7.                       Water Quality Assessment


7.1                   Introduction


7.1.1             The works of this Project are located in the Kam Tin valley, an area with a large number of watercourses, wetlands and fishponds. Potentially significant impacts may occur at these waterbodies.


7.1.2             This Chapter details the current condition within and around the proposed KT13 channel alignment, and describes the potential impacts on water quality during the construction and operation stages of the project. Mitigation measures for each type of impact are proposed and the residual impacts (following implementation of these measures) are evaluated.


7.2                   Sensitive Receivers and Baseline Water Quality Conditions


Field Observations


7.2.1             Several water courses and a number of fishponds still exist within and around the Study Area. Many of the former fishponds have been recently in-filled. The direction of flow of all of the watercourses is ultimately towards Kam Tin Main Drainage Channel, and Inner Deep Bay. Since 2000 the landscape of the area is completely altered by the new presence of West Rail and its depot, as well as the new Kam Tin Main Drainage Channel. Currently the civil and structural works for West Rail and the nearest sections of Kam Tin Main Drainage Channel have been completed and these are unlikely to constitute cumulative environmental impacts with the construction works of KT13. Figure 1.1 shows the location of KT13 with respect to other channelization projects in the North West New Territories. Figure 2.1 shows its position in the Kam Tin River Catchment.


7.2.2             Site visits took place over a number of days between February to July 2000 and September 2002 and 2005. Observations are detailed in the Table 7.1 below. Chainage positions are shown on Figure 2.2a and b respectively.


Table 7.1

Field Observations at Existing Streams to be Modified under Ma On Kong


Stream Section

Stream Description

Water Quality Observations in 2000

Water Quality Observations in 2002 and 2005

Upper Section (Correspond to Section B CH0-CH298)

The stream is very well vegetated and extends from an area of pig farms at the upper extent towards the Ho Pui egretry.

Water quality is bad, there is plenty of pig waste in the upper end of the stream. Water is very grey, turbid and odorous.

Water observed to be clear and generally free of pig waste or debris.

Middle Section (Correspond to proposed Box Culvert CH0-CH400)

From the small footbridge (KT13b) the stream passes though Ho Pui egretry and through a village area. The channel is well vegetated.

Water quality is also bad, dark grey and very turbid with high SS loading, Odorous sediments accumulate around the small meanders.

Water observed to be clear and free flowing. Generally free of pig waste or debris but strong odour still evident.

Lower Section (Correspond to proposed Section A CH0-CH400)

Part of the stream is already channelized around Ma On Kong village. Below the village the stream passes though a cultivated area and bank sides are well vegetated.

Water quality is slightly better at the upper end of this section. There are dark odorous sediments on the channel bed.

Water observed to be free flowing and clear. Generally free of pig waste or debris but strong odour still evident.


Further north the stream passes a few small dwellings and is partially channelized with less bankside vegetation. Water flow in the channel appears to be restricted.

There is plenty of debris dumped at the lower end. Water in the channel sometimes backs up and becomes stagnant at this point, the bottom sediments are very odorous and appear to be releasing gas. When the water then flows it exposes black odorous sediments. This may be due to water flow restrictions caused by the West Rail Works.




Baseline Conditions


7.2.3             In addition to these field surveys for KT13 a considerable amount of water quality data is available for water bodies the Kam Tin River Catchment and Deep Bay. A summary of EPD water quality data from 1998 to 2003 is shown in Tables 7.2a & b and 7.3a & b for streams and rivers within the study area.


7.2.4             Yuen Long Creek is 60 km long and has a catchment of 27 km². The river had high levels of E.coli and BOD5 and low dissolved oxygen. Similar to 2002, the overall WQO compliance in Yuen Long Creek in 2003 was low at 43%, in particular with the SS, COD and BOD5 parameters. The overall Water Quality Index (WQI) was “bad” mainly due to unsewered villages and illegal discharges from livestock farms in the catchment. The water quality of Yuen Long Creek remains unsatisfactory with very high E.coli counts.


