For information


Monitoring of three new uncontaminated disposal pits at
south of Tsing Yi, north of Lantau and East of Tung Lung Chau


At the Finance Committee (FC) meeting on 15 January 1999 when the captioned project was briefly discussed, the Chairman asked that the project be referred to a LegCo panel for further discussion. Given that FC Members' questions are mostly on environmental issues of the proposed backfilling, the Environmental Affairs (EA) Panel is considered the most appropriate. This supplementary paper summarises the key environmental aspects of the proposed project, which have been fully addressed in the Environmental Impact Assessment studies. The original paper to the FC is attached for information.


2. It is Government policy to leave mud in place whenever possible. Nevertheless, some mud must be dredged - this is not limited to essential maintenance dredging of ports, fairways and anchorages, but includes new works for construction of typhoon shelters and infrastructure development. The ability to dispose of uncontaminated mud is also vital to other projects such as the programmed flood protection works in the New Territories.

3. Uncontaminated mud generated from various projects has been disposed of in the open sea floor disposal grounds at south of Cheung Chau and east of Ninepins.

4. In November 1994, the Fill Management Committee endorsed the policy of backfilling exhausted marine sand borrow pits with uncontaminated mud, as the preferred alternative to disposal at the open sea floor disposal grounds. Besides providing much needed disposal capacity, the use of these empty pits has the following benefits:

  1. restoration of the natural hydrodynamic regime;

  2. recolonisation of the seabed by benthic communities similar to those which existed prior to sand dredging; and

  3. restoration of routine and emergency ship anchorage areas.

5. Sand from the marine borrow areas at south of Tsing Yi, north of Lantau and east of Tung Lung Chau was extracted for various land formation projects from 1992 to 1996. The proposed project is to reinstate the seabed at the three empty pits to original levels and conditions using uncontaminated mud.


6. Two separate Environmental Impact Assessment (EIA) studies were carried out to determine the environmental acceptability of using the empty seabed pits for disposal of uncontaminated mud. The EIA studies included assessments of water quality, marine ecology, fisheries, air quality and noise.

7. The studies predicted that under the proposed operations plan (paras. 12 & 13 below), the impacts of backfilling on suspended sediment (SS) concentrations, dissolved oxygen and nutrients would be in compliance with the adopted assessment criteria or Water Quality Objectives (WQOs).

8. The proposed backfilling operations would not result in any adverse impacts to the marine ecology in the vicinity of the pits because:

  1. soft bottom benthic communities in and around the pits do not contain rare species;

  2. water quality at nearby coral sites such as Ninepins Islands and Tung Lung Chau would comply with the adopted SS standard and the WQOs;

  3. the south of Tsing Yi and north of Lantau sites are not located within areas of frequent Chinese White Dolphins sightings and do not represent areas of preferred feeding; and

  4. the Finless Porpoise present near east of Tung Lung Chau would not be affected because water quality would be within the WQOs.

9. The study at south of Tsing Yi and north of Lantau concluded that the backfilling operations would not cause unacceptable impacts on capture fisheries, mariculture zones including Ma Wan, and fish fry collection areas. The assessment of fisheries impacts at east of Tung Lung Chau predicted that there would be no SS elevation at the nearby mariculture zones at Tung Lung Chau and Po Toi O, the impacts to capture fisheries would be localised and temporary, and the effects on the spawning ground would be very small and temporary.

10. The proposed backfilling would not cause any unacceptable air quality or noise impacts.

11. The findings and recommendations of the EIA studies were presented to the Advisory Council on the Environment. The Council endorsed the report for south of Tsing Yi and north of Lantau on 9 February 1996 and the report for east of Tung Lung Chau on 23 March 1998.


12. The studies also proposed an operations plan for the backfilling activities. The main features of the operations plan are as follows:

  1. South Tsing Yi is suitable for trailer- and grab-dredged mud at daily backfilling rates not exceeding 100,000 cubic metres;

  2. North Lantau is suitable only for grab-dredged mud at daily disposal rates up to 10,000 cubic metres; and

  3. East Tung Lung Chau - the northern pit is suitable for trailer- and grab-dredged mud at a maximum daily disposal rate of 50,000 cubic metres, while the southern pit is suitable for trailer-dredged mud at a maximum disposal rate of 25,000 cubic metres (only one pit will be used at any given time).

13. At East Tung Lung Chau, to minimise the effects of current direction and strength, disposal operations during the dry season (when currents typically flow to the southwest) will be restricted to the northern pit and during the wet season (when currents typically flow to the northeast) to the southern pit.


14. During backfilling, an environmental monitoring and audit (EM&A) programme will be implemented to verify the predicted results of the EIA studies and to provide an early indication of environmental degradation should any of the environmental control measures or practices fail to achieve acceptable standards. The EM&A programme meets the new international standards for managing mud disposal facilities adopted by the London Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter, which applies to Hong Kong.

15. The monitoring programme will include the following:

  1. bathymetric monitoring to measure backfilling levels, to ensure that mud placed in the pits is evenly spread and no high spots are formed;

  2. water quality monitoring to ensure compliance with acceptable levels of turbidity, concentrations of suspended sediments and dissolved oxygen; and

  3. ecological monitoring to study the number and variety of marine organisms, including worms (in sediments), fish and corals as appropriate.

16. Monitoring fieldwork will be undertaken by specialist contractors under the supervision of the Civil Engineering Department. An environmental consultant will be employed to assess the ecological monitoring results. All monitoring results will be reviewed by a Working Group comprising representatives of Environmental Protection Department, Agriculture and Fisheries Department and Civil Engineering Department.

17. The funding ($29.2 million) requested under FCR(98-99)64 will be used to implement the monitoring programme described in paras. 15 and 16 above at all three sites for a period of about five years. The average cost is therefore about $0.38 per cubic metre of mud disposed of. Apart from the consultants fees of $4.5M for assessing the ecological monitoring results, all the estimated expenses are for the engagement of survey vessels, monitoring equipment and qualified operators and personnel to collect bathymetric, water quality and marine ecological data.


18. Besides seeking Advisory Council on the Environment endorsement of the EIA studies, the project was presented at the Aquaculture Subcommittee and Capture Fisheries Subcommittees of the Advisory Committee on Agriculture and Fisheries. The Subcommittees did not object to this project. The Tung Lung Chau Mariculture Association indicated their support for the project in writing.

19. The proposal of backfilling at the east of Tung Lung Chau site was also presented to the Environmental Improvement Committee of the Sai Kung Provisional District Board and some members expressed concern about the potential environmental impacts resulting from the backfilling. To allay their concern, interested members will be invited to a site visit during the backfilling. This suggestion was well received.


20. Backfilling the empty sand borrow pits has the benefit of restoring the seabed to its natural state, and would not result in any adverse impacts to the marine ecology in the vicinity of the pits. We will implement an environmental monitoring and audit programme to ensure that the backfilling activities are carried out in compliance with internationally acceptable standards.

Works Bureau
30 January 1999