Paving compounds with interlocking stones, concrete can trigger flooding

Kunle Awobodu

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Managing Director of Reo-Habilis Construction Limited and the immediate past president of the Nigerian Institute of Building, Kunle Awobodu, tells DAMILOLA AINA on why compounds should not be fully floored to reduce flooding, among other related issues

 The National Emergency Management Agency said recently said interlocking blocks could cause flooding. What is your reaction to this as an expert?

Whenever it rains, water is expected to percolate into the soil so that there will be less natural runoff that can flood the environment. However, in a situation where the compound floor is concreted, percolation of rainwater into the unsaturated zone is hindered. Hence, the water escapes through the external drainage adjoining the compound.

Consequently, the volume of runoff from many compounds laid with impervious flooring materials could flood the whole area, especially when the street has equally been tarred or concreted for smooth movement of vehicles.

For instance, I observed, some time ago, a natural process of reducing excess water in the compound. At a site in Ogun State, during the installation of a roller or sliding gate, there came a sudden downpour. The outlet for the runoff was to be constructed immediately after the gate was installed.

The apprehension of flooding was doused by the internal drainage assisted with the hydraulic conductivity of the soil. The excess rainwater from the roof was absorbed by the soil. If the floor of the compound had been laid with concrete or paved with interlocking stones, flooding would have taken place.

The experience remains etched in my memory. It demonstrated the water retention process in the ground. It underscored the danger inherent in creating barriers between water and the soil. When the ground is deprived of porosity, flood develops.

Assuming the building had been completed and floored with concrete or interlocking blocks, the water could have gone into the street drainage and from there be transported to lower parts and when the volume of water is quite massive, it will flood the lower part. I think that’s the problem NEMA was trying to explain, which is true, to be frank.

But then, this type of problem occurs majorly in developed areas and what should be done is to create clusters of gardens. The earth needs rain to absolve moisture as not all water should be allowed to flow into the drainage system. The absence of this simple method is a major problem that leads to flooding.

Also, residential development in an area at times can prevent the natural absorption of water into the ground. For example, when the drainage is concreted, the roads are concreted or tarred and many compound floors are constructed with interlocked bricks, the little narrow path that should be reserved for flowers and kerbs for large flower paths or trees are destroyed.

If you look at the pattern of colonial buildings even in Lagos, they have a large compound and hardly concrete all the space but leave large areas for flowers, trees and the like. It is only the road within the compound that they will either tar or concrete so that the soil can absolve rainwater that can flood the environment.

What should a property owner do to avoid flooding of his compound after rainfall?

The natural and ecological system is an effective method to prevent flooding in a locality. Therefore, it is important to expose part of the ground in the compound to retain the umbilical cord to the groundwater reserves. Flood introduces dampness in the foundation of the building which could cause high moisture content in walls through capillary action. This undermines the structural integrity of the building.

The inclusion of gardens or green areas in the landscape of the compound is crucial to the enhancement of the environment. A damp-proof course or electro-osmosis damp-proofing system will be inevitable in places where flooding has become perennial.

Do you think the use of interlocking bricks should be stopped in houses built around flood-prone areas?

I don’t think so. Interlocking blocks are also good in sandy soil especially where asphalt may likely fail because there will be no space for water to percolate into the ground. Interlocking blocks in some areas can still allow water to percolate if they are not underlaid with polythene materials. They can also help to fill the ground with water.

The worst of them is to floor every part of your compound with concrete and not allow water from the roof to escape into the ground but into the general drainage. The implication is that the ground will not be saturated with water and thereby depriving the crust of the process it used to undergo before development. It is a simple analysis. It should not be stopped but there are other alternative ways to resolve the issue.

Should interlocking blocks be used in all sections of a house or a part like walkways and driveways?

What I am saying is that within a compound, let there be some sort of garden, where water can percolate into the ground and the whole compound should not be totally floored or covered with concrete so that water can seep into the ground. At the same time, it can be very messy if the compound is not tarred; it can be a difficult terrain to drive on. It can also be an ugly sight but aesthetically for neatness, you need to cover a reasonable part of the compound with solid materials that will protect the ground from being messy, especially a clay compound.

You can just leave a certain percentage of your compound so that water can percolate inside the ground and the drive-in environment will not be flooded. Walkways and driveways are the best part to cover and that is why we say there is a need to have gardens. Interlocking bricks or blocks laid on the compound ground, driveway or street cannot deprive the ground of water if the use of polystyrene or nylon membrane is eliminated. The need to prevent weeds from growing in the Interlocking blocks necessitated the introduction of the membrane that negates the effectiveness of herbicides.

Beyond aesthetic reasons, interlocking blocks can help water to percolate into cohesionless soil such as sandy soil. Contrarily, infiltration of water into interlocking blocks underlain on cohesive soil such as clay will only increase the moisture content of the soil, subjecting it to swelling and subsequent shrinkage, which will result in deformation.

What are the rules concerning the amount of space a house should occupy on a piece of land?

People at times like to use almost all the airspace for other things. Some persons want their building to occupy almost 80 per cent of the land which is against the regulations. The regulations say that a house is supposed to be built after at least six meters setback from your fence to the building in front, at the flanks, you are supposed to leave a remnant of three meters; same as the rear, minimum of three meters.

If this guideline is followed, part of vacant lands should be used to create gardens so that water can find a way through rather than allow water from your roof to flow into the drainage. Naturally, drainages should be cleared at intervals or regularly. Another problem I have noticed is that drainages constructed along the streets are not properly or professionally scooped.

A good drainage system must not retain water at the bottom, so when your drainage is frequently covered by water, you won’t be able to know on time when drainage is full for you to evacuate and then the concomitant effect on our health is possible as mosquitoes thrive in such areas. To avoid stagnant water, we must pay attention to the construction methodology that would make the drainage slope and have the required gradient so that water drains off as quickly as possible when it rains.

Does flooding weaken the building foundation and will you recommend the integrity testing of such houses?

Flood is water in excess and it is not good for water to occupy the surrounding of a building for long, especially foundations that were not specifically constructed for a wet area. Such buildings constructed on faulty foundations can never be friendly to water. Buildings constructed on waterways are bound to experience differential settlement and if care is not taken after some years, due to incidents of water, it can give way and that is why when you enter into some buildings, you will see paints peeling off from the wall. That is a sign that betrays the aesthetic of the buildings meaning the house is no longer habitable.

Such buildings, if they survive the flooding period, need to be re-examined for stability after the flood has subsided because during the time of flooding, there would have been some level of encroachment into the foundation and the soil under the foundation may also have been eroded. This can lead to defects in the building. So, there is a need to establish that the soil around the building and underneath the foundation is not eroded. These are paramount preliminary tests that should be done so that occupants of the building will not be subjected to unnecessary danger.

Also, there is this fear of chemical reactions. When there is flooding, water carries different chemicals, oil from chemical workshops, fertilizers, etc. After some days of erosion, chloride and sulphuric attack can react on buildings. So, these things need to be checked before occupants are allowed to go back to their buildings.

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