Floods refer to huge amount of water reaching land in a short span of time, causing land surface to be submerged under water at places, where, land surface is usually not covered with water.
Floods could be caused due to natural causes, or, human activities, or, a combination of both. Floods are caused by discharge of huge volume of water in a short span of time, at a rate, such that the water can not be carried away from the scene of discharge.
Some of the possible reasons for such huge discharge of water could be:
- very heavy rainfall (say: due to cyclones, typhoons etc.) in a short span of time. It should be noted that the amount of rainfall itself is not a sufficient cause, the duration within which the rainfall is receive is equally important contributor
- breach in levy, dams etc
- very high tidal waves (sometimes in the aftermath of a seismic activity, e.g. earthquakes) etc. also called tsunamis
Usually, flooding impacts a large area, wherein entire district or states might be flooded. However, sometimes, flooding is very local, i.e. limited to just one city, or, parts of it. Most often, the localized flooding is caused due to human activities, rather than natural phenomenon. A natural phenomenon might seem like the immediate trigger, but, in reality, this is caused by human activity.
There are some places, which get flooded almost every year. One such example is Bangladesh. Some of the other places which had incidents of bad flooding in the recent past include:
- Florida, in the aftermath of hurricane Katrina (2005)
- Myanmar (2008)
- Portions of Coastal India get flooded almost each year
Among various kinds of disasters, flooding is unique in the sense that it has a very high degree of predictability, both in the short term, as well as long term. In most situations, flood prone areas are quite known in the sense that they have a history of flooding. Only in very rare situations, a place might be flooded without having any past history of flooding. Even in such cases, a careful study of the area could give an indication of possible flooding.
Flood Prone Areas
The areas, which are prone to flood-risks are:
- places, which have a history of flooding (most important)
- area receiving heavy rainfall, with not much naturally sloping landscape
- areas at the lower levels of naturally sloping landscape where, the higher areas are receiving heavy rainfall
- areas around sea-coasts, or, river banks
- areas downstream of dams etc. As water level upstream of dams might rise, the dam authorities might be forced to release water (to safeguard the dam) which might cause flooding of downstream areas
- areas on the other side of levies (in case, the levy gets breached)
- low-lying areas (say: foot of an overbridge etc.)
Loss due to Flooding
The most common kinds of loss that are caused during flooding include:
- Lack of water: Its an irony, that a disaster which mean water everywhere, results in lack of water to drink and sanitation. Lack of proper drinking water and sanitation causes widespread outbreak of diseases.
- Lack of food: Most of the food items get damaged, causing a severe shortage of food. This shortage could be for the food to be consumed in the near future, or, even standing crops could be damaged, causing long-term food shortage.
- Lack of utilities: Utility services might have to be turned off, for the fear of electrocution, as, there is water everywhere.
- Widespread damage to structure
- Drowning: People, livestock, goods etc. might get drowned.
- Snakes and other creatures: Some of the dangerous creatures which usually stay underground would be forced to come up, as their natural habitat becomes unlivable. These could prove dangerous to human beings and cattle.
- Submerging of vehicles and other equipments: Vehicles and other equipments might get permanently damaged as they remain submerged under water for prolonged duration.
Because of wide-spread impact of such floods, the suffering could be long-drawn, besides the immediate impact as mentioned above.
Indicators of Possible Flooding
Usually, any of the following situations should indicate the possibility of flooding:
- heavy rainfall in/around the vicinity, especially, if the specific location falls in the pathway of the water-discharge system from the area receiving heavy rainfall
- if there is heavy rainfall/flow of water/accumulation of water, on the other side of a boundary, e.g. across a dam, across a levy, side of a river-embankment etc., because, these boundaries might get breached
As can be seen, both the above situations can be predicted to a reasonable degree. These days, the meteorological predictions are accurate enough for upto 4-5 days. Hence, its usually possible to know about the possibility of heavy rainfall about 4-5 days in advance.
Also, areas which are prone to heavy rainfall, cyclones, typhoons etc. are also well-known. Hence, the predictability is very high even in long-term, in the sense, that certain areas are known to be flood-prone. The advantage of long-term predictability is that people might be able to take long-term precautionary measures also requiring heavy investments.
Also, for situations, where, there is a boundary between huge mass of water, and, your living place, again, keeping an eye on the following two situations should be a good indication of the possibility of flooding:
- increase in the volume/mass of water being built up on the other side of the boundary
- general maintenance and upkeep of the boundary
General level of civic maintenance is a good indication of the possibility of flooding, during rainfall. If the drains and streets are generally clean, the possibility of flooding gets reduced; on the other hand, if the drains and streets are generally choked or dirty, the chances of flooding (atleast at the local level) gets increased.
Now, that we know, how can we figure out the possibility of flooding, lets look at the possibility of preventing it.
