel and the non-gazetted beach at Tai Long Wan. The concentrations drop significantly once the activities are complete,. which, for each cable, takes about 15 hours (150 m/h). In about 5-6 hours after cessation of activities, the concentrations in the plume have reduced to significantly less than 5 mg/L and from the time histories it is apparent that restoration to comply with WQO generally takes 2-4 hours. An exception is the station at Po Yue Wan which is rugged coastline and not a water sensitive receiver where the duration for restoration is slightly longer (except for the wet season spring tide scenario). For the spring tide this will last somewhat longer due to the higher current velocities. The plumes for the spring tide cover a larger area and are more elongated than for the neap tide which is a reflection of the tidal excursions and defines the extent of the impacts in the shorte. term.

It has been demonstrated by the models that by application of the direct burying method significantly more sediments could be resuspended compared to dredging, therefore leading to significantly higher concentrations near the activity with a sediment plume of the order of 1 km or less in which concentrations may exceed about 50 mg/L as peak values which only last 2-4 hours. The extent of the plume with concentrations between 10-15 mg/L can be several km in length and confined to the Adamasta Channel and Tai Long Wan non-gazetted beach. No fish culture zones or fish fry areas are adversely affected.

The plumes ofgenerated as a result of the dredging are less well defined, with significantly

lower concentrations in the vicinity of the dredging activities than for the direct burying. This is due to the assumed release rate. Concentrations exceeding 10 mg/L only occur occasionally and for a few stations and for relatively short periods. Examples of such behaviour are the monitoring locations nearest the dredging activities (R11, R12, R15 and R16). These monitoring locations are all located within all represent the the Adamasta Channel Some potential excedances are evident during the wet season at Tai Kwia Wan and as a result further investigation of the actual peak values is required for which reference is made to Table 4.9..

Numerical Data

According to the results, it was noted that in some of the contour plots and time series plots, sudden peaks in the suspended sediment concentration could be are observed at various locations away from the actual dredging location. This effect is due to the resuspension of sediment particles that have settled at these locations at earlier times. When the shear stress at a given location exceeds the critical shear stress for erosion, the resuspension routine in DELFT3D-PART causes all of the settled particles to instantaneously go back into suspension at this location. Subsequently, the resulting concentrations probably represent an over-estimation of the actual values that could be expected at these locations particularly in the lower layers, but indeed affecting the overall depth-averaged concentrations.

It has been demonstrated by the models that by application of the direct burying method significantly more sediments could be resuspended compared to dredging, and therefore leads to significantly higher concentrations near the activity with a sediment plume of the order of 1 km or less in which concentrations may exceed about 50 mg/L. The extent of the plume with concentrations between 10-15 mg/L can be several km in length. However, tThe effects of direct burying are visible for significantly shorter periods than dredging. The plumes from the dredging method show general concentrations increases concentrations ofin between 5-10 mg/L over an area of the order of 1-2 km (wet season, 1 trench), but last for the entire dredging period, which will be in the order of 3 months.

Modelling ResultsNumerical Data

The results of the Ttwelve scenarios of the water quality assessment for laying the submarine cables for the Project have been predicted. Details of the predicted modelling results are given in Appendix F with the average concentration of .

The 90 percentiles of suspended solids at the representative monitoring locations for direct burying and dredging are summarised in Tables 4-9 & 4-10 and Tables 4-110 & 4-12, respectively. These results present the impacts predicted for the release into the water column of a predefined volume of sediment (ie 400,000 particles) using the direct burying technique over a seven day periods4-8… Table 4-9…

.

TAs indicated earlier in this section the WQO’s for SS requires that thatany elevation must be less than 30% above the natural ambient level which in this case has been taken to be . For subzones the annual median is used. Following a review of the contour plots the raw data were used to determine whether the elevation of SS is acceptable due to direct burying. First the average values of suspended solids, over the seven day period (as the duration of the works is around 6 days and the models were run for 7 day cycles), were related to the increment “allowed” in accordance with the WQO (3.9mg/l). Secondly the time history plots were interpreted to determine the extent and duration of the peak concentrations and to identify whether, using this and the contour plots together, there would be breaches of WQO’s at the sensitive receivers.

