chapter 5

5. WATER QUALITY IMPACT ASSESSMENT

5.1 Introduction

5.1.1 This Chapter details the current condition of the water bodies within and around the Study Area, and describes the potential impacts on water quality during the construction and operation stages of the proposed secondary channels: MUP03, MUP04A, MUP04B, MUP05 and LMH01 (the Project). Mitigation measures are proposed and the residual impacts after implementation of these measures are evaluated.

5.2 Environmental Legislation, Policies, Standards and Criteria

5.2.1 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 EIA Process (TM-EIA). Under the WPCO, Hong Kong waters are classified into Water Control Zones (WCZs) and statutory Water Quality Objectives (WQOs) are specified for each WCZ. This Project falls within the catchment of Deep Bay WCZ.

5.2.2 The other relevant guideline is the Practice Note for Professional Persons (ProPECC Note) PN 1/94 on Construction Site Drainage which provide guidelines for the handling and disposal of construction discharges. This ProPECC Note is generally applicable for control of site runoff and wastewater generated during the construction phase of the Project.

5.2.3 The Technical Memorandum on Standards for Effluents Discharged into Drainage and Sewerage Systems, Inland and Coastal Waters under the WPCO defines acceptable discharge limits to different types of receiving water in Hong Kong. Site effluent discharge during the construction phase will be required to comply with the terms and conditions of a discharge licence issued by EPD.

5.3 Baseline Water Quality Conditions

Field Observations

5.3.1 The direction of flow of all of the watercourses except Lin Ma Hang stream is towards River Indus, ultimately towards Shenzhen River, and then the Inner Deep Bay. Lin Ma Hang stream drains directly to Shenzhen River. The area of River Indus basin is currently undergoing many changes as a result of construction works for other project.

5.3.2 Site visits took place over a number of days between February, July and October 2004. Observations are detailed in Table 5.1 below.

Table 5.1

Field Observations at Watercourses in the Study Area

Stream Number	Stream Description	Water Quality Observations
MUP03	A small road side concrete drain next to Sha Tau Kok Road. Several sections are obstructed by rubbish and vegetation.	No water flow observed during site visit.
MUP04	Flows through agricultural lands and nurseries. Small sections have been dammed with water being pumped to nearby agricultural land for irrigation. The stream section close to Sha Tau Kok Road has been affected by nearby construction of village house, village access road and land formation for a nearby horticulture shop. Construction waste can be found at the stream bank.	Clean, free flowing and relatively undisturbed at the upstream portion, middle portion potentially nutrient rich from surrounding agricultural lands, downstream portion (close to road) is adversely impacted by construction waste dumping and domestic discharge.
MUP05	The upstream portion (about 300 m) runs parallel to Sha Tau Kok Road. It is a rectangular concrete channel and is overgrown with vegetation at both sides. Several sections are obstructed by rubbish. At the middle portion, where the river meanders about 40 m away from the road, some patches of mostly disturbed riparian vegetation can be found. The stream banks consist of natural and concrete materials, while the stream bed is rocky or muddy. Rubbish can be seen where stream passes near village or industrial development. Near the western downstream end, the banks are formed with concrete, between 8 m to 20 m wide, with rocky / muddy bottom. Construction activities on the channel across Wo Keng Shan Road can be seen.	At the upstream portion, water is stagnant at some locations and has a greyish colour but has no bad odour. Water quality at the middle portion appears better with the appearance of small fishes. Clear flowing water at the western end, fishes (tilapia) can be seen.
LMH01	Natural stream with riparian vegetation. Rocky / sandy / muddy bottom at different locations. Rubbish tipping at the stream banks adjacent to the main village area.	Water is clean, free flowing and relatively undisturbed at most location. Algae and small fish can be observed at several locations.

Baseline Conditions

5.3.3 A considerable amount of water quality data is available from EPD for water bodies within the Study Area. A summary of EPD water quality data from 2002 to 2004 is shown in Table 5.2 for streams and rivers within the Study Area.

5.3.4 River Indus (Ng Tung River) is one of the largest rivers in the Deep Bay WCZ. The river and its tributaries are 49 km in length, covering a catchment area of 43 km².

5.3.5 In 2004, River Indus attained a WQO compliance of 63%. The two upstream monitoring stations IN2 and IN3 were less polluted with a “Good” water quality index and E. coli levels at 2,200 – 4,000 cfu/100mL. However, the water quality at the downstream station IN1 was “Bad” with a much higher E. coli (190,000 cfu/100mL). This may be related to the pollution from unsewered villages and livestock farms in the lower River Indus, as well as runoff from the Sheung Shui and Fanling Town Centres (EPD 2005).

5.3.6 In order to further reduce pollution in the River Indus, new public sewers would be constructed for two villages in the River Indus catchment by 2005 under the North District Sewerage Master Plan (SMP).

