3.1.1
Yuen Long Town Nullah (YLTN)
was constructed in the 1960s for alleviation of flooding problem. With the
increasing aspirations for a better living environment, the local residents of
Yuen Long are requesting the Government to improve the early design of the
YLTN, which is an old concrete-lined nullah and considered no longer in line with the modern with greenery changing
townscape of Yuen Long. Moreover, odour from expedient connection / illegal discharge / dry-weather
contaminated water (e.g. from street cleansing) discharging into the nullah would
cause nuisance to nearby residents. To improve the environment, this Project is proposed to resolve the odour problem and prevent
the DWF flowing into the nullah in the first stage of improvement works.
Later the landscaping and beautification works will be constructed in Barrage
Scheme soon after the completion of DWFI system.
3.1.2
Some
drainage projects including building fabric dam at the
downstream section, the Yuen Long Bypass Floodway and other drainage
improvement works were carried out
in 1990s to
improve the flooding problem at Yuen Long Town Centre. Upon completion of the drainage projects,
part of the flow from upper catchment has been diverted away from Yuen Long
Town. The flow in Yuen Long Town has
been greatly reduced and some of the nullahs become polluted open channels with
minimal base flow.
3.1.3
In view of the requests from
the District Council and members of the public to look for opportunities to
improve the drainage system and enhance the environment, the Drainage Services
Department (DSD) commissioned the Agreement No. CE 39/2006 (DS) “Rehabilitation
of Yuen Long Town Nullahs - Feasibility Study” to investigate different options
for improving the environmental conditions of the YLTN. The Study identified that rehabilitation of
the Town Centre Section is the top priority as high pedestrian flow compounded
with severe water quality problem in this area have created an undesirable
environment. The findings of the
Study provide the basis of scope of improvements for YLTN for the current
Project.
3.1.5
Regulation
enforcement (e.g. Water Pollution Control Ordinance)
is one of the strategies to resolve the expedient connection / illegal discharge and
subsequent water
pollution and odour problems at
sources. The whole Yuen Long catchment consists of 5
main streams with an approximate area over 13 km2.
The expedient connection/illegal discharge may come from various activities and
buildings in this large area. Although identification of expedient connections
is being carried out for subsequent enforcement works, the pollution sources
could not be completely eliminated due to difficulties in finding sources and
existence of some non-point sources. To control the polluted flow entering the
nullah, DWFI system is considered the most direct and effective measures. Given there would be some constraints of the expedient connection identification and
execution,
downstream control measures (i.e. DWF interception and treatment) are also
required. DSD and EPD will work together to improve the water quality and
minimize odour problem of YLTN with this two-pronged
approach.
Alleviation of Odour Nuisance
3.2.1
Odour have been a
problem of YLTN due to the polluted water in recent decades. With the
interception of partial DWF in YLTN under this Project, the odour
nuisance to the residents nearby will be alleviated.
Water Quality Improvement of Downstream Watercourse
3.2.2
The polluted DWF in the YLTN
is originally towards Shan Pui River and subsequently to Inner Deep Bay without
any treatment that may deteriorate the water quality of these ecological
sensitive water
bodies. By intercepting the polluted DWF for proper treatment, the water quality of ecologically sensitive Shan Pui River and
Inner Deep Bay can be improved.
Greening Enhancement
3.2.3
Green
roof, vertical greening and shrub / tree planting will be provided for
the DWF pumping station.
The maintenance access and footpaths of the pumping station will be paved with
soft paving materials (i.e. Grasscrete).
Blue-green Infrastructure
3.2.4
Less
polluted upstream DWF (i.e. from Kung Um Road Nullah) is selected to maintain
water flow in YLTN which will be integrated with the landscaping and
beautification works conducted in a separated DSD’s project “Yuen Long Barrage Scheme – Investigation,
Design and Construction” as blue-green elements for YLTN revitalization.
The revitalized YLTN would be a blue-green infrastructure and promoted as a
scenic and water-friendly place for leisure for the residents and public.
3.3.1
The “Without-Project”
scenario considers not implementing the recommended improvement of YLTN.
3.3.2
More
polluted DWF generation is expected due to population growth
and development in Yuen Long. If the proposed
Project is not implemented, more polluted DWF will be discharged to the nullah resulting
in further worsening of water quality and odour problems
as well as deterioration of the nullah appearance.
3.3.3
Based upon the above, the “Without-Project” option is not preferred due to the consideration
of potential worsening of odour and water quality problems.
