The engineering design is
principally the means to control water levels to close tolerances in this
largely flat area, with a minimum of mechanical assistance. Construction must
not be disruptive to the surrounding habitat. In addition, the ground in which
the ponds are located comprises a significant depth of mud, such that moderate
surcharge loads could cause significant short and long term settlement. This must be guarded against in the
construction of the mitigation area.
3.2.1
The
main constraint on the design of the wetland mitigation scheme is the highly
seasonal rainfall in Hong Kong. Typically there is a period of water deficit
(i.e. rainfall is exceeded by evapotranspiration) from September to March.
Without an additional source of water it is not normally possible, to maintain
permanent shallow wetlands, such as those required in the marshland mitigation
area. Permanent wetlands may be retained by storing sufficient water to
overcome the deficit period, but such wetlands are then inundated by water which
is too deep for many of the wetland target species over the much of the annual
cycle.
3.2.2
Potential
water sources include the local watercourses, but these are highly polluted
(see below) and / or underlying aquifers. Due to the presence of marble caverns
in the underlying rocks in the Lok Ma Chau area, boreholes are not allowed in
the area for engineering safety reasons. It is also possible that any
underlying aquifers may be brackish.
3.2.3
There
are no data currently available on groundwater levels within or nearby the
mitigation areas. Therefore it is not known whether the proposed wetland areas
would be potentially subject to water inputs or losses. Further site
investigation data are awaited before this can be evaluated.
3.2.4
There
are also potential constraints on the wetland design regarding drainage options
as a result of the tidal conditions of the water courses and frequent local
flooding. The top of the bunds in the area are generally at +3.5mPD and in
periods of heavy rain there is flooding in the area. This will be alleviated
with the construction of the San Tin Eastern Main Drainage Channel, which will
protect the locality from upstream events. The ponds will not flood as
frequently as has been the case to date. Rainfall intensities greater than that
of the annual event will likely cause the ponds to be inundated due to high
watercourse levels.
3.2.5
The
outfall into the channel which receives run-off from the station roof must not
cause this channel to be over capacity. The channel has been sized to allow for
a 1 in 200 year event.
3.2.6
The
outfall to the natural watercourse which provides drainage for the area is
tidal. Constructing an outfall will be difficult, and the watercourse banks and
bed would have to be protected from erosion. Any collapse of the bank or loss
of cross section will make the area more susceptible to flooding. An outfall
into this watercourse would require a flap valve. If this were to become wedged
open with debris, the tidal range could send saline water into the pond system.
In view of this all water will be need to be taken to the watercourse to the
east of site (Watercourse B).
3.3.1
As
described in Section 1.4 the soils present at the site consist of poorly
drained marine clays. These are sufficiently impermeable to maintain wetland conditions
or fish ponds over the site as demonstrated by the current presence of such
habitats, which are in the main solely maintained by direct rainfall inputs.
3.3.2
The
soils are, however, acid sulphate soils which can lead to highly acidic
conditions when these are disturbed and dried. Nevertheless vegetation such as Phragmites does spontaneously recover on
fishpond bunds and therefore the maintenance of basic vegetation cover is
unlikely to be constrained. However, care with soil handling or remedial measures
(e.g. application of lime) may be needed to establish more sensitive and
diverse wetland vegetation of high ecological value within the marsh areas of
the site.
3.4
Water Quality
3.4.1
Information
on the water quality in the smaller channels flowing through or adjoining the
site is not available. However, it is likely that these water courses contain
water with highly polluted eutrophic water similar to other channels in the
area.
3.4.2
The
use of nutrient rich water in maintaining wetland conditions may be helpful in
raising productivity levels in wetlands, thereby maintaining levels of food
resource for target species. However, it is considered that the hyper-eutrophic
conditions of the nearby water courses, especially during low flows, when the
additional water supply is most critical, are too poor for these sources to be
suitable for the target wetland habitats and many of the
target species.
3.4.3
The
maintenance of wetlands conditions and achievement of target water levels will
therefore be dependent on the storage of direct rainfall.
3.5.1
Access
to the edge of the mitigation area may be gained by the service road to the Lok
Ma Chau station. However, access to other areas of the fish ponds and other
parts of the mitigation area will only be possible by the fish pond bund
tracks. These vary in width and their capacity for supporting heavy or large
vehicles such as earth moving plant.
3.5.2
The
site is within the Closed Border Area.
3.6
Adjacent Habitats / Land-uses
3.6.1
The
area is mostly surrounded by fish ponds that are contiguous with those of the
Inner Deep Bay Ramsar site and MPNR. To the north the area is bounded by the
Shenzhen River. These habitats / land uses provide no constraints on the
proposed wetland mitigation.
4.