5.3.7 Sewers for some 16 villages in the Ta Kwu Ling area would also be provided under the North East New Territories (NENT) Landfill Village Sewerage Plan by 2007. Improvement of river water quality is expected with continuous enforcement of pollution control legislation and gradual provision of sewer facilities to the unsewered villages.

Table 5.2

Summary of EPD Water Quality Data for River Indus from 2002 to 2004

Parameters

IN1

IN2

IN3

2002

2003

2004

2002

2003

2004

2002

2003

2004

(mg/l)

3.1

(1.8 – 6.3)

5.2

(2.5 – 11.8)

3.9

(1.4 – 7.2)

7.4

(3.3 – 9.1)

10.1

(7.7 – 16.1)

8.8

(6.0 – 15.6)

8.3

(7.7 – 10.2)

8.3

(7.1 – 10.5)

8.0

(5.5 – 9.1)

7.2

(6.9 – 7.4)

7.2

(6.9 – 7.9)

7.1

(6.7 – 7.3)

7.2

(6.6 – 7.7)

7.8

(7.0 – 9.4)

7.4

(6.7 – 8.8)

7.4

(7.1 – 8.5)

7.3

(6.9 – 8.2)

7.3

(6.7 – 7.9)

(mg/l)

(13 – 110)

(9 – 77)

(8 – 440)

(22 – 330)

(7 – 150)

(1 – 41)

(3 – 32)

(1 – 210)

(2 – 92)

BOD₅

(mg/l)

(4 – 39)

(4 – 20)

(5 – 32)

(1 – 15)

(1 – 6)

(3 – 21)

(2 – 9)

(1 – 14)

(2 – 10)

COD

(mg/l)

(15 – 63)

(12 – 44)

(17 – 78)

(5 – 29)

(8 – 22)

(7 – 42)

(8 – 32)

(7 – 17)

(5 – 36)

E.coli

(cfu/100ml)

390,000

(5,000 – 4,100,000)

78,000

(11,000 – 3,000,000)

190,000

(25,000 – 2,900,000)

16,000

(1,600 – 90,000)

1,800

(120 – 16,000)

4,000

(170 – 48,000)

4,700

(800 – 100,000)

2,700

(200 – 16,000)

2,200

(130 – 13,000)

NH₃-N

(mg/l)

7.75

(1.20 – 17.00)

2.45

(0.41 – 11.00)

5.85

(0.98 – 18.00)

1.05

(0.29 – 4.10)

0.41

(0.20 – 1.10)

0.82

(0.40 – 1.70)

0.82

(0.23 – 7.30)

0.46

(0.16 – 1.30)

0.82

(0.31 – 2.90)

Source:

EPD (2003) River Water Quality Monitoring in Hong Kong in 2002

EPD (2004) River Water Quality Monitoring in Hong Kong in 2003

EPD (2005) River Water Quality Monitoring in Hong Kong in 2004

5.3.8 The ultimate receiving water body for water from these streams is Inner Deep Bay where poor water quality is of great concern because of its ecological importance. The shallow nature and poor flushing of Inner Deep Bay traps pollutants easily.

5.3.9 In addition to data available from EPD, sampling has been carried out under this Study to assess the sediment quality of the streams in the Study Area.

5.3.10 An updated baseline water quality should be established before the commencement of the construction phase of the Project to allow appropriate action and limit levels to be determined for impact monitoring and audit.

5.4 Water Sensitive Receivers

5.4.1 The Water Sensitive Receivers (WSRs) include the stream course immediate downstream of the proposed channel, River Indus and Shenzhen River.

5.5 Potential Impacts During Construction Phase

Site Preparation / Clearance

5.5.1 Preparation of the land for construction of the drainage channels will involve removal of surface vegetation and excavations, removal and import of construction materials. Construction of some temporary infrastructure may also be necessary such as drainage, bunds and access roads.

5.5.2 The permeability characteristics of the sites may change, reducing infiltration into surface layer. This may lead to increased surface runoff with high suspended solids loadings during the wet season. Erosion of soil enriched in organic matter may also release nutrients into the nearby watercourses.

De-Watering of Streams and Sediment Removal

5.5.3 During excavation, the water in the streams will be temporarily diverted to provide a dry working stream bed prior to the channel works. This may lead to disturbance of organic rich bottom sediments which will release nutrients and suspended solids into the water.

5.5.4 A sediment characterisation survey was conducted in January 2005 to determine whether there are any contaminants in the sediments. The results indicated that the sediments are “uncontaminated” (Category L). Details of the sediment testing and its recommended disposal route can be found in Chapter 6 – Waste Management.