Table 4-9 Summary of Average Concentration 90%-ile of Suspended Solids at the Representative Sensitive ReceiversMonitoring Locations (Direct Burying)

(All units in mg/L unless otherwise stated; all figures are upper limits unless otherwise indicated)

Note: Boldfaced value represents the elevated level of suspended solid exceeding the WQO.

Table 4-10 Summary of Average Concentration Suspended Solids at the Representative Assessment Points (Direct Burying)

By comparing the predicted results in Table 4-9 with the maximum allowable elevated level (3.9 mg/L), the depth average concentration of the suspended solids over the seven days cycles at all the representative monitoring locations except for R24 comply with the relevant WQO. However Exceedances of the elevated SS occur at R24 (Po Yue Wan) for both advancing rates (150 and 400 m/h) during the wet season only. Since it is a rugged coast but not a gazetted beach, it is not a water sensitive receiver. Ppeak values may be observed which exceed the WQO’s (in Appendix F) but these are short duration (~ 2-4 hours) and do not directly affect water sensitive receivers.

It should also be noted that the model assumed 100% release of sediment to the water column, which is conservative and thus the results predicted are more severe than would be expected even under the worst case scenario. Therefore, the water quality impact is expected to be acceptable. Moreover, with the implementation of general mitigation measures and pollution prevention measures, the water quality impact caused by the direct burying method will be minimal.

It is observed that the elevated SS levels at the Cheung Sha Wan Fishery Culturale Zone and fish fry nursery area are predicted to be in the range between 0.0 to 0.57 mg/L so the water quality impacts at these sensitive receivers are may be surprised surmised to be minimal for both advancing rates of work..

The highest average elevated SS levels at Tung Wan (a gazetted beach) are only 0.1 and 0.2 mg/L for “slow” and “fast” modelling rates respectively. Therefore, the water quality impact is expected to be minimal. Some differences occur in the predicted peak values of the “slow” and “fast” rates which suggest that the model is exhibiting the effects of longer release times in a grid cell therefore the peak is more exaggerated in some cases for the “slow” rate of burying combined with flow factors and re-suspension rates.

For all other non-gazetted beaches in Table 4-10, the highest predicted average elevated SS levels are 1.9 and 2.2 mg/L for “slow” and “fast” advancing rates so the water quality impacts are acceptable.

Anomalies in the results predicted for the For the Cheung Chau typhoon shelter, the predicted results show that the advancing rate of 150 m/h generated higher elevated SS levels than that that 400 m/h ddid. However, the range of the results is between 0.0 and 30.8 mg/L so the water quality impact at the typhoon shelter is also falls within the requirements of WQOminimal. The reason is probably due to the complex hydrodynamic regime in the vicinity of the breakwater and the influence of resuspension during the slow advancing rate.

The predicted SS levels at the gazetted beach, Tung Wan, are well below the acceptable levels and the predicted results at all other monitoring locations along the coast except for Po Yue Wan are well within the WQO requirement.

The predicted elevated SS levels at the Adamasta Channel vary between 0.0 and 1.54 mg/L for “slow” advancing rate of 150 m/h and between 0.0 and 2.01.9 mg/L for the “fast” advancing rate 400 m/h. No exceedance of elevated SS levels is expected.

For all other non-gazetted beaches in Table 4-10, the highest predicted average elevated SS levels are 1.9 and 2.2 mg/L for “slow” and “fast” advancing rates so the water quality impacts are acceptable.

Table 4-10 Summary of Average Concentration 90%-ile of Suspended Solids at the Representative Monitoring Locations (Dredging)

(All units in mg/L unless otherwise stated; all figures are upper limits unless otherwise indicated)

Note: Boldfaced value represents the elevated level of suspended solid exceeding the WQO.

Table 4-12 Summary of Average Concentration Suspended Solids at the Representative Assessment Points (Dredging)

(All units in mg/L unless otherwise stated; all figures are upper limits unless otherwise indicated)

In Table 4-10, it is observed that no exceedances of SS occur at any of the monitoring locations for dredging one trench. However, there are predicted exceedances at Cheung Chau typhoon shelter (R22 and R23), Po Yue Wan (R24) and Adamasta Channel (R11, R15 and R16) for dredging three trenches simultaneously during the wet season. The exceedances are only slightly above the maximum allowable elevated level (3.9 mg/L) based on background data which needs to be confirmed by baseline monitoring before marine works commencing on this contract so the water quality impacts are not substantially adverse. On this basis, general mitigation measures and pollution prevention measures are thus would be recommended in order to reduce the SS level complying with the WQO requirement. Should dredging three trenches simultaneously be the option selected by the contractor.

