Introduction
3.1 Lung Kwu Chau is located within the Sha Chau and Lung Kwu Chau Marine Park. Water quality is one of the key environmental issues. This chapter presents an assessment of the potential impacts on water quality associated with the construction and operation of the proposed jetty at Lung Kwu Chau. The environmental acceptability of these potential water quality impacts is assessed, with a view to identifying appropriate mitigation measures to reduce any identified adverse impacts to acceptable levels. A morphological impact assessment has also been undertaken for the Project to address any potential impacts on the beach located to the immediate north of the proposed jetty.
Environmental Legislation and Standards
3.2 The criteria for evaluating water quality impacts in this EIA Study include:
· Technical Memorandum on Environmental Impact Assessment Process (Environmental Impact Assessment Ordinance) (EIAO TM);
· Water Pollution Control Ordinance (WPCO);
· Technical Memorandum on Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters (TM-DSS);
· Hong Kong Planning Standards and Guidelines (HKPSG); and
· Practice Note for Professional Persons (ProPECC), Construction Site Drainage (PN 1/94).
Environmental
Impact Assessment Ordinance (EIAO), Cap. 499, S.16
3.3 This Project is a Designated Project under Schedule 2 of the EIAO. The EIAO TM was issued by the EPD under Section 16 of the EIAO. It specified the assessment method and criteria that was followed in this Study. Reference sections in the EIAO TM provide the details of assessment criteria and guidelines that are relevant to the water quality assessment, including:
· Annex 6 – Criteria for Evaluating Water Pollution; and
· Annex 14 – Guidelines for Assessment of Water Pollution.
Water Quality Objectives
3.4 The Water Pollution Control Ordinance (Cap. 358) provides the major statutory framework for the protection and control of water quality in Hong Kong. According to the Ordinance and its subsidiary legislation, Hong Kong waters are divided into ten Water Control Zones (WCZs). Corresponding statements of Water Quality Objectives (WQOs) are stipulated for different water regimes (marine waters, inland waters, bathing beaches subzones, secondary contact recreation subzones and fish culture subzones) in the WCZs based on their beneficial uses. The proposed jetty is located within the North Western Water Control Zone and the corresponding WQOs are listed in Table 3.1.
Table 3.1 Summary of Water Quality Objectives for the North Western WCZ
|
Parameter |
Objective |
Part(s) of Zone |
|
Aesthetic Appearance |
Discharge
shall not cause objectionable odour or discolouration No tarry residue, floating wood, articles made of grass,
plastic, rubber or any other substance Mineral oil
not visible on the surface. Surfactants shall not give rise to a lasting foam No
recognizable sewage-derived debris No floating, submerged or semi-submerged
subjects likely to interfere with the free movement or damage of material Not to contain
substances which settle to form objectionable deposits |
Whole Zone Whole Zone Whole Zone Whole Zone Whole Zone Whole Zone |
|
E. coli |
Annual
geometric mean not to exceed 610/100 mL Geometric mean
not to exceed 180/100 mL during March to October inclusive in 1 year; sample
should be taken at least 3 times in 1 calendar month at intervals of between
3 to 14 days Geometric mean
of the most recent 5 consecutive samples taken at intervals of between 7 and
21 days not to be less than 1/100 mL Geometric mean
of the most recent 5 consecutive samples taken at intervals of between 7 and
21 days not to exceed 1000/100 mL |
Secondary
contact recreation subzones Bathing beach
subzones Tuen Mun (A)
and Tuen Mun (B) Subzones and Water gathering ground subzones Tuen Mun (C) Subzone and other
inland waters |
|
Colour |
Not to exceed
30 Hazen units Not to exceed
50 Hazen units |
Tuen Mun (A)
and Tuen Mun (B) Subzones and Water gathering ground subzones Tuen Mun (C)
Subzone and other inland waters |
|
Dissolved Oxygen within 2 m of seabed |
Not less than
2 mg/L for 90% samples |
Marine waters |
|
Parameter |
Objective |
Part(s) of Zone |
|
Depth averaged Dissolved Oxygen |
Not less than
4 mg/L for 90% samples Not less than
4 mg/L |
Marine waters Tuen Mun (A),
Tuen Mun (B) and Tuen Mun (c) Subzones, Water gathering ground subzones and
other inland waters |
|
pH value |
Within the
range 6.5 to 8.5 units; change due to human activity not to exceed 0.2 unit Within the
range 6.5 – 8.5 units Within the
range 6.0 - 9.0 units Within the
range 6.0 to 9.0 units for 95% of samples collected during the year; change
due to waste discharge not to exceed 0.5 unit |
Marine waters
excepting Bathing Beach Subzones Tuen Mun (A),
Tuen Mun (B) and Tuen Mun (c) Subzones, Water gathering ground subzones Other inland
waters Bathing Beach
Subzones |
|
Temperature |
Change due to
waste discharge not to exceed 2oC |
Whole zone |
|
Salinity |
Change due to
waste discharge not to exceed 10% of natural ambient level |
Whole zone |
|
Suspended solids |
Waste
discharge not to raise the natural ambient level by more than 30%, nor cause
the accumulation of suspended solids which may adversely affect aquatic
communities Annual median
not to exceed 20 mg/L Annual median
not to exceed 25 mg/L |
Marine waters Tuen Mun (A),
Tuen Mun (B) and Tuen Mun (C) Subzones, Water gathering ground subzones Other Inland
waters |
|
Un-ionized ammonia |
Annual mean
not to exceed 0.021 mg/L |
Whole zone |
|
Nutrients |
Not to be
present in quantities that cause excessive growth of algae or other aquatic
plants Annual mean
depth-average inorganic nitrogen not to exceed 0.3 mg/L Annual mean