7.2.5             The situation is expected to gradually improve with the continued enforcement of the Livestock Waste Control Scheme and the implementation of sewerage system under the Yuen Long and Kam Tin Sewerage Master Plan. Table 7.2 a & b show a summary of EPD’s water quality data for Yuen Long creek from 1998 to 2003.


Table 7.2a

Summary of EPD Water Quality Data for Yuen Long Creek from 1998 to 2000















DO (mg/l)


























































SS (mg/l)





























BOD5 (mg/l)





























COD (mg/l)





























E.coli (cfu/100ml)

695,666 (200,000-5,900,000)

474,000 (140,000 - 2,000,000)

263,000 (49,000 - 820,000)


371,000 (75,000 - 2,000,000)

90,200 (35,000 - 340,000)

3,290,235 (1,700,000- 5,500,000)

2,700,000 (1,000,000- 5,300,000)

1,120,000 (240,000 - 4,300,000)











NH3-N (mg/l)





11.3 (1‑58)











17.0 (2.2‑40.0)

NM : Not measured.

Table 7.2b

Summary of EPD Water Quality Data for Yuen Long Creek in 2001 to 2003















DO (mg/l)

(1.9 – 8.1)

(3.5 – 7.8)

(2.0 – 7.8)

(6.1 – 10.0)

(5.2 – 10.5)

(2.5 – 9.8)

(1.6 – 7.6)

(2.8 – 6.4)

(2.0 – 5.8)


(6.4 – 7.9)

(6.8 – 7.7)

(7.0 – 7.8)

(6.9 – 8.2)

(6.9 – 7.9)

(6.9 – 7.6)

(7.1 – 8.0)

(6.8 – 7.6)

(7.0 – 7.7)

SS (mg/l)

(13 – 140)

(8 – 58)

(12 – 300)

(10 – 45)

(9 – 94)

(11 – 48)

(15 – 270)

(12 – 270)

(20 – 130)

BOD5 (mg/l)

(1 – 180)

(7 – 300)

(5 – 72)

(1 – 11)

(5 – 15)

(4 – 25)

(3 – 130)

(17 – 110)

(9 – 110)

COD (mg/l)

(9 – 250)

(15 – 420)

(8 – 190)

(6 – 38)

(14 – 31)

(11 – 45)

(10 – 170)

(28 – 110)

(19 – 200)

E. coli (cfu/100ml)

(25,000 – 1,800,000)

(90,000 – 1,700,000)

(56,000 – 980,000)

(23,000 – 170,000)

(39,000 – 410,000)

(15,000 – 170,000)

(310,000 – 5,100,000)

(420,000 – 2,100,000)

(260,000 – 1,100,000)

NH3-N (mg/l)

(0.02 – 90.00)

(3.30 – 110.0)

(2.30 – 63.00)

(0.72 – 24.00)

(2.60 – 14.00)

(2.30 – 14.00)

(1.30 – 39.00)

(4.70 – 21.00)

(3.00 – 31.00)




7.2.6             The main receiving water body for flow from KT13 is the Kam Tin River which is 50 km long and serves a 44 km² catchment. The portion of the Kam Tin River that will receive flow from the streams to be modified under KT13 has been channalised, in the form of a trapezoidal channel.  It was rated as bad by EPD in 2003 and had low levels of dissolved oxygen, and very high BOD5, E.coli, and ammoniacal nitrogen. Although some long-term improvements in DO and other pollution parameters were observed in 2003, the compliance with SS, COD and BOD5, WQOs remained very low, ranging between 0% and 4%. Tables 7.3a & b give a summary of EPD’s water quality data for Kam Tin River from 1998 to 2003.


7.2.7             The ultimate far field monitoring station TSR1 is in Tin Shui Wai nullah which was rated as “bad” in terms of water quality in 2003. The overall compliance of the Tin Shui Wai Nullah was 63% in 2003, with a notable decrease in BOD5 objective. The E.coli levels remained very high. Further reduction in pollution loading is expected from continue enforcement of legislation control on pollution and new sewerage network under the Yuen Long and Kam Tin Sewerage Mater Plan. Tables 7.3a & b give a summary of EPD’s water quality for Tin Shui Wai Nullah in 1998 to 2003.