5.5.5 Excavation of the sediments in the channels will be required for channel formation. Excavated sediments may have to be stored on-site before being sorted, reused or transported to disposal sites. If stored as open air stockpiles and left uncovered during rainfall, there is a potential risk of the excavated sediments being washed away, back into the stream water, thereby causing sudden increase of oxygen demand and turbidity during rainy condition.

Concreting Work

5.5.6 Concreting work will be required for part of the channel formation and other ancillary infrastructure. If concrete spillage, washdown or concrete curing water is inadvertently introduced into the water course, the pH level will be raised and may result in contamination of the water or creation of toxic conditions for the aquatic life. Turbidity and suspended solids levels will also increase if contaminated by runoff containing waste concrete. Other possible sources of concrete enriched waste water include water from wheel wash facilities, and washout from concrete lorries/pumps.

5.5.7 With the use of gabion linings and retention of natural stream bed, the use of concrete will be substantially reduced. The small scale bank improvement works at LMH01 would not require the use of concrete.

Site Workshop or Depot

5.5.8 Works areas and site offices will be provided for site staff and contractors, which may include workshop or depot to provide maintenance and repair services for the equipment on site. The workshop may store engine oil, lubricants, chemicals and waste oil/materials which may create impacts if accidental spillage occurs. Waste oil may infiltrate the surface topsoil and contaminate the ground. Stormwater runoff from the site may wash the oil spillage into adjacent waterbodies.

Presence of Additional Population (Workers)

5.5.9 The presence of site staff and construction workers will generate sewage and domestic refuse. Impacts include waste and wastewater generated from eating areas, temporary sanitary facilities and waste disposal areas. Sewage arising from on-site construction workforce would have the potential to cause water pollution if it is discharged directly into adjacent water bodies without any appropriate treatment.

General Construction Works

5.5.10 The channel construction works are scheduled to start in end 2007 for completion in mid 2011. General construction works will be continuous throughout both wet and dry seasons. During the wet season, silty runoff from the sites will be generated from areas cleared of vegetation. If the site runoff is not adequately managed, it may enter the streams and increase the suspended solids and turbidity levels of these water bodies.

5.5.11 The various construction activities and their associated impact on water quality described in the above sections are summarised in Table 5.3

Table 5.3

Summary of Impacts from Construction Activities

Construction Activities	Possible Impacts
Site preparation / clearance	Increase of site runoff from exposed surfaces. Increase in suspended solids, nutrient levels and turbidity in watercourses leading to a decrease in dissolved oxygen level.
De-watering of streams and sediment removal	Runoff during dewatering works into local streams or channels, resulting in an increase in suspended solids, turbidity and nutrient loading.
Concreting work	Generation of concrete washings, increase in pH value, leading to elevated un-ionised ammonia levels (potentially ecotoxic). Increase in suspended solids and turbidity levels.
Site workshop or depot	Accidental spillage of chemicals and waste oils into watercourses and groundwater. Stormwater runoff containing oil and grease into watercourses.
Additional population from workforce	Generation of rubbish and additional sewage.
General construction works	Silty site runoff resulting in increase in suspended solids and turbidity levels during wet season.

5.6 Mitigation Measures for Construction Phase

5.6.1 The main potential impacts from the construction stage of the Project include an increase in suspended solids, pH value, oil & grease and general site effluent entering adjacent water bodies. The following sections discuss the proposed mitigation measures for the potential water quality impacts identified above.

General

5.6.2 The contractor shall observe and comply with the Water Pollution Control Ordinance (WPCO) and its subsidiary regulations. The contractor shall carry out the works in such a manner as to minimise adverse impacts on the water quality during execution of the works. In particular the contractor shall arrange his method of working to minimise the effects on the water quality within and outside the site and on the transport routes.

5.6.3 The contractor shall follow the practices, and be responsible for the design, construction, operation and maintenance of all the mitigation measures below and as specified in ProPECC PN 1/94 – Construction Site Drainage. In particular, the contractor shall submit and implement an Erosion Control Plan (as part of the Environmental Management Plan[1]) which shall incorporate details of the mitigation measures recommended below to reduce water quality impacts arising from construction works. The design of the mitigation measures and the Plan shall be submitted by the contractor to the Engineer for approval.

Site Surface Runoff

5.6.4 Proper construction site drainage management measures shall be implemented to control site runoff and drainage, and thereby prevent high sediment loadings from reaching downstream sections of the river and adjacent agricultural land.

5.6.5 Turbid water from construction sites must be treated to minimise the solids content before being discharged. Advice on the handling and disposal of site discharge is given in the ProPECC Note PN 1/94 - Construction Site Drainage.

