The causes of flooding are : 

1. Intense rainfall

2. Topography of the catchment

3. Sedimentation of rivers and reservoirs 

4. Obstruction in the river flow

5. Failure of dam

6. Failure of river embankment

7. Inadequate cross drainage works 

8. Contraction of waterway

1) Intense rainfall :

An intense rainfall can occur only over a comparatively small area. 

Whenever such a precipitation bursts over a small watershed, it results in a high surface runoff, which reaches he main channel faster than it can be discharged, flooding the low lying area along the stream. 

The seriousness of the flood will depend upon the following factors :

  • The intensity and duration of precipitation.
  • Surface slopes of the watershed. 
  • Nature of the surface soil and covering vegetation

The intense precipitation sometimes also known as cloud burst.

If the rainfall is very heavy and the storm duration is longer, the resulting surface runoff will be more causing flood like situation.

2) Topography of the catchment :

For mountainous regions the relation of intensity of rainfall and slope of the surface have been found to have a good relationship. 

The same intensity of rainfall produces more discharge in a mountainous region than plain area. 

The shape of the catchment also affect the surface runoff and flood. 

For a fan-shaped catchment, the time of concentration will be less and hence the peak flow will be more.

But, in case of fern-leaf type (elongated) catchment of the same area and same storm, the time of concentration will be more. 

All the tributaries does not reach the stream at the same time, thus the peak flow will be comparatively less. 

The more pervious the soil, the greater is its water retention power and longer the period of saturation, resulting in lesser floods.

3) Sedimentation of rivers and Reservoirs:

If the top soil layer in the catchment area is loose and the vegetation cover is less, the tributaries flowing through such areas carry heavy sediment load. 

This sediment load is deposited in the river bed and the reservoir in which the river merges. 

Thus, the cross section of the river become shallow resultion in overtopping of water during heavy rains. 

Due to siltation in reservoirs, the storage capacity of the reservoirs decreases, resulting in flooding of the downstream area.

4) Obstruction in the river flow :

Sometimes, due to heavy landslides in the river, the natural flow of water is obstructed resulting in flood situation on the upstream of the river. 

When the obstructions in the river are removed due to heavy water pressure accumulated on the upstream, flooding is caused on the downstream of the river.

5) Failure of dam :

 A dam is constructed to store huge quantity of water in the reservoir created on the upstream of the dam. 

Due to failure of the dam the large quantity of water stored in the reservoir instantly released causing flood in the downstream of the dam.

A dam may fail due to the following reasons:

Piping, Erosion, Foundation failure, Earthquake, etc.

6) Failure of river embankment :

Sometimes, the river embankment may fail due to gradual erosion by flowing water, resulting in flowing of flood water in the surrounding areas and flooding.

7) Inadequate cross drainage works :

Sometimes at the crossing of canal and river, it may be required to pass the river below the canal, by constructing a aqueduct. 

Due to the construction of an aquduct, the depth of flow is reduced. 

Thus, during high flood, water level may rise on the u/s side and may submerge the surrounding area.

8) Contraction of Waterway :

Due to construction of railway or highway bridge across the river, the natural waterway of river is contracted due to bridge piers and abutments. 

Thus, water rises on the u/s side due to insufficient passage and the upstream area may get flooded.

Ill-effects of floods

The ill-effects of floods can be classified into the following three groups :

1. Inundation of low lying areas

2. Erosion of river banks

3. Change of river course

1) Inundation of low lying areas: 

The following factors may cause the inundation of low lying areas: 

(i) excessive runoff and inadequate channel capacity

(ii) Presence of obstructions in the stream

(iii) Inadequate waterway due to rail, road or canal crossing 

(iv) Congestion of confluences

(v) Tides resisting flood flows

2) Erosion of river banks : 

The fast flowing current may eat away the banks, formed of easily erodable soil and

thus may affect vast stretches of agricultural land.

3) Change in river course : 

The change in the course of the river may develop due to the following reasons :

(i) Due to high velocities of the river water

(ii) Due to abrupt variation in the bed gradient and heavy sediment charge.

(iii) Due to oscillatory bed of the river.

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