Benthic Construction Assessment
Direct Impacts
No construction activities are proposed in the vicinity of any hard substrate habitat where corals are present. There will
however be a loss of soft seabed habitat from the following construction activities:
Regarding nearshore transmission cable dredging, a summary of these works is presented here. Approximately 135,000m3
of soft marine muds are proposed to be dredged over a period of ~6 weeks. The total surface area of dredging works shall be
some 80,000 m2, with this area of seabed infauna habitat temporarily lost. The upper part of the dredged trench will be filled
with rock armour to the same level as the adjacent seabed. It can be expected that the voids around the rock armour will re-fill
with soft sediment under the natural hydrodynamic regime within a short period of time.
The significance of the dredging works on the benthic infauna community in Junk Bay is considered to be low, as the
community is disturbed, is dominated by polychaetes, and as recent past survey has revealed the absence of infauna species
of conservation interest in the immediate area (Maunsell, 2005).
Previous study suggests that the disturbed seabed will naturally be re-colonised by species recruited from adjacent seabed
areas within a short period. For example, Valente et al (1999) found that 2-3 years after cessation of dredged material
disposal in Eastern Waters a benthic epifauna community had established that possessed a significantly greater abundance
and diversity than those at nearby reference stations. The results suggest that the variety of dredged material disposed at the
site effectively increased habitat diversity relative to the adjacent homogenous seabed habitat.
A summary of the procedure for offshore transmission and array cable jetting is presented in sub-section 2.7.6. These
proposed works would involve the displacement of soft sediment under high-pressure jets of ambient water, resulting in the
temporary loss of an approximately 0.4m wide strip of seabed per cable. It is estimated that some 21 km of transmission
cable shall be installed over a period of approximately 2 months at a maximum installation rate of 150m / hour. There shall
also be approximately 40 - 50 km of array cable installation, resulting in a cumulative area of temporary habitat loss of ~28,000
m2.
As seabed jetting is proposed from the relatively shallow Tathong Channel, through increasingly deep and exposed waters
south of Tung Lung Chau and the Ninepin Islands group to offshore Eastern Waters, variations in habitat character inevitably
occur.
The baseline ecological surveys and data analysis presented under sub-section 5.5 were able to distinguish three ‘groups’ of
infauna community according to similarity in key sediment character and species composition criteria, with baseline field
survey suggesting that the grouping of stations 2 - 3 was the most biodiverse.
Stations 2 - 3 also represent an area of some conservation significance due to the wet season presence of juvenile
amphioxus Branchiostoma belcheri. Three individuals of amphioxus were recorded from 2 of 5 replicate samples at station 2,
with 32 individuals from 4 of 5 replicate samples at station 3. Amphioxus was not recorded from these (or any other) survey
stations during the dry season. As stations 2 - 3 are remote from the core amphioxus habitat area at offshore Tai Long Wan it
is considered that the small number of individuals present arose through the random settlement of planktonic larvae.
Accordingly, it is concluded that the seabed at stations 2 - 3 is not a major habitat for amphioxus.
During cable jetting benthic infauna may be damaged by the water jet, and / or may be exposed to predation while suspended
in the water column. Such impacts will be highly localised given the jetting width of just 0.4m. As the water jet passes over the
seabed the cable shall sink through the fluidised mud under its own weight, with the sediment consequently reconsolidating
as the jetting machine passes.
It is noted that the proposed jetting zone has a high edge / area ratio, in the order of 2.5:1.0. Various ecological studies have
proven that such areas are likely to be re-colonised more rapidly than those with a small ratio because of the greater area of
adjacent donor sites available from which to recruit, although a longer period may be required for full ‘recovery’.
With reference to past studies of benthic re-colonisation of seabed areas subject to mechanical disturbance, pollution or
anoxia, it can be expected that initial re-colonisation will occur with a period of weeks to months following jetting (WBM, 2004;
Santos and Simon, 1980). Field survey data for this EIA Study indicates that the characteristics of the benthic infauna
community are broadly consistent across the five replicate samples at each survey station. As such, the narrow width of the
jetting zone disturbed by jetting zone will thus facilitate re-colonisation by species from adjacent seabed areas.
Overall, impacts on the benthic infauna community from cable installation are anticipated to be moderate / low in the vicinity of
of stations 2 - 3, and of little or no significance elsewhere along the cable route.
As regards offshore foundation construction, these proposed works differ from the proposed dredging and jetting activities in
that there would be a direct loss of habitat due to the presence of permanent (~20 year lifespan) of fixed structures. These
proposed works would occur in exposed offshore water where surveys conducted for this EIA Study indicate a moderately
biodiverse infauna community may exist, although no species of conservation significance were recorded.
At this planning stage it is assumed there shall be no more than 67 turbines and one offshore transformer station. Each of
these components shall have a single foundation system comprised of four suction caissons each with a diameter of
approximately 15 metres. This generates a cumulative area of permanent benthic habitat loss of ~48,000m2.
Compared with cumulative soft sub-tidal habitat area in the wind farm of ~1,500 hectares, this represents an insignificant
habitat loss of ~0.3% of total seabed in the wind farm area. The total area of soft sub-tidal habitat in the Study Area is > 45,000
ha. Given the habitat area to loss ratio, and the absence of any specific conservation value, the impact on the benthic infauna
community is negligible.
Indirect Impacts
Indirect impacts would result from changes in water quality as a result of construction work in some cases mitigation and
monitoring are required. This is assessed in the water quality section.
The results of the water quality modelling can be viewed in 3D here.
