The basic concepts of flood
10/09/2014 08:53:03

 The flood in the central region usually happens between August and November every year and causes terrible loss of lives and properties. This writing will share the basic knowledge about flood with the readers so they can have the overview of this kind of natural disasters.

1) WHAT IS FLOOD?

 

 Flood is a phenomenon which is caused by the rising of the river in a specified period of time and then reduces this gradually.
 
2) WHAT IS THE RISING FLOOD WATER, THE FALLING FLOOD WATER, THE FLOOD PEAK, THE RISING LIMB, THE FALLING LIMB? 
 

- The rising flood water: the time when flood waters begin to rise (the level of water begins to rise high (Hcl) or the flow of water increases). 

 

- The flood peak: the highest water level or highest flow of water (Hđ)

 

- The falling flood water: flood water goes down to lowest level which is roughly equal the level when water begins to rise (Hcx).

 

- The rising limb: the time from the foot of flood to the peak of flood (tl).

 

- The falling limb: the time from the peak of flood to the foot of flood (tx).

 

- The flood stage: the time from the rising flood water to the falling flood water:

T = tl + tx


- The amplitude of the rising flood water: the difference between water level at the peak of flood and water level at the foot of rising flood water.

The amplitude of flood in the mountainous areas can reach 10 -20 meters, especially, it can reach over 25 meters in some areas (Lai Chau). In the urban areas, it usually reaches 3-8 meters. 

 
3) WHAT IS SINGLE FLOOD?
 
The single flood is the flood that has only one highest peak caused by rainfall in river basin.
 
4) WHAT IS COMPOUND FLOOD?
 
Compound flood is the flood that has many peaks such as two or three peaks caused by two or more non-stop rainfalls.
  
 
Severe flood is running up the houses
5) WHAT IS WATER LEVEL?
 
Water level is the elevation of free river surface which is calculated from national standard reference height (Average water level at Hon Dau station) and expressed in H and in cm (centimeter) or m (meter). 
 
6) WHAT IS THE LITTLE FULLNESS FLOOD?
 
The little fullness flood is caused by cloudburst in the period of grain full (at the end of May) every year.
 
The little fullness flood is not severe but it is the important water source for production because in this period the weather is usually hot and has little rain, the plants must be watered regularly. However, sometimes the little flood is quite severe and causes important damages (the flood in May, 1986) in Southern Central Vietnam.
 
 
7) WHAT IS THE FLOOD OF THE MAIN CROP
 
The flood of the main crop happens in the heart of flood season, is usually the biggest flood in a year so it is easy to cause flooding and terrible loss of lives and properties. In the northern rivers, the flood of the main crop usually happens in July and August. In the central river, it usually happens in October and November. In the southern rivers, it usually happens in September and October
 

8) WHAT IS THE FLOOD OF THE LATE CROP

 
The flood of the late crop usually happens in the end of autumn and is usually not severe. However, sometimes the flood of late crop is the biggest flood of the year. Flood can happen late in November in northern river, in December or January of the next year in the Central areas and Central Highlands, in November in southern
 
9) HOW ARE THE FLOOD CLASSIFIED ACCORDING ON THE MAGNITUDE OF FLOOD PEAK?
 
According to the magnitude of average flood peak in many years, we can divide the level of flood as follows:
 
Little flood:  the peak flood level is lower than the average peak flood in many years;
 
Moderate flood:  the peak flood level reaches the average peak flood in many years;
 
Severe flood: the peak flood level is higher than the average peak flood in many years; 

Extremely severe flood: the peak flood level is uncommonly high in many monitoring periods;
 
Historical flood: the peak flood level is highest in many monitoring and investigative periods.
 
                                                                                                                                            Thuy Dung
 
 
 
 
 
 
 
 
 
 
 
 
 
              
 
 
 
 
 
 
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