Selection of DWF for Interception and Treatment
3.4.1
The
proposed discharge standard of Yuen Long Effluent Polishing Plant (YLEPP) was
reviewed as reference to examine the necessity for DWF interception, as
extracted in Table 3.1:
Table 3.1 Proposed Discharge Standard of Yuen
Long Effluent Polishing Plant
|
Parameter
|
Unit
|
Concentration
|
Remarks
|
|
5-day
Biochemical Oxygen Demand (BOD5)
|
mg/L
|
10
|
95
Percentile
|
|
Total Suspedned Solid (TSS)
|
mg/L
|
10
|
95
Percentile
|
|
Ammonia
Nitrogen (NH3-H)
|
mg/L
|
2
|
Annual
average
|
|
Total
Nitrogen (TN)
|
mg/L
|
10
|
Annual
average
|
|
Total
Phosphorus (TP)
|
mg/L
|
1
|
Annual
average
|
|
E. coli
|
Count/100mL
|
100
|
Monthly
Geometric Mean
|
3.4.2
In accordance with the River
Water Quality in Hong Kong published by the Environment Protection Department
(EPD), the Water Quality Index (WQI) is based on the assessment level of
dissolved oxygen, ammonia-nitrogen content and the Biochemical Oxygen Demand.
The water quality is classified in the range from “Very Bad” to “Excellent”. The
water sampling of DWF in YLTN has been conducted in 2017. The results and the
corresponding WQI at each monitoring location is summarized in
Table 3.2.
Table 3.2 Water Sampling Results
|
|
Kung Um
Road Nullah
|
San Hui
Nullah
|
End of
Town Centre Section
|
Western Nullah
|
Eastern Nullah
|
|
WQI (Water Quality)
|
9
(Fair)
|
11
(bad)
|
13
(bad)
|
11
(bad)
|
15
(very bad)
|
|
DO(mg/l)
|
5.46
|
3.80
|
1.86
|
2.38
|
1.28
|
|
SS (mg/l)
|
14.70
|
7.45
|
14.60
|
4.39
|
31.1*
|
|
Ammonia as N (mg/l)
|
3.55
|
12.77*
|
3.79
|
1.79
|
8.80*
|
|
Total Kjeldahl Nitrogen
(mg/l)
|
4.41
|
13.90
|
5.01
|
2.94
|
11.33
|
|
Total Phosphorus (mg/l)
|
0.53
|
2.59*
|
0.64
|
0.32
|
1.15
|
|
BOD (mg/l)
|
8.47
|
7.15
|
14.20
|
6.20
|
64.4*
|
Note: *Water quality exceed
the proposed discharge standard of YLEPP
3.4.3
As
illustrated, the water quality of DWF in San Hui Nullah, Town Centre Section
and Eastern Nullah is relatively poor. They are recommended to be intercepted
and treated at YLEPP. The water quality of DWF in West
Nullah is better than the proposed treatment standards of YLEPP. Therefore,
it is recommended to be intercepted but not to be treated in YLEPP. The water
quality of DWF in Kung Um Road Nullah is the best among the branches. It is
recommended as flowing water remained in YLTN.
Alternative Location of Discharge Points of the
Proposed DWFI System
(a)
Option 1 – Discharge all DWF
to YLEPP (original scheme)
(b)
Option 2 – Discharge 11,000 m3/day DWF
to YLEPP and 7,000 m3/day to San Wai Sewage Treatment Works (SWSTW)
(c)
Option 3 – Discharge all DWF to SWSTW with
upgrading of existing sewage pumping station (SPS)
(d)
Option 4 – Discharge all DWF
to SWSTW via a proposed pumping station
3.4.5
Options 2, 3 and 4 are proposed
to convey part or all DWF to SWSTW. However, these options would impose
significant cost impact and interface issues with the existing sewerage
facilities due to the extensive upgrading works required and are thus not
recommended. The potential impacts on
existing sewerage system due the proposed options are summarized in Table 3.3 below:
Table 3.3 Impacts on Existing Sewerage System
|
Impact on
|
Option 1
|
Option 2
|
Option 3
|
Option 4
|
|
YLEPP
|
·
Additional
operation cost (18,000 m3/d)
|
·
Additional operation cost (11,000 m3/d)
|
N/A
|
N/A
|
|
SWSTW
|
N/A
|
· Upgrade treatment capacity by 7,000 m3/d
· Additional operation cost
|
· Upgrade treatment capacity by 18,000 m3/d
· Additional operation cost
|
· Upgrade treatment capacity by 18,000 m3/d
· Additional operation cost
|
|
Yuen Long South SPS (YLS SPS)
|
N/A
|
· Upgrade pumping capacity by 7,000 m3/d
· Additional operation cost
|
N/A
|
N/A
|
|
Ping Shun Street SPS (PSS
SPS)
|
N/A
|
N/A
|
· Interface with MTR Development
· Upgrade pumping capacity by 18,000 m3/d
· Additional operation cost
|
N/A
|
|
Ha Tsuen SPS (HT SPS)
|
N/A
|
· Interface with HT SPS expansion works
· Upgrade pumping capacity by 7,000 m3/d
· Additional operation cost
|
· Interface with HT SPS expansion works
· Upgrade pumping capacity by 18,000 m3/d
· Additional operation cost
|
· Interface with HT SPS expansion works
· Upgrade pumping capacity by 18,000 m3/d
· Additional operation cost
|
|
Existing rising main and
sewer
|
N/A
|
· Between YLS SPS and HT SPS (34 km)
· Between HT SPS and SWSTW (1.9 km)
|
· Between PSS SPS and HT SPS (34 km)
· Between HT SPS and SWSTW (1.9 km)
|
· Between HT SPS and SWSTW (1.9 km)
|
3.4.6
In
addition, it is anticipated that the quantity of the DWF, particularly from the
upstream of YLTN (at the junction of Kiu Hing Road and
Tai Shu Ha Road West), would be subsequently improved after completion of the
sewerage works at the area. Eventually,
the DWF at upstream (i.e. Kung Um Road Nullah and San
Hui Nullah) is expected to be cleaner and probably less DWF is required to
be treated. In view of this, upgrading
of substantial existing sewerage facilities to handle the DWF might not be a
cost-effective option.