Table 7.3a

Summary of EPD Water Quality Data for

Kam Tin River and Tin Shui Wai Nullah in 1998 to 2000



KT2 (Kam Tin River)

TSR1 (Tin Shui Wai Nullah)







DO (mg/l)



5.5 (1.9‑14.4)

4.9 (3.2‑9.2)

6.5 (0.4‑11.0)








7.3 (6.9‑8.2)

7.3 (6.8‑7.8)







SS (mg/l)



65 (21‑100)



21 (3‑170)





BOD5 (mg/l)













COD (mg/l)



57 (31‑240)

42 (23‑100)







E.coli (cfu/100ml)

579,089 (3,200 - 1,700,000)

9,680 (900 - 260,000)

15,100 (4,000 - 48,000)

1,792,885 (230,000 - 17,000,000)

2,160,000 (90,000 - 65,000,000)

608,000 (33,000 - 23,000,000)

NH3-N (mg/l)

20.5 (9.6‑79.0)

14.0 (2.3‑21.0)

6.7 (3.5‑16.0)

6.65 (1.5‑8.8)





Table 7.3b

Summary of EPD Water Quality Data for

Kam Tin River and Tin Shui Wai Nullah in 2001 to 2003



KT2 (Kam Tin River)

TSR1 (Tin Shui Wai Nullah)







DO (mg/l)

(3.2 – 11.7)

(1.8 – 6.3)

(1.7 – 4.0)

(2.9 – 10.9)

(2.3 – 8.1)

(3.3 – 11.3)


(6.6 – 8.2)

(7.0 – 7.6)

(7.1 – 7.7)

(7.0 – 8.2)

(7.2 – 8.1)

(7.1 – 8.7)

SS (mg/l)

(41 – 130)

(18 – 340)

(26 – 370)

(10 – 590)

(8 – 290)

(8 – 610)

BOD5 (mg/l)

(1 – 30)

(8 – 390)

(10 – 200)

(5 – 220)

(3 – 42)

(4 – 58)

COD (mg/l)

(17 – 120)

(21 – 630)

(36 – 700)

(8 – 54)

(8 – 58)

(8 – 80)

E. coli (cfu/100ml)

(1,300 – 29,000)

(220,000 – 9,100,000)

(390,000 – 5,600,000)

(120,000 – 18,000,000)

(100,000 – 26,000,000)

(15,000 – 3,600,000)

NH3-N (mg/l)

(0.38 – 16.0)

(6.10 – 72.0)

(8.90 – 70.0)

(1.00 – 8.80)

(0.65 – 8.30)

(0.87 – 8.5)



7.2.8             Overall water quality of the streams to be channelized and those in the Study Area remains poor. The major pollution source was from livestock waste and unsewered villages. This is however becoming less evident from the site visit in 2002, during when all the KT13 stream sections were observed to be relatively clear and free flowing.


7.2.9             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 of Inner Deep Bay traps pollutants easily.


7.2.10         Water and sediment quality in the Deep Bay Water Control Zone are regularly monitored by EPD. Monitoring results for 1999 to 2003 (EPD, 2000-2004) indicate that Inner Deep Bay was affected by pollution to a greater extent than Outer Deep Bay. BOD5, SS, E.coli and inorganic nutrients were all greater in Inner Deep Bay. Dissolved oxygen was observed to have decreased in Inner Deep Bay over the last few years. The changes are considered to be threatening the delicate ecosystem of Inner Deep Bay.


7.2.11         In addition to the EPD data, sampling was carried out in 2000 under this Study to assess the water and sediment quality of KT13. The sampling locations (W1, W2, W3 and W4 for water quality and KT13A, KT13B, KT13C and KT13D for sediment) are shown in Figure 7.1. Figure 7.1 also shows the new KT13 alignment, which in addition to the bypass culvert, there are some slight modification to the gabion sections since the water and sediment quality sampling were carried out. The results of the water quality and sediment sampling are shown in Appendix G1.