depth-average inorganic nitrogen not to exceed 0.5 mg/L |
Marine waters Castle Peak
Bay Subzone Marine waters
excepting Castle Peak Bay Subzone |
|
5-Day Biochemical Oxygen Demand |
Not to exceed
3 mg/L Not to exceed
5 mg/L |
Tuen Mun (A),
Tuen Mun (B) and Tuen Mun (C) Subzones, Water gathering ground subzones Other Inland
waters |
|
Parameter |
Objective |
Part(s) of Zone |
|
Chemical Oxygen Demand |
Not to exceed
15 mg/L Not to exceed
30 mg/L |
Tuen Mun (A),
Tuen Mun (B) and Tuen Mun (C) Subzones, Water gathering ground subzones Other Inland waters |
|
Toxic Substances |
Not to be
present at levels producing significant toxic effect, carcinogenic, mutagenic
or teratogenic effects in humans, fish or any other aquatic organisms, with
due regard to biologically cumulative effects in food chains and to
interactions of toxic substances with each other Not to cause a
risk to any beneficial use of the aquatic environment |
Whole zone Whole zone |
|
Phenol |
Not to be
present in such quantities as to produce a specific odour, nor to exceed 0.05 mg/L as C6H5OH |
Bathing beach
subzones |
|
Turbidity |
Not reduce
light transmission substantially from the normal level |
Bathing Beach
Subzones |
Hong Kong Planning Standards and Guidelines (HKPSG)
3.5 The Hong Kong Planning Standards and Guidelines (HKPSG), Chapter 9 (Environment), provides additional information on regulatory guidelines against water pollution for sensitive uses such as aquaculture and fisheries zones, bathing waters and other contact recreational waters.
Technical Memorandum
3.6 Besides setting the WQOs, the WPCO controls effluent discharging into the WCZ through a licensing system. A Technical Memorandum on Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters (TM-DSS) was issued under Section 21 of the WPCO that gives guidance on the permissible effluent discharges based on the type of receiving waters (foul sewers, storm water drains, inland and coastal waters). The limits control the physical, chemical and microbial quality of effluents. Any effluent discharges from the proposed construction activities should comply with the standards for effluents discharged into the inshore waters or marine waters of the North Western WCZ, as shown in Table 10a and Table 10b, respectively, of the TM-DSS.
Practice Note
3.7 A practice note for professional persons was issued by the EPD to provide guidelines for handling and disposal of construction site discharges. The ProPECC PN 1/94 “Construction Site Drainage” provides good practice guidelines for dealing with ten types of discharge from a construction site. These include surface runoff, groundwater, boring and drilling water, bentonite slurry, water for testing and sterilisation of water retaining structures and water pipes, wastewater from building constructions, acid cleaning, etching and pickling wastewater, and wastewater from site facilities. Practices given in the ProPECC PN 1/94 should be followed as far as possible during construction to minimise the water quality impact due to construction site drainage.
Sediment Quality
3.8 Dredged sediments destined for marine disposal are classified according to a set of regulatory guidelines for designation of sediments (Management of Dredged/Excavated Sediment, WBTC No. 3/2000) issued by the Works Bureau and Planning, Environment and Lands Bureau in April 2000. These guidelines comprise a set of sediment quality criteria, which include organic pollutants and other toxic substances. This set of guidelines are applicable to any dredging work commencing after 31 December 2001 and, therefore, are applicable to this Project.
3.9 The requirement for the marine disposal of sediment is specified in the Works Bureau Technical Circular No. 3/2000 and No. 12/2000. Marine disposal of dredged materials is controlled under the Dumping at Sea Ordinance 1995.
Baseline Conditions
Water Quality Monitored by EPD
3.10 The EPD water quality monitoring station NM5 in the North Western WCZ is located in the vicinity of the Project area (Figure 3.1). A summary of the published EPD monitoring data (in 2000) collected at this station is presented in Table 3.2.
Table 3.2 Summary Statistics of Marine Water Quality in the North Western WCZ at Station NM5 for 2000
|
Parameter |
|
EPD Monitoring Station
(NM5) |
WPCO WQOs (in marine
waters) |
|
Temperature
(oC) |
|
23.6 (17.6 – 28.1) |
natural daily level ± 2 oC |
|
Salinity
(ppt) |
|
27.7 (20.4 – 32.2) |
natural ambient level ± 10 % |
|
Dissolved
Oxygen (DO) (%
saturation) |
|
82 (52 – 108) |
- |
|
|
Bottom |
79 (40 – 110) |
- |
|
DO
(mg L-1) |
|
6.0 (3.7 – 8.6) |
³ 4 mg L-1 |
|
|
Bottom |
5.7 (2.8 – 8.8) |
³ 2 mg L-1 |
|
pH
value |
|
7.9 (7.7 – 8.3) |
6.5 - 8.5 (± 0.2 from natural range) |
|
Secchi
disc (m) |
|
1.5 (1.0 – 2.5) |
- |
|
Turbidity
(NTU) |
|
15.3 (4.7 – 39.9) |
- |
|
SS
(mg L-1) |
|
11.1 (2.0 – 26.1) |
£ natural ambient level + 30% |
|
Silica
(as SiO2) (mg L-1) |
|
1.6 (0.1 – 3.6) |
- |
|
BOD5
(mg L-1) |
|
0.8 (0.2
– 2.0) |
not
applicable to marine waters |
|
Nitrite
Nitrogen (mg L-1) |
|
0.05 (0.01
– 0.13) |
- |
|
Nitrate
Nitrogen (mg L-1) |
|
0.31 (0.10 – 0.82) |
- |
|
Ammonia
Nitrogen (mg L-1) |
|
0.15 (0.09 – 0.20) |
- |
|
Unionised
Ammonia (mg L-1) |
|
0.005 (0.002 – 0.014) |
£ 0.021 mg L-1 |
|
Total
Inorganic Nitrogen (mg L-1) |
|
0.51 (0.27 – 0.99) |
£
0.5 mg L-1 |
|
Total
Nitrogen (mg L-1) |
|
0.68 (0.45 – 1.20) |
- |
|
Ortho-Phosphate
(mg L-1) |
|
0.03 (0.01 – 0.04) |
- |
|
Total Phosphorus (mg L-1) |
|
0.06 (0.04
– 0.08) |
- |
|
Chlorophyll-a
(µg L-1) |
|
3.8 (0.2 – 25.0) |
- |
|
E. coli (cfu per 100 mL) |
|
480 (170
– 2300) |
<
610 cfu per 100 mL for keeping live seafood |
|
Faecal
Coliform (cfu per
100 mL) |
|
1000 (340 – 5300) |
- |
Notes:
1.
