Advisory Council on the Environment

Status Report on Sand Dredging at West Po Toi Marine Borrow Area


(ACE-EIA Paper 6/2001)
For information


In our regular "Status Report on Sand Dredging and Mud Disposal in Hong Kong", ACE Paper 7/2001 dated February 2001, we informed Members, inter alia that the West Po Toi marine borrow area had been allocated as a supplementary sand source to the Penny's Bay project. Although the recent studies are still under discussion and some of the conclusions are not shared by all parties, the purpose of this paper is to inform Members as fully as possible at this stage about the following :-

  • the history of the borrow area and the previous dredging there,
  • the computer modelling study undertaken prior to the recent dredging,
  • the comprehensive EM&A programme to control the dredging, and
  • the results of an investigation on the sedimentation at a coral site.

Establishment and previous use of the West Po Toi Marine Borrow Area (WPTMBA)

2. In the early 1990's, when the then Fill Management Committee (now the Marine Fill Committee) was examining a number of potential marine borrow areas each potential borrow area was assessed under the staged environmental assessment process used at that time. This process commenced with an Environmental Review and if necessary was followed by more detailed study. The Environmental Review covering WPTMBA was completed in 1991 and concluded that sand extraction would have only limited potential for detrimental effects provided the works were controlled by an EM&A programme.

3 On the basis of the Environmental Review, the then Fill Management Committee agreed that West Po Toi should be designated a marine borrow area; in 1992, it was authorized under the Foreshore and Seabed (Reclamations) Ordinance. The WPTMBA is therefore exempted under the EIA Ordinance which came into effect in 1998. No Environmental Permit is required for its continued use.

4. Between 1993 and 1995, nearly 30Mm3 of sand were extracted from the WPTMBA to provide sand for the West Kowloon Reclamation and for construction of the airport at Chek Lap Kok. At its peak, nearly 1Mm3 of sand was dredged per week - a satellite image taken at the time is shown in Attachment A. Apart from the project proponents' routine water quality monitoring during this dredging period, we also undertook an intensive monitoring exercise to measure the extent and decay rate of the suspended sediment plumes. This work confirmed that the plumes of very high suspended solids concentrations were short-lived and essentially limited to the immediate vicinity of the dredging.

5. In 1996, the then Fill Management Committee reserved WPTMBA for use in what was then proposed to be Lantau Port development located at Penny's Bay. The Committee, however, imposed a condition that if there were any concurrent dredging works in the vicinity a focussed water quality assessment had to be undertaken to demonstrate that adverse cumulative impacts would not result. The need for such a focussed assessment would, however, be waived if there were no other concurrent works.

6. Following completion of the earlier dredging, various estimates have been made of how much economic sand remains in WPTMBA. The current estimate is that at least 14Mm3 might be available.

Present use of WPTMBA

7. As explained in ACE Paper 7/2001, the Marine Fill Committee allocated WPTMBA to the Penny's Bay project as a supplementary sand source in November 2000. This allocation was made with the same condition as originally laid down in 1996 for the Lantau Port development, namely that a focussed assessment be undertaken to establish whether there would be adverse cumulative impacts related to other concurrent works. This assessment has been undertaken, a comprehensive EM&A programme has been put in place and dredging commenced on a single-dredger-only trial basis on 28 May. Further details are provided below.

Focussed water quality impact assessment

8. The Penny's Bay contractor HAM, appointed consultants ERM - Hong Kong to undertake the required assessment of the possible cumulative water quality impacts from other concurrent works. This assessment used computer modelling to examine water quality impacts related to the release and dispersion of sediment. Key inputs for such computer modelling are the daily extraction rate, the sediment loss rate during dredging, and the settling velocity of suspended sediment particles. The values used were based on likely dredging scenarios and data from previous studies (including measurements taken during the previous West Po Toi dredging).

9. The sensitive receivers identified during the study included areas of ecological interest (particularly corals), fish culture zones, seawater intakes and gazetted beaches. The model was used to predict suspended sediment impacts at these locations.

10. Although many sensitive receivers have been considered, the key concern has been the potential impact of sedimentation on some high ecological and conservation value table coral communities which are about two kilometres east from the WPTMBA. Although acceptability criteria of a sedimentation level of 0.1kg/m2/day and an elevation of suspended sediment concentration of 10mg/l have both been examined, it must be borne in mind that the accurate prediction of settlement of sediment on such corals is actually beyond the capabilities of the computer model. This has resulted in considerable difficulty in interpreting the modeling results. Nevertheless, it has been concluded that with appropriate mitigation methods built into the dredging programme it should be possible to avoid adverse impacts at the important coral sites.