7.2.12         Results obtained in 2000 indicate that water quality at all four sampling locations in 2000 was bad, all showing low levels of DO and high Total Organic Carbon (TOC) levels. Sampling location KT13C (W3) failed to meet the WQO for DO of 4.0mg/L. Visual inspection in September 2002 to up stream areas (where KT13A and B were located) however shows free flowing water which possibly reflects recent changes to the local livestock population or improved waste control but strong odour can still be noticed.  Refuse or livestock waste were not visible. An updated baseline should be established nearer the construction phase of the project to allow appropriate action and limit levels to be determined for impact monitoring and audit.


7.2.13         A Sediment Quality Report (SQR) was undertaken in 2005 to ascertain the volume and degree of contamination of the sediment such that appropriate marine disposal sites can be allocated by the relevant authorities.  Details are presented in Chapter 8 and Appendix G2.


7.3                   Identification and Evaluation of Potential Impacts


7.3.1             There are potential impacts on the water quality of the streams during construction of secondary drainage channel KT13. The following sections identify the likely impacts that would arise due to the implementation of this DP. Discussion on the absence of cumulation impacts with other progress and during operation is provided.


7.4                   Potential Impacts During Construction Phase


Site Formation


7.4.1             The site formation stage will involve excavation and disposal of spoil materials. Construction of some temporary infrastructure may also be necessary such as drainage, bunding and access roads. This is particularly relevant to the area where the bypass culvert is proposed since most of the surplus fill is likely to arise from the formation of this part of the works.


7.4.2             The permeability characteristics may also change, reducing infiltration into the surface layers. This may lead to increased rate of sediment removal from Kam Tin Catchment into Deep Bay with higher suspended solids loadings than previously undisturbed state.


Stream Diversion and Dredging of the Stream


7.4.3             Contaminated sediments disturbed during excavation will increase oxygen demand and suspended solids levels, which in turn will cause decrease of oxygen levels.


7.4.4             Excavation of stream bed sediments at KT13 will be required for channel formation. Excavated sediments, whether clean or contaminated, may have to be stored on-site before being sorted, re-used 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


7.4.5             Concreting work will be required for part of the channel formation. 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, caused by dissociation of ammonia compounds in the livestock waste. Also turbidity and suspended solids levels will 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.


Oil and Grease from Construction Equipment


7.4.6             Spillage, hydraulic leakage and runoff from the surface of standby construction equipment during rainy conditions may release oil and lubricants to the greater environment, when these contaminates reach the stream, the impact will reduce COD levels and dissolved oxygen levels.


Site Workshop or Depot


7.4.7             Works areas will be provided for site staff and contractors, which will include, a workshop to provide maintenance and repair services for the equipment on site. The workshop will store engine and hydraulic oil, lubricants and waste oil/materials which have the potential to create impacts if spillage occurs. Waste oil may infiltrate the surface topsoil and contaminate the ground. Stormwater run-off from the site may wash the oil spillage into adjacent waterbodies.


Presence of Additional Population (Workers)


7.4.8             The presence of construction workers will generate sewage and domestic refuse. These waste have to be managed and disposed of appropriately or they may present a potential source of pollution to the environs of KT13 stream.


7.4.9             The construction activities and their associated impact on water quality described in above sections are summarized in Table 7.4.


Table 7.4

Summary of Impacts from Construction Activities


Construction Activity

Possible impact

Site formation

Increase of site run-off from exposed surfaces. Potential increase in suspended solids, and turbidity, BOD and COD levels, leading to a decrease in dissolved in dissolved oxygen level in water courses.

Stream Diversion/Dredging of streams

Runoff during dewatering works into local streams or channels, resulting in an increase in nutrient loading and BOD.

Release of heavy metals from contaminated stream bottom sediments.

Increase in SS and turbidity in the channel.

Runoff from wet mud containing nutrients and BOD into the local channel or streams and hence Inner Deep Bay.

Concreting Works

Generation of concrete washings, increase in pH value, leading to elevated un-ionised ammonia levels (potentially ecotoxic).

