Information Paper for Legislative Council
LegCo Panel on Planning, Lands and Works
Meeting on 13 May 1999
59CD - West Kowloon drainage improvement, stages 2 and 3
This paper informs Members of a strategy for solving the flooding problem in West Kowloon and the development of an implementation programme. In June 1999, we will seek PWSC's approval for upgrading part of 59CD - West Kowloon drainage improvement, stage 2 phase 1 works to Category A.
2. West Kowloon, a major residential and commercial area, is susceptible to frequent flooding due to the inadequate capacity of the existing drainage system in the area.
3. West Kowloon comprises Sham Shui Po, Mong Kok, Yau Ma Tei, part of Tsim Sha Tsui and part of Kowloon City. The drainage catchment is large and extends to the natural hillsides behind the urban area as indicated on the plan at Enclosure 1. The existing drainage system was designed and built many years ago. Its capacity can no longer cope with the recent developments in West Kowloon.
4. Due to rapid developments and changes in land use in Kowloon over the years, natural ground and slopes have been paved and become impermeable. Rainwater which would previously dissipate naturally through ground infiltration can no longer do so. This has resulted in increase in surface runoff overloading the drainage system. As a result, many areas of West Kowloon are prone to frequent flooding during heavy rainstorms.
5. A preliminary study by the Drainage Services Department in 1990 indicated that most of the existing trunk drains in West Kowloon were inadequate in capacity. Since then, we commenced implementing a strategy to progressively upgrade the capacity of the drainage system to cope with development needs and to alleviate the risks of flooding.
6. In December 1993, we upgraded 59CD "West Kowloon stormwater drainage improvement" to Category B. In June 1994, we upgraded part of 59CD to Category A, as 65CD to employ a consultant to examine the deficiencies of the drainage system in West Kowloon and to develop a drainage master plan as a blue print for improvement of the system. The study was completed in December 1995. The study identified that some 103 km of the existing drains did not meet the current standards for flood protection and recommended upgrading works to be implemented in a three-stage improvement programme as below:
|Stage||Drains to be Upgraded (km)||Completion (Year)
7. Stage 1 improvement works are for those drains which are critically under-capacity and lie in the same roads where the stormwater drainage improvement works will be carried out in conjunction with the sewerage improvement works under 4SS "North West Kowloon sewerage stage III phase II". Stage 2 works are for those drains which are critically under-capacity but fall outside the stage 1 works boundary. Stage 3 works are for those drains which are moderately under-capacity.
8. We upgraded, in June 1997, part of 59CD to Category A as 89CD with an approved project estimate of $464 million to implement the stage 1 drainage improvement works. Construction commenced in April 1998 in conjunction with the sewerage improvement works under 4SS for completion in 2003. This arrangement avoids multiple opening of roads, thereby minimizing traffic impacts and inconvenience to the public.
9. We also upgraded, in April 1997, part of 59CD to Category A as 88CD to engage consultants to carry out site investigations and detailed design for the stage 2 drainage improvement works. The design consultancy started in June 1997 for commencement of construction in December 1999.
10. The revised drainage improvement strategy for solving flooding problem in West Kowloon is illustrated on the plan in Enclosure 2. The original drainage improvement strategy recommended in consultancy study under 65CD in 1995 is shown on the plan in Enclosure 3.
11. As pointed out in paragraph 6, West Kowloon drainage improvement stages 2 and 3 originally comprise the upgrading of 94 km of drains and culverts. To minimize the opening of roads and disruption to the public, the original scope of stages 2 and 3 has been critically examined and revised as follows:
|Stage 2, |
|23 km of drains and culverts||2004
|Stage 2, |
|Tai Hang Tung Flood Storage |
Scheme & Kai Tak Transfer Scheme
|Stage 3||25 km of drains and culverts||2007
12. The part of the project we intend to seek PWSC's approval in June 1999 for upgrading to Category A is the stage 2 phase 1 works to improve the drainage system in Sham Shui Po, Mong Kok, Yau Ma Tei and Tsim Sha Tsui as shown on the plan in Enclosure 4. The detailed scope of works comprises the construction of:
- about 22 km of drain pipes of diameters ranging from 300 mm to 2100 mm; and
- about 0.7 km of single-cell or two-cell box culverts, of cell size up to 4800 mm x 3500 mm.