· Direct Burying

Peak concentrations of SS predicted at the sensitive receivers for each tide and season are given in Table 4-.8 from which it can be observed that full compliance with the WQO’s can be achieved regardless of the time of year the direct burying works take place. Interpretation of the data given in the time histories which illustrates the peak concentrations, the duration and significance in relation to impacts on water sensitive receivers are discussed in the following paragraphs.

Using the raw data given in the time histories contained in Appendix F, it is evident that the peak values occur mostly in the Adamasta Channel (R9, R10, R15, R32, R36 and R37) with a duration of generally between 2 and 4 hours although up to 8 hours has been simulated.

Predicted peak values in Cheung Chau Typhoon Shelter can be discounted because although the model simulated the effects of direct burying in this typhoon shelter the work will be carried out by hand by divers which will be significantly less disruptive to the seabed conditions.

At R24 (Po Yue Wan) two peak exceedances of up to 24 mg/l for 2 hours and 14 mg/l for up to 2 hours were predicted. This is not a water sensitive receiver but rather a rugged coastline. No exceedances of SS were predicted at the sensitive receivers and with the exception of the predicted elevations at Po Yue Wan and Pak Tso Wan the effects of direct burying are confined to the works area in the Adamasta Channel.

However, Aas noted earlier in t hhis section, the actual rates for direct burying are much slower than those simulated in the model. Taking into account the various factors which affect the levels of suspended sediments in the water coulumn it has been surmised that the actual if the rate of work is slower than that simulated is the models, the actual average concentrations of suspended solids would comply with the WQO’s (as the volume of sediment released is the same, albeit the sediment is released over the same time but at a slaver rate). The peak concentrations can be divided by however could be divided by a factor which represents the slower rate of work ie 6 days rather than say 6 hours.

Although the results are not exactly linear, by decreasing the rate by 1/24th, the peak results would be generally more representative of those for a slaver rate of work. The model over-predicts the rate of re-suspension (apparent on slower rates) and as such the submission of the peak values by say x 20 to get the revised maximum concentrations arising from the direct burying method would still be expected to be conservative. The focus of attention has been placed on the “Fast” rate of direct burying as this is around 20 times the actual speed of the works to be undertaken. The results given in Table 4.11 indicate that no

Even if the average concentrations of suspended sediments of the unadjusted sediment concentrations are considered, as given in Tables … and … are used then it may be seen that the re are no exceedances of the WQO’s (assuming 3.9mg/l is the compliance threshold).

No exceedances are predicted at the sensitive receivers even at these peak concentrations..

Table 4-8 Summary of Peak Concentration Suspended Solids at the Representative Sensitive Receivers (Direct Burying)

Direct Burying		Advancing Rate = 150 m/hr (Worst Case Scenario)
Sensitive Receiver	Assessment Point	Dry Season		Wet Season
		Neap	Spring	Neap	Spring
Cheung Sha Wan Fish Culture Zone	R1	0.0	0.0	0.0	0.5
Tai Kwai Wan	R13	1.0	3.9	0.0	0.5

The predicted peak elevated SS levels at Cheung Chau typhoon shelter are in the range between 0.0 and 7.2 mg/L for “slow” advancing rate and between 0.0 and 14.8 mg/L for “fast” rate. As stated previously, the submarine cables within the typhoon shelter will be laid by divers manually instead of using direct burying machine. Therefore, the predicted water quality impact will be greatly reduced and the exceedances at the typhoon shelter can be discounted.

The predicted peak elevated SS levels in the Adamasta Channel are in the range of 0.0 and 8.2 mg/L for “slow” advancing rate and between 0.0 and 19.9 mg/L for the “fast” rate. Generally Eexceedances of elevated SS levels only occur for a short period (~2-4 hours) and as the average results demonstrate compliance then water quality can be generally assumed to be acceptable.