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Direct Impacts
No construction activities are proposed in the vicinity of any hard substrate habitat where corals are present. There will
however be a loss of soft seabed habitat from the following construction activities:
- Nearshore transmission cable dredging: temporary habitat loss in Junk Bay
- Offshore transmission and array cable jetting: temporary habitat loss in Eastern Waters
- Offshore foundation construction: permanent habitat loss from installation of wind turbine and transmission station
foundations
Regarding nearshore transmission cable dredging, a summary of these works is presented here. Approximately 135,000m3
of soft marine muds are proposed to be dredged over a period of ~6 weeks. The total surface area of dredging works shall be
some 80,000 m2, with this area of seabed infauna habitat temporarily lost. The upper part of the dredged trench will be filled
with rock armour to the same level as the adjacent seabed. It can be expected that the voids around the rock armour will re-fill
with soft sediment under the natural hydrodynamic regime within a short period of time.
The significance of the dredging works on the benthic infauna community in Junk Bay is considered to be low, as the
community is disturbed, is dominated by polychaetes, and as recent past survey has revealed the absence of infauna species
of conservation interest in the immediate area (Maunsell, 2005).
Previous study suggests that the disturbed seabed will naturally be re-colonised by species recruited from adjacent seabed
areas within a short period. For example, Valente et al (1999) found that 2-3 years after cessation of dredged material
disposal in Eastern Waters a benthic epifauna community had established that possessed a significantly greater abundance
and diversity than those at nearby reference stations. The results suggest that the variety of dredged material disposed at the
site effectively increased habitat diversity relative to the adjacent homogenous seabed habitat.
A summary of the procedure for offshore transmission and array cable jetting is presented in sub-section 2.7.6. These
proposed works would involve the displacement of soft sediment under high-pressure jets of ambient water, resulting in the
temporary loss of an approximately 0.4m wide strip of seabed per cable. It is estimated that some 21 km of transmission
cable shall be installed over a period of approximately 2 months at a maximum installation rate of 150m / hour. There shall
also be approximately 40 - 50 km of array cable installation, resulting in a cumulative area of temporary habitat loss of ~28,000
m2.
As seabed jetting is proposed from the relatively shallow Tathong Channel, through increasingly deep and exposed waters
south of Tung Lung Chau and the Ninepin Islands group to offshore Eastern Waters, variations in habitat character inevitably
occur.
The baseline ecological surveys and data analysis presented under sub-section 5.5 were able to distinguish three ‘groups’ of
infauna community according to similarity in key sediment character and species composition criteria, with baseline field
survey suggesting that the grouping of stations 2 - 3 was the most biodiverse.
Stations 2 - 3 also represent an area of some conservation significance due to the wet season presence of juvenile
amphioxus Branchiostoma belcheri. Three individuals of amphioxus were recorded from 2 of 5 replicate samples at station 2,
with 32 individuals from 4 of 5 replicate samples at station 3. Amphioxus was not recorded from these (or any other) survey
stations during the dry season. As stations 2 - 3 are remote from the core amphioxus habitat area at offshore Tai Long Wan it
is considered that the small number of individuals present arose through the random settlement of planktonic larvae.
Accordingly, it is concluded that the seabed at stations 2 - 3 is not a major habitat for amphioxus.
During cable jetting benthic infauna may be damaged by the water jet, and / or may be exposed to predation while suspended
in the water column. Such impacts will be highly localised given the jetting width of just 0.4m. As the water jet passes over the
seabed the cable shall sink through the fluidised mud under its own weight, with the sediment consequently reconsolidating
as the jetting machine passes.
It is noted that the proposed jetting zone has a high edge / area ratio, in the order of 2.5:1.0. Various ecological studies have
proven that such areas are likely to be re-colonised more rapidly than those with a small ratio because of the greater area of
adjacent donor sites available from which to recruit, although a longer period may be required for full ‘recovery’.
With reference to past studies of benthic re-colonisation of seabed areas subject to mechanical disturbance, pollution or
anoxia, it can be expected that initial re-colonisation will occur with a period of weeks to months following jetting (WBM, 2004;
Santos and Simon, 1980). Field survey data for this EIA Study indicates that the characteristics of the benthic infauna
community are broadly consistent across the five replicate samples at each survey station. As such, the narrow width of the
jetting zone disturbed by jetting zone will thus facilitate re-colonisation by species from adjacent seabed areas.
Overall, impacts on the benthic infauna community from cable installation are anticipated to be moderate / low in the vicinity of
of stations 2 - 3, and of little or no significance elsewhere along the cable route.
As regards offshore foundation construction, these proposed works differ from the proposed dredging and jetting activities in
that there would be a direct loss of habitat due to the presence of permanent (~20 year lifespan) of fixed structures. These
proposed works would occur in exposed offshore water where surveys conducted for this EIA Study indicate a moderately
biodiverse infauna community may exist, although no species of conservation significance were recorded.
At this planning stage it is assumed there shall be no more than 67 turbines and one offshore transformer station. Each of
these components shall have a single foundation system comprised of four suction caissons each with a diameter of
approximately 15 metres. This generates a cumulative area of permanent benthic habitat loss of ~48,000m2.
Compared with cumulative soft sub-tidal habitat area in the wind farm of ~1,500 hectares, this represents an insignificant
habitat loss of ~0.3% of total seabed in the wind farm area. The total area of soft sub-tidal habitat in the Study Area is > 45,000
ha. Given the habitat area to loss ratio, and the absence of any specific conservation value, the impact on the benthic infauna
community is negligible.
Indirect Impacts
Indirect impacts would result from changes in water quality as a result of construction work in some cases mitigation and
monitoring are required. This is assessed in the water quality section.
The results of the water quality modelling can be viewed in 3D here.
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