4    Prescription for the Recommended Facility


4.1    Introduction


This Section presents the information to be included in the specification for the detailed design, construction and operation of the proposed contaminated mud disposal facility at East of Sha Chau.  The projected decommissioning scenario is addressed and the actions required to restore the site to an acceptable level are presented.


4.2    General Description of the Proposed Disposal Facility


This study has identified a location referred to as East of Sha Chau in which a Confined Aquatic Disposal (CAD) facility could be constructed and operated in a manner compliant with environmental standards and legislation.  The facility will comprise up to three dredged pits, which together, would be able to accommodate a minimum of 8 Mm3 of contaminated sediments.  On completion of backfilling with contaminated sediments, the pits would be capped with at least 3 metres of uncontaminated dredged sediments and/or natural uncontaminated soil in order to isolate the contaminated material from the marine environment.

The pits are to be dredged to the base of the soft marine deposits but may be extended into the underlying alluvium if these materials are sufficiently soft to permit economic dredging operations.  Backfilling with contaminated sediments is permissible up to a level of no less than 3 metres below the lowest seabed level on the periphery of each pit.  Following completion of backfilling, the contaminated materials are to be covered by a layer of at least 3 metres of uncontaminated dredged mud and/or natural uncontaminated soil.  Further backfilling with uncontaminated materials may be undertaken in order to restore the original seabed levels.


4.3    Design Requirements


This study has indicated that the required 8 Mm3 disposal capacity appears attainable but there exists insufficient geological data to finalise a pit layout and design.  Further ground investigations will be required to establish in more detail the depths of the soft deposits in which the pits will be dredged.

In addition, the initial pit layouts have been based on an assumed pit slope of 1:3.  A steeper slope would increase the available storage capacity of the area and would improve the ratio between the disposal volume and the volume of the cap.  The ground investigations should, therefore, be designed to obtain the information required to design the dredged slopes.

The overall design of the facility should maximise the disposal capacity of the area and minimise the volume of dredging required to form the pits.  This will be achieved by optimising the dredged slopes, the shape of the pits and the spacing between the pits. 


4.4    Construction of Facility


Due mainly to constraints on access, the pits will be dredged using grab dredgers.  The dredged materials will be loaded into barges for onward transport to the disposal site.  The disposal pits are to be dredged in sequence and in such a manner as to:

1)      ensure continuity of disposal of contaminated sediments during the lifetime of the facility, and

2)      minimise environmental impacts on the surrounding areas;

3)      minimise the requirements for off-site disposal of the materials dredged to form the pits.


4.5    Continuity of Operation


In order to ensure continuity of disposal operations, it will be necessary to dredge the first pit in advance of the time when disposal operations are due to commence in the East of Sha Chau area and to ensure that subsequent pits are dredged, in turn, before the preceding pit is completely filled with contaminated materials.


4.5.1    Minimisation of Environmental Impacts


Environmental impacts arising from sediment release during dredging, disposal and capping operations can be minimised by programming construction so that dredging and capping operations extend over the maximum length of time available, thus minimising the number of dredgers, and rate of dredging and capping, that are required.

For example, if it is anticipated that a new pit will be required two years after commencement of backfilling operations in a pit, the dredging of the new pit should utilise as much of that time as is practical and economic so that the daily rate of dredging is minimised.  Dredging production rates should be monitored so that, in the event that there is risk of a delay to completion of the new pit, additional plant can be mobilised at an early stage.  This approach would reduce the risk that the dredging effort has to be suddenly greatly increased just before a pit is required for disposal operations.

4.5.2    Minimisation of Off-Site Disposal of Dredged Materials


In order to minimise the need to dispose of dredged material off-site, as much as possible of the material dredged to create the pits should be used for:


1)      the capping of other pits in the East of Sha Chau area;

2)      capping of Pit IVc in the East of Sha Chau area, and

3)      topping-up of other pits in the East of Sha Chau Area where consolidation of placed materials may have resulted in seabed depressions.


Excess dredged material that cannot be used for the above purposes are to be disposed in areas allocated by the Marine Fill Committee.


4.5.3    Capping of Contaminated Sediments


When a pit has been filled to capacity with contaminated sediments, a cap of uncontaminated sediments and/or natural uncontaminated soil is to be placed in order to isolate the contaminated sediment from the environment.  The cap will be at least three metres thick but should not result in the formation of areas where the seabed level is higher than the seabed that existed prior to the construction of the facility.


