LCQ13: Incidents of severe flooding on HK Island
************************************************
Question:
It has been reported that on the 19th of last month, flooding occurred in extensive areas in Hong Kong due to severe rainstorm. Many places on Hong Kong Island (such as Chai Wan, Wan Chai and Tai Tam Road) experienced severe flooding, causing great inconvenience to members of the public walking through the water. Moreover, floodwater poured into a number of shopping malls, resulting in pecuniary losses to quite a number of shop operators. In this connection, will the Government inform this Council:
(1) of the number of flooding reports received by the Drainage Services Department (DSD) and the number of flooding blackspots which experienced flooding on that day; whether the DSD has investigated the causes for flooding on that day at locations which have not been classified as flooding blackspots; if the DSD has, of such causes;
(2) as the Hong Kong Observatory had forecasted that rainstorms would be brought about by two severe typhoons hitting Hong Kong one after the other within a few days' time, whether the authorities put in place corresponding measures to prevent flooding in light of such forecast; if they did, of the preventive measures implemented by the authorities at the flooding locations prior to the onset of flooding; if not, the reasons for that;
(3) of the maximum hourly rainfall that the existing flood prevention facilities on Hong Kong Island can handle; how the flood relieving capacity of such facilities compares with the relevant international standards; whether the rainfall on Hong Kong Island on that day had exceeded the flood relieving capacity of such facilities; and
(4) whether, in light of the aforesaid severe flooding occurred on Hong Kong Island, it has conducted any comprehensive review to ascertain if there is any need to enhance the flood relieving capacity of the flood prevention facilities; if it has, of the details and the measures in place to prevent the recurrence of severe flooding on Hong Kong Island?
Reply:
President,
The Drainage Services Department (DSD) has been implementing various drainage improvement works. A number of drainage tunnels and stormwater storage schemes have come into operation. These flood prevention infrastructures have operated effectively during the past rainy seasons and significantly reduced the flooding risk in the areas concerned. The Government will continue to review the Drainage Master Plans for all districts in the territories in light of their latest developments. The objective is to examine the flood relieving capacities of existing stormwater drainage systems and recommend improvement proposals to suit the actual situation.
My reply to the various parts of the question is as follows:
(1) During the persistent severe rainstorm on October 19, Amber, Red and Black Rainstorm Warnings were issued in succession by the Hong Kong Observatory (HKO) after 1pm and stayed in force for 6.5 hours until 7.30 pm in the evening on the same day. The Black Rainstorm Warning issued was also the first ever issued in the month of October since the rainstorm warning system came into operation in 1998. The total rainfall recorded on that day reached 223 millimetres. The rain came suddenly and concentrated in a few areas, particularly in the Hong Kong Island East and the Southern District. The hourly rainfall exceeded 100 millimetres, which was far higher than the threshold level of 70 millimetres for Black Rainstorm Warning. The DSD received 14 confirmed flooding cases at locations that have not been classified as flooding blackspots. Eleven of them involved minor flooding, which quickly subsided after discharge. The other three more serious cases occurred at the roundabout of Chai Wan Road, Tai Tam Road and Repulse Bay Road. On receipt of these flooding reports, the DSD and the Highways Department (HyD) immediately deployed staff to clear the inlets on the spot to help the traffic at these locations resume normal flow as soon as possible.
A detailed analysis of the flooding cases that occurred on that day by the DSD is currently underway. For the three relatively serious cases, preliminary findings indicated that the causes for flooding may be attributed to inadequate flood relieving capacities and blockage of upstream channels. During the year, Hong Kong has been affected by a number of typhoons. The HKO hoisted Tropical Storm Signal No. 1 and No. 3 during the period from October 16 to 18. A considerable amount of tree branches and leaves were torn down by strong winds and accumulated on the slopes upstream. They were washed down subsequently into the catchwaters and stormwater drainage systems downstream by the severe rainstorm on October 19, resulting in blockage of the drainage systems, and hence undermining their overall flood relieving capacities. The DSD, in collaboration with the Water Supplies Department and the HyD, are implementing immediate improvement measures, including stepping up inspections of drainage channels at hillsides and catchwaters, provision of additional road gullies, etc. Formulation of long-term improvement measures for stormwater drainage systems is also underway. For its Drainage Master Plan Study on Northern Hong Kong Island, the DSD will focus on examining and formulating recommendations for the existing drainage systems in the Eastern District of Hong Kong Island.
(2) The DSD regularly attends pre-wet season inter-departmental meetings and engages relevant departments in discussion and implementation of flooding prevention measures in all districts. Apart from conducting regular patrols and clearance of stormwater drainage systems, the DSD also steps up inspections and clearance of major drainage channels and inlets (particularly at flooding blackspots), both before the onset of typhoons and afterwards, to ensure that the drains are free from obstruction. The DSD has also set up an Emergency and Storm Damage Organisation to handle emergencies and flooding problems. It will also activate its Emergency Control Centre and deploy additional staff and resources to cope with flooding cases as well as emergency clearance of blocked drains and channels when a Red or Black Rainstorm Warning signal is hoisted, a special announcement on Flooding in the Northern New Territories is issued, or the Tropical Cyclone Warning Signal No. 8 or higher is in force. Furthermore, contingency teams are deployed to stand by at the locations prone to flooding during severe weather to ensure that inspections and drain clearance will be conducted in time to reduce flooding risk. During the severe rainstorm on October 19, the DSD Emergency Control Centre was activated according to the established mechanism and over 40 teams were mobilised.
(3) The design standards of stormwater drainage systems in Hong Kong are developed with reference to the relevant international standards and with due consideration for land use, consequences of flooding and other relevant factors. Generally, the flood relieving capacities of drainage systems in the urban areas are designed to withstand rainstorms of a 50-year return period. While the rainfall recorded on October 19 did not exceed the threshold of the return period, certain locations at low-lying areas with ageing or unimproved drainage systems and with inlets susceptible to occasional blockage by refuse might still become flooded during the severe rainstorm. The DSD would dispatch staff to carry out emergency flood relief measures at these spots as soon as possible to facilitate an early return to normal after the passage of severe rainstorm.
(4) The Drainage Master Plan Studies for all districts across the territory were completed in 2000 and their proposed construction projects were also completed in succession. Over the past years, new land use planning has brought about new developments, which have in turn brought significant changes to the drainage characteristics of catchment areas. Climate changes also pose new challenges to stormwater drainage systems, rendering old hydraulic models and related analysis cannot fully reflect the actual situation. In view of this, the DSD has commenced studies to review the stormwater drainage systems for all districts across the territory by stages since 2008. The study for northern Hong Kong Island is currently underway. The latest flooding cases will be analysed and appropriate improvement measures will also be developed to enhance the flood prevention levels in the areas concerned.
Ends/Wednesday, November 23, 2016
Issued at HKT 11:30
Issued at HKT 11:30
NNNN