5.6.6 In general, surface run-off from construction sites should be discharged into waterbodies via adequately designed sand/silt removal facilities such as sand traps, silt traps and sediment basins. Channels or earth bunds or sand bag barriers should be provided on site to properly direct stormwater to such silt removal facilities. Perimeter channels at site boundaries should be provided to intercept storm run-off from outside the site so that it will not wash across the site (or into the proposed channel works area). Catchpits and perimeter channels should be constructed in advance of earthworks.

5.6.7 Silt removal facilities, channels should be maintained and the deposited silt and grit should be removed regularly, at the onset of and after each rainstorm to ensure proper functioning of these facilities at all times.

5.6.8 Measures should be taken to prevent the washing away of construction materials, soil, silt or debris into the nearby waterbodies. Open stockpiles susceptible to erosion should be covered with tarpaulin or similar fabric and provided with containment such as bunds, sand bag barriers or equivalent measures, especially during the wet season (April – September) or when heavy rainstorm is predicted. Runoff to watercourses should be reduced by minimising flat exposed areas of permeable soil, and by forming pits or diversion channels into which runoff can flow to suitable treatment facilities before discharge.

De-watering / Excavation of Streams and Removal of Sediment

5.6.9 The use of containment structures such as earth bund or sand bag barriers wrapped with goetextile fabric or similar material or diversion channels is recommended to facilitate a dry or at least confined excavation within watercourses. Schematic diagram of typical temporary drainage measures during construction is shown in Figure 5.1.

5.6.10 Excavation works at the existing stream section of MUP05 should be programmed to be carried out during periods of low flow (dry season from 1^st October to 31^st March) to minimise impacts on downstream water quality and sensitive receivers. For the ecologically sensitive stream of LMH01, the restriction period should be further extended for an additional month (i.e. excavation works allowed from 1^st November to 31^st March) to protect the aquatic fauna from silty runoff due to possible heavy rain during the transitional period of the wet / dry seasons.

5.6.11 In addition, the excavation works should be carried out in sections to reduce the area of exposed surfaces as described below. For MUP05, the first 300m upstream portion will have no restriction as the works involve small scale reconstruction of existing concrete drains/channel. The associated area of excavation is expected to be small. For the remaining sections of MUP05 (within existing stream course), the length of excavation would be restricted to 300m at any one time. For MUP04A, a 100m restriction should be imposed for the entire stream works area to cater for potential cumulative impact on MUP05.

5.6.12 As for LMH01, given its relatively small scale works but sensitive nature of the stream, it is recommended that only either one portion of bank improvement works or one vehicular crossing reconstruction should be carried out at any one time.

5.6.13 No restriction is necessary for MUP03 and MUP04B as works involve small scale reconstruction of existing culvert and roadside concrete drains.

5.6.14 After dewatering of the streams, the sediments should be allowed to dry before excavation (yet still maintain a moist state to avoid dust nuisance). This will facilitate excavation of the sediments and also minimise the risk of drained water flowing back into watercourses as the sediment is handled. Where time or weather constraints require handling of wet sediment, care should be taken in the removal of sediment and the storage area should be bunded to prevent silty runoff entering watercourses. Given its small quantity, all excavated sediment should be reused on-site as backfilling material.

5.6.15 Excavated sediment will likely be temporarily stored on-site for reuse as backfilling material. This should be stored in a bunded area and covered during wet season or when rainstorm is forecasted to avoid inadvertent release of silts and suspended solids to nearby water bodies.

5.6.16 Regular monitoring of suspended solids and turbidity should be conducted during excavation works. Any exceedance of water quality in the nearby water bodies caused by inadvertent release of site runoff should be rectified in accordance with EM&A programme for this Project.

Concreting Work

5.6.17 Runoff should be carefully channelled to prevent concrete-contaminated water from entering watercourses. Adjustment of pH can be achieved by adding a suitable neutralising reagent to wastewater prior to discharge. Re-use of the supernatant from the sediment pits for washing out of concrete lorries should be practised.

5.6.18 Any exceedance of acceptable range of pH levels in the nearby water bodies caused by inadvertent release of site runoff containing concrete should be monitored and rectified under the EM&A programme for this Project.

5.6.19 To protect the sensitive stream of Lin Ma Hang, no concrete should be used during bank improvement works at LMH01.

Site Workshop or Depot

5.6.20 Any contractor generating waste oil or other chemicals as a result of his activities should register as a chemical waste producer and provide a safe designated storage area for chemicals on site. The storage site should be located away from existing water courses.