Increase in SS and turbidity levels.

Oil and Grease

Increase of SS and turbidity in watercourses. Elevation of pH value by concrete washings. Change of physical hydrology of the streams.

Site workshop and maintenance facilities

Spillage of waste oils and their infiltration into groundwater.

Stormwater runoff containing oil and grease into water courses.

Additional population from workforce

Generation of rubbish and additional sewage.



7.5                   Mitigation Measures for Construction Activities


7.5.1             The following paragraphs discuss the proposed mitigation measures for the potential water quality impacts identified in Table 7.4.






7.5.2             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.


7.5.3             The Contractor shall follow the practices, and be responsible for the design, construction, operation and maintenance of all the mitigation measures as specified in ProPECC PN 1/94 - Construction Site Drainage. The design of the mitigation measures shall be submitted by the Contractor to the Engineer for approval.


7.5.4             As part of the ecological mitigation measure to minimise impact on the existing stream course, a restriction to minimise the working area to 75m length of stream has been proposed. Such restriction would also reduce the likely water quality impact.


               Site Formation


7.5.5             Temporary earth bunds and/or sand bag barriers should be used to direct stormwater run-off to temporary settlement area. The settlement area should be within the channel itself. A cofferdam should be formed to keep the working area dry. The channel will be dug out to a depth of around 1 – 2m for a length of approximately 12m, to form a sedimentation area. The volume will be approximately 50m3 (with a channel width of 3.5m). A sketch showing the typical section of the settlement area is shown in Figure 7.2.


7.5.6             Sediment flowing downstream should settle in this settlement pond, while run-off from the surface should be channel through a local site drainage system into the settlement area. The settlement area should be maintained and the deposited materials should be removed regularly, at the onset of and after each rainstorm to ensure proper functioning at all times. No sediment removal shall be allowed in rainy weather.


7.5.7             Open stockpiles susceptible to erosion should be covered with tarpaulin or similar fabric, especially during the wet season (Apr-Sep) or when heavy rainstorm is predicted.



               Stream Diversion and Dredging of Streams


7.5.8             The Contractor should provide temporary drainage diversion during construction to ensure continuous water flow to the unmodified portion of the stream.  A schematic design for temporary drainage diversion is shown in Figure 7.3.


7.5.9             The use of containment structure such as temporary earth bunds, sand bags, sheetpile barriers or other similar techniques is recommended to facilitate a dry or at least confined excavation within watercourses.


7.5.10         Excavated sediment from streams and channel is likely to be wet and contaminated. The material should be stored in covered impermeable skips and disposed on the same day, or within 1 day, to avoid both odour and inadvertent release of contaminants to nearby water bodies.


               Concreting Work


7.5.11         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.


7.5.12         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.


               Site Workshop or Depot


7.5.13         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 storage area for chemicals on site. The storage site should be located away from existing water courses. Hard standing compounds should drain via an oil interceptor. 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. Disposal of the waste oil should be done by a licensed collector. Oil interceptors 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. 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.


               Presence of Additional Population (Workers)


7.5.14         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.


7.5.15         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.


7.5.16         Proper waste handling, storage, collection and disposal measures as recommended in the Waste chapter should be implemented by the Contractor.


Summary of Mitigation Measures for Construction Phase


7.5.17         The potential impacts on water quality during the construction phase of the Work and the associated recommendation of mitigation measures are summarized in Table 7.5.


Table 7.5

Summary of Mitigation Measures for Construction Impacts


Impact from Construction

Mitigation Measures

General water quality impacts.

To strictly follow the requirements in ProPECC PN 1/94. Minimise working area to 75m length of stream.

Increase of suspended solids and turbidity from silty site run-off during site formation.

After diversion of stream water, section of channel under construction will be used as a settlement area for construction site runoff. Sediments will be cleared from this settlement area before and after each rainstorm. No sediment removal will be allowed in rainy weather. Open stockpiles will be covered with tarpaulin during wet season.

