Understanding The ‘Why’, ‘What’ And ‘How’ Of Flood Mitigation And Prevention: A Scientific Approach To The Liberian Case


The increasing occurrence of flood continues to permeate the world. From increasing frequencies and intensities in Asia and Africa to hurricanes resulting floods in the Americas and back to our own situation here in Liberia.

By David D. Wounuah, BSc Civil Engineering, MSc Regional Planning,[email protected] 


The fact of the matter is that it is the activities of man which contributes the largest to flooding. Nevertheless the fact that flooding is now an annual phenomena, no systematic approach appears to be in the works. It appears that we have resigned to our fate.

The fate that for several days every year we will have to close down our only international airport due flooding on the highway. The fate that emergency response institutions will have to distribute food and non-food items to flood victims. The fate that members of the legislature will have to be prepared to elk out resources for distribution to flood victims. The fate that we can only afford mediocre responses to the symptoms of the problem and not the problem itself. Let it be established here that flooding in Liberia is not a problem in itself but it is a symptom to a bigger problem.

Therefore unless we zoom-in on the bigger problem using scientific principles, mathematical models and the dictates of sustainable development as tools to solve this problem our response will continue to remain reactive, temporary and politically driven. Sustainable development is the process through which development initiatives are viewed through a lens that attempts to meet the social and infrastructure needs of mankind while at the same time maintaining the ability of the natural environment to provide the ecosystem services on which society depends for stability.

This article will explain and attribute human’s factors contributing to flood. Attributions will be based on the elementary scientific and mathematical concepts of hydrology and land use. The article will conclude with an attempt to propound recommendations of proactive steps to policy makers, politicians and the general public on actions which can be taken in the short term, medium term and long term to reduce and subsequently advert the current trend in the local flooding patterns.

Now to the matter: Flood is defined by Oxford dictionaries as an overflow of a large amount of water beyond its normal limits, over what is normally dry land. Low lands which are most susceptible to flooding and located adjacent to large water bodies are called flood plains. Swamps or wetlands are areas of low laying land not necessarily adjacent large water bodies where storm water collects, stores and are subsequently absorbed into the earth or is discharged through streams to larger water bodies. Swamps are composed of mostly sedimentary, spongy and highly porous soils. As storm water (rain water) is conveyed at varying velocities from higher grounds to lower grounds on the earth’s surface it increases in volume and velocity, thereby causing erosion in its path as it moves towards the its final storage point. Floods subsequently occurs when storm waters recharges at a rate higher than the discharge rate. To maintain a stable, efficient and functional storm conveyance systems recharge rates needs to be less than or equal to discharge rates.

Q 1 Recharge Rate (net) ≤ Q2 Discharge Rate (net)

Let’s apply some engineering to this discussion and set the basis of the flood science using an elementary but fundamental theory in hydrology; the Rational Equation.

Rational Equation:  Q=c * I * A, where; Q = Peak discharge, c = runoff coefficient,

i = Rainfall intensity, A = Drainage area,

The key take away here is that the rate at which water discharges into the flood plains or wet lands is a function of the area being drained, the rainfall intensity and the nature of the land use. For our Liberian situation we know what the annual average rainfall is, we can measure the catchment area using simple topography survey techniques. Rainfall and catchment parameters we cannot control but we can control how we use our land.

When we convert the land usage we change the run off coefficient (c). For instance when we cut down rubber farms and use the land for the construction of homes, we increase runoff by 30%. This increase in runoff means storm water can now move 30% faster than it used to when the land was being used as a rubber farm. Faster flowing storm water recharges low lands faster than they can discharge this results in flood.

The case above demonstrates how the actions we individually take contributes to flood. Other examples of similar cases would include: the conversion of forested land to an oil palm plantation; also the conversion of a grass lawn in our yards to concrete pavements also contributes to increased run off. Even when a personal decision is made to hoe out or grub all the grass from our compound and bring in gravel we have also contributed to increase in flood levels. Do not get me wrong land use conversion is an integral part of development which must happen, however, as positive as development is they do impose significant environmental impacts which needs to be acknowledged, analyzed , planned for, and mitigated. Based on the afore mentioned discussions on the tendency to increase runoff simply by changing land uses, this articles establishes that one major contribution to flooding in Liberia especially in sub-urban and rural farming communities is the rapidly changing land use which significantly increases run off and increases the likelihood of flooding.

Another contributing factor is the uncontrolled construction of residences in low lands, swamps and within flood plains. Wet lands are not waste lands. Wetlands provide habitats for thousands of species, wetland provide locations for cultivation of food, and wetlands serves as natural water storage. Wetlands are also referred to as nature’s sponge for stabilizing water resources, and recharging our groundwater aquifers, the atmosphere’s humidity and rivers. These natural reservoirs, (we called swamps/wet lands/low lands/flood plains) absorb excess storm water into the ground and also store excess water. When humans decide to reclaim these natural reservoirs to build physical infrastructure, we stimulate unnatural chain reactions which among other things result to flooding. Every square inch of construction we erect in a lowland, decommissions (takes away) a percentage of the natural water storage capacity.