Data
are provided by EPD.
2.
Except
as specified, data presented are depth-averaged data.
3.
Data
presented are arithmetic means except for E.
coli and faecal coliforms that are geometric means.
4.
Data
enclosed in brackets indicate ranges.
5.
Shaded
cells indicate non-compliance with the WQOs.
3.11 Results indicated that the mean value of the depth-averaged total inorganic nitrogen (TIN) exceeded the WQO. According to the “Marine Water Quality in Hong Kong 2000”, all monitoring stations in the North Western WCZ met the WQO for TIN apart from station NM5. As reported by EPD, upward trends in TIN and orthophosphate phosphorus were detected at station NM5 which is more impacted by the Pearl River flow.
3.12 The geometric mean of E. coli in 2000 complies with the WQO. However, according to the EPD report, an increasing trend of E. coli and faecal coliform counts was observed. This long-term increase might be related to the increased sewage discharge from the North West New Territories (NWNT) sewage outfall.
3.13 Full compliance with WQO for bottom dissolved oxygen (DO) was achieved in 2000. The biochemical oxygen demand (5 day) and suspended solids levels in the North Western WCZ remained similar to those in 1999.
3.14 Apart from the TIN, it is worth noting that full compliance with WQOs for dissolved oxygen and unionized ammonia were achieved. Levels of nitrogen and phosphorus nutrients in 2000 were reported to be similar to those in the previous year.
Water Quality of Sha Chau and Lung Kwu Chau Marine Park
3.15 Agriculture, Fisheries and Conservation Department (AFCD) conduct routine monitoring on the water quality within and around the Marine Park in order to provide good baseline information of the state of the Marine Park.
3.16 Water quality has been measured at four stations within the Marine Park (shown in Figure 3.2), namely station 1 (N Lung Kwu Chau), station 2 (N Sha Chau), station 3 (SE Sha Chau) and station 5 (Pak Chau), and fall within the assessment area.
3.17 The following table summarizes the monitoring results in the period 1997-2000.
Table 3.3 Summary Statistics of Water Quality of Sha Chau and Lung Kwu Chau Marine Park in 1997 – 2000
|
Parameter |
|
Monitoring Station |
WPCO WQOs (in marine waters) |
|||
|
|
|
SE Sha Chau |
Pak Chau |
|
||
|
Temperature (oC) |
|
24.0 (10.0 – 30.3) |
24.1 (10.0 – 30.0) |
24.2 (10.0 – 34.0) |
24.2 (10.0 – 31.0) |
natural daily level ± 2 oC |
|
Salinity (ppt) |
|
26.5 (3.0 – 36.9) |
26.4 (3.0 – 36.2) |
26.7 (3.0 – 36.2) |
26.3 (2.5 – 36.9) |
natural ambient level ± 10 % |
|
DO (mg L-1) |
Surface |
6.9 (4.4 – 8.5) |
6.9 (4.4 – 9.0) |
6.9 (4.6 – 8.9) |
6.9 (4.4 – 8.8) |
³ 4 mg L-1 |
|
|
Bottom |
5.6 (1.9 – 8.6) |
6.0 (3.5 – 9.0) |
6.1 (3.1 – 8.7) |
6.0 (2.4 – 8.9) |
³ 2 mg L-1 |
|
pH value |
|
8.1 (7.6 – 8.4) |
8.1 (7.6 – 8.4) |
8.1 (7.7 – 8.4) |
8.1 (6.7 – 8.3) |
6.5 - 8.5 (± 0.2 from natural range) |
|
Turbidity (NTU) |
|
15.6 (1.0 – 146.0) |
9.67 (0.5 – 42.1) |
13.22 (0.7 – 85.0) |
11.8 (0.85 – 44.0) |
- |
|
SS (mg L-1) |
|
13.0 (3.0 – 113.0) |
11.5 (2.0 – 36.0) |
12.1 (3.0 – 35.0) |
13.4 (3.0 – 39.9) |
£ natural ambient level +30% |
|
BOD5 (mg L-1) |
|
0.80 (0.01 – 2.27) |
0.80 (0.05 – 1.99) |
0.80 (0.04 –2.31) |
0.83 (0.14 –2.14) |
not applicable to marine waters |
|
Ammoniacal Nitrogen (mg L-1)* |
|
0.17 (0.02 – 0.64) |
0.17 (0.01 – 0.76) |
0.17 (0.02 – 0.76) |
0.16 (0.03 – 0.72) |
- |
|
Total Inorganic Nitrogen (mg L-1)* |
|
1.56 (0.03 – 8.29) |
1.61 (0.04 – 8.87) |
1.56 (0.04 – 7.68) |
1.61 (0.04 – 8.87) |
£ 0.5 mg L-1 |
|
Total Phosphorus (mg L-1)* |
|
0.10 (<0.02 – 0.31) |
0.10 (<0.02 – 0.62) |
0.08 (<0.02 – 0.35) |
0.10 (<0.02 – 0.65) |
- |
|
Chlorophyll-a (µg L-1) |
|
2.94 (0.21 – 16.3) |
2.8 (0.5 – 22.4) |
6.5 (0.3 – 16.0) |
2.6 (0.5 – 19.7) |
- |
|
Unionized Ammonia (mg L-1)** |
|
0.04 (0.01 – 0.09) |
0.04 (0.0 – 0.09) |
0.04 (0.01 – 0.11) |
0.04 (0.01 – 0.11) |
£ 0.021 mg L-1 |
|
E.coli (colonies/ 100ml)** |
|
90.2 (0.0 – >200) |
63.5 (0.0 – >200) |
65.4 (0.0 – >200) |
39.4 (0.0 – 97.5) |
|
Notes:
3.18 Results indicated that the mean value of all measured parameters comply with the WQOs apart from TIN and unionised ammonia. This was reported to be the result of unusually high levels of TIN and unionised ammonia recorded in the year 2000. The mean depth-averaged values for the TIN level at all four monitoring stations in the year 1999 showed compliance with the WQO. Data for the unionised ammonia level is available for the year 2000 only. Similar to the findings of EPD, 100% compliance of bottom DO criterion is observed.