Release of contaminated and uncontaminated sediments into stream water during stream diversion and dredging of the stream.

Ensure continuous water flow to the stream by providing temporary drainage diversion during construction.  Use of containment structures during excavation. Excavated material to be stored in covered impermeable skips and transported away from site on the same day or within 1 day.

Elevation of pH, ammonia and suspend solids  in water courses from concrete washings.

Monitoring and control of pH levels in nearby water bodies as part of the EM&A programme.

Release of oil and grease washings used in the site workshop.


The Contractor shall register as chemical waste producer if waste oil is generated. Waste oil and other chemical waste shall be collected by licenced contractors.

Increase of sewage and other wastewater from additional workers.

Provision of mobile chemical toilets and the collection and disposal of sewage by licenced contractors to government sewers. Implement proper on-site waste management.


Cumulative Impacts from Concurrent Projects during Construction Phase


7.5.18         A number of projects are identified which also have cumulative impacts on Inner Deep Bay. These are listed in Table 2.1. The projects with the greatest potential to impact water quality are those which are directly related to water bodies, such as sections of the Main Drainage Channels the cumulative impact of runoff and polluting discharges from these is potentially high. However, with the implementation measures described in this report for the construction stages and those committed for other projects, the overall cumulative impacts should be reduced. It is essential that all projects within the catchment of Inner Deep Bay implement appropriate and adequate mitigation measures to minimise impacts on downstream water quality.


7.5.19         KT13 channel is the furthest upstream of the concurrent projects in the area. As most major civil projects in the catchment are largely completed, there is limited possibility for cumulative impact. Nevertheless, small scale construction projects on private lots may be carried out at the same time as this project in the nearby villages, resulting in cumulative water quality impacts if construction site runoff and concreting works are released into KT13 or other channels in the same catchment.


7.5.20         To prevent any potential cumulative construction impacts, mitigation measures described in this report include the retention of runoff in a settlement tank and neutralisation of concrete washings, if necessary, before discharge should be enforced. Similar measures should be used for all works areas where there is a potential for polluting adjacent watercourses.


7.5.21         This project has no control over the implementation and effectiveness of other concurrent projects. However, the need for an understanding by the contractor of the implications of not only pollution from their site works, but the combined impact from several project sites, is paramount to minimising overall cumulative impacts on water quality and associated ecology.


7.6                   Potential Impacts During Operation Phase


Identification, Prediction and Evaluation of Impacts


7.6.1             Pollution loads entering secondary drainage channel KT13 during operation phase include:


                     surface runoff from upstream hinterland, Ma On Kong and Ho Pui, which could be contaminated with livestock waste.

                     domestic sewage.

                     Run-off from Route 3.


The operation of secondary drainage channel KT13, however, does not in itself generate any new pollution load to the catchment.


7.6.2             The project provides an opportunity for removing the contaminated sediments from the stream course. 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 continued enforcement of Livestock Waste Control Scheme by relevant authority. The stream currently receives some domestic waste discharges which will continue once KT13 works are complete. The level of impact will be similar to the current situation and the long term solution will be to provide sewers connecting the village households to treatment works. Provisions of sewers to villages in Kam Tin Catchment however is outside the current scope of this DP.


7.6.3             Vegetation removes dissolved and particulate pollution through absorptive, filtration and biological mechanisms. An all concrete drainage environment will not provide such functions. This impact can be mitigated through appropriate design of the channels. In line with the recommendation under the ecological assessment of the EIA, gabion base and banks will be installed instead of concrete channel, vegetation will be allowed to re‑establish along the banks.


7.6.4             Another potential impact is the temporary release of odour from polluted water and sediments in the channel during low flow conditions. As a combination of removing contaminated sediments during construction and maintenance of the channel, and long term reduction of pollution loading to the channel, will lead to a long term reduction of this nuisance. The regularised channel presents an opportunity for regular removal of accumulated sediment. This is in contrast to the natural channel which has no access for such maintenance. The new channel therefore represents an opportunity to improve the nutrients conditions with respect to odour.