Let’s illustrate this: imagine that the low land is a tub partly fill with water. Imagine what happens when one attempt to insert a solid object (say a piece of block) in the half full tub of water. The water level in the tub moves upward. Yes, this happens because by inserting a solid object in the tub of water we actually take away water storage capacity which increases the water level. This is exactly what happens when we construct in low lands. This causes flood.

When we reclaim swamp lands for construction, we are simply inviting storm water to be stored on higher grounds which in human habitats are our homes and communities. Therefore, another observed contributor to Liberian flooding is our practice consistent practice of taking away/removing of natural water storage thereby inviting water to our homes. Yes, our actions flood our own homes, roads and communities.

We also contribute to flooding when we disobey the most elementary principle of fluid mechanics. Water flows downhill. It always did, it does now and always will flow downhill. When we obstruct this flow we increase the likelihood of flood. We can obstruct this flow by throwing thrash in the drains, building physical structures in the drains, not maintaining the drains or simply by not building the drains. The network of channels which safely collects, conveys and discharges runoff is referred to as the storm conveyance systems or drainage system. Storm conveyance systems are either natural or artificial. Natural conveyance systems includes gutters, streams, flood plains and wetlands. Artificial conveyance system includes road side ditch, closed and open collector drains, trapezoidal drains, and rectangular drains, V-ditches or U-drains. Hybrid conveyance systems can also be constructed when we merge natural drainage channels or storage which artificial drains. The conveyance system is either by natural design or through hydrological design are intended to carry a specific amount of water at an estimated speed. This is referred to the designed flow. Obstructions reduces the conveyance system capacity to handle the design flow, this leads to overflowing and subsequently floods.

Manning’s equation is used to design and size constituent drains in a storm conveyance system.

Where: Q = Flow Rate, v = Velocity, A = Flow Area, n = Manning’s Roughness Coefficient, R = Hydraulic Radius, S = Channel Slope

Key take away on this point is that, when we obstruct flow in the drainage system or simply do not built or maintain an adequate drainage systems we contribute to flood.

To conclude this article three major contributing factors to flooding in Liberia have been identified and discussed: 1) unregulated conversion of land use 2) consistent decommissioning of natural water storage, by constructing in wetlands and floodplains, 3) obstruction , lack of maintenance and inadequate storm conveyance systems.

Having established the above major contributors a series of short term, medium term and long term interventions are recommended.

Short term interventions:1) An extensive public awareness campaign to educate the local populace on the relationship between land use and floods 2)Massive, regular and sustainable efforts to remove all obstructions from the storm conveyance system in every urban and suburban community must be initiated.3)Land use conversions within sub-urban areas must to be controlled. Greenfield construction must to be temporarily suspended until the appropriate land use regulatory regime is instituted.4) Disaster response institutions must develop early flood warning systems, identify communities susceptible to flood and maintain a stock of supplies to rapidly respond to flood victims. 5) Reclaiming of wet lands for construction needs to be immediately halted.

Medium intervention includes: 1) a comprehensive regulatory regime around land use planning and land use conversions inclusive of clear guidelines and regulations governing the management of water resources and ecosystem services needs to be developed. 2) There need to be a full review of the current capacity of storm conveyance systems in urban and sub-urban areas to determine current discharge and recharge capacities and identified needs for additional drainage infrastructure. 3) Local building codes and standards need to be developed and must incorporate flood resilient design principles and standards.

Long-term actions includes: 1) every lowland, swamp, wetland and floodplain must be located, mapped, and their uses thereof strictly regulated. 2) Every residence, community or physical infrastructure located in identified lowland, floodplain or wetland must be relocated. 3) The principles and concepts of sustainable development must be incorporated into our infrastructure development planning regime. 4) The national human resource development strategy must incorporate emphasis on training in land use, regional planning, water resource management, hydrology and environmental engineering. 5) A rigorous implementation and monitoring regime must be launched alongside policy actions around the themes of land use, water resource management, urban and regional planning. 6) The storm conveyance system of each major Liberia city must be accessed to determine the current capacity, gaps in the existing systems identified and capital investment projects planned to respond to the identified gaps.

It is said that when we plan, we solve tomorrow’s problems today. Well as a practicing infrastructure engineer and development practitioner it is our professional and moral duty to educate, enlighten and put forward recommendations on issues of development significance. We cannot claim full command and understanding of the laws of nature and its applicable principals, but we must stimulate positive thinking using science. Remember, our solutions need to be based on scientific principles and backed by the political will. The reverse approach never works.

My two cents.

This article was first published in October 2017