Prevention of Flood
Sometimes, it might not be possible to prevent a flood, even if we know that its about to get flooded. However, there are certain actions that can be taken to reduce the impact significantly, or, to reduce the possibility of flooding:
- The first step is to keep the drainage system clean. This allows water to be carried down very fast. Choked drains cause a significant reduction in the ability and speed of the water to be drained away. In most situations of urban flooding this is a major cause. The drains might get choked due to throwing of solid-wastes inside storm drains. These solid-wastes might include construction material, plastics, paper etc. This is a clear example, how human activity can amplify the process of flooding. Drains might also get choked due to falling tree-leaves etc.
- General clean-up of streets is also important. As rain-water falls down the street, it rushes into the storm drains. if the streets are not clean, the rain water trying to go into the drain carries solid wastes into the drain with itself, which then obstructs the flow of water by the drainage system.
- Rain water harvesting system: As more rain-water tries to flow down the drains, it puts that much more stress on the drainage system. Instead, if there are several rain-water harvesting systems, the rainfall falling in that much area would try to go to the sub-soil of the region locally, rather than straining the drainage system. Lower is the amount of water trying to go through the drainage system, the easier it is for the drainage system to drain off the water.
- Desilting: The drains should be desilted before the onset of the rainy season. This prevents the drains from getting choked. And, it also inceases the holding capacity of the drain, as, accumulated silt prevents that much more water from being accumulated in the drains.
- Inspection and repair of dams, levees, embankments etc: Before the onset of seasons causing accumulation and/or carrying of heavy volume of water (such as rainy season), these structures should be thoroughly inspected for possible weak-spots, and, these should be repaired.
- Afforestation: Forestation helps in binding the loose soil. The most major impact of this is, as flood-water races through, it might take loose soil with it. This loose soil will now choke the drains, as well as water-harvesting systems, thus, rendering both of these as ineffective. On the other hand, trees will prevent soil to flow with the water, as, the roots of the trees will act as binding force. Another major impact that afforestation provides is by reducing the impact of flowing water. This has impact on large-scale flooding, such as overflowing river. As water charges forward, its speed is reduced to some extent due to resistance offered by trees. This can reduce the force of the charging water thereby, reducing structural damage due to weakening in the force with which water hits various structures.
- Local lowlands (say: foot of an overbridge) should have storm drains, so that water does not get accumulated there. These drains should have some kind of mesh covering, so that only water can flow in. Leaves and other solid debris should not go in these drains.
- Local embankments around low-lying houses etc: Lets say, for some reason, your house is at a level lower than its vicinity (e.g. road-level). This can happen, because, say: you have constructed a basement which is obviously lower than the road-level, or, over a period of years, the road-level has risen due to repeated tarring etc. In such cases, you should create a local embankment between the street/road and your property, so that water can not flow down from the street/road inside your house. These embankment might be permanent in the form of concrete structure.
Besides impacting the process of flooding itself, most (not all) of these factors also have an immense impact on the rate at which water levels might recede after the source of the flooding has been removed. e.g. Lets say a city got flooded, after heavy rainfall. Now, once the rainfall is stopped, the water levels in the streets etc. might tend to recede. At this stage, once again, the rate at which water levels can recede is dependent on the ability of the storm drains to carry the accumulated water, as well as the total amount of water that has been accumulated which needs to be drained out.
People who stay in flood-prone areas should construct their houses using material which does not get damaged severely due to flood-water. Also, since, there is a strong risk of structural damage (for large-scale flooding), the material used to construct the house should be such that it can withstand high impact due to the charge of flowing water.
One should prefer areas, which are slightly elevated. These could be local elevations, i.e. higher parts of the city etc.
There should be strong embankments along all entrances of the houses so that flood water does not enter the house easily.
Cement bags, covered with plastic sheets might be used to keep the flood water from entering the houses.
Besides, long boots should always be kept, so that one does not run the risk of being bitten by snakes and/or other insects that might also be trying to save themselves from the twirling flood-waters.
One should keep arrangements for raising the height of items, which might get damaged in water, e.g. put a few pieces of bricks below the legs of the furniture, such as bed etc. to raise its height.
Important document should always be kept on higher shelves.
As water, food and utilities would not be available and that too for possibly several days, one should also take measures towards General Preparedness
Macro Level Efforts
While some of the steps mentioned above need to be taken at municipal/city level, and, some at individual level, there are some other techniques which have been tried/used at some places. However, these require efforts at a much larger level. Some of these steps include:
- Identified flood diversion areas: Flood waters are diverted to these unpopulated areas, so that populated urban areas may be protected.
- Construction of dams etc. at strategic locations
- Levees, embankments around cities lying along river/sea coasts. The flooding of New Orleans in the aftermath of Katrina hurricane was due to a breach in such a levee.
- Sea walls
- Beach nourishment: The sea-beaches are widened, so that they can absorb the impact of flood-waters due to rise in sea-levels.
- Conversion of flood-prone areas into wetlands, where, urbanization is not allowed, i.e. one can not construct residential houses, or, any other permanent structures etc.
As can be seen, such efforts require a very high degree of financial commitment, not just for constructing the system, but, also for maintaining it.