5.6.21 All compounds in works areas should be located on areas of hard standing with provision of drainage channels and settlement ponds where necessary to allow interception and controlled release of settled/treated water; and provision of bunding for all potentially hazardous materials on site including fuels. Hard standing compounds should drain via an oil interceptor. The oil interceptor should be regularly inspected and cleaned to avoid wash-out of oil during storm conditions. A bypass should be provided to avoid overload of the interceptor's capacity. To prevent spillage of fuels or other chemicals to water courses, all fuel tanks and storage areas should be sited on sealed areas within a bund of a capacity equal to 110% of the storage capacity of the largest tank. Where temporary storage of chemicals or fuel drums outside the storage area is necessary, drip tray should be provided. Disposal of the waste oil should be done by a licensed collector. Good housekeeping practices should be implemented to minimise careless spillage and to keep the storage and the work space in a tidy and clean condition. Appropriate training including safety codes and relevant manuals should be given to the personnel who regularly handle the chemicals on site.

5.6.22 The contractor should prepare an emergency contingency plan (spill action plan) for the Project to contain and remove all accidental spillage of chemicals and all hazardous materials on-site including fuels at short notice and to prevent or to minimize the quantities of contaminants entering the stream water and affecting the habitats. The contractor should submit the emergency contingency plan to the ET for review & comment and the Engineer for approval. The Plan should include, but not limited to, the following:

(i) potential emergency situations

(ii) chemicals or hazardous materials used on-site (and their location)

(iii) emergency response team

(iv) emergency response procedures

(v) list of emergency telephone hotlines

(vi) locations and types of emergency response equipment

(vii) training plan & testing for effectiveness.

General Guidance for Handling of Spillage / Leakage

5.6.23 In the event that accidental spillage or leakage of hazardous substances / chemical wastes takes place, the response procedures as listed below should be followed. It should be noted that the procedures below are not exhaustive and the contractor should propose other response procedures in the emergency contingency plan based on the particular types and quantities of chemicals or hazardous substances used, handled and stored on-site.

· Oil leakage or spillage should be contained and cleaned up immediately. Waste oil should be collected and stored for recycling or disposal in accordance with the Waste Disposal Ordinance.

· Instruct untrained personnel to keep at a safe distance well away from the spillage area.

· If the spillage / leakage involves high toxic, volatile or hazardous waste, initiate emergency evacuation and call the emergency service.

· Only trained persons equipped with suitable protective clothing and equipment should be allowed to enter and clean up the waste spillage / leakage area.

· Where the spillage/ leakage is contained in the enclosed storage area, the waste can be transferred back into suitable containers by suitable handheld equipment, such as hand operated pumps, scoops or shovels. If the spillage / leakage quantity is small, it can be covered and mixed with suitable absorbing materials such as tissue paper, dry soft sand or vermiculite. The resultant slurry should be treated as chemical waste and transferred to suitable containers for disposal.

· For spillage / leakage in other areas, immediate action is required to contain the spillage / leakage. Suitable liquid absorbing materials such as tissue paper, dry soft sand or vermiculite should be used to cover the spill. The resultant slurry should be treated as chemical waste and transferred to suitable containers for disposal.

· Areas that have been contaminated by chemical waste spillage / leakage should be cleaned. While water is a soluble solvent for aqueous chemical wastes and water soluble organic waste, kerosene or turpentine should be used for organic chemical wastes that are not soluble in water. The waste from the cleanup operation should be treated and disposed of as chemical waste.

· In incidents where the spillage / leakage may result in significant contamination of an area or risk of pollution, the Environmental Protection Department should be informed immediately.

Presence of Additional Population (Workers)

5.6.24 Sewage arising from the additional population of workers on site should be collected in a suitable storage facility, such as portable chemical toilets. An adequate number of portable toilets should be provided for the construction workforce. The portable toilets should be maintained in a state that will not deter the workers from using them. The collected wastewater from sewage facilities and also from eating areas or washing facilities must be disposed of properly, in accordance with the WPCO requirements. Wastewater collected should be discharged into foul sewers and collected by licensed collectors.

5.6.25 Either chemical toilets or other types of sewage treatment facilities without local discharge of wastewater shall be used to handle the foul water effluent arising from the project sites.

Summary of Mitigation Measures for Construction Phase

5.6.26 The potential impacts on water quality during the construction phase of the Project and the associated recommendation of mitigation measures are summarised in Table 5.4.

