Earlier this year heavy rains and thunderstorms caused havoc in Lagos, Nigeria’s economic nerve centre and one of Africa’s most populous cities. Residents woke up in many parts of the city to find their streets and homes flooded and their property, including cars and other valuables, submerged.

Pictures and videos later posted online showed dramatic and even bizarre scenes of flooding in the city, including the capture of a crocodile in the floodwater. Another video, which went viral, was one of a man kayaking in floodwater on one of the streets.

Lagos has not been alone. Suleija, a town near the capital city Abuja hundreds of kilometres away, suffered its own flooding challenge in early July. Heavy rains washed houses away and caused others to collapse, trapping occupants. Thirteen people were reported to have died.

Some of the worst flooding in recent memory happened five years ago in March 2012 when 32 of Nigeria’s 36 states were affected, 24 severely. More than 360 people were killed and almost 2 million people were displaced.

The seriousness of the flooding was attributed to a combination of two events: very heavy local rainfall and the release of excess water from the Lagdo Dam in nearby Cameroon.

Although the degree and seriousness of flooding in Nigeria fluctuates, flooding remains a recurring phenomenon in most parts of the country. The first factor aggravating flooding is climate change, which has been shown to contribute to more extreme storms and rainfall. Another factor contributing to flooding in cities is that Nigeria has experienced rapid urban growth and planning is poor.

The problem of flooding is not peculiar to Nigeria alone. In 2007, floods affected 1.5 million people across several countries in Africa, including Uganda, Sudan, Kenya, Ghana, Ethiopia and Niger. Alluvial flooding is common for major rivers - such as Nile, Niger, Benue, Orange, Zambezi - in Africa. Major cities in Africa are also susceptible to fluvial flooding which occurs when excessive rainfall, over an extended period of time, causes rivers to overflow.

Why Nigeria suffers

Rainfall patterns in Nigeria (1978 to 2007) suggests that rainstorms are getting more intense. The data show that there are fewer rainy days, yet the total yearly amounts of rainfall have not changed much from previous decades. This means that more rain is falling on the days that there is rain, which in turn means that rain storms in the city are getting more intense, increasing the threat of flooding.

In addition to more intense rain storms, the other possible cause of flooding in coastal regions is rising sea levels. Although up-to-date data on the rising sea levels in Nigeria are scarce, it’s believed that if nothing is done, this is likely to aggravate flooding in the future, particularly in coastal cities.

Areas at risk include Lagos, which is on the coast, as well as the Niger Delta region which has many low-lying towns and villages. Being on the coast also makes these places more susceptible to storm surges. While these areas are no stranger to floods, evidence suggests that floods have become increasingly common and intense in recent times.

In the northern parts of the country, heavy rains are likely to cause rivers to overflow their banks and cause flooding in the adjoining states. The changes in rainfall patterns, particularly in frequency and intensity, have meant that these events have begun to happen more frequently.

In Nigeria’s cities, the most common cause of flooding after excessive rains is poor drainage systems that can’t cope. This is called pluvial flooding. Lagos provides a good case study.

Lagos as a case study

Lagos has been urbanising rapidly. By some estimates there will be 19 million in the city by 2050, making it the 11th most populous city in the world. It is also home to most of the country’s industrial, commercial and non-oil operations.

Urbanisation and industrialisation increase the number of roads and buildings. This in turn increases the proportion of surface area where water cannot be absorbed into the ground, leading to rapid runoff which then causes flooding during storms. And in cities that manage their infrastructure well, storm water drainage systems are built so that water can be directed to rivers efficiently and quickly.

Lagos has not kept up with its infrastructure needs. The growth and expansion of the city has been largely unregulated. The has resulted in inadequate and poor housing, the development of slum areas and inadequate water supply and waste disposal, among other problems.

What’s complicated the situation for Lagos is that many parts of the city were originally low-lying mangrove swamps and wetlands, which have been reclaimed and settled, mostly by poorer communities and more recently through concerted efforts by the government.

These low-lying areas are particularly at risk of flooding, and the situation is complicated by buildings being constructed on water ways, and bad waste dumping habits which block the drains.

70% of the population of Lagos live in slums, with the density of people being as much as 120,000 people per square kilometre. (The average population density of New York City is 10,384 people per square kilometre.)

What must be done

It’s clear Nigeria needs to take measures to cope with flooding. This will require both local and international interventions, and could include early warning and rapid response systems, flood data gathering and modelling, proper urban and spatial planning, flood emergency preparedness and political will.

The country can learn from others. For example, in Mumbai, India various measures have been implemented to reduce the impact of flooding. These have included an emergency control centre, automated weather stations, removal of solid waste from storm water drains and the development of emergency response mechanisms. Nigeria must invest in these measures, and sustain them.