3.19 The Hong Kong Institute of Education (HKIED) was also appointed by AFCD to conduct comprehensive biological monitoring surveys within the Sha Chau and Lung Kwu Chau Marine Park. The Pearl River, Deep Bay, residual flow from Victoria Harbour and the Urmston Road outfall are identified to be the four acknowledged pollution sources to the Marine Park.
3.20 According to the Monitoring Report[1], the relatively high organic loadings recorded are in line with the trends of increasing organic pollution documented over the past 10 years by EPD. The major source of pollution was stated to be from the Pearl River which has a very large influence on all the western coastal waters of Hong Kong. Locally, discharge from the Urmston Road outfall has a measurable impact on water quality within the Marine Park.
3.21 Non-compliance of E. coli recorded in the Monitoring Report further indicates that the waters in the Marine Park are contaminated by domestic sewage and livestock waste. The highest values were observed in the north-eastern area which is adjacent to the Urmston Road outfall discharge.
Water Sensitive Receivers
3.22 In order to evaluate the water quality impacts during the construction and operation phases, the proximity of Water Sensitive Receivers (WSRs) to the Project area must be considered.
3.23 In accordance with the HKPSG and EIAO TM, the Marine Park itself is identified as the WSR. There is no salt water intake or cooling water intake within the assessment area.
3.24 Marine ecological and fishery resources such as Chinese White Dolphin and the artificial reefs at Sha Chau would be sensitive to any deterioration in water quality. Potential impacts on these resources are addressed in Sections 4 and 5.
Assessment Methodology & Criteria
Hydrodynamic Model
3.25 Numerical modelling of hydrodynamics has been conducted for the Project by DHI Water & Environment (DHI). The methodology for the study approach has previously been described in the “Methodology Report on Water Quality Modelling and Assessment Approach for Lung Kwu Chau Jetty” (February 2002). The set-up of the local hydrodynamic model used for simulation of currents and salinity was documented in the Key Issue Report on “Local Model Set-up” (March 2002). The simulations are used for assessment of the impact from the proposed jetty on the hydrodynamic regime, and form the basis for subsequent sediment spreading simulations.
3.26 A local model has been set up to enable simulations of currents and salinity in the Pearl River estuary based on the model suite applied in the Pillar Point Study.[2] The model has been extended to include a subset grid containing a nested 75m-25m-8.3m dynamically linked grid for simulations of currents around the proposed jetty. As part of these simulations, two culverts are included in the catwalk, which form part of the jetty. 3D simulations have been performed for both dry and wet season hydrodynamics. The dry season is simulated as a period during 1993, where the model has previously been calibrated against WAHMO data sets. The wet season is simulated as a period during 1996, where SSDS EIA data has been used for model calibration. The simulations have been compared to previous results obtained in the Pillar Point Study, and good agreement is observed. The results imply that the model can be used for description of the hydrodynamic regime around Lung Kwu Chau. Details of the model set-up and verification are presented in Appendix 3.1.
3.27 Baseline and operation phase scenarios have been modelled using the local model to examine the effects of the proposed jetty structure on the tidal flow.
Construction Phase
Sediment Spreading Simulation
3.28 During construction phase, the suspended solids (SS) concentration in the water column may increase as a result of dredging. In this study, sediment spill modelling has been conducted to assess the impact of the SS elevation in the Marine Park associated with the dredging activities for the approach channel and jetty and catwalk foundation.
3.29 The simulations of spreading of suspended solids from dredging activities are performed by DHI using the PA, Particle Dispersion model. The PA model is an add-on module to the hydrodynamic module and simulates spreading, sedimentation and re-suspension of particulate matter based on a pre-calculated flow field. A detailed description of the PA model background is provided in the “Methodology Report on Water Quality Modelling and Assessment Approach for Lung Kwu Chau Jetty” (February 2002). Details of the sediment spill modelling methodology and the parameters used for spreading simulation are given in Section 6.3 of Appendix 3.1.
