Surface run-off is the primary source of flood. When the flow amount increases than the outflow amount in the river or stream. then the excess water accumulates in the river course and increases the water level. If this phenomenon continues for a longer duration, the excess water spreads in the adjoining regions causing flood. Flood occurs when the water in the river channel overflows its banks and inundates the outlying regions. They are mostly caused by two factors. One of them is the excessive melting of snow or glacier ice or sudden bursting of glacier lakes. Another major cause of flood is the high run-off resulting from rainfall. The rainfall of very high intensity lasting for a short duration may give rise to high run-off. Similarly, a lower intensity rainfall lasting for a longer duration of time may also yield high run-off. Both of these instantaneously cause high discharge in the river channels. As a result, if the discharge exceeds the channel capacity, it overflows the banks and causes floods.

The main causes of flood can be summarized in the following five points.

(a) Heavy rainfall is one of the major causes of flood. When it rains heavily for a long time then the earth's surface cannot absorb all the water and it flows downward. When all the collected water doesn't get adjusted in rivers, lakes and streams then the water covers the nearby lands and causes flood.

(b) Very large amount of water is also stored in the glacier lakes of the Himalayan regions. Sometimes, the lakes burst out from the weak surrounding and take the form of the flood.

(c) Sometimes, landslides that occur in the stream and rivers block the flow of water in them. When the blocked water is forced out suddenly, then the flood occurs.

(d) Countries near the oceans face floods due to sea storms (cyclones).

(e) Tsunamis are produced by earthquake waves in ocean. It is also a type of flood that occurs in the coastal regions.

Flood forecasting; For the estimation of flood, different methods can be employed. They are as follows:

(a) Increasing the observed maximum flood by a certain percentage.

(b) Envelop curve

(c) Empirical flood formulae

(d) Rational method

(e) Unit hydrograph application and

(f) Frequency analysis

The first four methods listed above can give only the flood peaks. The fifth method gives the complete hydrograph but cannot be used for a very large area. The sixth method can be to construct the flood hydrograph by making its peak discharge equal to the estimated design flood and the other feature to resemble the observed hydrographs in the available records.