2 Options Selection

2.1 Design Return Period

The relevant legislation and guidelines in both Mainland China and HKSAR will be followed in setting design criteria for the Project. In accordance with the PRC National Standard Flood Prevent Standard (GB 50501-94) and Urban Flood Prevent Engineering Design Standard (CJJ50-92), the design return period for Shenzhen River should be between 20 to 50 years. Whist in Hong Kong, the Stormwater Drainage Manual published by the DSD recommended that the design return period based on flood levels should be 50 years for main rural catchment drainage channels. With reference to the above relevant legislation and guidelines and the design criteria adopted in the Stage 1, 2 & 3 regulation works, the governments of the two cities have agreed that the flood prevention measures to be implemented for the Stage 4 regulation works shall be designed to attain the drainage capacity of a 50-year return period.

2.2 Flood Prevention Measures Analysis

With an attempt to increase the hydraulic capacity of the concerned section of the Shenzhen River within the Project site, the following flood prevention measures have been considered during the feasibility study:

· Flood storage: construct of flood storage tank / pond upstream or retardation pond along the relevant sections of the river;

· Flood diversion: construct of a floodway to divert significant amount of rainwater away from the overflowed river; and

· River modification works: such as river widening, raising of river embankment and river deepening to increase the sectional area and hence the drainage capacity of the river.

A comparison of these flood prevention measures are summarized in Table 2.1.

Table 2.1 Flood Prevent Measures Comparison

Flood Prevent Measures	Effect on Flood Prevention	Limitation	Recommendation
River modification works	Significant	Require additional land along the river	A combination of river widening, deepening and raising of river embankment is recommended
Flood Storage Pond along the river	Low – cannot attain the drainage capacity of a 50-year return period if use alone	Require additional land along the river	Could be used in conjunction with river modification works
Flood Storage Tank at upstream	Low/medium – cannot attain the drainage capacity of a 50-year return period if use alone	Involve construction works outside the relevant section of Shenzhen River; impacts on ecological resources and land resumption implication could be significant	Not recommended for this Project
Flood Diversion to outer tributaries	Significant	Involve construction works outside the relevant section of Shenzhen River; expensive; may involve change of landuse; impacts on ecological resources and land resumption implication could be significant	Not recommended for this Project

Based on the preliminary assessment of different flood prevention measures, a combination of river modification works and flood retardation pond was taken forward to study the preferred option for the Stage 4 regulation works.

2.3 Options Evaluation

An options evaluation exercise has been conducted with the purpose of identifying a technically feasible, cost-effective and environmentally & socially acceptable option of the Stage 4 regulation works. A hydraulics model, MIKE 11 developed by the Danish Hydraulics Institute (DHI), was used to assess the effects of different design options on the hydrology and hydraulics of Shenzhen River.

2.3.1 Flood Prevention Options

The design principle of the works is to protect the livelihood of residents on both sides of the river with an ecological sensible design, which should maintain the naturalness of the river and riparian habitats and hence protect the biodiversity. The design and layout planning of the works should be compatible with the future LT/HYW BCP. Two design options were then developed for further study:

Option A: The existing alignment of the concerned section of the Shenzhen River will be maintained as far as practical and massive excavation, dredging or filling will be avoided. Different designs such as trapezoidal, vertical or compound channel will be used with reference to the existing conditions. The centerline of the concerned section of the Shenzhen River will be about 4.5 km in length and a flood retardation pond will be constructed with a storage capacity of 80,000 m³. The river side slope will be at the ratio of 1:2 to 1:5 and the standard bottom width of the riverbed will be in a range of 14 to 32m.

Option B: The concerned sections of the Shenzhen River will be straightened and widened. The centerline of the concerned section of the Shenzhen River will be about 4.5 km in length and a flood retardation pond will be constructed with a storage capacity of 80,000 m³. The regulation works will mainly in the form of trapezoid open channel with side slope at the ratio of 1: 3. The standard bottom width of riverbed will be in a range of 14 to 16m.

The alignment and land requirement of both options are shown in Figure 2.1.