3.30 The simulation is performed for the dry season for an unmitigated construction phase scenario. The dredging will be undertaken using one closed grab dredger of small capacity. The unmitigated dredging scenario comprises a dredging rate of 250 m3/day and a spill of 20 kg/m3 removed mud is assumed (refer to Section 6.3 of Appendix 3.1 for calculation of spill release rate). It is considered not necessary to perform the simulation for the wet season as the contribution of suspended solids due to dredging outside a silt curtain would be incomparable to the Pearl River wet season discharge in the region.
3.31 The resulting output of the simulation is the calculated time dependant excess concentrations of sediment in suspension and areal coverage of the plume and net and gross sedimentation rate in kg/m2/day.
3.32 Any sediment plume generated during dredging activities will cause the ambient suspended sediment (SS) concentrations to be elevated and the extent of elevation will determine whether or not the impact is adverse or not. The determination of the acceptability of any elevation is based on the WQOs. The WQO of SS is defined as being an allowable elevation of 30% above the background. EPD maintains a flexible approach to the definition of ambient levels, preferring to allow definition on a case-by-case basis rather than designating a specific statistical parameter as representing ambient. As directed in previous studies of the environmental impacts of released SS, the ambient value is represented by the 90th percentile of reported concentrations.
3.33 The depth-averaged SS reading and 90 percentile are summarised in Table 3.4. These values are derived from the marine water quality monitoring results of the four AFCD routine monitoring stations located within the Sha Chau and Lung Kwu Chau Marine Park for the period between 1997 and 2000. As stipulated by the WQOs for the North Western WCZ, the 30% allowable elevation of depth-averaged SS above the ambient level will be 5.5 mg/L.
Table 3.4 Depth-Averaged Suspended Solids Concentrations For Monitoring Stations In Sha Chau And Lung Kwu Chau Marine Park
|
Parameter |
Monitoring Station |
Overall Average |
|||
|
SS (mg/L) |
SE Sha Chau |
Pak Chau |
|
||
|
Mean |
13.0 |
11.5 |
12.1 |
13.4 |
12.5 |
|
Range |
3.0
– 113.0 |
2.0
– 36.0 |
3.0
– 35.0 |
3.0
– 39.9 |
|
|
90%ile (ambient level) |
20.4 |
16.0 |
17.1 |
19.5 |
18.2 |
|
30% increase of ambient
level |
|
|
|
|
5.5 |
Data source: AFCD
routine water quality monitoring programme for Marine Parks for period 1997 to
2000.
Contaminant Release during Dredging
3.34 As the sediment may contain organic and chemical pollutants, the loss of sediment to suspension during dredging may lead to the release of contaminants to the receiving waters. As part of the marine site investigation works for this Project, laboratory testing of sediment sampled from the area of the proposed approach channel has been undertaken to characterise the sediment quality. A full description of the laboratory testing and the classification of the sediment quality is contained in Section 7.
3.35 The chemical oxygen demand (COD) of the sediment sample collected from the marine site investigation will be used to determine the reductions in dissolved oxygen (DO) concentration, based on the predicted increases in suspended solids concentrations for the construction phase scenario. The predicted oxygen demand will then be subtracted from the ambient DO levels to estimate the effects of the increases in SS concentrations on the DO level.
Operation Phase
3.36 As mentioned before, the effects of the proposed jetty structure on the local tidal flow will be examined based on the hydrodynamic simulation results. However, effect on water quality is considered to be insignificant as there is no discharge associated with the operation phase of the Project.
3.37 The potential impact that construction of a jetty and dredging of an approach channel to a level of –2.5mCD in front of the berth at Lung Kwu Chau, Hong Kong, may have on the morphology of the pocket beach located to the north of the project site has been assessed.
3.38 The morphological assessment has been based on the analysis of the changes that construction of the jetty and dredging of the approach channel will introduce in the wave conditions existing in front of the beach. The rationale behind this approach is that:
· Unchanged wave conditions will preserve a stable beach as it exists today; and
· Significant gradients in wave height along the beach due to the sheltering of the incident waves by the jetty will generate currents capable of transporting sediment, thus impacting on the beach morphology by changing its equilibrium alignment.
3.39 Wave conditions in front of the beach were determined through a two-step approach. First, the yearly-mean wind wave climate off the eastern coast of Lung Kwu Chau was established by applying MIKE 21 NSW together with yearly wind statistics at Lau Fau Shan. Secondly, the wave conditions in front of the beach were computed using MIKE 21 PMS for five selected wave cases (combination of significant wave height Hm0, peak period Tp and mean direction of wave propagation MWD). The simulations considered both the existing situation and following construction of the jetty and dredging of the approach channel.
3.40 Temporal impact during maintenance dredging would be similar to those predicted in the construction phase. The impact will be assessed based on the construction phase simulation results.
Identification of Environmental Impacts
Construction Phase
3.41 The main impact during construction phase is the release of sediments to the water column during dredging works, as well as the potential release of any hitherto bound contaminants. The demolition of the existing jetty will take place after completion of the proposed jetty and the works are expected to have limited impacts on water quality. It is recommended that the demolition and removal works of the submerged portion of the existing jetty be carried out around low tide to minimise the duration of works taking place within the marine waters.