Detailed
design and construction methods
4.1.1
The mitigation area
will consist of two main habitats: fishponds (most of which will be under
active traditional fish pond management) and a new area of created marshland
with a mixture of wetland vegetation and open water pools (see Figure
4.1).
4.1.2
The
fish pond area currently consists of 22 existing fishponds, 7 of which will be
joined to create larger ponds (because these are favoured by many water birds):
Ponds 3, 6, 8, 9, 10, 13 and 14.
4.1.3
The
ponds (1 – 15) within the fish pond compensation area will be managed as a
mixture of active fish pond, shallow lakes, naturally regenerated marsh and
reedbed.
§
Ponds
1 and 2 will be managed to create moderate-shallow rain-fed lakes primarily as
duck habitat.
§
Pond
5 will be maintained as a reedbed with open pools.
§
Pond
12 will be maintained as a shallow marsh, with managed naturally regenerated
vegetation.
4.1.4
All
other ponds will be used in a 3-year rotational sequence of fish farming / shallow set-aside ponds (with no fish
stocking). Thus approximately 1/3 of ponds will be set-aside from
production in any one year, one of
which will be used as a contingency reservoir pond.
4.2.1
As the area already
consists of fish ponds the mitigation works would be restricted to the modification
of existing ponds, rather than construction of new ponds. These modifications
will consist of the joining of ponds to reduce enclosure effects and to provide
shallow margins and islands (Figure 4.1).
4.2.2
Where ponds are joined
the bund material currently separating them will be lowered and side cast
to create shallow sloping margins to islands and / or shoals as well as interconnecting
channels. This will create additional areas of muddy shallows that will provide
suitable feeding areas for herons, egrets, spoonbills and, during low water
levels, smaller wading birds. The levels to be attained on each modified bund
are indicated in Figure A3.1a in Appendix 3.1.
4.2.3
The
purpose of reprofiling the selected pond bunds is to create as shallow a slope
as feasible (given the properties of the bund materials) between the Initial
Operating Level (IOL) and Maximum Operating Levels (MOL) and over the range of
the Low Operating Levels (LOL; see Table 5.1).
4.2.4
To
facilitate vehicle access to all parts of the site, for water level management
and other activities, some works on the remaining bunds will be necessary. The tops of the bunds may require some
vegetation clearance and strengthening with a geofabric to enable them to be
used as accessways. Current practice is to use some stone fill to form the
running surface and it is proposed to extend this where necessary.
4.2.5
The
work to lower the bund between ponds 13a and 13b will require the removal of
some electricity poles.
4.2.6
Consideration
will be given to the use of earth covered floating platforms seeded with Paspalum distichu. These could be placed in each operating
fish pond to provide additional foraging areas for herons and egrets etc and
potential breeding sites for some water birds (such as Little Grebe) as well as
shelter for fish and aquatic invertebrates.
4.2.7
The marsh area around
the station will principally consist of a shallow basin sloping gradually
away from the station buildings, to produce a zoned wetland with drier terrestrial
habitats on the upper slopes and permanent wet marshland
and pools at the lower end (see Figure 4.2).
4.2.8
A
clay curtain will be placed at locations where tranverse flows from the marsh
could take place, causing a loss of water. Levels will be controlled by adjustable sluices, whilst the
addition of water will generally be by a small pump.
4.2.9
A
feature of this area is the likely consolidation of ground if it is surcharged
over and above current pressures, and the length of time taken to achieve
ultimate consolidation. This has a bearing on the design of the water control
structures and the fill levels during construction. For example, draining and
filling a pond to the top of the bund is likely to lead to a settlement of the
order of 500 mm. Whilst the first 300 mm ma take place in the first month,
after 10 years the settlement would be 420 mm. To overcome this movement there
are three options:
• A
significant initial surcharge to achieve faster consolidation
• The
installation of vertical drains to accelerate consolidation
• Design
of the fill and water control structures to allow for the long term
consolidation.
4.2.10
Of
these three, the last option will be implemented with the first item depending
on the availability of excess materials arising from the construction of Lok Ma
Chau StationTerminus.
4.2.11
After
completion of the site formation, according to the design drawings and
specifications described above, further site formation works will be carried
out to provide fine scale modifications to the topography, e.g. creation of
uneven edges to ditches, small pools (0.5 m deep) and hummocks and variations
in slopes, to increase structural diversity across each mitigation area. These
works will be directed by an on-site wetland habitat creation ecologist.
4.3.1
It
is assumed that ground water levels will be sufficiently low during the dry
season that all of the wetland mitigation areas will potentially subject to
water losses through seepage. It is therefore essential that the loss of water
through seepage is kept at negligible levels. Within the entire mitigation area
it is assumed that seepage is within acceptable levels as the long term
persistence of fish ponds indicate that rainfall and water retention is
adequate to maintain fish production without the requirements for external
water supplies (see Section 1.4).
