Rehabilitation of 450Mm Diameter Sewage Rising Mains at Deep Water Bay Stage 1 – Section at Island Road Near Deep Water Bay Beach
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DDHC Paper No. 13/2018 (for discussion on 19.7.2018) Southern District Council District Development and Housing Committee Rehabilitation of 450mm Diameter Sewage Rising Mains at Deep Water Bay Stage 1 – Section at Island Road near Deep Water Bay Beach Purpose The purpose of this paper is for the Drainage Services Department (hereinafter referred to as “our department”) to brief the District Development and Housing Committee of the Southern District Council about the proposed rehabilitation works at Island Road (Stage 1) and invite members’ views on the implementation and the environmental mitigation measures of the proposed rehabilitation works, such that our department could commence the project works in time and to enhance the reliability of a 450mm diameter rising mains at Deep Water Bay. Background 2. At present, the sewage from Shouson Hill, Deep Water Bay and Repulse Bay areas is collected by the Repulse Bay Main Sewage Pumping Station (RBMSPS). The collected sewage is then conveyed to Aberdeen Preliminary Treatment Works (PTW) via an existing single 450mm diameter rising mains (DN450) which is sub-divided into seven sections along its alignment as below: I. a section of about 600 m long buried pipeline between RBMSPS and Seaview Promenade (near No. 38 Island Road); II. an exposed pipeline of about 190 m long installed within the utility trough between Seaview Promenade (near No. 38 Island Road) and Deep Water Bay Beach; III. a section of about 340 m long buried pipeline underneath Island Road; IV. a section of about 320 m long buried pipeline underneath the drainage reserve within Hong Kong Golf Club; V. an exposed section of about 680 m long pipeline supported by concrete piers beneath Mills and Chung Path; VI. an exposed section of about 630 m long pipeline accommodated within a 1 tunnel between Mills and Chung Path and Wong Chuk Hang; and VII. about 1 640 m long buried / exposed pipeline between the tunnel portal at Wong Chuk Hang and Aberdeen PTW. These sewage collection and treatment facilities serve to provide a clean environment for the community and help maintain the quality of the public bathing beaches. A layout plan showing the routing of this rising mains is at Annex 1. 3. The rising mains was put into service in 1994 and has shown signs of deterioration. 4. Since July 2012, several leakage incidents had occurred to the rising mains at Deep Water Bay. There were two leakages at the Island Road section in March and September 2015. To repair the leakages, our department had to close off a traffic lane of in Island Road during daytime non-peak hours to conduct the inspection and repair. There were also minor leakages at the Mills and Chung Path section between 2013 and 2017. Please refer to Annex 1 for the leakage incident records. 5. Besides leakages due to pipe aging, the rising mains at Mills and Chung Path were also damaged in two other occasions, during the passage of Typhoon Vicente in July 2012 and Typhoon Hato in August 2017. 6. In all occasions, our department promptly repaired the damaged mains. During the rising mains repair works, there was a need to temporarily close individual bathing beaches. To ensure that the water quality was suitable for swimming before reopening the beaches, the Environmental Protection Department (EPD) had also closely monitor the water quality and collected and tested seawater samples on a daily basis. In addition, our department also installed a temporary treatment plant at RBMSPS in mid-2016 to further reduce the environmental impacts during the repair works. 7. Due to the increasing number of the rising mains leakage incidents, our department proposes to undertake preventive repair to the entire rising mains. 8. Our department considers that the rehabilitation works for the rising mains at Island Road (Stage 1) is most critical and should be carried out first in 2018. Please refer to Annex 2 for the section to be rehabilitated at Island Road near Deep Water Bay Beach. 9. Our department will carry out the rehabilitation works of the remaining sections of the rising mains in phases from 2019 and onwards. These remaining sections are located 2 within the drainage reserve inside Hong Kong Golf Club, beneath the Mills and Chung Path, inside service tunnel and the utility trough along Seaview Promenade or buried under footpath or roads with low traffic flow, where temporary diversion is technically feasible and would only cause minimal environmental and traffic impacts. Our department targets to complete the rehabilitation works for the entire rising mains by end of 2023. Proposed Stage 1 Rehabilitation Works 10. The Stage 1 rehabilitation works at Island Road will be carried out in non-bathing season, between December 2018 and February 2019. Trenchless technique will be adopted for the Stage 1 rehabilitation works at Island Road. It will involve the insertion of a continuous folded