11. A copy of ERM's assessment report is attached. Some key conclusions, assuming a weekly extraction rate of 730,000m3, are as follows :-

  • Interaction between suspended sediment plumes from WPTMBA and from other concurrent works is not expected around the WPT region. Any predicted effects on the sensitive receivers around that area are the result of dredging at WPTMBA
  • seawater intakes and gazetted beaches will experience negligible impacts, as a result of dredging at WPTMBA,
  • fish culture zones, including that at Po Toi Island, will not be adversely affected as a result of dredging at WPTMBA,
  • the multi-layered computer model indicates that communities of the high ecological and conservation value table coral at Po Toi should not be adversely affected if proper mitigation measures are implemented,
  • the water quality criterion of 10mg/l elevation in suspended sediment might be exceeded in areas of lesser ecological value where there is very low and patchy cover by a low diversity assemblage of corals.

EM&A Manual

12. Concurrently with the modelling study, ERM prepared an EM&A Manual designed to ensure protection of the sensitive marine sites, particularly the sensitive table corals. As part of the EM&A programme a baseline water quality monitoring and coral dive survey were undertaken before the recent trial dredging started. It is important to note that the living decades-old table corals recorded during the baseline survey were clearly not subject to smothering sedimentation during the previous dredging between 1993 and 1995.

13. Using the baseline data, an EM&A programme was devised and put in place for the start of dredging. Since then, further refinements have been made to the EM&A Manual. A copy of the latest update of the Manual is attached. The key components of it are :-

  • water quality monitoring at the perimeter, upstream and downstream of the dredging area,
  • water quality monitoring at the coral impact and control stations,
  • weekly dive surveys at impact coral sites and monthly full ecological survey at all impact and control coral sites, and
  • an event and action plan.

14. For effective environmental control and auditing of the dredging, various parties are involved. The environmental monitoring is undertaken by the contractor's Environmental Team, Maunsell Environmental Management Consultants Ltd (the ET), and the ET's reports are reviewed and checked by Mouchel Asia Ltd, the Independent Environmental Checker (the IEC) who is employed by the project controller, the Civil Engineering Department. In addition, because of the specialist nature of the assessments of coral health, the Agriculture, Fisheries and Conservation Department also employs an independent coral specialist, Ms Denise McCorry, to audit the dive survey results.

Impacts during the first two weeks of dredging

15. Dredging on the single-dredger-only basis effectively limited the weekly dredging production to about 170,000m3 of sand. Results of the EM&A from 28 May up until the 4 June indicated no adverse impacts resulting from the dredging.

16. In addition to the monitoring carried out as part of the EM&A, we undertook two days of intensive plume monitoring at WPTMBA. The preliminary analysis of the results of the work on 1 and 6 June are very similar to the results obtained in 1995, and earlier this year during similar monitoring of sand dredging at East Lamma Channel. The results indicate rapidly decaying sediment plumes which are essentially confined to the immediate vicinity of the dredging works.

17. A high level of turbidity and suspended solids were recorded at NE Po Toi on 5 June and sedimentation and bleaching on some table corals was recorded at the same site on 7 June. The dredging works were then stopped on 8 June to allow time to establish the cause of the sedimentation.

18. A dive survey on 18 June, jointly conducted by AFCD's coral expert and the ET, has confirmed that the sedimentation of corals recorded on 7 June has disappeared and that apart from about 3% of the previously sedimented area, there has been no damage. In the nature of things it is not possible to be conclusive as to the cause of the sedimentation but the ET's Incident Report on the 7 June survey concludes that dredging-related sedimentation was unlikely to have been the cause. It has been accepted by ET and IEC that definite conclusions cannot be drawn based on the data available. The IEC, upon assessing the data in the ET's draft report, and application of the event and action plan to the dive survey data, has recommended that dredging can be resumed with the control provided by the EM&A and its event and action plan and subject to completion of the ET's report. A copy of the ET's report is attached.

19. Subject to endorsement of the IEC's recommendation, we will resume sand dredging work at WPTMBA. Government highly values the importance of protecting the marine environment and we will continue to closely monitor the table corals and the water quality in the vicinity of West Po Toi. We will also strictly implement an agreed EM&A programme and event and action plan and necessary mitigation measures to control impacts on the environment.

Civil Engineering Department
July 2001


Attachment A

Satellite image from December 1993 showing dredging at Po Toi during the stage when weekly production at West Po Toi was nearly 1Mm3 per week

Attachment B

Focussed cumulative water quality impact assessment of sand dredging at the West Po Toi Marine Borrow Area : Final report dated 28 June 2001 : Report by ERM - Hong Kong Ltd

Attachment C

Focussed cumulative water quality impact assessment of sand dredging at the West Po Toi Marine Borrow Area : Environmental monitoring & Audit Manual - Updated Manual dated 28 June 2001 : Report by ERM - Hong Kong Ltd

Attachment D

Penny's Bay Reclamation Stage I : West Po Toi MBA : Incident Investigation Report (28.5.01 - 08.06.01) : Report by Maunsell Environmental Management Consultants Ltd


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