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FAQ >Flooding >

Glossary-Flood Prevention

Glossary (Flood Prevention)
1. Village Flood Pumping Schemes

Village flood pumping schemes involve the construction of bunds around existing villages and pumping of stormwater from within the bunded area to an outside channel during rainstorms. These schemes are necessary where the villages are so low-lying that floodwater cannot effectively be drained by gravity to the drainage network outside. See also



2. Rainfall Return Period

Return period is a statistical means to describe the severity of rainfall. The higher the severity of the rainfall, the less likely will be its recurrence. The rainfall return period is defined as the average period of time expected to elapse between occurrences of rainstorm events at the particular location with the described severity or higher. The use of return period to quantify rainstorm severity has been widely adopted internationally and is the yardstick to measure flood control standards in China, Hong Kong and other developed countries. See also


3. Flood Warning System (or Flood Siren)

As interim measures to minimize flood loss before the completion of long-term drainage improvement works, local flood warning systems have been installed at flood prone villages to inform the villagers when the floodwater reaches a predetermined alert level. The warnings are disseminated through flood sirens or through automatic telephone calls to the village representatives. See also


4. Flooding Blackspots

Flooding blackspots are locations susceptible to flooding of one or more of the following types:


(a)    extensive flooding as a result of inundation of a natural floodplain;

(b)   occasional localized flooding with serious consequences such as causalities, serious traffic disruption or substantial property damage; or

(c)    repeated localized flooding with less serious consequences such as inconvenience to the public or less property damage.


DSD will pay special attention and carry out preventive maintenance to these locations.  With the progressive completion of major drainage improvement projects, more and more flooding blackspots will be removed. See also


5. Flood Proofing Measures

Flood proofing is a simple and inexpensive means of reducing potential damage to dwellings located in flood prone areas.  Basic flood proofing measures that can be implemented by individuals to protect properties and belongings under flood conditions, particularly during small rainstorms, may include :


(a)    Wiring, power outlets, switches, etc. to be installed above the maximum floodwater level, otherwise submersible-type installations should be used.

(b)   Construction and interior decoration materials to be relatively less liable to damage under flood conditions.

(c)    Walls, doors and windows to be watertight.

(d)   Flood-barriers do be erected during floods to form watertight seal in front of all building openings.

6. Flood Prone Areas

Flood prone areas are susceptible to flooding under heavy rainfall or high tide due to one or more of the following causes:

(a)    low-lying topography;

(b)   inadequate drainage; or

(c)    drainage paths for overland flow being obstructed.


7. Inflatable Dam

Inflatable dam is a tubular hydraulic structure constructed across the river course.  It is anchored at the bottom and at both ends.  When inflated, it will form a weir and control the water flow.  For tidal channel where the downstream receiving water body is polluted or for some other reasons that the channel at upstream has to be kept dry, an inflatable dam together with a low flow pumping station installed near the downstream end of the channel are used by DSD to prevent tidal water from flowing into the channel.


The operation of the inflatable dam is automatic according to the pre-set conditions. Under normal weather conditions, the dam will be inflated to prevent tidal water from flowing into the channel. The dry weather flow collected at upstream of the dam will be removed by pumping to the downstream receiving water body. During severe rainstorm, the inflatable dam will be deflated and the operation of the pumping station will be suspended.

8. Flap Valve (or Non-return Valve)
A flap valve is a flow control device that, in principle, only allows water to flow through it in one direction. It is used to prevent backflow from the downstream water body with a higher water level. A flap valve usually consists of a flat plate that is hinged at the top of a culvert outfall. When the water elevation is higher on the downstream side, it will force the flap against the face of the culvert to block the flow and prevent water flowing from the downstream into the drainage network. When the water elevation is higher on the upstream side, the water pressure will force the flap valve to open and release water.
9. Trunk Drainage

Different components of a drainage system are categorized according to the nature of catchment they serve and the hierarchical grouping of such components within the drainage system. Trunk drainage serves large catchments and the failure of which will lead to regional flooding and is therefore assigned with a higher flood protection standard.

10. Stormwater Runoff
When the rain falls on the ground, part of it will be absorbed into the ground, part of it will be retained in some depressed areas, and the remain will flow over the land.  Stormwater runoff is the unabsorbed water that flows off the man-made and natural surfaces during rainstorm. The water is collected via catchment pits, gutters, roadside gullies, stormwater drains, drainage channels or natural rivers before finally being discharged into the sea.
11. River Rehabilitation Projects

Rehabilitation works are small scale drainage improvement work carried out at locations of the river to reduce flood losses. The river rehabilitation works mainly involve removal of drainage bottlenecks, bank stabilization, construction of access roads for maintaining the river, and local widening, straightening or deepening of river sections.