Operation Phase
3.42 During the operation phase, the structural form of the jetty and the new bathymetry may impact on the tidal flow and sediment transport regime in the area. The impact of the proposed jetty on the nearby beach from any changes in local hydrodynamic conditions will also be assessed. In addition, potential impacts on water quality may arise during maintenance dredging. The frequency of maintenance dredging is estimated to be 5 years. Maintenance dredging would be carried out at areas where results of sounding survey show that the seabed level is too shallow for navigation.
3.43 As discussed in Section 2, the proposed design option for the jetty would be beneficial in terms of minimizing disruption to the existing water circulation pattern. The provision of two culverts beneath the catwalk would enable a flow through the structure.
Evaluation of Impacts
Construction Phase
Sediment Spreading Simulation
3.44 Dredging of approximately 5,550 m3 marine deposits is required for the jetty foundation and approach channel. The loss of fines from the laying of rock bedding in the foundation trenches is expected to be insignificant taking into account the coarse nature of the rock fill and the shallow water depth. No sand fill will be used for the foundation of the jetty and catwalk.
3.45 The results of the calculated instantaneous concentration of SS for the simulation of the unmitigated dredging scenario are depicted in Figures 7.1 and 7.2 of Appendix 3.1 during north-going and south-going current, respectively. The calculated statistical maximum values of SS concentrations are depicted in Figure 6.20 of Appendix 3.1. The concentration level signifies the maximum value attained at any point during the entire simulation period, and does not represent an occurrence of the simultaneous maximum value.
3.46 The results show that the highest levels of concentration in the sediment plume remain relatively close to the Island. The results shown in Figures 7.5 and 7.7 indicate the area of the mixing zone in which the predicted elevation in SS concentration is higher than 30% above the calculated ambient level (i.e. 5.5 mg/l). For the unmitigated scenario, an SS elevation of higher than 5.5 mg/l is predicted in an area of 500m by 100m in the immediate vicinity of the dredger for less than 5% of the time during the dredging period (Figure 7.7). The maximum value of the calculated statistical maximum SS concentrations was 110mg/l at the spill release point and represented the location of the grab. The value of 110mg/l was found at a single point only and the SS concentration in the immediate surrounding area was lower with values of around 80mg/l.
3.47 Mitigation measures have been proposed to minimize the water quality impact. The mitigated dredging scenario comprises a dredging rate of 500 m3/day and the implementation of a silt curtain around the closed grab dredger (refer to Section 6.3 of Appendix 3.1 for calculation of spill release rate for the mitigated case). A higher dredging rate is proposed to reduce the duration of impact of the dredging works on the marine ecological receivers, in particular the Chinese White Dolphin which is the key sensitive receiver in the Marine Park.[3] With a dredging rate of 500 m3/day, the works can be completed in 2 weeks, on comparison to a 4 week dredging programme for a rate of 250 m3/day.
3.48 The calculated statistical maximum values of SS concentration from the simulation of dredging spill for the mitigated scenario are depicted in Figure 6.21 of Appendix 3.1. The results of the calculated instantaneous concentration of SS are depicted in Figures 7.3 and 7.4 of Appendix 3.1 during north-going and south-going current, respectively. During mitigated dredging the sediment plume would be very narrow (20-60 m) and even in the plume, the SS concentrations would in general be only slightly elevated (1-4 mg/l). Figure 7.6 shows that the calculated statistical maximum concentrations in the sediment plume remain very close to the source. The maximum value of the calculated statistical maximum SS concentrations was 45mg/l and occurred at a single point only at the spill release point. In the immediate works area in the mixing zone, it is predicted that an SS elevation of higher than 5.5mg/l would occur for less than 10% of the time during the dredging period (less than 1 day) in an area of 150m by 25m (Figure 7.8 of Appendix 3.1 refers). Although a mixing zone would inevitably occur in the immediate works area of the dredger, the impact of dredging on the ambient water quality would occur only on a very local scale and over a short time. Furthermore, the sediment load created during the dredging works would be well within the range of natural fluctuation of SS concentrations measured in the assessment area.
3.49 The calculated sedimentation rate during the entire simulation period is shown in Figures 6.23 and 6.25 for the mitigated scenario. The main part of the sedimentation remains close to the Island and is located south of the dredging area. The area just to the south of the location of dredging is subjected to increased levels of sedimentation relative to the area north of the location of dredging. A maximum sedimentation rate of less than 0.1 kg/m2/day is shown near the headland of the bay to the south. An evaluation of the impact of the dredging works on the marine ecological receivers, including corals, is given in Section 4. The ecological assessment concluded that no significant impacts were predicted on the Chinese White Dolphin due to elevated SS levels during the dredging works for the mitigated scenario, and the predicted increases in sedimentation rates on the identified soft corals in the assessment area were minor and would not cause any significant impact.
3.50 The dredging of the approach channel may be carried out in the dry season. In case the baseline monitoring of marine water quality during the EM&A programme indicates low ambient levels of suspended solids i.e. less than 12.5mg/l (the calculated overall mean value for the Marine Park), it is recommended that additional precautionary measures be implemented during the dredging works to further minimize the impact on water quality. The proposed additional mitigated measures comprise the implementation of a second silt curtain around the project site and a lowering of the dredging rate to 250 m3/day to reduce the spill loss during dredging. An illustration of a typical configuration of a two-layer silt curtain arrangement is given in Figure 3.3.