4.3.2
As
described above, the existing pond clays in the marshland area will be overlain
with a layer of imported sand (see above) as a surcharge and to bring the marsh
up to its required levels. Water will readily permeate this layer but will be held
within the wetland mitigation area by the clays underlying the ponds and in the
existing pond bunds.
4.3.3
Within the marshland
area the sand layer will be overlain with a layer of top soil as required
to raise the ground levels to those indicated in the land formation plan (Figure
4.2). The soils will consist of a mix of pond bund material, sand and
completely decomposed granite as described in the specification provided in
Appendix 3.2.
Fish Ponds – Wet Season Controls
4.4.1
The engineering inputs
to the LMC ponds are principally to install a water management system and
to provide access for management. The proposed water management system comprises
an arterial pipe system to transfer water between ponds in the dry season
when the major draining down will take place, plus a series of adjustable
sluices to allow fine control of water levels in the wet season (Figure
4.1). The sluices would automatically allow through excess water from
a group of ponds to the lowest pond in the group, from where it will be pumped
to a watercourse. The pump will be located in a manhole constructed in the
bund; Appendix 3.3 refers.
4.4.2
Current
practice is to drive small vehicles around the ponds, and this will be the case
under the new regime. The vehicles will transport the pump and other small
equipment.
4.4.3
A bridge over the watercourse
to gain access to Ponds 1 and 2 will be constructed to carry a pipe to remove
excess water from these ponds. The bridge superstructure, which will be constructed
in timber, will be founded on a hollow concrete structure. There will be no
access for personnel across the bridge – Figure 4.3
refers.
4.4.4
All
wood structures will be designed to BS 5268 Part 2 and treated in accordance
with the recommendations of BS 5268 Part 5.
4.4.5
The
excess water drainage facility comprises a catchpit with an adjustable weir,
and pipe through the bund to the receiving pond. A difference in waterlevels
between ponds of 200 to 300 mm is necessary to ensure this can operate.
Draining of the ponds is likely for the construction of these elements.
4.4.6
To
drain wet season excess water from the ponds at low operating levels, they will
be grouped as follows:
• Group
1 Ponds
9b, 9a, 6b, 6a, 3b: 3a
• Group
2 Ponds
4, 7: 8b: 11: 8a
• Group
3a Ponds
10b, 10a: 13b: 13a
• Group
3b Ponds
14a, 14b: 15
4.4.7
For
one year in 3 a group will be at low operating level, with rotation of groups
on a 3 year cycle. Ponds in a group separated by commas flow one to the other
to reach the collection pond. The last pond (in bold) in each group is drained
by pumping. All other ponds drain by pipes and weirs.
4.4.8
The
same groupings apply in the years when the ponds are at normal operating
levels, but with different collection ponds. These are the pond numbers which
are underlined.
4.4.9
The
water management system is based on a uPVC pipe system buried in the bunds,
which is also carried by the bridge.
Fish
Ponds – Dry Season Controls
4.4.10
In
the dry season water can be a scarce commodity, and the operation of the ponds
requires that a considerable quantity of water can be transferred from pond to
pond in a relatively short time. It is assumed that drain down from one
pond/filling of the receiving pond should take approximately one week. Interim
storage in a reservoir is not considered to be feasible. On this basis the
maximum weekly flow is approximately 20,000 m3 a week, or around 80
litres/second assuming daytime operation for six days of the week. Flows will
be in opposite directions according to need.
4.4.11
This
is a significant quantity of water. There are two options for the design of the
system: a gravity system or a pumped system. Given the constraints of a gravity
system and the flow requirements, and also the wet season excess water
controls, a pumped system is necessary.
4.4.12
The
pumped system would be sealed, and a portable pump lowered into the appropriate
bund manhole. The pump would be connected by a flexible pipe to the permanent
system, and water directed through the arterial system to the appropriate
manhole from which it can be directed into the receiving pond. The required
pump rating is approximately 15kW.
4.4.13
At
the receiving pond the water would outfall onto a precast concrete slab on the
bed of the pond. Reducing turbidity to a minimum is a desirable feature of the
system.
4.4.14
Power
for the pumps would come from a supply installed around the ponds, such that no
generators are necessary. Both systems will require some concrete catchpits,
manholes etc, but these will be below ground and will be constructed of precast
items.
4.4.15
The
system has parallels with commercial practice and the practices adopted by WWF
at Mai Po, pond 20.
4.4.16
Watercourse
A (to the east of the site) will receive rainfall runoff from the station roof.
