Rainfall is extremely important for life on earth, but an excess of it can result in floods and damage to livelihood and life. Floods can be natural occurring due to topography of the region or man-made due to insensitive building practices near river beds. The extent of flood damage is determined by many factors such as the time duration for which the building was flooded, the depth of the water in the flooded building, etc. The damage is not limited to the period of the flooding, rather continues even after the water recedes. The causes and effects of flood damage on buildings is discussed in detail in the article.
Table of Contents
1. What are the damages Caused by Flood on Buildings?
1.1. Damage Due to Forces of Floodwater
Upwards and sideways thrust created by water on structural members of the house damage it. The buildings are also damaged from the floating objects like timber logs, cars, etc. The force of water on the wall if it reaches the eave level is approximately equal to the load of 8 cars stacked on top of each other.
1.2. Flood Damage on Materials
Building in contact with water over a long period of time can lead to damage and loss of material strength, especially earth walls and construction of burnt brick walls made with mud mortar.
1.3. Structural Failure from Soil Movement
The foundation damage and structural failure in buildings may occur due to change in soil conditions and movement beneath.
1.4. Flood Damages to Homes due to Differential Pressure
If the doors and windows of a building are closed, there will be a difference in water level between the inside and outside of the flooded buildings. This difference in level will exert a pressure on the walls and may lead to damage. The internal rate of flow depends on the amount of water getting inside the building through openings. The difference in water level creates pressure on the wall.
2. Factors Influencing Flood Damages
The flood water damages will depend on certain factors.
2.1. Depth of Water
More the depth means more water pressure on the structure thus increasing the chances of the walls getting collapsed/ cracked.
2.2. Velocity of Floodwater
When a certain volume of water moves through narrower areas, the velocity of water flow increases intensifying the damage further.
2.3. Period of Submersion
Longer the duration of submersion, greater the effects of flood on the buildings since prolonged exposure will lead to both absorption and pressure exertion. It will lead to both structural and aesthetic deterioration.
2.4. Type of Material
Certain materials such as conventional earth buildings (which are not stabilized) will get more damaged when compared with burnt brick or concrete buildings.
2.5. Water Contamination
In most cases, the flood water will be contaminated with drainage, dead bodies, chemicals, etc. and will contain many floating objects. Even though the contamination is diluted by the huge volume of water, the salt in the water impacts the buildings.
3. Negative Effects of Flood on Buildings and How to Identify Flood Damage
3.1 Dampness
It is difficult to find out the source of water in the case of dampness in buildings. There are various ways through which water may enter the buildings and damage it. In case of buildings completely immersed in flood water, multiple damages can be caused.
But the effects of dampness can have serious consequences in concrete and timber members of the building, especially roof. If concrete is left damp, the reinforcement will start corroding and the slab will degrade beyond repair, losing its structural strength. The English proverb "a stitch in time saves nine" is apt in the case of repairing concrete components of a building.
In the presence of water, the salts in the wall or in the ground will crystallize and damage the walls and the surface decoration. On further seepage the plaster may start to flake off as well. Paint finishes may start flaking off as dampness enters the building. Many defects hidden all these years may surface with the floods.
3.2. Rising Dampness
If a porous walling material such as brick or stone is in contact with damp soil, moisture will be drawn into the pores by a physical process called capillary action. Until a balance is created between the rate of evaporation and the rate of action of capillary forces, the height of moisture absorbed will rise in the wall. The rising damp height will vary in accordance with the level of the water table of the soil. The more the thickness of the wall, the more water will rise.
Rising dampness usually contains salts carried up from the soil or dissolved from the walling material. Some of these salts are hygroscopic; i.e. they absorb moisture from the air. Their presence will maintain dampness even after the rising damp has been eliminated.
The signs of rising damp usually extend from 3 cm to 100 cm above floor level and exhibit a sharp change from wet to dry on the wall. Moisture will be present throughout the wall thickness. Contaminating salts may be seen as white deposits or crystals. Presence of a damp proof layer will prevent the ground water from rising in the walls through capillary action.
3.3. Salt Crystallization
Salt crystallization is the precipitation of salts such as chlorides, sulphates and carbonates in the building material. These salts are activated only when a suitable agent is present. Water plays a key role as an agent either in the form of moisture, which causes dampness, or subsoil water rise in the structure; or seepage of water accumulated in the structure due to cracks and crevices. When water is drawn from soil, it carries salts present in the soil.
When evaporation takes place on the surface, the salts get deposited near the surface and the mechanical forces exerted during crystallization cause the plaster or brick to disintegrate over a period of time. The floodwater has many salts present in it and this salt damage to buildings is observed in many places.
3.4. Cracks in the Buildings
Many cracks appear in the existing buildings due to the pressure exerted by flood water and also due to the absorption of water by the various building materials.
Some cracks may continue to appear in the buildings even after the initial building cracks repair are done. One of the major reasons for this is the clayey soil in flood prone areas, which shrinks while drying (due to changes in the water table). There will be dimensional changes in the volume of building materials with variation in moisture content and temperature. This change leads to minor cracks in the building.
Although most cracks are not dangerous, it is important to fill them to avoid penetration of rainwater.
Cracks in the buildings can be repaired by grouting if the cracks are small. In case of larger cracks repairing can be done through stitching the cracks. Rebuilding of cracked wall elements is the last resort in repair.
Clayey soil has high water retention capacity hence it changes its shape drastically when the water table settles. This may result in settlement of the foundation. Hence for a house resting on clayey soil, it is quite natural for cracks to develop after the floods. The width of the cracks may also vary.
3.5 Dampness in Timber Members
Fungal attack on timber can lead to destruction of the timber, if it is persistently wet and has moisture content of over 20%. It may take some time for the timber members to dry, after the flood water has receded. There are more chances of timber being attacked by termites when it is wet. There should be maximum ventilation inside the building including the roof so that the timber members dry as soon as possible.
3.5.1. How Do Fungi Attack Timber?
Fungal attack on timber starts from "spores". They are the equivalent of plant seeds, minute and can float in the air. They are disseminated by air, water, animals or man. They occur in such large numbers and when conditions are suitable for fungal germination on timber, the timber decay starts.
3.5.2. How to Prevent Fungal Attack on Timber?
Although fungal spores are common in the air, they cannot develop and attack timber unless it has moisture content in excess of about 20 per cent by weight. The dry timber will not rot. The first and foremost step is the simple act of keeping the wood dry enough.
In a new house, the moisture content of timber may for a while be as high as 20 per cent, this is soon reduced in normal circumstances to some 12-14 per cent. So, in fact a sound and dry building has nothing to fear.
Fungal damage in timber occurs more when the area is not ventilated and the dampness persists for a longer time. Timber embedded in walls will get affected by fungi because water in the wall takes a long time to dry.
Conclusion
As a result of climate change and resultant sea level rise, the occurrence of floods is thought to become even more common. Public awareness is to be created on the causes and effects of flood damage so that the disaster’s aftermath becomes easier to control. The need of the hour is to work towards changing the outlook of flood damage and its control. Please read the blog from the Flood Manual Series :