2.3.2 Evaluation Aspects

Land Requirement

The construction of flood retardation ponds and river widening works would require extra land along the relevant sections of the Shenzhen River within the Project site. At the planning and feasibility study stage of the Project, attempts have been made to confine the footprint of the Project to be with the area within 60m from both side of the river will be reserved for the regulation works. The land available in the Shenzhen side is constrained by the existing buildings and boundary patrol facilities. On the other hand, most of the existing land uses on Hong Kong side, besides the boundary patrol facilities, are abandoned farmland. However, the use of land in Shenzhen and Hong Kong should maintain a reasonable balance as far as practical.

The design of the regulation works should avoid the need of demolition of existing patrol roads and minimize the potential impacts on the adjacent villages such as Heung Yuen Wai and Chuk Yuen and the associated Fung Shui Woodland and graves.

The land requirements under the two options are summarized in Table 2.2. Option A would require less additional land then Option B and can achieve a more balance approach in term of land use in both Hong Kong and Shenzhen sides.

Table 2.2 Land Requirement Summary

Option	Land Requirement (m^2)
	Shenzhen Side	Hong Kong Side	Total
Option A	129,190 (44%)	163,250 (56%)	292,440
Option B	129,160 (41%)	182,500 (59%)	311,660

Environmental Consideration - Ecological Friendly Design

Option A will minimize the alternation of the existing river alignment and the meandering nature of the river will be preserved. In addition, the existing channel, river side land and embankments will be maintained as far as practical which will provides different natural aquatic and riparian habitats to support local ecology. This option will also avoid massive excavation and filling works and hence will minimize potential dust and noise impacts during the construction phase.

On the other hand, Option B will involve widening, straightening and sectional modification of the concerned river sections. The alternations will reduce the current winding nature of the river. The river will be mainly in the form of trapezoid channel and the river bank habitats are expected to be man-made in nature after the modification works.

The slope works on the river embankment will be integrated with landscape design to re-establish the natural riparian ecosystem with the aims to enhance the local biodiversity, protect the water resources and avoid erosion.

Environmental Consideration - Fill Balance

The expected quantities of excavated materials and filling requirement of two options are summarized in Table 2.3. Based on the preliminary study, it is estimated that about 610,000 m³ and 651,300 m³ of materials will be excavated and about 102,000 m³ and 143,500 m² of those materials can be reused on-site as backfilling materials for Option A and Option B respectively. The preliminary sediment quality investigation results show that Option A and Option B would require disposal of 508,000 m³ and 507,800 m³ of surplus materials, respectively.

Table 2.3 Fill Balance in Two Options

	Item	Option A (m^3)	Option B (m^3)
1	Excavated materials	610,000	651,300
2	Backfilling materials required	102,000	143,500
3	Surplus materials for off-site disposal
	- Uncontaminated soil	410,000	416,700
	- Contaminated sediments	98,000	91,100
Note: (a) These quantities were estimated during the option evaluation stage. The updated fill balance quantity for the preferred option after preliminary design consideration shall refer to Section 9 of this EIA Report.

Environmental Consideration - Air and Noise Impacts during Construction

The works area and the quantity of the excavated materials in Option A will be smaller than Option B. It is also expected that the number of construction plants needed under Option A, and hence the potential construction dust and noise impacts during construction, will be less than Option B.

Environmental Consideration - Landscape and Visual Considerations

Compare with Option B, Option A will minimize the alternation of the existing river alignment and the meandering nature of the river will be preserved.

Both options will consist of a flood retardation pond. The pond will be designed with distinctive landscape features including three small isolated islands to create a winding flow of water. Proper landscape will be provided on these islands.

Management and Maintenance during Operation

It is expected that the regular clearing of refuse and desilting will be required to maintain the normal operation of the river. The design of the inlet and outlet of the flood retardation ponds are based on overflow weir or gates with automatic control system and it is anticipated that limited management resources would be needed. Patrol roads connecting the main access road and the rest of Shenzhen River will be re-provided. The management and maintenance of the Project operation for both options would be simple and the costs are relatively low.

Cost

Based on the preliminary design of the two options, the estimated costs (excluding the cost for land resumption) are summarized in Table 2.4.

Table 2.4 Costs of Two Options

	Major Work Items	Option A (RMB ¥)	Option B (RMB ¥)
1	River Regulation	486.5 million	512.5 million
2	Patrol road and boundary fence - Shenzhen side (includes CCTV)	19.8 million	19.8 million
3	Patrol road and boundary fence – Hong Kong side	77.6 million	76.5 million
	Total	586.9 million	608.8 million

2.3.3 Construction Sequence and Methodology

The reprovisioning of boundary patrol road on Hong Kong side will commence before the river modification works for security reason. The road sections near the LT/HYW BCP development will be constructed first in order to tie in with the BCP development.