To separate the station area run-off from the pond system a weir will be constructed,
with a crest level of +3mPD. Given the potentially high flows associated with a
storm, the watercourse , as part of the pond system, will receive half of the
roof run-off, whist the other half and all station groundwater run-off will
outfall directly into the Shenzhen River. The banks forming the channel are at
+3.5mPD. Under a 10 year storm event water levels rise to +3.5mPD, that is the
area is inundated, although the channel itself can theoretically sustain a 1 in
200 year event. The excess water resulting from an annual event will not
adversely impact the channel because:
• There
is spare capacity under an annual event,
• The
channel has been designed assuming a catchment which includes half the ponds.
4.4.17
Water
levels in the marshland mitigation area will be controlled by an adjustable
sluice allowing flow from Water Course A out of the marsh, or by pumping in
using a small pump, depending on seasonal water levels.
Fish Ponds
4.5.1
All
fish pond water will be obtained by direct rainfall and will be retained and
recirculated during drain-down periods as necessary. One pond will also be used
in each year as a reservoir pond as a contingency measure to ensure that pond
water levels are kept within tolerance levels (see below). No surface or
groundwater water supplies will be used for fish pond operations.
4.5.2
The water requirements
for the marshland area will be met by direct rainfall supplemented with runoff
from the station roof area, which will be stored in Water Course A (Figure
4.1).
4.5.3
A
water budget with predicted water levels for the marsh area based on average
monthly rainfall and evaporation rates between 1990 and 1999 is provided in
Appendix A3.4. This indicates that over the last 10 years the available water
from direct rainfall and the station roof runoff would have been sufficient to
maintain water levels at target levels (see Appendix A3.4). These target levels
allow restricted falls in water levels to provide newly exposed and wet margins
to the wetland areas, which provide good feeding areas for many wetland birds.
Vegetation
to be Retained
4.6.1
All
of the semi-natural areas are readily re-creatable and there are no individual
plant species or specimens which should be conserved in situ or transplanted. No restrictions requiring the protection
of existing vegetation are, therefore, necessary; indeed in many instances the
deliberate destruction of existing vegetation of limited ecological value will
be required.
Vegetation
to be Established in the Fish Pond Area
4.6.2
A
list of the species to be established on the fish pond bunds is provided in
Table 4.1 below. Reed-fringed areas will provide suitable habitat for Ardeids
(especially bitterns Ixobrychus
spp.), rails and passerines. Paspalum
distichum will provide loafing areas for Common Teal as well as food for
this and other duck species.
4.6.3
Phragmites
will be planted in Pond 5 and along the eastern margins of Ponds 13a and 13b
and the southern margins of Ponds 14a and 15. Paspalum will be planted along
all the bunds of Ponds 1 and 2. On the remaining bunds natural regeneration of
vegetation will be allowed, but with 25% of these kept as largely bare banks to
provide foraging and/or loafing areas for larger Ardeids, Black-faced
Spoonbills and Red-billed Starlings. The areas kept bare will be rotated on an
annual basis.
Table 4.1
Wetland Species to be Established in the Fish Pond Mitigation Area
|
Species |
Approx. density (no. m-2) |
%
cover |
Quantity(no. hectare) |
Growth form |
Approx. height for planting (cm) |
|
Phragmites
australis |
4 |
100 |
|
Monostands |
>20 cm |
|
Paspalum
distichum |
N.A. |
0 – 100* |
Broadcast seed |
Monostands |
N.A. |
Notes: * Paspalum distichum seed to be broadcast
on permanently dry and seasonally wet areas. Overall percentage of cover along
punds to be 25 – 75% around each pond.
4.6.4
A
list of the species to be established in the marshland mitigation areas is
provided in Table 4.2 below. The planting list is derived from those native
species known from Luk Keng Marsh, Liu Pok Marsh and recently established freshwater
wetland areas at Mai Po and San Tin. There are no suitable lowland freshwater
wetlands in Hong Kong of sufficient size, diversity and naturalness to provide
a template for a suitable planting list or species mix as all such wetlands
(even the most natural such as Luk Keng) are highly modified by man and/or
domestic animals. The proposed list is, therefore, made up of native species
and contains redundancies to accommodate the likelihood that not all species
selected will necessarily thrive in the particular conditions to be created.
Habitats to be created will cater for requirements of target species detailed
in Section 2 above.
Table 4.2
Species for establishment in Lok Ma Chau Freshwater Marsh (numbers below refer to planting zones shown in Appendix 3.5)
|
Mix
/ species |
Size |
Shoots |
Density
/ sq m |
%
Cover |
Block
size |
||
|
1. Lightly vegetated
with bare areas (permanently damp). |
|||||||
|
Bacopa
monnieri |
20 |
5 |
25 |
10 |
50 |
||
|
Hygrophila
salicifolia |
10 |
5 |
25 |
10 |
50 |
||
|
2.