13. The remainder of 59CD comprises the drainage improvement works under stage 2 phase 2 and stage 3.
14. We have critically reviewed the overall drainage improvement strategy for both stages 2 and 3 works with due consideration given to constraints, such as non-availability of space, conflicts with underground structures and utilities of substantial sizes which cannot practically be diverted, and traffic disruptions which will arise during the execution of the works in the heavily built-up areas of West Kowloon.
15. Flooding incidents at Nathan Road, Mong Kok, during heavy rainfalls have been primarily due to the under-capacity of the nullah under Nullah Road. During heavy rainstorms, stormwater spills over the top of the nullah at the junction of Flower Market Road/Sai Yee Street, flows down the nearby streets and floods the section of Nathan Road between Boundary Street and Nullah Road. The above junction is the confluence point of the two main tributaries of the catchment - the Tai Hang Tung and the Waterloo Road tributaries.
16. The original strategy to solve the flooding problem is to build new drains to convey and discharge the flood water to the sea. This would have required the construction of two 4800 mm wide x 3500 mm deep box culverts through the heavily built-up area of Mong Kok. All possible routings for the culverts have been investigated and this strategy has been proved highly disruptive due to the constraints discussed in paragraph 14 above. The only alternative available is diverting part of the flow from entering the nullah and temporarily storing the overflow in a large storage tank.
17. The above review recommends revision of the implementation strategy which will include the construction of a flood storage tank underneath the Tai Hang Tung Recreation Ground (Tai Hang Tung Flood Storage Scheme), a stormwater transfer tunnel from Kowloon Tong to San Po Kong (Kai Tak Transfer Scheme) and a second tunnel intercepting hillside stormwater away from West Kowloon (Lai Chi Kok Transfer Scheme), so as to reduce the extent of drainage upgrading works in West Kowloon. The revised strategy will reduce the overall length of drains to be upgraded under the original stages 2 and 3 from 94 km to about 48 km, a reduction of about 50% in length. The extent of reduction of drain upgrading is illustrated on the plan in Enclosure 5.
18. Accordingly, we propose to revise the packaging of works for stages 2 and 3 works as follows:-
- Stage 2 Phase 1 Works
Construction of approximately 23 km of drains in Mong Kok, Sham Shui Po, Yau Ma Tei and Tsim Sha Tsui, for substantial completion by 2004;
- Stage 2 Phase 2 Works
Construction of the Tai Hang Tung Flood Storage Scheme and the Kai Tak Transfer Scheme, for completion by 2004; and
- Stage 3 Works
Construction of approximately 25 km of drains in Yau Ma Tei and Sham Shui Po, Yau Yat Tsuen, Ho Man Tin and Kowloon Tong, for completion by 2007.
19. The consultants have substantially completed the design for the stage 2 phase 1 works. We intend to seek PWSC's approval in June 1999 to upgrade the stage 2 phase 1 works to Category A and start construction in December 1999 for substantial completion in 2004.
20. We have commenced the planning/design work for the Tai Hang Tung Flood Storage Scheme and the Kai Tak Transfer Scheme. The Tai Hang Tung Flood Storage Scheme and the Kai Tak Transfer Scheme have been programmed to start in late 2000 for completion in 2004. Together with the completion of drainage upgrading works under stage 1 and stage 2 phase 1, flooding problem currently experienced at Mong Kok will basically be resolved.
21. Stage 3 works are currently under design. We plan to start the construction of the stage 3 works in 2001 for completion in 2007.
22. We have also commissioned a feasibility study on the second stormwater transfer tunnel (i.e. Lai Chi Kok Transfer Scheme) to intercept rainwater from the upper catchment of West Kowloon.
23. We estimate the capital cost of the revised stage 2 and stage 3 works to be $4000 million at December 1998 prices, made up as follows -
|(a) Stage 2 phase 1 works||1,575
|(b) Stage 2 phase 2 works||900
|(c) Stage 3 works||1,525
24. We presented the findings and recommendations of the consultancy study under 65CD to the Sham Shui Po and Kowloon City District Boards on 9 November 1995 and to the Yau Tsim Mong District Board on 16 November 1995. The District Boards supported the implementation of the entire West Kowloon drainage improvement project.
25. We consulted and obtained the support of the Yau Tsim Mong Provisional District Board and the Sham Shui Po Provisional District Board on 26 November 1998 and 11 February 1999 respectively on the revised implementation strategy and the associated temporary traffic arrangements and environmental mitigation measures during construction.
26. We consulted the Kowloon City Provisional District Board on 4 March 1999 and the Wong Tai Sin Provisional District Board on 16 March 1999 on the proposed Kai Tak Transfer Scheme. Both District Boards supported the Scheme.