In Table 4-14, no exceedances were identified at any of the non-gazetted beaches. The only exceedance in the peak concentration is at Po Yue Wan, which is rugged coast and is not a sensitive receiver. In addition to which the average SS concentration complies with the WQO at this location.

It should also be noted that in addition to the fact the results represent the worst case it is not believed that 100% of sediment would be put into suspension because the sediments which are finalised and at the bottom on the trench do not enter the water column and are not available for dispersion off-site.

Notwithstanding this, on the basis the aim is to protect water quality and marine life, it is proposed that a ‘rest’ period of contour be used between finishing the burying operation (by boat) and commencing the next cable laying operation. The work done by divers (by hand) in the Cheung Chau typhoon shelter should be exempted from this vest period as there will be minimal disturbance to the seabed during hand operations. This rest period has been assumed on the basis of the time taken for sediments to resettle in the Adamasta Channel (refer to Appendix F). The time taken for material to settle, as predicted by the model, is generally around 2-4 hours but as some longer periods of peak concentrations are simulated it is considered that 6 hours would be an appropriate rest period noting that in many cases it is apparent that the settling rate is much faster.

· Dredging

The predicted peak concentrations of suspended solids for dredging are illustrated in Table 4-.9.

The dredging method will not be used for laying the entire cable, but rather will be used only at the ends of the cable where the water is too shallow to use the direct burying technique.

Where a single trench is dredged the model predictions indicate full compliance with the WQO’s (ie over the year) at the Cheung Sha Wan Fish Culture Zone ( R1). At Tai Kwai Wan beach (R13) the dredging works can be carried out without any non-compliance during the dry season, but exceed the WQO’s if dredging is carried out during the wet season.

No mitigation is therefore required if dredging takes place during the dry season assuming only a single trench is dredged.

Where dredging is proposed to take place for a single trench during the wet season the reduction in suspended solids concentrations needs to be around 55% to ensure compliance with the WQO at the Tai Kwai Wan beach.

In the event that dredging is carried out for three trenches simultaneously, then according to the model predictions breaches of the WQO’s may be expected during both wet and dry seasons at R13 (Tai Kwai Wan). At the Cheung Sha Wan Fish Culture Zone (R1) the model predicts compliance during the dry season but excedance of the WQO’s during the wet season.

The most effective mitigation measures would like be to limit the dredging in dry season if the works programme allows.

For the situation where three trenches are proposed to be dredged simultaneously, a reduction of 30% of the suspended solids concentration is required during the dry season. During the wet season the mitigation measures need to effect a reduction of 85% (ie 21.6mg/l) to ensure full compliance with the WQO’s at the sensitive receivers.

No It is found that all Only minor ’s, and these all take place in the wet season. The current programme as shown in Section 2 is to undertake the dredging works during the dry season therefore no problems are anticipated. Should the Contractor decide to alter his programme to undertake dredging during the dry season then mitiation measures would be required. As the modelling scenario assumes no mitigation then it can be surmised that with the application of the mitigation measures provided in Section 4.6 the impacts would eassily be reduced by half. This would ensure compliance. It should furthermore be noted that prior to consrtuction the predicted exceedances are taking place during the wet season. One way to avoid potential adverse water quality impacts is to install the submarine cables during the dry season only. However, it may affect the construction programme. Alternatively, the baseline at the wet season can be used to determine the WQO compliance threshold. This may allow a higher ceiling for the elevated levels. In fact, a baseline water quality monitoring will be carried out prior to the commencement of the construction. This is more useful to reflect the conditions of the marine environment. and thus the works could be carried out either in the wet or dry .season should the contractor decide to adopt this method.

Table 4-9 Summary of AveragePeak Concentration Suspended Solids at the Representative Sensitive Receivers (Dredging)

(All units in mg/L unless otherwise stated; all figures are upper limits unless otherwise indicated)

Dredging		Single Trench		Three Trenches
Beneficial Use	Monitoring Station	Dry	Wet	Dry	Wet
Cheung Sha Wan Fish Culture Zone	R1	0.8	2.3	2.4	6.8
Tai Kwai Wan	R13	1.8	8.5	5.4	25.5

Note: Boldfaced value represents the elevated level of suspended solid exceeding the WQO.

