Estuary Processes and Managment - River Dynamics and Integrated River Management - page 478

Hong Kong at the Pearl River mouth are often hit by rainstorm flooding. Thunderstorms and lightning
can damage electrical installations, start fires and cause death by electrocution. Waterspouts are rather more
common and may capsize small boats in nearby waters and damaging coastal facilities. Hail sometimes
forms in well-developed thunderstorm clouds. Hailstones are hard pellets of ice which are usually a few
mm in diameter. Larger hailstones have alternate rings of clear ice and frost. Large hailstones can damage
crops, particular young fruits and vegetables, and can break windows, glass houses and car windscreens.
Flooding occurs when the rainfall rate is so large that natural or artificial drainage is insufficient to drain
away the fast accumulating water on the ground. Floods are usually fairly transient in the urban area but
may last longer in those rural areas with large catchment and gentle slopes.
Hong Kong is a harbor city and an international center of, finance, commerce and trade. The long term
average annual rainfall in Hong Kong is 2,200 mm; about 80 percent of the rain falls between May and
September. The wettest month is August, with an average monthly rainfall of 391 mm. The topography of
drainage basins consists typically of steep upland terrain and relatively flat valley floor in the downstream
semi-rural or densely populated “new towns” or metropolitan/urban areas. Over the past forty years,
changing land use patterns (in particular the conversion of farm land to fish ponds and increasing urban
areas) have resulted in the loss of flood plain storage and increased runoff. For example, agriculture land
(cultivated and fallow) have decreased from 2830 ha in 1963 to 925 ha around 1990; on the other hand,
the fish pond area increased from 665 ha to 990 ha, and developed urban area from 460 to 1465 ha
respectively (Lee et al., 2002). In addition, the 1990s witnessed several unusally warm years, which seems
to be correlated with record high rainfalls. For example, 1997 was the fourth warmest year on record, with
an average temperature of 23.4 ć ; an extremely high annual rainfall of 3,343 mm was recorded, with
extensive flooding in West Kowloon. The rainfall also exhibited significant spatial and temporal variability.
In 2001 (the fourth wettest year), the annual rainfall was 3,091 mm; 1,368 mm of rain (62%) fell on the
Northern New Territories in June 2001 alone, causing wide spread flooding (HKDSD, 2001).
A variety of measures have been developed to protect semi-rural areas, new towns, and metropolitan
areas from flooding—including real time warning systems, village polder schemes, river training, storm-
water diversion and storage schemes. Many of these flood control schemes share certain characteristics:
high intense storms and inflows, the need to design conveyance systems under tight space constraints,
proximity to densely populated areas and congested underground utilities, and enhanced backwater effect
due to coastal reclamation. For instance the Yuen Long Bypass Floodway was designed under difficult
land availability constraints—its success depends crucially on the use of the jet principle to lower water
levels at a critical river junction, thus enabling the required flow diversion into the floodway (Lee et al.,
1998). The design of these flood control schemes often involve highly complex three-dimensional flows:
complicated subcritical-supercritical transitions, free surface, surcharged and two-phase flows, and spatially
varied flow at 3D junctions.
The significant spatial and temporal variability of rainfall and rapid urbanization resulted in an insufficiency
of drainage capacity and the solutions are very site-specified. For instance, as shown in Fig. 8.15, the
upstream storm-water runoff from Tai Hang Road and Tat Chee Road are conveyed in underground storm
drains; the combined flow passes through a steep culvert downstream to join with the Boundary Street
Nullah in an open conduit. The total flow is discharged into a steep 1.5 km long culvert under Nullah Road
leading to the Hong Kong Harbour. Due to rapid urbanization, the existing drainage system (designed for
10-years storm) failed to cope with severe rainstorms in recent years ( Q max = 110 m 3 /s for 50-year storm);
there are a number of flooding “black spots”.