The river modification works will be carried out in four phases, commencing from the downstream section of the river. Two phases will be carried out in parallel. This approach will strike a balance between the need to control the magnitude of environmental impact at the same time while not hindering the overall programme of the Project. Heavy rains during wet and typhoon seasons will increase the quantity of site runoff during construction. In order to minimize the potential water quality impacts, river dredging works will be arranged to be conducted in dry seasons.

River sediment will need to be dredged during the river modification work. Due to the site constraints and the shallow water depth, the use of dredging barge in this Project is not feasible. Land based dredger will be used.

With regard to river bed excavation, instead of directly excavating the river bed, which will generate sediment plume release to the river water throughout the excavation period, the excavation of river bed for this Project will be carried out within a cofferdam made of hessian bags. By implementing this method, the cofferdam will block the sediment released into the river during the excavation work (ie work in dry condition), thus minimising the water quality impact. River diversion work will be carried out before the commencement of excavation and construction works. When the excavation and placement of embankment foundation are carried out on one side, the river course on the other side will be used as diversion channel. This method will ensure the seamless flow of water along the river and minimise the disruption of hydrology of the river.

The construction sequence and methodology for both options will be the same.

2.3.4 Summary of Options Evaluation

A summary of the option comparison is presented in Table 2.5. The environmental benefit and disbenefits of both options are also summarised in Table 2.6.

Table 2.5 Summary of Options Evaluation

Issue / Criteria	Option A	Option B	Preferred Option
Land Requirement	29 ha of which 44% in Shenzhen side and 56% in Hong Kong side	31 ha of which 41% in Shenzhen side and 59% in Hong Kong side	Option A
Environmental Consideration - Ecological Friendly Design	Minimize the alteration of the existing river alignment; provides diversified river and riparian habitats to support the local ecology along the modification section	Widening, straightening and sectional modification of the relevant river section; will provide similar river and riparian habitats along the modification section	Option A
Environmental Consideration - Fill Balance	About 0.508 Mm^3 surplus materials required off-site disposal	About 0.508 Mm^3 surplus materials required off-site disposal	No significant difference (but less earthwork for Option A)
Environmental Consideration - Air and Noise Impacts during Construction	Less construction plants needed and hence less air and noise impacts during construction	More construction plants needed and hence more air and noise impacts during construction	Option A
Environmental Consideration - Landscape and Visual Impacts	Minimize the alternation of the river and hence preserve the existing landscape character area.	Extensive modification works will cause permanent loss of existing landscape character area	Option A
Management and Maintenance during operation	Management and maintenance would be simple and in low costs	Management and maintenance would be simple and of low costs	Same
Cost	586.9 million	608.8 million	Option A

Table 2.6 Summary of Environmental Benefits and Dis-Benefits of the Design Options

Options	Environmental Benefits	Environmental Dis-Benefits
A	· Follow the existing alignment as much as possible and hence preserve the existing landscape character better · Less earthworks (excavation and fill) required and thus use less construction plant and hence less dust and noise impacts · Smaller land requirement area which means less direct impact on existing habitats	· Will disturb the existing habitat but the impact will be localised · Nuisance during construction period but will be mitigatable
B	-	· Will disturb the existing habitat but the impact will be localised · Nuisance during construction period but will be mitigatable · More earthworks required · Use more construction plant and hence more dust and noise impact · More habitat loss

Both Option A and Option B are designed to attain the drainage capacity of a 50-year return period. The management and maintenance of both options will be simple and of low costs and the design and layout planning of both options will be compatible with the proposed LT/HYW BCP.

Although the alignment under both options have no significant difference, the alignment of Option A follows closer to the existing river alignment and hence performs better in terms of maintaining the river profile and landscape character. Option A also requires less additional land than Option B especially the use of land in Hong Kong and involves slightly less earthwork. The capital cost of Option A is 4.3% less that that of Option B.

As a result, Option A is preferred in terms of land requirement, environmental consideration and cost implication and will be adopted as the design scheme for detailed design and EIA Study.