7.7                   Mitigation Measures for Operation Phase


Maintenance Considerations


7.7.1             Testing of the existing sediments accumulated in the KT13 streams revealed Category M contamination. While this DP provides an opportunity for the proper recovery and disposal of these sediments during the construction phase, suitable measures need to be carried out to prevent the excavation of the contaminated sediments from polluting the stream water.


7.7.2             The most important feature for KT13 is the prospect of suitable revegetation 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. Ecological benefits of gabions are discussed in Chapter 4.


7.7.3             In addition, the use of a gabion or rock fill base for the bed of the channel has the benefit of providing uneven surfaces and cavities for sediment to accumulate, to support regenerated stream benthos. Sediment will accumulate in the new bed within the hydraulic profile. This will need to be removed periodically and this can be managed without proving fatal to the newly established benthic community.


7.7.4             Two sections of the KT13 channel, upstream and downstream of the egretry, will be constructed using gabion blocks close to the existing alignment of the stream. Engineering feasibility studies have indicated that channelisation of these sections of watercourse is required to reduce the risk of flooding.  A twin box concrete culvert will be constructed to connect the two channelized sections, with an alignment to avoid the egretry. The middle section of the original stream course will be left unmodified except for a small section of the meander for the protection of egretry and the surrounding habitats. The twin box bypass culvert will allow for flood flows to be passed down stream, but will retain the dry weather flow in the original natural channel, to ensure the current supply of water to valuable habitats will continue after the implementation of KT13.


7.7.5             No low flow channel will be provided for KT13.  The idea being that water will naturally develop its own low-flow channel.  The channel should be permitted to find (and adjust) its own natural low flow channel and natural changes in the deposition of silt, sand and rock should be tolerated.  Cleaning can be carried out during the dry season when flows are minimal.


7.7.6             The gabion channel lining will collect sediment that passes down the watercourse, as mentioned above, and 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 300mm thick, and sediment is likely to be washed downstream in heavy storms. 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.


7.7.7             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.


7.7.8             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 to channel bank stabilisation and re-vegetation.


7.7.9             Maintenance of the plants in the channel bottoms is relatively simple and they can be cut prior to the rainy season to prevent washing into Inner Deep Bay by DSD’s maintenance team. The recommended vegetation will take up both nutrients and pollutants and should be disposed to landfill. At the same time as grass cutting, sediment may be removed to prevent this being washed into Deep Bay.


Summary of Mitigation Measures for Operation Phase


7.7.10         A summary of the mitigation measures is shown in Table 7.6.


Table 7.6

Summary Mitigation Measures for Minimizing Operational Impacts


Potential Impact

Mitigation Measure

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 channel. 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 re-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 Inner Deep Bay

Sediment will be carried 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.


7.8                   Residual Impacts


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


7.9                   Monitoring and Audit Requirements


7.9.1             Monitoring and audit requirements for establishing baseline conditions and impact monitoring during construction phase for water quality is detailed in Chapter 11.


7.10               Summary


7.10.1         The current condition of stream KT13 is observed to be free of debris or rubbish, the water is free flowing, clear but still has noticeable odour.  It continues to receive some domestic wastewater from the nearby un-sewered villages houses and surface runoff from the nearby livestock farms.


7.10.2         The construction of the channel could adversely impact the water bodies through silt-laden site runoff, disturbance of stream sediment during excavation and concreting works. These impacts can be readily mitigated through the construction of a suitable drainage system with silt traps, good site management practices and careful working practices when excavating contaminated sediments and proper sewage collection and disposal system.


7.10.3         Environmental monitoring and audit requirements have been identified for water quality to establish the up to date baseline conditions of the potentially affected water bodies, and during the construction phase, which among other purposes, will check the effectiveness of the implementation of the proposed mitigation measures.


7.10.4         With the effective implementation of the mitigation measures, impacts on the water quality of receiving water bodies due to the operation of the project are expected to be negligible.





EPD (2000-2004) River Water Quality Monitoring in Hong Kong in 1999-2003


EPD (2000-2004) Marine Water Quality in Hong Kong in 1999-2003