Conclusion
and Recommendation for Location of Discharge Point
3.4.7
It
is considered that conveying all DWF from YLTN to YLEPP, i.e. Option 1, would
be the most appropriate choice as the final scheme in this Project.
Construction Activities
3.5.1
Construction
of the rising mains, DWFI system and pumping station generally involves
construction activities such as site clearance, excavation, erection of
formwork, steel fixing, concreting and backfilling.
Comparison of the
Environmental Benefits and Dis-benefits of Alternative Construction Methods
Pumping Station
3.6.1
Potential
alternative construction method includes offsite precast structures for pumping
station. The benefit of offsite precast
structures is that there will be less environmental nuisance to nearby
sensitive receivers during construction.
3.6.2
Offsite precast
method will be proposed to most part of the pumping station structure to
minimize the environmental impacts. The remaining small parts will be cast in-situ where environmental
impacts to air quality, noise and water quality, the duration and extent of the
impacts are generally temporary and localised. It is anticipated that the environmental
impacts can be mitigated by appropriate measures.
Dry Weather Flow Interception (DWFI) System
3.6.3
Major
parts of the DWFI system are u-channel cast-in together with concrete mass for
nullah wall stabilization. Since the concrete mass is 5m in width and about
1.5m in depth, the size of the precast concrete mass will be too large for
transportation through the limited and narrow access into the nullah. Off-site precast units
are considered not practical and cost effective. Although in-situ
construction could pose environmental nuisance such as constructional noise,
dust and water quality impacts, it is expected that the duration and extent of
the impacts are temporary and localized given the limited section to be
constructed. It is anticipated that the
potential environmental impacts can be alleviated by proper implementation of
appropriate mitigation measures.
Therefore, conventional construction method of in-situ construction of DWFI system is considered more suitable and
practical and will be adopted.
Rising Main
3.6.4
The
use of ‘no-dig’ trenchless technique such as pipe jacking was considered
necessary below existing box culverts and central divider because open trench
techniques would require major modification works of the structures on top (Figure
2.1).
3.6.5
Outside
existing structures, the use of ‘no-dig’ trenchless technique was considered
not necessary/practicable as the Project Site is highly developed and is in close
proximity to various types of high risk underground
utilities. Open cut construction method is considered for the sections outside
existing structures and the associated potential environmental impacts can be
mitigated by adopting standard mitigation measures.
Consideration of Alternative
Construction Sequence
3.6.6
The
construction works
of DWFI system, including u-channel and DWF interceptors, will not be undertaken simultaneously in adjoining sections to
reduce the construction impacts to nearby sensitive receivers. This approach aims to avoid heavy construction
concentrated in one area in order to strike a balance between the need to
control the magnitude of environmental impact while not hindering the overall programme of the Project.
3.6.7
It will be necessary to carry
out careful phasing of the works to re-connect the proposed DWFI system with existing pipes
to ensure satisfactory diversion of flows without the risk of backing up of
drain. It is thus recommended that the
connection with existing pipes should be carried out during dry season to reduce
the risk.
3.6.8
With regard to nullah
excavation, instead of directly excavating the nullah bed which will generate
sediment plumes in river water and lead to water quality impacts, the excavation
of nullah bed for this Project will be carried out within an enclosed area
surrounded by concrete blocks,
sandbag
barriers or other appropriate measures. By adopting this method, sediments generated
from the excavation works will be confined within the enclosed area, thus reducing
the potential water quality impacts due to dispersal of suspended
sediments. Temporary diversion works of
flow will be carried out before the commencement of excavation works within the
nullah. To further reduce the potential
water quality impacts, excavation works in the nullah will only be undertaken
in dry condition.