Table 4-10 Summary of Peak Concentration Suspended Solids at the Representative Sensitive Receivers (Dredging with Mitigation Measures)

(All units in mg/L unless otherwise stated; all figures are upper limits unless otherwise indicated)

In addition, it is anticipated that the conventional advancing rate for direct burying method will be much slower (say 400 m/day) than what has been assumed in the models. The predicted modelling results are based on an accelerated advance rate which is ten times faster than the usual rate of work which shows no exceedances of SS levels at the water sensitive receivers.

4.6 Mitigation Measures

Direct Burying

Interpretation of the model results indicates that no mitigation measures are required for direct burying regardless of the tide and season. Full compliance with the WQO’s at the sensitive receivers are predicted.

Dredging

It is recognised that although the majority of the cables will be laid by direct burying, at either end minor dredging works may be required as the shallow depth of water is unsuitable for the direct burying machine to operate. Such minor dredging works would best be carried out during the dry season with a single cable trench being dredged at any given time to ensure compliance with the WQO’s.

In the event that dredging takes place during the wet season then additional mitigation measures to be adopted include the use of closed and sealed grabs. This will reduce the release of sediment by at least 50% and coupled with a controlled rate of lifting (can be up to 50% reduction in suspended solids release) of the dredged material will be adequate to ensure compliance with the WQO’s will be achieved at the sensitive receivers.

In the event that dredging three trenches simultaneously is considered the use of closed and sealed grabs will be adequate to achieve compliance with the WQO’s at the sensitive receivers during the dry season. During the wet season the combined suite of mitigation measures such as closed and sealed grabs, controlled lifting rate and a reduction in the rate of dredging (by 40%) will be needed to reduce the release of suspended solids to enable compliance with the WQO’s.

IIn Summary

The effects of applying mitigation measures to dredging activities are shown in Table 4-.10 which illustrates that compliance with the WQO’s can be achieved for both wet and dry seasons.

Simulated worst possible cases to allow effect of mitigation (if required) were assessed. The water qualityMitigation assessment has concluded that mitigation measures may also be required to minimise the effects of construction on receiving water quality. These include the control of direct burying/dredging cable trenches, the through the control of control of surface runoff from sites, during construction and the provision of appropriate collection, treatment and disposal facilities for the wastes (liquid and solid) which are generated during the implementation of the works.

It should also be noted that the extent of mitigation measures will vary depending upon the nature of the contract. In order to provide a cost effective package of mitigation measures which can be included in the environmental requirements of each contract, a suite of general measures have been proposed for the protection of receiving water quality.

Table 4-10 Summary of Peak Concentration Suspended Solids at the Representative Sensitive Receivers (Dredging with Mitigation Measures)

(All units in mg/L unless otherwise stated; all figures are upper limits unless otherwise indicated)

Dredging		Single Trench		Three Trenches
Beneficial Use	Monitoring Station	Dry	Wet	Dry	Wet
Cheung Sha Wan Fish Culture Zone	R1	-	-	-	3.4
Tai Kwai Wan	R13	-	2.1	2.7	3.9

Remarks: -‘-’ complies with WQO without mitigation measures being applied.

General Mitigation Measures

· All waste water generated on the Site shall be collected, and rremoved from Site via a suitable and properly designed temporary drainage system and disposed of at a location and in a manner that will cause neither pollution nor nuisance.

· The Contractor shall construct, maintain, remove and reinstate, as necessary, temporary drainage works and take all other precautions necessary for the avoidance of damage by flooding and silt washed down from the Works. He shall also provide adequate precautions to ensure that no spoil or debris of any kind is allowed to be pushed, washed down, fall or be deposited on land or on the seabed adjacent to the Site.

· The Contractor shall not permit any sewage, waste water or other effluent containing sand, cement, silt or any other suspended or dissolved material to flow from the Site onto any adjoining land or allow any solid waste to be deposited anywhere within the Site or onto any adjoining land and shall have all such materials removed from the Site.

· The Contractor shall be responsible for temporary drainage, diverting or conducting of open streams or drains intercepted by any works and for reinstating these to their original courses on completion of the Works.

