1. DRAINAGE IMPACT ASSESSMENT (DIA) - Hung Shui Kiu New ...

Agreement No. CE 2/2011 (CE)Hung Shui Kiu New Development AreaPlanning and Engineering StudyExecutive Summary ofDIA, SIA, WSUIA and Site Formation Assessment1. DRAINAGE IMPACT ASSESSMENT (DIA)1.1 Three major drainage catchments (Tin Shui Wai, Tuen Mun and Lau Fau Shan Catchments)are reviewed. Only Tin Shui Wai Catchment is affected by the proposed development. Thedrainage impact due to the change of land uses and the change of overland flow path hasbeen assessed.1.2 After reviewing the catchment characteristics and drainage systems of HSK NDA, provisionof a stormwater storage tank at the downstream of the NDA near Tin Shui Wai Channel isrecommended as a mitigation measure for temporary storage of additional runoff arisingfrom the development.1.3 Drainage improvement works area proposed to upgrade the existing trunk drainage channelfrom trapezoidal to rectangular shape. These channels include Lo Uk Tsuen Channel, HungShui Kiu Channel and the downstream section of Ha Tsuen Channel.1.4 Site formation levels are proposed according to the hydraulic model results. To comply withthe flood protection standard of Stormwater Design Manual, a minimum 500mm freeboardhas been provided when designing the formation levels adjacent to drainage channels while300mm freeboard is adopted for other sites.1.5 Polder scheme is also proposed for Shek Po Tsuen, San Uk Tsuen, Kau Lee Uk Tsuen andTin Sam Tsuen. An underground water storage tank is proposed for flood storage.2 SEWERAGE IMPACT ASSESSMENT (SIA)2.1 In accordance with the PODP, the existing Ha Tsuen Sewage Pumping Station will beremoved to facilitate residential development. Three new pumping stations are proposed toconvey the sewage to San Wai Sewage Treatment Works (STW).2.2 The treated effluent from STW will be conveyed to the 9km NWNT effluent tunnel andeventually discharged to the Urmston Road Submarine Outfall.2.3 For assessment of the impacts on the existing and planned sewerage systems, sewagetreatment and disposal facilities, three flow scenarios (namely Year 2024, Year 2027 andYear 2034) were considered. Existing San Wai STW would not have enough capacity tomeet the increased sewage treatment demand, upgrading works is anticipated to benecessary.2.4 The proposed sewerage works will be implemented in three phases in line with theproposed development programme for the HSK NDA.3 WATER SUPPLY AND UTILITIES IMPACT ASSESSMENT (WSUIA)Water supply3.1 The fresh water demand is estimated based on the planned population and land use.3.2 Au Tau Water Treatment Works has been identified as a possible source with sufficientcapacity to feed fresh water to HSK NDA.3.3 Currently, there is no salt water supply in HSK NDA for flushing purpose. Since HSK NDAis not fully covered by the NWNT salt water supply system, a number of potential flushingwater supply options were discussed, including upgrade Lok On Pai Salt Water PumpingStation and expand the storage volume of Tan Kwai Tsuen Salt Water Service Reservoiri July 2013

Agreement No. CE 2/2011 (CE)Hung Shui Kiu New Development AreaPlanning and Engineering StudyExecutive Summary ofDIA, SIA, WSUIA and Site Formation Assessment(TKTSWSR); reuse treated sewage effluent; constructing a new salt water pumping stationat Lau Fau Shan; and using fresh water for flushing purpose. The flushing water supplysystem for HSK NDA is subject to further study.Utilities3.4 There is a group of 400kV overhead cable crossing the southern portion of HSK NDA,which may hinder the residential and commercial development. The feasibility of divertingthe 400kV to underground is being investigated by CLP.3.5 In addition, 3 nos. of 132kV electricity substation are proposed to be constructed to copewith power demands within HSK NDA.3.6 Based on existing gas mains record, diversion/abandonments of existing low and mediumpressure gas mains are identified. However, the high pressure gas mains along Yuen LongHighway will not be affected.3.7 According to the information provided by various telecommunication companies, there areexisting cables running along Castle Peak Road, Hung Shui Kiu Main Street and Hung TinRoad, and they are willing to expand network for future HSK NDA.3.8 Common utility ducts are recommended for accommodating the gas and telecommunicationservices so as to minimize road opening.4 SITE FORMATION ASSESSMENT4.1 With reference to the minimum formation levels recommended in the DIA Report, thedemand for fill and excavation works were estimated and a deficit of fill materials ispredicted. Fill material required to be imported from other parts of Hong Kong.4.2 The extent of contaminated lands would be subject to the outcome of the contamination teststo be carried out. The method of treatment to the contaminated lands depends on the level ofcontamination as confirmed under contamination test.4.3 The implementation programme will also take into account the quantities of fill andexcavated materials in the development sites so as to minimize transporting suitablematerials off-site during excavation.4.4 Temporary drainage system would be provided for the formed sites to collect surface runoffand within the sites before permanent drainage systems are in place. At the end oftemporary sand traps will also be provided before discharging into the drainage network tominimize impact on water quality.ii July 2013