Flooding is a major problem in Nigeria. The 2012 Nigeria floods began in early July 2012, killed 363 people and displaced over 2.1 million people as of 5 November 2012. According to the National Emergency Management Agency (NEMA), 30 of Nigeria’s 36 states were affected by the floods. The floods were termed as the worst in 40 years, and affected an estimated total of seven million people. The estimated damages and losses caused by the floods were N2.6 trillion ($6.75 billion). There are 2 major areas to which attention of flood prevention measures are to be addressed. These focus areas are those as pertain to occurrences due to persistent rains and flash floods.
Figure 1 showing 2 Major Flood Types in Nigeria
Whenever there is a rainfall in any place, there is a certain volume of water that falls to the ground. This rain water takes 3 basic means by which they are transformed and return to the atmosphere. The first is by evaporation of the rain water back to the sky. The second is by infiltration into the ground that turns to groundwater, while a part of it goes to rivers, streams etc. And the third means is by runoff on the surface of the ground.  See figure 2 showing a schematic of the rain cycle. Ultimately, these runoff waters terminate within rivers, streams, manmade water collectors, which end up as evaporation and return to the sky. Flooding in Nigeria may be due to inadequate routing and insufficient containment of the volume of this resulting runoff water.
In some other cases the runoff ends up flowing towards residential properties rather than within a gutter, which is due to poor routing of the drainage system. Drainage systems and gutters are some of the most underestimated and neglected design features. We often feel that the runoff will move on its own towards the termination point on its own by following the down slope. What we fail to realize is that by us humans constructing buildings and other infrastructure the ecosystem has inadvertently been distorted. Even in the area of road construction, a highway constructed with drainage systems on the sides last 50% longer which leads to a 21.9%  savings in the long run when compared with a highway constructed without a drainage system. Thus, it is a better decision to include a drainage system in every road construction. Drainage systems are a necessity than an option, proper siting of drains will reduce flooding.
Figure 2 showing a typical rainfall cycle
During a persistent rain, there are times when there is a temporary flood for up to 4 hours, this flooding is a result of the capacity of the drainage infrastructure being insufficient. The total volume of runoff water left to drain along the ground surface are hydrological parameters which need to be provided by engineers. These hydrological parameters are what is used in the design of the sizes of drainage cross-sections and profiles. Temporary inundations of this nature could affect visibility on the roads and could be embarrassing when it occurs in highbrow geographical locations.
To that end, all stakeholders involved in town planning and land development need to collaboratively address these issues. A segment of stakeholders are vested with the responsibility of allocating segments of a site to individual investors. A site is a large mass of land comprised of several plots of land along with roads, drainage facilities amongst others collectively being managed by a property developer or a group of developers who may choose to sell smaller parcels of land to individuals. Examples of property developers are real estate management companies, and families which possess large expanses of land. Other stakeholders are the government, engineers, surveyors, and urban planners. Collectively they can reach meaningful agreement on the height levels of the site surface in relation to the major roads, rivers and streams in the locality. The following are some important considerations for property developers.
- The height of the finished level of the proposed roads within the site should be established in conjunction with Local governing authorities involved in town planning.
- The height of the local roads within inner streets or site areas should be of the same height or preferably higher than the road levels of major roads which link between towns, cities or different site areas.
- These height differences are created to ensure that flood waters easily drain from within residential areas through the major road routes towards the rivers, streams or other water bodies to which all gutters deposit their content.
- For flood waters not to affect houses, it is recommended that the ground floor of buildings are a minimum of 450 – 600 millimeters or 1.5 – 2 feet above the top of the inner roads for these site areas.
- Runoff waters always goes from high ground levels to low ground levels by the influence of gravity.
- Any other combinations where the houses are below the roads to which there is no special drainage design is a recipe for flooding of the houses within that site area.
Actions to take in order to minimize the impact flooding due to persistent rainfall
- Government at all levels need to provide property developers with a datum reference for the proposed road in every locality.
- Drainages are to be constructed as a necessary infrastructure.
- Ensure that property owners earth fill the surroundings of their property prior/during construction where necessary.
Flash floods are more extreme occurrences of flooding. These kinds of floods could reach heights in excess of 900mm or 3 feet above the ground. Flash floods may be the result of release of water from a dam, collapse of a dam or other artificial water body as well as storms from the ocean. In most cases, flash floods are more catastrophic than those due to persistent rains. The solutions to these kinds of inundations is more capital intensive. These could be resolved by use of protective engineering structures or the creation of artificial lakes.
Waterfront structures are some examples of protective engineering structures. They are in the most case utilized in restraining storm forces from the ocean. Several of these can be found in any coastal country of the world, including Nigeria, as can be seen at the Bar Beach and other beaches in Lagos State. The Dutch have perfected the art of preventing flash floods due to storms. This they were able to accomplish following a catastrophic incident that led to the loss of 1,800 lives through the North Sea floods on the 1st of February 1953.  During this incident, billions of dollars were lost. Protective engineering structures are usually expensive to build, however the probable value of the catastrophe often justifies the investment in such project.
In Nigeria, the flash floods which have caused the most damage in the past are due to release of water from dams and persistent rains. There are times whereby the stored water behind the dam is in excess of its capacity, thus there is need to drain some. The combination of high river tide and the release of water from dams in neighbouring countries has led to damage of several properties which are along the River Niger/Benue axis. Nigeria has come to a point where artificial lakes need to be created in order to hold these excess water due to flash floods. This has become necessary as we observed during the flood in 2012 whereby the ocean, rivers and stream could not take up fast enough all of the flood waters released from the dams and due to rain combined.
To that end, government need to explore further the need for adequate artificial lakes. Prior to the development of such lakes, stakeholders as ecologists, hydrologists, engineers, environmentalists, farmers, residents need to be engaged. This is to ensure that all opinions and reservations are aired and solutions proffered. Typically, an artificial lake for flood control could cost up to $5bn for a 120 Sq. Kilometre lake (land, design, construction and filling).  As much as this is a huge investment, the losses incurred in 2012 and other years that followed is a justification for the need to invest these huge capital. The creation of artificial lakes alone should not be the solution as they also have some shortcomings.  See figure 3 showing Lake Sidney Lanier, USA. 
Figure 3 showing a an artificial lake in the USA
The following are some suggestions for management of these artificial lakes to ensure they serve more than only flood control.
- Prevention of contamination
- Purification of water
- Piping of the Water for supply and re-use
- Tourism potential
Photo Gallery of Man-made Lakes
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The time to start applying the right approach is upon us…
- Controlling watersources – https://en.wikipedia.org/wiki/Reservoir
- World Meteorological Organisation, “Guide to Hydrological Practices”, Volume I, Hydrology – From Measurement to Hydrological Information WMO-No. 168, 6th Edition, 2008, Chapter, pp. 25
- Roadex Network. https://www.roadex.org/e-learning/lessons/drainage-of-low-volume-roads/introduction-why-drainage-is-important/
- Matt Rosenberg, “How the Netherlands Reclaimed Land From the Sea”, September 2019, https://www.thoughtco.com/polders-and-dikes-of-the-netherlands-1435535
- The Federal Government of Nigeria, World Bank, EU, UN, “NIGERIA Post-Disaster Needs Assessment 2012 Floods”, June 2013