· Any proposed temporary diversions to stream courses or nullahs shall be submitted to the Engineer for agreement one month prior to such diversion works being commenced. Diversions shall be constructed to allow the water flow to discharge without overflow, erosion or washout. The area through which the temporary diversion runs is to be reinstated to its original condition when the temporary diversion is no longer required.

· The Contractor shall not discharge directly or indirectly (by runoff) or cause or permit to be discharged into any public sewer, storm-water drain, channel, stream-course or sea, any effluent or foul or contaminated water or cooling water without the prior consent of the relevant Authority who may require the Contractor to provide, operate and maintain at the Contractor's own expense, within the premises or otherwise, suitable works for the treatment and disposal of such effluent or foul or contaminated or cooling or hot water.

· The Contractor shall at all times ensure that all existing stream courses and drains within, and adjacent to the Site are kept safe and free from any debris and any excavated materials arising from the Works. The Contractor shall ensure that chemicals and concrete agitator washings are not deposited in watercourses.

· All Contractor's Equipment shall be designed and maintained to minimise the risk of silt and other contaminants being released into the water column or deposited in other than designated locations.

General Pollution Prevention Measures to be Adopted During Dredging

mechanical grabs shall be designed and maintained to avoid spillage and shall seal tightly while being lifted; all vessels shall be sized such that adequate clearance is maintained between vessels and the sea bed at all states of the tide to ensure that undue turbidity is not generated by turbulence from vessel movement or propeller wash. ;

· Dredging rate should not exceed 573cu.m/day

· use of closed and sealed grabs to be used to reduce sediment release rate by at least 50%;

· mechanical grabs shall be designed and maintained to avoid spillage and shall seal tightly while being lifted; all vessels shall be sized such that adequate clearance is maintained between vessels and the sea bed at all states of the tide to ensure that undue turbidity is not generated by turbulence from vessel movement or propeller wash;

· .d Dredging rate must be lower than 54784 m³/day during dry season

· dredging rate must be lower than 3218wet season and shall not exceed 1094 m³/day during wetdry season (if three trenches dredged simultaneously);;

· dredging should preferably be carried out during the dry season. In the event that simultaneous dredging of three trenches is proposed then the sediment losses to the water column must be reduced by 40%;

Dredging should preferably be carried out during the dry season. In the event that simultaneous dredging of three trenches is proposed then the sediment losses to the water column must be reduced by 40%If rescheduled to be carried out during the wet season then mitigation measures may need to be put in place to reduce the rate of release of sediment to the water column by 50%

· the marine works shall cause no visible foam, oil, grease, scum, litter or other objectionable matter to be present on the water within the Site or dumping grounds;

· all barges shall be fitted with tight fitting seals to their bottom openings to prevent leakage of material;

· excess material shall be cleaned from the decks and exposed fittings of barges before the vessel is moved;

· loading of barges and hoppers shall be controlled to prevent splashing of dredged material to the surrounding water and barges or hoppers shall not be filled to a level which will cause overflowing of material or polluted water during loading or transportation;

;

loading of barges and hoppers shall be controlled to prevent splashing of dredged material to the surrounding water and barges or hoppers shall not be filled to a level which will cause overflowing of material or polluted water during loading or transportation;

· the Engineer may monitor any or all vessels transporting material to ensure that no dumping outside the approved location takes place. The Contractor shall provide all reasonable assistance to the Engineer for this purpose;

· all vessels used for marine works must be currently registered as such with the marine department; and

· water quality monitoring shall be carried out by the Contractor during the dredging. Requirements and extent of monitoring will be agreed with DEP and the Engineer.

Specifically: In addition to the foregoing general protection measures the following control measures for dredging may also apply in the event that dredging of three trenches is proposedwhen exceedances of SS levels at the monitoring locations are identified:

· use of closed and sealed grabs to reduce release rate by at least 50%;

· if programme permits schedule dredging works during dry season;

· control release of suspended sediments to achieve 50% reduction; anduse of closed and sealed grabs to reduce release rate by at least 50%;

· dredging works should be scheduled to take place during the dry season wherever possible; and

· dredging rate should be lower than 32218 m³/day during wet season.

dredging rate should be lower than 218down 84achieve 50% reduction. and and

·dredging one trench at a time.

