Flood Warning for Northern New Territories

The public should be aware that when the Amber Rainstorm Warning Signal is issued, it is likely that there will be flooding in some low-lying and poorly drained areas. The Red and Black Rainstorm Warning Signals indicate more serious flooding which could cause major disruptions. A special announcement on flooding due to heavy rain in northern New Territories will be issued whenever significant flooding is expected to occur or is occurring in the low-lying plains of northern New Territories. This special announcement is broadcast by radio and television to the public. The announcement will be updated at regular intervals until heavy rain ceases. Occasionally, isolated thunderstorms bring about sudden flooding in very localised regions. The public should therefore be on the lookout when they cross water courses in the countryside on thundery days. Points to Note: Residents in northern New Territories, who are likely to be affected, are advised to take necessary precautions to avoid possible flood damage. Residents in northern New Territories should pay attention to the flood sirens if they are nearby. Farmers and fish pond owners should take necessary precautions to minimise losses like checking and clearing the drainage system and around the farm/fish ponds to ensure that all the drains would not be blocked. Fish pond operators, where possible, should reduce the water level of ponds which are likely to be flooded.

Landslip Warning

A warning of landslips due to prolonged heavy rain will be issued by the Hong Kong Observatory whenever heavy rain has occurred, the rain is expected to continue in the next few hours and landslips are considered likely by the Geotechnical Engineering Office. On the issuance of such a warning, a Landslip Special Announcement will be sent to the local radio and television stations for broadcast to the public and the announcement will be updated hourly until the likelihood of landslips has diminished. Flood and landslip warnings supplement routine weather forecasts by drawing attention to floods and landslips due to heavy rain. They are intended to assist engineers, contractors or others who are likely to suffer losses from flooding or landslips. These warnings also alert the relevant government departments and organizations to take appropriate actions, e.g., opening of temporary shelters, stand-by for search and rescue operations, etc. The flood and/or landslip warnings are issued irrespective of whether tropical cyclone signals are displayed. Like all forecasts, a warning of floods or landslips represents an assessment of the most likely development in the weather based on the latest information available at the time. There will unavoidably be false alarms as well as occasions when heavy rain which may lead to flooding or landslips develops suddenly and affects parts of Hong Kong before a warning can be issued. Points to Note: Cancel non-essential appoinments, stay at home or remain in a safe shelter. Pedestrians should avoid walking or standing close to a steep slope or retaining wall. Motorists should avoid driving in hilly areas or on sections of road with the landslip warning sign until the rain has eased. Listen to radio or television broadcasts on the latest warnings. Report any sign of landslip danger to the police immediately. Take appropriate measures to avoid possible damage or loss of life due to flooding and landslips. Farmers and fish pond owners should take necessary precautions to minimize losses. If you have received a notice to evacuate or you think your home is endangered by an unstable slope or by overhanging boulders you must make immediate arrangement to move to a safe shelter. Temporary shelters will be opened when a landslip warning is issued. Please call Home Affairs Department at 2835 1473 to find out the telephone numbers of district offices and where temporary shelters are.

Rainfall in Hong Kong

The mean annual rainfall at the Hong Kong Observatory is about 2,200 millimetres with some variations over the territory. Mean annual values range from around 1,300 millimetres at Waglan to more than 3,000 millimetres in the vicinity of Tai Mo Shan.

Close to 80 per cent of the annual rainfall occurs between May and September. Although heavy rainstorms are not uncommon at any time of the year, most of them happen during the summer months. From past records, hourly rainfall amounts of 50 millimetres or more occurred about twice a year. For example, the hourly rainfall amount at the Hong Kong Observatory is 109.9 millimetres measured during the rainstorms on 8 May 1992.

Figure 1: Daily Rainfall Distribution of a heavy rain case (2 May 1989) showing the rain was heaviest over eastern Kowloon.

Flood and Landslip

Every year heavy or prolonged rain causes floods and landslips in Hong Kong. Engineers, architects, cotractors and other concerned should take all necessary precautions.

Flood occurs when the rainfall rate is so large that natural or artificial drainage fails to drain away the accumulated water. Floods are usually fairly transient in the urban areas but may last up to a few hours in rural areas with large catchment and gentle slopes.

Hong Kong is mostly of hilly terrain. Under certain conditions, especially when influenced by heavy rain, slopes may fail giving rise to landslips which pose considerable threat to people dwelling on the foothills and on the slope itself. In June 1966 three rainstorms resulted in the loss of 86 lives and in June 1972 a total of 148 people died in landslips caused by heavy rain.

Meteorological Watch

In order to issue timely warnings to the public, the Hong Kong Observatory keeps a continuous watch of the weather in and around Hong Kong. Readings from a network of more than 90 automatic rain-gauges, operated by the Hong Kong Observatory, the Geotechnical Engineering Office and the Drainage Services Department, are telemetered to the Hong Kong Observatory Headquarters to provide up-to-the-minute information essential for assessing the likelihood of flooding and landslips. Water-level and rainfall information from the northwest New Territories are also relayed directly to the observatory headquarters for real-time monitoring of floods in these areas. In addition to conventional meteorological observations, the digital radar system of the Hong Kong Observatory provides a good means to continuously monitor the movement and development of rainstorms. The system can also generate short term objective rainfall forecasts at selected places over the territory. Hourly high resolution satellite imageries are received from the Japanese Geostationary Meteorological Satellite, providing a bird's eye view of the cloud pattern around Hong Kong. The forecaster can utilize special enhancement techniques to track intense convective activity in the region.

Figure 2: Satellite Picture at 2 p.m. on 2 May 1989 showing a narrow band of rain-bearing clouds covering Hong Kong.