flexible high pressure pipeline liner inside the existing mains. The liner will be unfolded under high pressure and form a new pipe inside the existing mains. Environmental Implications for Stage 1 Rehabilitation Works 11. The existing rising mains is a single mains and is in operation at all time. During the Stage 1 rehabilitation works, it is necessary to suspend the operation of the rising mains for 7 days. However, as the rising mains is pumping about 5 000 m3/day, it is not feasible to arrange the tankering away or the temporary diversion of the sewage during the suspension period. Our department has therefore installed a temporary treatment plant at RBMSPS to treat and disinfect the sewage so that the treated effluent may be discharged into the surrounding waters of RBMSPS during these 7 days without causing unacceptable environmental impact. 12. As a precautionary measure, Deep Water Bay Beach will be temporarily closed during the suspension period of the rising mains. EPD will closely monitor the water quality by collecting and testing seawater samples on a daily basis and ensure that the water quality is suitable for swimming before reopening the beach. 13. Our department has conducted a Water Quality Impact Assessment to analyze the environmental impacts arising from the temporary discharge of disinfected effluent from RBMSPS. Modelling results indicate that the impact to the water quality is transient and reversible, and that the water quality will resume normal in one to two days after the cessation of the effluent discharge. 14. Since the proposed works will not involve open trench excavation, existing trees would not be affected. For any works to be carried out near existing trees, our department 3 will conduct detailed assessment and submit comprehensive tree survey report to relevant government departments for comment prior to the commencement of the works. 15. Our department has consulted the stakeholders in the vicinity including the Ocean Park, the Victoria Recreation Club, the Aberdeen Boat Club and the Royal Hong Kong Yacht Club. They have no objection in principle to the rehabilitation works. Our department will maintain close liaison with them before the commencement and during the course of the rehabilitation works. Traffic Implications for Stage 1 Rehabilitation Works 16. No road opening will be required for the proposed rehabilitation works and therefore no traffic lane closure is anticipated. All works will be carried out within the limited area at the car park of Deep Water Bay (downstream end) and the pedestrian path (upstream end). Loading and unloading operations will be restricted during non-peak hours and temporary traffic arrangement in place to minimize traffic impacts. 17. Besides, our department will require the contractor to implement appropriate temporary traffic arrangement to minimize traffic impacts. (a) Prior to the commencement of works, the contractor shall submit the temporary traffic arrangement proposal to the Traffic Management Liaison Group (TMLG), comprises representatives from the relevant government departments (including the Transport Department, Road Management Office of the Hong Kong Police Force and the Highways Department) for consideration and agreement. If there are other road works proposed in the same period, the combined effect of the temporary traffic arrangement schemes would be taken into consideration during the assessment; and (b) In general, there will be no open trench excavation involved in the proposed works. Therefore, construction plants, equipment and the temporary traffic arrangement can be removed quickly for resuming the traffic if there is unexpected traffic condition occurred. Remaining Rehabilitation Works 18. Our department will carry out the remaining rehabilitation works in phases between 2019 and 2023. 4 19. As it is technically feasible to arrange the temporary diversion of sewage from the remaining sections of the rising mains to enable their rehabilitation works, there is no need to suspend the operation of the rising mains for more than several hours, and the discharge of disinfected effluent from RBMSPS will be minimised. Environmental mitigation measures similar to Stage 1 rehabilitation works will also be implemented. 20. There will be no open trench excavation at public road involved in the rehabilitation works of the remaining sections of the rising mains. Therefore, traffic impacts will also be minimal. Mitigation measures for traffic impacts similar to Stage 1 will be also implemented. Coordination with Other Works 21. Our department will coordinate with government departments for other works in-progress or proposed works in the same area. Our department will closely liaise with the concerned departments and flexibly arrange works programme of the proposed rehabilitation works to further minimize impacts to the public arising from the project. Drainage Services Department July 2018 Annexes: Annex 1 – Location Plan and Leakage Incident Records Annex 2 – Location Plan for Proposed Stage 1 Rehabilitation Works at Island Road 5 .