Hong Kong is on the common track of tropical cyclones and can experience very heavy rainstorms at times. The annual average rainfall is about 2400 millimeters, one of the highest among the cities in the Pacific Rim. During particularly heavy rainstorms, flooding in the rural low-lying areas and natural flood-plains in the northern part of the territory and in parts of the older urban areas may occur. Some rainfall records in Hong Kong are shown below:
|Maximum hourly rainfall||145.5 mm||7 June 2008|
|Maximum daily rainfall||534.1 mm||19 July 1926|
|Maximum monthly rainfall||1346.1 mm||June 2008|
|Maximum annual rainfall||3343.0 mm||1997|
Rapid urban development in the eighties resulted in intensive development in the rural areas including the flood-plains. This had turned large areas of natural ground into hard paved areas and as a result, rainwater which formerly was retained, now quickly becomes surface flow. The expansion of built-up areas in close proximity to the major watercourses had also reduced their flood carrying capacities and had further aggravated the flooding problems. In recent years, flooding threats in the New Territories have been significantly reduced subsequent to the completion of main drainage channels and village flood protection schemes under our flood prevention strategy.
The expansion of the built-up urban areas had also aggravated flooding risks in the old urban areas, where aged stormwater drainage systems were built decades ago to the older protection standards. The flooding risks in the urban areas have been improved gradually over the years through implementation of various measures, such as interception, stormwater storage and pumping, identified and adopted under the flood prevention strategy.
With our continuous efforts, flooding risks have been much reduced but cannot be entirely eliminated. For example, flooding may occur when there is an exceptional rainstorm whose intensity is more severe than that assumed in our design standards. Also the drainage system may not function as designed if some sections and in particular inlets such as catchpits are blocked by rubbish, fallen trees or landslide debris. Besides, flooding at low-lying coastal areas may occur when there is exceptionally high tide level caused by storm surge during the passage of a typhoon.