3.51 With the implementation of two silt curtains and a lowering of the dredging rate to 250 m3/day, a spill release rate of 0.026 kg/s is expected (assuming 11-hour working day as was assumed for the calculation of spill release rate for the unmitigated and mitigated cases given in Section 6.3 of Appendix 3.1). The implementation of two silt curtains around the closed grab dredger would reduce the release of sediment by at least 80% (a reduction factor of greater than 80% has been demonstrated for the Yam O Reclamation project). The maximum values of the calculated statistical maximum SS concentrations released from the grab dredger would range from 8 to 18 mg/l. The upper value of 18 mg/l would occur at a single point only at the spill release point. In the immediate works area in the mixing zone, it was predicted that an SS elevation of higher than 5.5mg/l would occur for less than 5% of the time during the dredging period (less than 1 day) in an area of 150m by 25m. This transient exceedance is considered acceptable given that no significant impacts on the marine ecological receivers were predicted for the mitigated scenario comprising a higher dredging rate of 500m3/day with the implementation of one silt curtain, as discussed above. An environmental monitoring and auditing programme would be required to ensure the strict and full implementation of the recommended water quality mitigation measures during the dredging works (Section 3.65).
Contaminant Release during Dredging
3.52 The sediment testing results show that the marine sediments to be dredged for the Project are uncontaminated, with no exceedance of the Lower Chemical Exceedance Level recorded (Section 7). Hence, it is considered that the potential release of metals and organic compounds into the water column is not an issue of concern and adverse impacts on water quality during the dredging works due to contaminant release are not anticipated. Provided that the dredged materials are properly disposed of in the designated marine dumping grounds and with the adoption of best management practices, no significant impact on water quality is anticipated to be associated with the dumping activities.
3.53 An assessment of dissolved oxygen depletion during dredging has been made in relation to the results of the predicted elevation in SS concentrations and the sediment quality data for the project area. The predicted maximum elevations in SS concentrations for the mitigated scenario were used to calculate the effects of these increased SS concentrations on DO levels. In the calculation, it is assumed that all of the chemical oxygen demand is exerted. This is a conservative assumption and would likely result in an over-prediction of the potential impacts.
3.54 In order to determine compliance with the water quality criteria, the background water quality data for the Sha Chau and Lung Kwu Chau Marine Park are referred (see Table 3.3). The results of the analysis for DO is presented in Table 3.5 below.
Table 3.5 Calculation of the Effects of Increased Suspended Sediment Concentrations on Dissolved Oxygen Concentrations
|
Elevation in SS concentration (mg/L) |
COD in Sediment (mg/kg) |
Maximum DO depletion (mg/L) |
Background DO (mg/L)(1) |
Resultant DO (mg/L) |
|
50 |
11000 |
0.55 |
5.9 |
5.35 |
Notes: (1) Mean value taken from routine water quality monitoring data for Marine Park (1997-2000)
3.55 As shown in the above table, the predicted DO level would comply with the WQO during the dredging works.
Operation Phase
3.56 The effects of the proposed jetty structure on the local tidal flow have been examined based on the hydrodynamic simulation results. The presence of the jetty would block a part of the north-south going water flow in the vicinity of the coast (as described in Section 6.2 of Appendix 3.1). A current difference in the order of 0.5-1.0 m/s is shown in the area around the jetty periodically during spring tide conditions (Figure 6.15 of Appendix 3.1). It is concluded that the simulations with the jetty and culverts demonstrate that the jetty has a limited effect on the overall flow regime, and no major disruption to the tidal flow is expected.
3.57 In order to assess the impact of the proposed jetty on the flushing of the small bay north of the jetty, a numerical tracer experiment was performed (as described in Section 6.2 of Appendix 3.1). It is concluded that the jetty structure has a small effect on the flushing of water in the nearby bay.
3.58 Maintenance dredging is anticipated to be substantially less intensive than the construction phase dredging with an estimated total dredged quantity of 750 m3. It is recommended that the dredging rate employed for maintenance dredging does not exceed 500 m3/day. Based on the results of the construction phase sediment spill modelling, the sediment plume generated during maintenance dredging is expected to be very localized and remain close to the Island, and the impact on water quality would be very short-term. With the adoption of the recommended mitigation measures as for construction phase dredging, the maintenance dredging works are not anticipated to result in unacceptable impacts on water quality.
3.59 The results of the morphological assessment indicate that the construction of the jetty and dredging of the approach channel would not significantly change the wave conditions in front of the nearby beach for the predominant (easterly) direction. For waves approaching from more southerly directions (150°N and 170°N), the jetty would have a relatively larger impact. However, the beach is to a large extent sheltered from these waves by the rocky headland existing on its southern side. Furthermore, large waves from southerly directions have a very low probability of occurrence. It is concluded that the construction of the jetty and dredging of the approach channel would not impact negatively on the morphology of the beach located to the immediate north of the proposed jetty. The morphological impact assessment is presented in detail in Section 8 of Appendix 3.1
Mitigation Measures
Construction Phase
3.60 In order to minimize the impacts on water quality due to dredging, the implementation of the following measures is recommended:
· Use of closed grab dredgers with a maximum daily production rate of 500 m3/day; and
· Deployment of silt curtain to enclose the grab while dredging works are in progress. This layer of silt curtain is a frame type silt curtain and is designed to enclose local pollution caused by the grab dredging. It consists of floats, curtain and ballast at the bottom. Mid-ballast may be added as necessary. The Contractor should install the silt curtain prior to dredging and should remove it upon completion of dredging. In case the baseline water quality monitoring data indicates low ambient suspended solids concentrations of less than 12.5 mg/l, a second silt curtain should be deployed around the project site (i.e. hanging type silt curtain) and the dredging rate lowered to 250 m3/day. The second layer of silt curtain should be formed from tough, abrasion-resistant permeable membranes, supported on floated booms in such a way as to ensure that egress of turbid waters from the enclosed dredging area shall be restricted.
