Present Flood Situation In Manipur
- Part 1 -

Salam Gunakanta Singh *

Heavy Flooding at Kangpokpi on 15 June 2017 :: Pix - Lamdamba Oinam

Natural Disasters - Floods

If you thought that floods are natural disasters that happened only in poorer continents like Africa and Asia, then you are wrong. First, not all floods are caused by nature. They are man-made too. And second, even countries like the USA and the UK get floods that cause huge losses to life and property. To understand what causes floods and how man's activities affect floods, visit the links mentioned below.

For a simple and scientific explanation of floods and different types of floods, visit this link. The language is simple and the concepts are clearly explained in this site. See the diagrams to understand how the landscape changes when floods occur.

Causes of floods

To understand how floods are caused, the extent to which they can be considered as "acts of God" and how the activities of man can make floods more or less severe, read this comprehensive guide here.

It is written mainly for insurance companies, builders and other professionals and talks mostly of flooding in UK. But it is very informative as the basic facts remain the same- the change in landscape, land use, how the size of the river basin and the flood plain affects the severity of floods, and how we can prepare for floods in a flood prone area.

1998: A year of floods

One of the nature's most turbulent years was 1998. Floods wreaked havoc in North America, South America, Asia and Europe. Though the El Nino and La Nina were thought to be responsible for these floods, scientists have now found out that these factors combined with global warming caused by pollution and made the disasters more severe.

Deforestation and global warming are cited as two of the main man-made causes that led to such a disastrous year with loss of lives and huge damage. Read these reports from BBC network on some of the worst floods and the devastating effect of Hurricane Mitch - all testimonies to the result of human activity interacting with natural phenomena.

Are man-made floods possible?

A debate around the world is that nature is to blame for natural hazards while one section says that man causes or triggers these hazards by deforestation and pollution. In China, a flood affected the lives of 250 million people and caused large scale devastation and the El Nino was cited as the main cause.

But after some analysis, it has been found that it was caused by large scale deforestation leading to loose soil and therefore huge floods. Read this article to know more about this debate.

How are floods caused by man? How do man-made changes in a river basin affect floods? In Washington, floods are caused by man-made actions. The rapid urbanization of Washington has led to more concrete surfaces that seal the surface and do not allow water to seep in.

Dikes can help contain the water but they also cut off the river from the floodplain. The floodplain helps to retain some of the floodwater but cutting the river off from it means that this natural storage is gone. The consequent pressure on the environment leads to such disasters.

What causes Floods?

Flooding occurs most commonly from heavy rainfall when natural watercourses do not have the capacity to convey excess water. However, floods are not always caused by heavy rainfall. They can result from other phenomena, particularly in coastal areas where inundation can be caused by a storm surge associated with a tropical cyclone, a tsunami or a high tide coinciding with higher than normal river levels. Dam failure, triggered for example by an earthquake, will result in flooding of the downstream area, even in dry weather conditions.

Other factors which may contribute to flooding include:
o volume, spatial distribution, intensity and duration of rainfall over a catchment;
o the capacity of the watercourse or stream network to convey runoff;
o catchment and weather conditions prior to a rainfall event;
o ground cover;
o topography; and
o tidal influences.

Types of Flood

Three Common Types of Flood Explained

One of the keys to understanding flood risk is understanding the nuances of the type (or types) of flood you face. Why? A flood's a flood, right? Wrong. There are several different kinds of flood, and each one bears a different impact in terms of how it occurs, the damage it causes, and how it is forecasted. Here's a crash course on three common types of flood to help you better assess your risk.

Coastal (Surge Flood)

A coastal flood, as the name suggests, occurs in areas that lie on the coast of a sea, ocean, or other large body of open water. It is typically the result of extreme tidal conditions caused by severe weather. Storm surge — produced when high winds from hurricanes and other storms push water onshore — is the leading cause of coastal flooding and often the greatest threat associated with a tropical storm. In this type of flood, water overwhelms low-lying land and often causes devastating loss of life and property.

Coastal flooding is categorized in three levels:
o Minor: A slight amount of beach erosion will occur but no major damage is expected.
o Moderate: A fair amount of beach erosion will occur as well as damage to some homes and businesses.
o Major: Serious threat to life and property. Large-scale beach erosion will occur, numerous roads will be flooded, and many structures will be damaged. Citizens should review safety precautions and prepare to evacuate if necessary.

The severity of a coastal flood is determined by several factors, including the strength, size, speed, and direction of the storm. The onshore and offshore topography also plays an important role. To determine the probability and magnitude of a storm surge, coastal flood models consider this information in addition to data from historical storms that have affected the area, as well as the density of nearby development.

Fluvial (River Flood)

Fluvial or riverside flooding, occurs when excessive rainfall over an extended period of time causes a river to exceed its capacity. It can also be caused by heavy snow melt and ice jams. The damage from a river flood can be widespread as the overflow affects smaller rivers downstream, often causing dams and dikes to break and swamp nearby areas.

To be continued....