Floods have become a severe problem nowadays due to many factors. Every year, many parts of Globe are suffering from the problem of flooding. It is responsible for many life losses. Between 1998-2017 almost 2 billion people were affected by floods worldwide.
What is flood?
Flood is a process of over bank flow of water. The water surface has reached an elevation which can cause damage to the personal property on flood plains. Flood plains are simply the plain area surrounding a water reservoir.
Types of flood
Broadly speaking there are five types of floods.
- River flooding.
- Coastal flooding.
- Pluvial flooding.
- Flash flooding.
- Urban flooding.
Assam flood, 2020
The state of Assam in India is experiencing the attack of flood. Almost 15 lakhs people are affected by that. 23 districts are under its effect. The worst hitted place is Barpeta district where almost 8.6 lakh people get affected. The South Salmara district is also greatly affected. 1.95 lakh people are affected due to it. The death toll has climbed up to 34.
Besides humans, animals and crops are also devastated. Almost 75700 hectares of cropland is being disturbed. The flood has severely attacked 20 lakhs domestic animals and poultry.
Causes of flood
Due to urbanization, the magnitude and frequency of flood has increased because of the so-called “impervious-cover”. In an urban areas two criterias are to be taken into consideration:
- Percentage of land covered by concrete structures.
- Area covered by storm sewers.
Because of the concrete cover less water infiltrates or seeps down and thus it increases the run off. Studies showed that urban runoff is 1.1 to 4.6 times pre urban runoff.
Storm sewers are extremely important as they serve as a passage for urban runoff from impervious surfaces to reach the stream.
Due to extreme rainfall and lower discharge capacity, such areas experience a dangerous form of river flooding known as “Flash Floods”.
|Minneapolis dam, U.S.A|
Failure of the dam
Failure of the dam can be due to several reasons such as due to engineering inaccuracy, heavy rainfalls and seismic activities.
In 2019, Tiwari dam in Ratnagiri, Maharashtra has experienced dam failure. The main factor behind dam failure was heavy rains. The flood has a destructive and catastrophic effect in the downstream areas.
Rapid melting of ice and snow in the spring
Rapid melting of the land ice is one of the consequences of Global warming. It enhances the sea level which is generally known as “Sea level rise”.
Sea level rise is one of the factors for coastal flooding. It increases the amount of water due to which the land becomes helpless and it has to get submerged under the water cover.
If the upper soil is completely filled with moisture i.e. if it is saturated, there is no more space available for the rain water to percolate thereby the runoff increases and produces flood conditions.
Amount and distribution of precipitation
The amount of rainfall is another obvious reason. Extreme amounts of precipitation is experienced during Cloudburst.
On 31 July 2019, the city of Vadodara, Gujarat went through a disastrous cloud burst. About 424mm rainfall was recorded in six hours only. The city experienced the flood stage conditions of Vishwamitri River for several days.
The amount of surface and subsurface runoff is greatly influenced by the geologic conditions of the drainage basin. If the terrain through which the water is flowing is steeper, the surface water will flow more readily. Also the subsurface water is allowed to flow down gradient thereby reaching the stream.
Permeability of soil
The rate of infiltration of water depends on the soil type. If the soil is porous and permeable, it will allow the water to percolate. And thus, the flood conditions will be reduced.
But if the permeable soil is already saturated with the water, the additional water from precipitation will become a part of the runoff.
Effects of floods
The flood can affect social, economical and environmental aspects. There are two types of effects of flood. It can have a primary or secondary impact. Further both primary and secondary effects can have a positive and negative impact.
Primary effects are the adverse changes caused by the floods. It includes injury and loss of life. Damages caused by swift currents and debris to the infrastructures. The sediments brought by the water in the landscape can harm crops and vegetation.
Moreover water can act as an eroding agent thereby increasing the problem of soil erosion. Disruption of communication and transportation services.
|Everything under water due to flood|
Secondary effects are the malfunction and disruption of the services and system associated with floods. It includes epidemics, hunger, short term pollution of rivers and drinking water.
Loss of livelihood, displacement of persons who lost their homes and fires due to short circuits and gas leakages.
Some positive impacts
The positive impacts include recharging of groundwater aquifers. Certain species are dependent on floods for their breeding events.
The sediments and nutrients brought by the water on the landscape enhances the soil capability and thereby increases the crop production.