Specifically; if dredging one trench at a time is proposed then the following control measures are proposed:

·use of closed and sealed grabs to reduce sediment release rate by at least 50%;

·the dredger or when dredging works during wet seasone.;

if programme permits schedule dredging works during dry season..

General Pollution Prevention Measures to be Applied During Direct Burying

· other than exerting due care when carrying out the works, direct burying can be carried out without any special mitigation measures being required to protect sensitive receivers regardless of the tide or season.

·other than exerting due care when carrying out the works, direct burying can be carried out without any special mitigation measures being required to protect sensitive receivers regardless of the tide or season.direct burying techniques should be carried out with a result period of 6 hours between completion of one cable and laying the next.

·Rr

·between completing one direct burying activity and commencing the next a rest period of 6 hours should be observed to allow the sediment levels to reduce to pre-works levels.

in the event that direct burying techniques generate suspended solids levels greater than 30% above ambient (to be defined in the baseline survey Ll) consideration may need to be given to working within a shield over the face of the direct burying machine as illustrated on Figure _____.. This is a proprietary piece of equipment which is essentially a hood placed over the face of the works, it suppresses the release of suspended solids into the water column by acting as a mobile “barrier” between the seabed and water column above.

General Pollution Prevention Measures to be Applied During Placing of Cables in Cheung Chau Typhoon Shelter

·use of silt screens may be required, either side of the cable laying iof the placing of cables causes elevation in suspended solids in excess of 30% above the ambient level as defined through the baseline water quality monitoring data. For the purposes of this assessment this is assumed to be 3.9 mg/Ll.._____.General Water Pollution Prevention Measures to be Applied if Drill and Blast Techniques are Used

·all wastewater generated must be treated to the appropriate standard given in the TM on “Standards”.

General Water Pollution Prevention Measures to be Applied if TBM’s Used

· all wastewater generated must be treated to the appropriate standard given in the TM on “Standards for Effluents Discharged into DraiangeDrainage and Sewerage Systems, Inland and Coastal Waters”.

· in the event that conventional sedimentation techniques are inadequate to treat wastewater generated, consideration should be given to use of alternative techniques such as mobile microfiltration plants.

General Pollution Prevention Measures to be aAdopted if Drill and Blast Method is Adopted

· all wastewater generated must be treated to the appropriate standard given in the TM on “Standards for effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters”.

· in the event that conventional sedimentation techniques are inadequate to treat wastewater generated, consideration should be given to use of alternative techniques such as mobile microfiltration plants.

4.7 Residual Impacts

Residual water quality impact during construction is not expected with the implementation of the pollution control measures.

4.8 Cumulative Impact

As discussed in Section 2.5 of this Report, it has been confirmed with relevant Government Departments that tinued that no other works projects are scheduled to overlap with the current Project and as such no cumulative impact assessment was carried out. t The EIA study for the project “Reclamation of Sai Wan Typhoon Shelter and Associated Engineering Works at Cheung Chau” is at the preliminary stage and it is thus more appropriate that the EIA study for the reclamation works at Sai Wan takes on board the findings of this EIA Report with respect to cumulative impacts.

4.9 Operational Phase

No water quality impact will be anticipated during the operational phase.

4.10 Environmental Monitoring & Audit

With the implementation of the water quality mitigation measures, no exceedance of WQO of suspended solids is anticipated at the monitoring locations. However, a robustTo confirm compliance with the WQO’s it is proposed that an environmental monitoring and audit programme is established. This will focus on confirmatory monitoring for the direct burying option, and compliance monitoring in the event dredging is proposed.

will be implemented to ensure that the general mitigation measures are effective and the actual water quality impacts during the installation of submarine cables and the temporary working platformreclamation works at Pui O Beach are acceptable.

Five wWater quality monitoring points should include locations at including Cheung Sha Wan Fish Culture Zone, Cheung Chau Typhoon Shelter, Tai Kwai Wan, Tai Long Wan, South of Hei Ling Chau Typhoon Shelter, and Pui O Beach and Adamasta Channel are proposed. The locations and number of the monitoring points shall be agreed with EPD before undertaking any works. Details of the water quality monitoring and their monitoring locations will be are presented in the section 4 of Environmental Monitoring and Audit Manual. .

