Why the Accra Floods Occur — Reasons From Scientific Studies
- By: George AgboKlu | Lanone Express
- Jun 14, 2015
- 5 min read
A week has passed since the night of Wednesday June 3, when Accra suffered severe flooding during several hours of heavy tropical rainfall, and gas explosion killed 100 people sheltering from the rains at the GOIL fuel station near Kwame Nkrumah Circle, and injured others.
The Ghana police and the National Disaster Management Organisation (NADMO) have recovered 160 bodies, though some feared the death toll may exceed 200. The president and people of Ghana, on Wednesday, June 10 attended a public memorial service for the dead.
The injured are receiving treatment at the military and police hospitals in the city. In the neighbourhoods affected, NADMO and a few socially responsive organisations continue to provide relief services to survivors.
Now, as the flood waters recede, residents are recovering from the trauma, and try to salvage property. Labone Express digs into several studies on Accra’s floods and why they occur. In fact, there are several good studies on Accra’s perennial floods. This one from UN Habitat, Accra Ghana: A City Vulnerable to Flooding and Drought-Induced Migration, and another, The Three Dimensional Causes of Flooding in Accra, Ghana are just two recent ones from 2011 and 2014 respectively.
At the start, let’s define what a flood or flooding is. According to geographers and scientists, a flood occurs when land which is normally dry is inundated with water. Rainfall is a very common, but not the only, cause of floods. That said, we may continue the discussion on the recurrence of the Accra floods.
Accra is Vulnerable to Flooding
According to the studies reviewed, Accra’s vulnerability to flooding has both hydrological and anthropogenic sources. Hydrological factors concern rainfall, its formation and the flow of water on the surface of the earth, while anthropogenic factors are those human activities that cause flooding.
Heavy Rainfall
Accra is located in the coastal savannah ecological zone, in the south-eastern end of Ghana. Although the zone receives the least rainfall annually, (600mm — 800mm), heavy rains exceeding 84mm in one day, and 91mm in two days do recur biennially. In recent times, the zone’s proclivity to such heavy rains is exacerbated by climate change and variability and its associated extreme weather events.
Removal of Vegetation Cover
Naturally, vegetation cover with its accompanying mulch of decaying leaves, barks and dead logs intercept rain water and slow the speed of run-off. In the case of Accra, most of the vegetation has been cleared to make way for buildings, car parks, roads, pavements, bus termini, playgrounds, etc. Consequently, there is heavy discharge of run-off water when it rains.
The Built Environment
The city’s massive, sprawling built-up and paved land surface has reduced the pervious surface available to allow rain water to percolate or sink into the ground. For this reason also, heavy rains produce high volumes of run-off and flush floods. For example, large volumes of run-off from the high population density settlements, Accra Newtown, Nima, Mamobi and Alajo feed the large Nima drain and the Odaw channel.
Silted River Courses
Being a city on the coast, Accra’s elevation is 92 ft above sea level. It’s much lower in some parts. A number of rivers and streams run through Accra, from the north and down into the sea in the south. The Odaw River is the biggest of them, and most of Accra’s floods occur in settlements close to its banks and those of its tributaries, from Dzowulu through Alajo, Avenor and Adabraka to the Korle Lagoon.
Flood-prone areas and types of floods in Accra | Credit Kagblor (2010)
The Odaw is in the maturity stage of its course. In that stage of the river’s course, gradient is most gentle and the speed of flow is slowest. Additionally, the volume of water is so much larger than it was in the youthful and middle stages, because smaller streams and open sewers join the it at different places along the course.
The maturity stage is a period of high siltation: the gentle gradient and slow current cause the river to lose its force to transport materials it may be carrying along any further. As a result the river deposits silt and debris it carried from the upper course here. It’s no wonder that sand banks develop in the channel from Odawna, near Kwame Nkrumah Circle, downstream into the Korle. These sand developments obstruct the river’s flow, and cause it to overflow its dykes to flood the surrounding lowlands.
Unfortunately, the silting of the Odaw channels is worsened by bad waste management practices and attitudes. Solid wastes discharged into drains impede water flow during storms and force the drains to overflow their banks or dykes into adjacent low-lying lands.
Informal and Uncontrolled Housing Developments
Reports from the 2010 population and housing census show that about 3 million people live in Accra. The city is experiencing rapid population growth, consistent with the United Nations’ projections of urbanisation trends in Africa. It is believed that rural-urban migration accounts for a considerable proportion of Accra’s growth.
A notable aspect of the city’s growth is the growth of slums on marginal lands, on or close to river banks and waterways. The population on those lands are identified to be very low income earners and the city’s underclass living in poverty. Sodom & Gomorrah in central Accra is a clear example. The pressures on those lands are not unrelated to Ghana’s 1.7 million housing deficit.
The residents construct shacks with cardboards, or thin plywood and iron sheets. In some instances, the builders fill-up dry stream channels with rubble to construct their houses. Such developments lack building permits and are never supervised by qualified technician. Those poverty enclaves are vulnerable to flooding almost always.
Poor flows in drainage networks
Accra has an underdeveloped drainage network. Several natural river and stream courses drain the city from the north to the south. Some short sections of these large river channels have been reinforced with concrete dykes. One can find them in the Nyaniba Estates, Nima, La and Mamponsee.
Besides these water courses, there are drains that run along most asphalt streets, and much smaller ones laid in some of the small, previously planned neighbourhoods, draining sewerage and run-offs.
In all, engineered drains are a small fraction of the city’s requirement. Apart from their insufficiency, many have been observed to be undersized, unconnected, or improperly channeled. As a result, some floods have been traced such faulty drains.
Conclusion
At this stage one can imagine clearly how the factors enumerated above act together to produce the floods. Besides these, another significant factor, which perhaps is the most critical cause, is what seems to be official indolence, if you may, towards executing adequate flood preparedness and mitigation schemes.
Even though researchers have documented the cycle of floods recurrence, the regular havoc they wrought on unfortunate residents catches the authorities, pants down most of the time. How long must the people of Accra and its businesses endure such avoidable disasters?
Source: laboneexpress.com
By: George AgboKlu