Table 5.4

Summary of Mitigation Measures for Construction Impacts

Impacts from Construction	Mitigation Measures
Increase of suspended solids and turbidity from silty site run-off.	Reduction of site run-off by diversion into temporary sedimentation or other silt removal facilities. Site management practices in accordance with ProPECC PN 1/94. Carry out works in dry season. Construction of bunds or barriers where there is potential for runoff to enter watercourses.
Release of nutrients/ silts into runoff during excavation of stream sediment.	Carry out works in sections and during dry season. Drying of sediment before excavation. Careful handling of sediment being excavated. Storage of sediment in bunded area and properly covered. Sediments to be reuse on-site as backfilling material to minimize off-site disposal. Avoid stockpiling, if unavoidable stockpile for the shortest duration possible. Ability to confine flow of water (using bunds, diversion channels, etc). Monitor suspended solids and turbidity in water courses.
Elevation of pH in water courses from concrete washings.	Close monitoring of pH in watercourses. Construction of bunds around the concreting work to collect spilt cement and concrete washings. Adjustment of pH in any water which has become contaminated with concrete. Restrict the use of concrete for bank improvement works at LMH01.
Release of waste oil or other chemicals used in the site workshop or depot.	Registration of contractor as a chemical waste producer. Provide a safe designated storage area for chemicals on-site. Removal of waste oil by licensed collectors. Siting the workshop away from water bodies. Installation and maintenance of oil interceptor in hard standing compound. Oil interceptor should have a bypass installed and be regularly maintained. Good housekeeping practices to minimise careless spillage. Prepare an emergency contingency plan to contain and remove all accidental spillage. Appropriate training given to the personnel to handle chemicals.
Increase of sewage and other wastewater from additional workers.	Provision of portable chemical toilets and proper collection and disposal of sewage and wastewater.

5.7 Potential Impacts During Operational Phase

Identification, Prediction and Evaluation of Impacts

5.7.1 Potential water quality impacts during operational phase of the proposed channels include:

• pollution loads entering the channels (from polluted upstream water, runoff from adjacent land and flood waters; and

• increase in suspended solids during maintenance of the proposed channels.

5.7.2 The operation of the proposed secondary channels, however, does not in itself generate any new pollution load to the catchment.

5.7.3 As the proposed channels are well over 15 km from Inner Deep Bay, the effect of brackish tidal influence from Inner Deep Bay is not expected. Mitigation measures such as tidal barriers / flow management devices are therefore considered not necessary.

5.7.4 Long term reduction of pollution load into the channels is required to improve the water quality in the channel. The first step would be the continuous enforcement of Livestock Waste Control Scheme by the relevant authority. The stream currently receives some domestic waste discharges and industrial effluent which will continue even after the Project is complete. The level of impact will be similar to the current situation and the long term solution will be to provide sewers connecting to village households to treatment works. Provision of public sewers to the unsewered villages is currently being implemented under the North District Sewerage Master Plan.

5.7.5 Vegetation removes dissolved and particulate pollution through absorptive, filtration and biological mechanisms. As concrete drainage environment will not provide such functions, gabion lined channel with existing bed retained is adopted. In line with the recommendations under the ecological assessment, gabions linings will be installed instead of concrete channel. Vegetations will be allowed to re-established along the banks. Natural stream bed will be retained where feasible.

5.7.6 Another potential impact is the temporary release of odour from polluted water and sediments in the channel during low flow conditions. A combination of removing sediments during construction and maintenance of the channel, and long term reduction of pollution loading to the channels, will lead to a reduction of this nuisance.

5.7.7 Maintenance would be necessary for the proposed gabion channels to remove excessive silts, vegetation, debris and obstructions in order to maintain its hydraulic performance and structural integrity. Siltation will generally be allowed to accumulate and removal of excess silt would be carried out at locations where it would impede water flow. Little or no maintenance will be necessary for the ‘two-stage’ channel at MUP05 and the natural stream sections of LMH01. The potential temporary impacts include an increase of suspended solids during removal of the accumulated sediments and disturbance to the aquatic communities from workers and equipment. The impacts associated with the maintenance works are not expected to create adverse water quality impacts.

5.7.8 There would be no chemicals or hazardous materials stored / used on-site during operation, as such the likelihood of accidental spillage is not expected. Emergency contingency plan for operation phase is therefore considered not necessary.

5.8 Mitigation Measures for Operational Phase

5.8.1 The most important feature of the proposed channels is the prospect of suitable re-vegetation of the gabion side slopes replicating existing riparian vegetation. The vegetation is not expected to be detrimental in any way to the structure. However, seasonal cutting and clearance of vegetation, particularly in advance of the wet season will be required. This mitigation measure has additional benefits of aesthetic and ecological value.

5.8.2 In addition, the use of gabion or rock fill base for the bed of the channel has the benefit of providing uneven surfaces and cavities for sediment to accumulate. Ultimately a sediment layer will build up on the gabion floor, forming a natural layer for development of the benthic community. Removal of the upper layer of this sediment will only be necessary once the layer thickness has built up to around 300 mm thick, and sediment is likely to be washed downstream in heavy storms. A minimum of 75mm thick sediment would be allowed to accumulate at the channel bed to permit recolonizing of benthic communities. Growth of vegetation within the gabion sections will inhibit washout of sediment and sediment removal can be carried out at the same time as vegetation harvesting during the dry season when flows are minimal.

