Agreement No. CE 2/2011 (CE) Hung Shui Kiu New Development Area Executive Summary of Planning and Engineering Study and Sewerage Impact Assessment

1 INTRODUCTION

1.1 The purpose of this report is to present the findings of Drainage and Sewerage Impact Assessment that has been carried out for the proposed Hung Shui Kiu New Development Area (HSK NDA) in the Northwest New Territories (NWNT).

2 DRAINAGE IMPACT ASSESSMENT

2.1 The analysis of the levels and flood protection design in the assessment followed the Drainage Manual of Drainage Service Department (DSD). The proposed drainage system serving the NDAs will be generally be designed for 1-in-200-year events. The design has also taken into consideration global warming and resulting rising sea level. 2.2 Three major drainage catchments (Tin Shui Wai, Tuen Mun and Lau Fau Shan Catchments) are reviewed. The proposed development will mainly affect the Tin Shui Wai Catchment. 2.3 The runoff within the Tin Shui Wai catchment to Tin Shui Wai Main Drainage through trunk channels, namely Hung Shui Kiu Channel, Ha Tsuen Channel and Lo Uk Tsuen Channel, and eventually to Deep Bay. 2.4 The drainage impact due to the change of land uses and the change of overland flow path has been assessed. The proposed development will increase and the peak flow in the trunk drainage channels. However, there will still be sufficient free-board for the existing channels. 2.5 The downstream end of Tin Shui Wai Main Channel has been assessed to simulate the peak flow condition. Provision of a stormwater storage tank, is proposed to store the additional runoff arising from the development temporarily. 2.6 Some low-lying villages may be prone to flooding risk after formation of adjacent sites for development. Polder schemes are therefore proposed for Shek Po Tsuen, San Uk Tsuen, Kau Lee Uk Tsuen and Tin Sam Tsuen. A system comprising of underground storage tank and balancing lake is proposed for the temporary stormwater storage. 2.7 With implementation of the mitigation measures, there will be no adverse impact to the drainage systems within the three drainage catchments.

3 SEWERAGE IMPACT ASSESSMENT

3.1 In accordance with the Recommended Outline Development Plan (RODP), four new pumping stations (SPSs) together with the existing Ha Tsuen Sewage Pumping Station will be required to serve HSK NDA. The new/existing rising mains will convey the sewage flow from the pumping stations to San Wai Sewage Treatment Works (STW) eventually. 3.2 To release the capacity of Ha Tsuen SPS for HSK NDA, the existing twin 800mm diameter rising mains from Tin Wah Road SPS to Ha Tsuen SPS will be diverted to a new pumping station. 3.3 San Wai STW will receive all the flows from the four new pumping stations, existing Ha Tsuen SPS and Tin Wah Road SPS where sewage will be treated by CEPT and partially by Tertiary Treatment process. 3.4 Existing San Wai STW with planned capacity will not be sufficient to meet the increased sewage treatment demand, and further expansion works is required.

i June 2015 Agreement No. CE 2/2011 (CE) Hung Shui Kiu New Development Area Executive Summary of Planning and Engineering Study Drainage and Sewerage Impact Assessment

3.5 The treated by CEPT from San Wai STW will be conveyed to the NWNT effluent tunnel and eventually discharged to the Urmston Road Submarine Outfall. Considering the limited capacity of Urmston Road Submarine Outfall as as the water quality impact to the receiving water body, it is proposed to partially upgrade the San Wai STW from CEPT to tertiary treatment level. The treated sewage effluent after tertiary treatment would be used as flushing water, irrigation or district cooling system in HSK NDA and/or other areas. 3.6 The proposed sewerage works will be implemented according to the proposed development programme for the HSK NDA.

ii June 2015