Construction of the cap is to commence as soon as practical after completion of backfilling with contaminated sediments.  Where possible, the materials used for the cap are to be sourced from other disposal pits that are being dredged.


Where possible, materials dredged to form the first and second pits are to be used to cap Pit IV in East of Sha Chau and to top-up any depressions over other previously-capped pits in the area.


Water quality modelling results were based on the use of uncontaminated mud as capping material.  This presents a worst-case scenario due to fines in uncontaminated mud being of a smaller size than those found in natural uncontaminated soil.  Therefore, the use of natural uncontaminated soil for capping would be acceptable due to suspended sediment levels being lower than those modelled for.  Additionally, it should be noted that this practice is taking place at present at the existing pits at East of Sha Chau and no adverse environmental impacts have been documented.


4.6    Environmental Monitoring of Construction Activities


The Environmental Monitoring & Audit requirements have been presented in Section 5.


4.7    Operation of Facility


4.7.1    Method of Disposal of Contaminated Sediments


The facility will be able to accept contaminated materials delivered either by barges or by trailing suction hopper dredgers.  Barges and tugs will be able to enter the pits either directly, if their draft is small, or via short dredged channels leading from the maintained channel to Tung Chung.  Barges will place the contaminated sediments in the pits by simple bottom discharge.


Trailing suction hopper dredgers are too large to enter the pits and will need to stand off in the deeper water to the north east of the area and pump the contaminated sediments to the pits using through a floating hose or a combination of floating hoses and a submerged pipeline.  The hose will terminate with a down-pipe which will ensure that the contaminated sediment is released at a depth that is below the level of the seabed surrounding the pit.


4.7.2    Rate of Disposal of Contaminated Sediments


The water quality impact assessment of this EIAFSS Report (Part 2, Section 2) concluded that disposal of contaminated sediments at a maximum rate of 26,700 m3 per day would not give rise to adverse environmental impacts.  This rate may be applied to both barges and trailer dredgers.


4.8    Management of Disposal Operations


The management system that is currently employed at East of Sha Chau should also be employed for the disposal operations at East of Sha Chau.  The future operation of the mud dumping operations will be almost identical to current activity, in that a target barge will be stationed on site and a workboat escort incoming split-hopper barges, one at a time to the site.   This operation ensures that marine activity at the site is controlled and not significant, however a suitable site for the temporary mooring of waiting barges, if any, will be required.


Prior to the commencement of disposal operations the Contractor should seek approval with CEDD by means of a Method Statement.  No work should commence until written approval has been received.  It is envisaged that due to the relatively weak currents in the area, in combination with the very shallow water, it will not be necessary to determine, based on real-time current measurements, the optimum disposal location for each barge.


The facility management barge should be anchored adjacent to the disposal area.  CEDD inspectors, as is the current practice, will check the documentation of incoming barges and register the disposal event.  The pit will previously have been divided into a number of disposal ‘target areas’, each approximately 75m in diameter.  Disposal events will take place in the target areas in rotation so as to ensure an even backfill level.  Periodic bathymetric surveys will be undertaken in order to check the backfill level.  The frequency of surveying will be determined on the basis of the actual rates of backfill.


4.9    Environmental Monitoring of Disposal and Capping Operations


The Environmental Monitoring & Audit requirements have been presented in Section 3.


4.10    Facility Decommissioning and Site Restoration


On completion of backfilling with contaminated sediments, and capping with uncontaminated sediments, it is likely that consolidation of the placed materials will continue for many years.  The consolidation will eventually give rise to depressions on the seabed.  The facility area should be periodically surveyed to monitor the extent and depth of the depressions, which should be backfilled using uncontaminated dredged materials (if available) of a type that are generally similar to the materials found on the surrounding seabed and/or natural uncontaminated soil.




4  Prescription for the Recommended Facility  1

4.1  Introduction  1

4.2  General Description of the Proposed Disposal Facility  1

4.3  Design Requirements  1

4.4  Construction of Facility  2

4.5  Continuity of Operation  2

4.6  Environmental Monitoring of Construction Activities  3

4.7  Operation of Facility  4

4.8  Management of Disposal Operations  4

4.9  Environmental Monitoring of Disposal and Capping Operations  55

4.10  Facility Decommissioning and Site Restoration  55