5.8.3 The main disadvantage in the use of gabions in the channel is the increase of hydraulic resistance. This will cause a slightly wider hydraulic section to be adopted in the channel design to maintain the same capacity for flood control.

5.8.4 Certain wetland plant species such as ‘sedges’ which have high flexibility and are easily deformed during flood conditions and present minimum drag to flow can be used without significantly compromising the flood capacity of the channel. Species such as vetiver are well documented as being hardy and able to grow under seasonally flooded conditions and are ideally suited for channel bank stabilisation and re-vegetation.

5.8.5 Maintenance of grass species in the channel bottoms is relatively simple and they can be cut prior to the rainy season to prevent washing into River Indus. The recommended vegetation will take up both nutrients and pollutants and should be disposed to landfill. At the same time as grass cutting, excessive sediment may be removed to prevent this being washed into River Indus. As the volume of excess sediment is expected to be minimal, this can be disposed to landfill along with the excess vegetation. The excess sediment should be allowed to drained and dried before disposal.

5.8.6 Odour reduction can be achieved through regular sediment removal during low flow (dry season).

Environmental Considerations for Maintenance of the Proposed Channels

5.8.7 Maintenance may be necessary for the proposed gabion channel at regular intervals to remove excessive silts, vegetation, debris and obstruction. Little or no maintenance will be necessary for the natural streams (e.g. Lin Ma Hang and ‘two-stage’ channel of MUP05). Good practice guides for the planning and execution of desilting and maintenance works on environmentally sensitive watercourses are recommended in the following sections.

5.8.8 Before proceeding with any desilting or maintenance works, except for emergency works, the maintenance engineer should check to ascertain if any of the proposed works will be located in or near an environmentally sensitive and/or ecologically important watercourses. In case of doubt, advice from EPD and AFCD or other relevant departments should be sought.

5.8.9 If the proposed works will be located inside or near one of the environmentally sensitive and/or ecologically important watercourses, careful consideration should be given to the proposed method of implementation so as to minimise any adverse environmental impact. Depending on the extent of the maintenance works, EPD and AFCD should be notified and/or consulted as appropriate on the proposed method and mitigation measures for executing the works. Their comments on necessary mitigation measures should be seriously considered and incorporated. Any difference in opinion on the right balance between flood protection and ecological conservation should be brought to the attention of the relevant Chief Engineer.

5.8.10 The following considerations should be included in planning for the maintenance works for the proposed gabion channels:

(a) Maintenance of the channels should be restricted to annual silt removal when the accumulated silt will adversely affect the hydraulic capacity of the channel (except during emergency situations where flooding risk is imminent). Desilting should be carried out by hand or light machinery during the dry season (October to March) when water flow is low.

(b) The management of woody / emergent vegetation should be limited to manual cutting, to be carried out during dry season and only when unchecked growth of such vegetation is very likely to impede channel flow.

(c) A minimum of 75mm thick sediment should be allowed to accumulate on the channel bed to permit recolonization of benthic communities.

(d) Phasing of the works should be considered to better control and minimize any impacts caused, and to provide refuges for aquatic organisms. Where possible, works should be carried out along half width of the watercourse in short sections. A free passage along the watercourse is necessary to avoid forming stagnant water in any phase of the works and to maintain the integrity of aquatic communities.

(e) Containment structures (such as sand bags barrier) should be provided for the active desilting works area to facilitate a dry or at least confined working area within the watercourses.

(f) Where no maintenance access is available for the channel, temporary access to the works site should be carefully planned and located to minimize disturbance caused to the watercourse, adjacent vegetation and nearby sensitive receivers by construction plants.

(g) The use of lesser or smaller construction plants should be considered to reduce disturbance to the channel bed where fish habitats are located and to the nearby sensitive receivers. Quiet construction plants should be used.

(h) The use of concrete or the like should be avoided or minimized.

(i) The locations for the disposal of the removed materials should be identified and agreement sought with the relevant departments before commencement of the maintenance works. Temporary stockpile of waste materials should be located away from the channel and properly covered. These waste materials should be disposed of in a timely and appropriate manner.

