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.
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.
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.
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.
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.
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.
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.
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.
The Environmental Monitoring & Audit requirements have
been presented in Section 5.
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.
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.
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.
The Environmental Monitoring & Audit requirements have
been presented in Section 3.
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.
Contents
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