· Use of small sized hopper barge and dredger for dredging works. The maximum draft of the dredger and hopper barge, when fully loaded, should not exceed 2m. The hopper barge should make more frequent trips to transport the dredged material to the allocated marine disposal ground, instead of remaining at Lung Kwu Chau until it is fully loaded, in order to avoid grounding of the barge during low tide.
3.61 Other good site practices that should be undertaken during the construction works include:
· mechanical grabs, if used, should be designed and maintained to avoid spillage and sealed tightly while being lifted;
· all vessels should be sized such that adequate clearance (i.e. minimum clearance of 0.6m) is maintained between vessels and the seabed in all tide conditions, to ensure that undue turbidity is not generated by turbulence from vessel movement or propeller wash;
· all pipe leakages shall be repaired promptly and plant shall not be operated with leaking pipes;
· the decks of all vessels should be kept tidy and free of oil or other substances that might be accidentally or otherwise washed overboard. Litter and loose objects, e.g. tools, must not be left lying around. Wires and ropes must be coiled and secured, so as to cause least obstruction. Cylindrical shape items (such as pipes), heavy objects/equipment and stagings/scaffoldings on deck should be lashed securely against the rolling movement of vessels. Doors must be properly secured to open or closed position to prevent swinging as the vessel rolls;
· adequate freeboard (i.e. minimum of 200mm) shall be maintained on barges to ensure that decks are not washed by wave action;
· all hopper barges and dredgers should be fitted with tight fitting seals to their bottom openings to prevent leakage of material;
· construction activities should not cause foam, oil, grease, scum, litter or other objectionable matter to be present on the water within the site or dumping grounds; and
· loading of barges and hopper barges should be controlled to prevent splashing of material into the surrounding water. Barges or hopper barges should not be filled to a level that will cause the overflow of materials or sediment laden water during loading or transportation.
3.62 It is recommended that the demolition and removal works of the submerged portion of the existing jetty be carried out around low tide to minimise the duration of works taking place within the marine waters.
Operation Phase
3.63 Adverse water quality impact associated with the operation phase of the jetty is not anticipated. During maintenance dredging, the recommended mitigation measures for the construction phase dredging should also be implemented to minimize the impacts on water quality.
Residual Environmental Impact
3.64 With the full and strict implementation of the recommended mitigation measures for the construction phase of the jetty and maintenance dredging, no unacceptable residual impacts on marine water quality are anticipated to arise. As adverse water quality impacts associated with the operation of the jetty are not anticipated, there would be no residual impact associated with the operation phase.
Environmental Monitoring and Audit Requirements
3.65 Environmental monitoring and auditing (EM&A) of marine water quality is recommended during the dredging works. An EM&A program will be required to ensure the implementation of the recommended water quality mitigation measures during the dredging works. Details of the EM&A procedures are presented in a separate EM&A Manual. If monitoring results indicate that the dredging works have caused an unacceptable water quality impact at the Marine Park, the construction program should be carefully reviewed to slow down the rate of dredging accordingly, so that the water quality complies with the water quality criteria.
Conclusion
3.66 The proposed jetty will be constructed within the Sha Chau and Lung Kwu Chau Marine Park. Marine water quality data from the nearest EPD monitoring station NM5 and the AFCD marine water quality monitoring data for the Marine Park itself indicated that the water quality in the Lung Kwu Chau area is mainly affected by the Pearl River and the discharge from the Urmston Road outfall, and is subject to periods of naturally high turbidity conditions.
3.67 During the construction phase of the proposed jetty, suspended sediment is identified as the most significant water quality parameter. The water quality impact as a result of dredging has been quantitatively assessed using a Particle Dispersion Model to simulate spreading, sedimentation and re-supension of particulate matter. The sediment plume is predicted to be very narrow and localized. The calculated maximum concentrations in the sediment plume for the mitigated dredging scenario are shown to remain very close to the source and are well within the range of natural fluctuations of suspended solids concentrations measured in the assessment area. The implementation of the proposed mitigation measures, including the deployment of a silt curtain around the closed grab dredger, will be required during the marine works to effectively minimize the sediment loss from dredging activities given the presence of ecological resources in the assessment area. It is concluded that with the adoption of the recommended mitigation measures, the construction works for the jetty are not anticipated to result in unacceptable impacts on water quality. An environmental monitoring and audit programme will be required to monitor and audit the implementation and efficiency of the proposed mitigation measures.
3.68 An assessment of the impact from the proposed jetty on the hydrodynamic regime has been made using a local model. With this quantitative modelling tool, impacts have been assessed for the dry and wet seasons, and for both seasons, the baseline and operation simulations have been compared. It is predicted that the jetty and catwalk structure has a limited effect on the flow regime and on the flushing of water in the nearby bay to the north of the jetty. A morphological impact assessment has also been undertaken to address any potential impacts on the beach located to the immediate north of the jetty. It is concluded that the construction of the jetty and dredging of the approach channel would not impact negatively on the morphology of the nearby beach.
[1] The Hong Kong Institute of Education, May 2000, “Shau Chau and Lung Kwu Chau Marine Park Monitoring Programme, Final Report”
[2] Pillar Point Outfall Study Report. Calibration of hydrodynamic models. DHI/Unisearch and Montgomery Watson, 2000. For Environmental Protection Department, HKSAR Government.
[3] A higher dredging rate of 500m3/day with a shorter dredging duration would be the preferred option, personal communication with Dr Tom Jefferson, April 2002.