5.8.11 A summary of the mitigation measures proposed is shown in Table 5.5.

Table 5.5

Summary of Mitigation Measures for Operational Impacts

Impacts from Operation	Mitigation Measures
Reduction in water quality of downstream water courses from introduction of pollutants upstream.	Pollutants present in run-off during operation can be reduced through an increase in permeable areas in sections where the run-off occurs to reduce the volume of pollutants entering the channels. Grasscrete and gabions can be used to achieve this.
Removal of permeable area presently covered in vegetation.	Use of gabions in sections of the channels mitigates this impact through allowing vegetative growth. Infiltration of water into the ground and passage through vegetation will provide partial treatment of the water in terms of trapping suspended solids and removal of BOD through aeration.
Increased sediment transport to River Indus and ultimately to Inner Deep Bay.	Sediment will be carried to River Indus and ultimately to Inner Deep Bay by faster flow velocities. Normal stream flow velocities can be reduced by the use of vegetation which will also trap sediment and encourage further vegetation growth. The use of gabions will also trap sediment and discourage transport downstream. Maintenance will be required when there is unacceptable excessive build-up of sediment, debris, obstruction in the channels.
Temporary release of odour from polluted water and sediments in the channel during low flow condition.	Odour reduction can be achieved through regular sediment removal during dry season.
Temporary release of suspended solids and waste during maintenance process, minor disturbance from maintenance equipment.	Implement good practice guides for the planning and execution of desilting and maintenance works such as works during dry season, phasing of the works, use of containment structures, sensitive use of construction plant to reduce disturbance, minimise the use of concrete, and proper storage and disposal of waste.

5.9 Residual and Cumulative Impacts

5.9.1 With the effective implementation of mitigation measures as described, residual impacts on water quality of the receiving water bodies due to the implementation of this Project are expected to be negligible.

5.9.2 A number of other secondary channels will be constructed within the Study Area. The Non-DP channels, MUP01 and MUP02, will be connected to MUP05. Only short sections of bank stabilisation / improvement works are proposed for MUP01 and MUP02. The main potential cumulative impact would be silty site runoff during construction phase. With the implementation of proper site drainage system as stipulated in ProPECC PN 1/94, cumulative impact is not expected.

5.9.3 In general, excavation in all the proposed secondary channels under this Project is to be executed in the dry season within a containment structure (see Figure 5.1 for typical example) and any groundwater outflow from the containment is pumped out by way of a settling basin. Any water courses or outfalls that would normally be served by the area of channel under improvement will bypass the works either in a piped system or a temporary mortar lined peripheral channel.

5.9.4 The implementation of a water quality monitoring and audit programme during the construction phase can ensure early detection of deteriorating water quality and timely action to rectify the situation as well as to ensure the proper implementation of the mitigation measures and to verify its effectiveness.

5.9.5 Based on the above justification, adverse cumulative works related water quality impacts are thus not expected.

5.10 Monitoring and Audit Requirement

5.10.1 Further information regarding water quality monitoring and audit are described in Chapter 10 of this Report. Key points are summarized below.

5.10.2 A water quality monitoring and audit programme will be formulated to check for compliance with permissible effluent standards, to ensure the proper implementation of the mitigation measures and to verify the effectiveness of the recommended mitigation measures. Monitoring locations will be proposed where the works are close to sensitive water bodies or different projects interface with each other (i.e. where cumulative impacts may occur). Monitoring should be provided at both downstream (as impact station) and upstream (as control station).

5.10.3 Baseline monitoring will be required prior to construction, to determine the appropriate action and limit levels for impact monitoring.

5.10.4 Monitoring of pH value, dissolved oxygen, suspended solids and turbidity will need to be carried out to ensure that any deteriorating water quality could be readily detected and timely action be taken to rectify the situation.

5.11 Summary

5.11.1 The current condition of the water bodies in the Study Area is relatively poor in the lower stretches with many of the streams receiving a variety of polluting inputs including livestock waste, industrial effluent, domestic wastewater and construction waste from local villages.

5.11.2 The construction of the channels could adversely impact the water bodies through silt-laden site runoff, disturbance of stream sediment during excavation, concreting works, runoff from workshops & depot and increased sewage and wastewater resulting from the additional workforce on site. These impacts can be readily mitigated through the construction of a suitable drainage system with silt traps, good site management practices, careful working practices when excavating sediments and proper sewage collection and disposal system.

5.11.3 The major operational impacts are a net increase in pollutant loadings to Inner Deep Bay due to the removal of vegetation from channel banks. This vegetation acts as a natural pollutant trap and filter and this naturally occurring process will be removed. The impacts can be mitigated through the incorporation of gabions, grasscrete or similar vegetated areas into the drainage channel design. These channels will require regular maintenance to ensure effective functioning and prevent the unacceptable build up of sediments. The routine maintenance of the channel may pose potential impacts if it is not properly executed. By following the recommended good practice guides during maintenance works, no adverse impacts are expected.

5.11.4 With the implementation of the recommended mitigation measures, no adverse residual water quality impact is expected during construction and operation phases of the Project.

5.12 References

EPD (2003) River Water Quality Monitoring in Hong Kong in 2002

EPD (2004) River Water Quality Monitoring in Hong Kong in 2003

EPD (2005) River Water Quality Monitoring in Hong Kong in 2004

[1] As required under ETWB TCW No. 19/2005 – Environmental Management on Construction Sites.