Lightly vegetated with bare areas (seasonally wet). |
|||||||
|
Bacopa
monnieri |
20 |
5 |
25 |
20 |
50 |
||
|
Cyperus
iria |
20 |
3 |
16 |
10 |
50 |
||
|
Fimbristylis
ferruginea |
20 |
5 |
25 |
10 |
50 |
||
|
Saururus
chinensis |
20 |
5 |
25 |
10 |
50 |
||
|
3.
Well vegetated (permanently damp). |
|||||||
|
Bacopa
monnieri |
20 |
5 |
25 |
10 |
100 |
||
|
Commelina
diffusa |
20 |
3 |
16 |
10 |
100 |
||
|
Chrysopogon
aciculatus |
20 |
5 |
16 |
10 |
50 |
||
|
Fimbristylis
complanata |
20 |
5 |
16 |
10 |
100 |
||
|
Fimbristylis
subbispicata |
20 |
5 |
25 |
20 |
100 |
||
|
Philydrum
lanuginosum |
40 |
3 |
16 |
10 |
100 |
||
|
Polygonum
glabrum |
20 |
5 |
16 |
20 |
100 |
||
|
Sagittaria
trifolia |
20 |
3 |
16 |
10 |
50 |
||
|
4. Well vegetated (seasonally wet). |
|||||||
|
Cyperus
malaccensis |
40 |
3 |
16 |
20 |
100 |
||
|
Cyperus
pilosus |
40 |
3 |
16 |
5 |
50 |
||
|
Fimbristylis
ferruginea |
20 |
5 |
25 |
10 |
50 |
||
|
Ludwigia
antipoda |
20 |
5 |
25 |
5 |
50 |
||
|
Polygonum
dichotomum |
20 |
5 |
16 |
10 |
100 |
||
|
Polygonum
hydropiper |
20 |
5 |
16 |
20 |
100 |
||
|
Ranunculus
scleratus |
20 |
3 |
16 |
5 |
50 |
||
|
Rumex
maritimus |
20 |
5 |
25 |
10 |
100 |
||
|
Sagittaria
trifolia |
20 |
3 |
16 |
5 |
50 |
||
|
Saururus
chinensis |
20 |
5 |
25 |
10 |
50 |
||
|
5. Tall marsh (seasonally wet). |
|||||||
|
Bacopa
monnieri |
20 |
5 |
25 |
5 |
50 |
||
|
Cyperus
malaccensis |
40 |
3 |
16 |
20 |
100 |
||
|
Eleocharis
plantagineiformis |
20 |
5 |
16 |
10 |
100 |
||
|
Juncus
prismatocarpus |
40 |
3 |
16 |
10 |
50 |
||
|
Polygonum
barbatum |
20 |
5 |
16 |
10 |
100 |
||
|
Ranunculus
scleratus |
20 |
3 |
16 |
10 |
50 |
||
|
Sagittaria
trifolia |
20 |
3 |
16 |
10 |
50 |
||
|
Saururus
chinensis |
20 |
5 |
25 |
5 |
50 |
||
|
Schoenoplectus
littoralis |
40 |
3 |
16 |
20 |
50 |
||
|
6.
Tall marsh (largely wet but partially dry in mid-winter). |
|||||||
|
Bacopa
monnieri |
20 |
5 |
25 |
5 |
50 |
||
|
Cyperus
malaccensis |
40 |
3 |
16 |
10 |
100 |
||
|
Eleocharis
plantagineiformis |
20 |
5 |
16 |
10 |
100 |
||
|
Equisetum
debile |
20 |
5 |
16 |
10 |
50 |
||
|
Ludwigia
adscendens |
20 |
5 |
25 |
5 |
50 |
||
|
Polygonum
glabrum |
20 |
5 |
16 |
20 |
100 |
||
|
Rumex
maritimus |
20 |
5 |
16 |
20 |
100 |
||
|
Sagittaria
trifolia |
20 |
3 |
16 |
10 |
50 |
||
|
Schoenoplectus
littoralis |
40 |
3 |
16 |
10 |
100 |
||
|
7. Tall marsh (permanently wet). |
|||||||
|
Cyperus
malaccensis |
40 |
3 |
16 |
20 |
100 |
||
|
Cyperus
pilosus |
40 |
3 |
16 |
20 |
100 |
||
|
Ludwigia
adscendens |
20 |
5 |
25 |
20 |
100 |
||
|
Utricularia
aurea |
20 |
3 |
16 |
10 |
50 |
||
|
Utricularia
bifida |
20 |
3 |
16 |
10 |
50 |
||
|
8. Short marsh (seasonally wet). |
|||||||
|
Bacopa
monnieri |
20 |
5 |
25 |
20 |
100 |
||
|
Blyxa
japonica |
20 |
5 |
25 |
10 |
50 |
||
|
Commelina
diffusa |
20 |
3 |
16 |
10 |
100 |
||
|
Cyperus
iria |
20 |
5 |
25 |
10 |
50 |
||
|
Eleocharis
congesta |
20 |
5 |
25 |
10 |
50 |
||
|
Eleocharis
plantagineiformis |
20 |
5 |
25 |
10 |
50 |
||
|
Eleocharis
spiralis |
20 |
5 |
25 |
10 |
50 |
||
|
Fimbristylis
cymosa |
20 |
5 |
25 |
10 |
50 |
||
|
Fimbristylis
ferruginea |
20 |
5 |
25 |
10 |
50 |
||
|
9.