4.11 Conclusions

It may becan concluded that for direct burying method, short term exceedances of peak elevated SS levels take place especially only at Po Yue Wan during drywet season season. At Po Yue Wan these occur on neap tide for both the “slow” and “fast” advancing rates although it should be noted that average results fully comply with the WQO’s. Since No short term exceedances occurred any of the sensitive receivers rather they are, not unexpectedly, observed in the Adamasta Channel and a Po Yue Wan which is not a water sensitive receiver, the predicted SS levels at all the water sensitive receivers comply with the relevant WQO. Therefore, .

Water quality impacts associated with the implementation of this Project are acceptable.

The construction programme has scheduled the cable laying work during the dry season. The results of the assessment confirm that cable laying work can be carried out without any adverse water quality impact during the dry season.

Direct burying is the preferred option for the cable laying as it is of shorter duration than dredging, has lesser and acceptable impacts than dredging and can fully comply with the WQO’s at the sensitive receivers all year round.

Full compliance with the WQO’s (throughout the year) has been predicted for the direct burying option, although confirmatory monitoring is proposed to ensure water quality at Tai Kwai Wan and Cheung Sha Wan Fish Culture Zone is not impacted by the cable laying works.

Dredging will not be carried out for laying the entire length of the cables. Instead, For the minor dredging works which need to be carried out for laying either end of the cables (due to the shallow depth of water). tThe results indicate that the WQO’s can be achieved if the single trench dredging option is carried out during the dry season.

In the event that dredging needs to take place during the wet season (eg for reasons of programming) then practical mitigation measures will be needed to ensure compliance with the WQO’s. For the single trench option mitigation measures including the use of closed and sealed grabs and through controlling the rate of lifting will ensure the WQO’s can be achieved.

Although the water quality impact is acceptable., it has been proposed that a “rest” period of 6 hours should be observed between the completion of one cable burying operation (by boat) and commencement of the next cable laying operation. The 6 hour period is based on the time taken for the sediments to resettle to acceptable levels as shown in the time history plots in Appendix F. Reference should be made to these plots relating to the Adamasta Channel and Po Yue Wan where peak concentrations are observed.

In the event that a three trench dredging option is considered then mitigation measures would need to reduce the impacts by 40% during the dry season (through the use of closed grabs) and by 85% during the wet season (using closed grabs, controlling the rate of lifting and reducing the dredging rate by a further 40%). All of the foregoing are practical measures which can be adopted to ensure compliance with the WQO’s.

For the high concentration observed within the typhoon shelter of Cheung Chau Wan the potential impacts are over-exaggerated because the work carried out in the typhoon shelter will be carried out by hand using qualified divers. Therefore the impacts will be significantly less than predicted by the model and no adverse water quality impact is expected.

The construction programme has scheduled the cable laying work during the dry season. The results of the assessment confirm that cable laying work can be carried out without any adverse water quality impact during the dry season. Even cable laying work is required during the wet season, the water quality can also comply with the WQO with the implementation of the above-mentioned mitigation measures.

The potential impacts from off-site runoff can be controlled to acceptable levels. There will be no adverse impact on water quality arising from the temporary working platform at Pui O. The facility is only required for the excavation of the tunnel. The working platform is small (180m²) and will be formed behind a seawall of concrete blocks with no gaps. Once the excavation has been completed the working platform will be removed and the shoreline reinstated.Work at Pui O in respect of the tunnel excavation will not have an adverse impact on water quality as the working platform is small, a temporary facility and the area will be formed behind a No exceedances of SS levels at all the monitoring locations are predicted with dredging one trench or three trenches but exceedances occur at Cheung Chau typhoon shelter, Po Yue Wan and Adamasta Channel when three trenches are dredged simultaneously. With the implementation of the general mitigation measures and pollution prevention measures, the water quality impacts at the water sensitive receivers are not expected.Therefore, the water quality impact is acceptable.seawall of concrete blocks with no gaps.

An environmental monitoring and audit will be implemented to ensure the general mitigation measures are effective and that the actual water quality impacts are within the acceptable levels during the installation of submarine cables and the reclamation temporary working platformworks at Pui O Beach.