Short marsh (usually wet but partially dry in mid-winter). |
|||||||
|
Bacopa
monnieri |
20 |
5 |
25 |
20 |
100 |
||
|
Blyxa
japonica |
20 |
5 |
25 |
10 |
100 |
||
|
Cyperus
iria |
20 |
5 |
25 |
5 |
100 |
||
|
Eleocharis
congesta |
20 |
5 |
25 |
10 |
100 |
||
|
Eleocharis
plantagineiformis |
20 |
5 |
25 |
20 |
100 |
||
|
Equisetum
debile |
20 |
5 |
25 |
10 |
100 |
||
|
Fimbristylis
ferruginea |
20 |
5 |
25 |
10 |
100 |
||
|
Fimbristylis
subbiscipata |
20 |
5 |
25 |
10 |
100 |
||
|
Limnophylla
chinensis |
20 |
5 |
25 |
5 |
100 |
||
|
10.
Bamboo clumps (planted at 50 cm centres). |
|||||||
|
Bambusa
vulgaris |
|
100 |
See plan |
||||
|
11.
Buffer tree/shrub mix (planted at 50 cm centres). |
|||||||
|
|
Growth
form |
%
Cover |
Block
size |
||||
|
Bridelia
tomentosa |
Tree |
10 |
Groups of 5 |
||||
|
Celtis
tetranda |
Tree |
20 |
Groups of 3 |
||||
|
Litsea
glutinosa |
Tree |
5 |
Groups of 3 |
||||
|
Melastoma
candidum |
Shrub |
20 |
Groups of 10 |
||||
|
Melastoma
sanguineum |
Shrub |
10 |
Groups of 10 |
||||
|
Morus
alba |
Tree |
5 |
Groups of 3 |
||||
|
Rhaphiolepis
indica |
Shrub |
10 |
Groups of 5 |
||||
|
Sapium
sebiferum |
Tree |
20 |
Groups of 5 |
||||
Notes: At least 5% of the areas
(except 10 & 11) need to be bare mud and water. The quantity per metre is
converted to the quantity required per hectare by the following equation: 36 x
4840 x 2.471 / 39.
4.7.1
The
ecological mitigation area will be fenced immediately after the site is handed
over to KCRC. Access will then be limited to authorized personnel.
4.7.2
The
mitigation area perimeter fencing will comprise 1.2m high railposts 2m apart,
with two bars across the rails, spaced 0.3m from the ground and the top of the
post respectively. Fencing is to be outdoor quality recycled plastic, ultra
violet light and fire resistant, self coloured dark brown or black. Fencing may
be pr-fabricated or final finishing may be undertaken on site (suppliers will
recommend appropriate construction / assembly methods). Contractors unfamiliar
with recycled plastic fencing are advised that numerous suppliers of these
materials may be sourced by Internet search. It is possible that it will be
necessary to source a supplier from overseas.
4.7.3
Recycled
plastic supplier and a sample of the proposed fencing material, together with
construction / assembly method should be provided for the approval of the
Resident Engineer or his representative prior to erection on site.
4.7.4
Access
to the wetland mitigation areas will be provided by means of lockable gates to
be located at appropriate positions.
4.7.5
The
station works area will be screened off from the rest of the site before the
start of any works, to reduce disturbance to wildlife in the surrounding area.
Similarly the marshland area must be screened off from the surrounding wetland
areas during the wetland construction works.
4.7.6
Screening
must consist of solid hoarding over at least 2m height.
4.8
Construction and Habitat Establishment
Work Programme
4.8.1
An outline list of
the main actions necessary for the creation of the marshland and enhancement
of the fish pond habitats is provided in Tables 4.3 and 4.4 below. General
procedures are listed in Appendix 3.6. Pond numbers referred to in the table
are indicated in Figure 4.1. Time periods, start
and completion dates for these actions are dependent on the overall railway
and station construction programme, which has yet to be approved and scheduled.
Dates will be added to this list when an Environmental Permit has been granted
and a construction programme established.
Fish Ponds
4.8.2
It
is important to avoid disturbance to the western ponds, because these will
provide mitigation for the disturbance occurring as a result of the station and
marshland mitigation works. Therefore, it is proposed that mitigation
construction works are carried out in two phases. The first phase will consist
of works within the eastern half of the fish ponds area (i.e. east of Ponds 9a,
10a, 10b and 11) and the marshland area. During this period works in the
western half of the mitigation area will be restricted to selective tree
felling (to open up enclosed areas), other vegetation management and traditional
fish pond management of Ponds 3a, 3b, 4, 6a, 6b, 7, 8a and 8b (but with half
taken out of production and part drained in each year in rotation). Ponds 1 and
2 will be drained and allowed to refill with rain water and maintained at
shallow to moderate water levels (by simple pumping when necessary) to provide
high quality habitat primarily for water fowl.
4.8.3
Pond
5 will be planted with reeds during the initial mitigation phase.
4.8.4
After
completion of the station and marshland construction the remaining mitigation
works will be carried out on one pond at a time to minimize disturbance impacts
on the other ponds and marshland areas nearby.
Table 4.3
Habitat enhancement work programme for the Lok Ma Chau Station fish pond area wetland mitigation
|
|
Action |
Start
date |
Completion
date |
Notes |
|
|
Preworks period; |
|
|
|
|
1 |
Site
handover |
|
|
|
|
2 |
Erection of boundary fencing and
works area hoarding |
|
|
|
|
|
1st Mitigation Phase during station & marshland
construction period: |
|
|
|
|
|
Pond 5 |
|
|
|
|
3 |
Partial
drainage and planting of pond with Phragmites |
|
|
|
|
|
Ponds 1 and 2 |
|
|
|
|
4 |
Drainage of
Ponds 1 and 2 |
|
|
|
|
5 |
Cary out
selective tree felling and vegetation management to open up habitat in
vicinity of western ponds |
|
|
|
|
6 |
Allow partial re-filing of ponds from rainfall |
|
|
Maintain water levels below target levels by pumping |
|
|
Ponds 9a/b, 10a/b, 11, 12, 13a/b, 14a/b & 15. |
|
|
|
|
7 |
Drainage of ponds and drying of
substrate |
|
|
Trash fish
etc to be transferred to other ponds, commercial fish to be sold. |
|
8 |
Construction of water pumping
system |
|
|
|
|
9 |
Lowering of
connecting bunds to form islands and shoals, and connection of ponds |
|
|
|
|
10 |
Reprofiling
of pond base and bunds |
|
|
|
|
11 |
Filling and
land formation with topsoil on vegetated islands |
|
|
|
|
12 |
Construction
of water control structures and jetties |
|
|
|
|
13 |
Final fine
scale land formation |
|
|
|
|
14 |
Initial
soil preparation for planting |
|
|
|
|
15 |
Fish farming preparations (liming, fertiliser
applications*etc) |
|
|
No toxic compounds for killing pathogens, to
be used. |
|
16 |
Planting / translocations of wetland vegetation |
|
|
Possibly including wetland plant rhizomatous
material |
|
17 |
Refilling to initial operational water level |
|
|
Check water quality |
|
18 |
Start fish farming operations |
|
|
(see Management below) |
|
19 |
Vegetation establishment management (e.g.
weeding, thinning, replacement planting, irrigation) |
|
|
|
|
|
2nd
Mitigation Phase: Post construction period (Ponds 1, 2, 3a, 3b, 4, 6a, 6b, 7,
8a & 8b) |
|
|
|
|
20 |
Drainage
of ponds and drying of substrate |
|
|
Trash
fish etc to be transferred to other ponds, commercial fish to be sold. |
|
21 |
Construction
of bridge across water course to Ponds 1 & 2 |
|
|
|
|
22 |
Completion
of water pumping system |
|
|
|
|
23 |
Lowering of connecting bunds to form islands
and shoals, and connection of ponds |
|
|
|
|
24 |
Reprofiling of pond base and bunds |
|
|
|
|
25 |
Filling and land formation with topsoil on
vegetated islands |
|
|
|
|
26 |
Construction of water control structures and
jetties |
|
|
|
|
27 |
Final fine scale land formation |
|
|
|
|
28 |
Initial soil preparation for planting |
|
|
|
|
29 |
Fish farming preparations (liming, fertiliser
applications*etc) |
|
|
No toxic compounds for killing pathogens, to
be used. |
|
30 |
Planting / translocations of wetland
vegetation |
|
|
Possibly including wetland plant rhizomatous
material |
|
31 |
Refilling to initial operational water level |
|
|
Check water quality |
|
32 |
Start fish farming operations |
|
|
(see Management below) |
|
33 |
Vegetation establishment management (e.g.
weeding, thinning, replacement planting, irrigation) |
|
|
|
Marshland
4.8.5
An
outline list of the main actions necessary for the construction and
establishment of the marshland habitats is provided in Table 4.4 below.
Table 4.4
Construction and habitat establishment work programme for the Lok Ma Chau Station marshland mitigation area
|
|
Action |
Start
date |
Completion
date |
Notes |
|
1 |
Site
clearance and handover, erection of hoarding |
|
[To be
advised] |
|
|
2 |
Underlying site land formation |
|
|
|
|
3 |
Infilling
with surcharge sand layer |
|
|
|
|
4 |
Filling and land formation with
topsoil mix |
|
|
|
|
5 |
Construction
of water control structures |
|
|
|
|
6 |
Testing of
integrity of marshland wetland unit |
|
|
Clay layer
to be used as contingency measure if necessary |
|
7 |
Final fine
scale land formation |
|
|
|
|
8 |
Boundary fencing |
|
|
|
|
9 |
Water
filling to initial level for planting using ‘seed’ water with required
invertebrate larvae and phytoplankton |
|
|
|
|
10 |
Initial
soil preparation for planting |
|
|
|
|
11 |
Planting of
wetland vegetation |
|
|
Incremental
increases in water level to track planting |
|
12 |
Removal of
hoarding |
|
|
|
|
13 |
Vegetation
establishment management (e.g. weeding, thinning, replacement planting,
irrigation) |
|
|
|
4.9
Mitigation Measures for Wetland
Construction and Establishment Works
4.9.1
During
the construction of wetland mitigation areas and any associated works
environmental impacts must not exceed those predicted in the EIA. In particular
measures must be taken to avoid or minimise the following potential ecological
hazards:
·
Noise
and visual disturbance;
·
Hydrological
disruption of surface watercourses;
·
Pollution
of watercourses;
·
Dust
deposition;
·
Soil
compaction; and
·
Soil
contamination.
4.9.2
Mitigation
measures that are related to the construction of the railway will be listed in
the Particular Specification.
4.10.1
A
number of measure have been included in the design, construction methods,
management and monitoring proposals that aim to ensure the successful
establishment and long-term sustainability of the wetland as effective
compensation habitat for key target species.
4.10.2
The
wetland designs also have in built contingency measures for key factors
affecting wetland establishment, especially regarding water supply, water
retention and plant establishment. These include:
·
The
collection and storage of the station roof run-off to supplement direct
rainfall supplies to the marshland area.
·
The
use of a fishpond as a reservoir pond to maintain water levels within target
tolerances if required (a practice not normally deemed necessary by fish pond
operators)Measures to reduce high acidity in pond water by adding neutralising
agents (for example, pigeon faeces or peanut waste).
·
The
use of a clay base liner if tests indicate that water losses from the marshland
wetland are outside tolerable limits. Further potential contingency actions if
required could include placement of impermeable membranes in specific problem
areas or insertion of vertical plates (or membranes) at the edge of the wetland
to reduce lateral seepage.
·
High
planting densities to accommodate poor plant survival (and reduce potential
weed growth).
4.10.3
Also,
as described in Section 5 routine construction and establishment phase and
long-term management actions, such as weed clearance, replanting, thinning and
water level control will also be undertaken. These also aim to ensure the
successful establishment and long-term sustainability of the wetland cells.
4.10.4
In
addition, specific contingency actions will be defined in a Contingency Action
Plan. This will be prepared by the Contractor (as part of the development of
the wetland construction Method Statement) and agreed by the Resident Engineer
and Wetland Habitat Creation Ecologist before commencement of works. As a
minimum, contingency measures will be prepared for potential:
·
inadequate
water supply
·
failure
of the mains pumping system (including pump and timer failure, breakage of
supply pipes);
·
damage
to sluices and drainage structures;
·
pollution
of water supply;
·
direct
pollution of wetland cells by toxic substances (e.g. from spillages / dumping);
·
unacceptable
plant survival rate or growth during the 2 year establishment phase;
·
invasion
by exotic or other undesirable plant species;
·
flooding
of the site and other potential effects from storm events; and
·
fire
damage.
4.10.5 As described in Section 6
detailed and intensive monitoring will be carried out of the physical and
ecological performance of the wetland mitigation area during the construction
and establishment phases and in
the long-term. The monitoring will be carried out in relation to defined
“Action Levels” and “Limits” which trigger appropriate actions to rectify
problems. Where necessary these will include actions defined in the Contingency
Action Plan.
