Introduction

Flooding is one of the most damaging natural disasters which is expected to be exacerbated in the future due to climate change, population growth, and intensive urbanization. It has been estimated that, there were 175 floods globally, with direct economic losses of $44,767 billion, affecting 577,066,696 people, and 7,910 deaths in 2022 alone.

In Ghana flooding has been a recurring phenomenon dating back from the 1930’s up to today. This natural disaster has brought about social, economic and environmental consequences with loss of life and properties running into several billions of Ghana cedi when it occurred. We can recall the June 3, 2015 flood incident in Accra where 154 people lost their lives and properties worth millions of Ghana cedi were not spared. It must be understood that flood cannot be stopped completely and creating resilience to flooding is therefore crucial in our preparedness to flooding.

Flood control or mitigation measures

Flood mitigation measures fall into two distinct categories. These are structural and non-structural measures.

Structural measures include the construction of dams, levees, flood control reservoirs, bank protection, and so on. The hydraulic structures are quite expensive to design, construct and maintain and can take years to complete the construction and their likelihood of the negative impacts to the environment. Additionally, these structures cannot be easily altered after construction.

It must be remembered in 2001 in New Orleans, because of failures of levees, there were 1,836 fatalities, mostly from flooding. In Ghana, the Volta River Authority had to spill excess water from the Akosombo and Kpong dams to avoid dam breaks owing to heavy rainfall and increased volume of water to the dams in September 2023 and this led to catastrophic consequences to downstream communities. These two scenarios show how structural measures if breached can lead to inundation of communities downstream of these hydraulic structures.

Non-structural measures include flood forecasting and warning, increasing the resilience of floodprone areas, implementation of Natural Flood Management (NFM), including low-impact development (LID) and Sustainable Drainage Systems (SuDS) and reducing risk through evacuation, and so on.

Flood forecasting and warning to city dwellers require the collaborative efforts of the Hydrological Services Authority (HSA), academia, private sector and the Meteorological Agency. I hope there is a team at the HSA coordinating the efforts.

The wetlands and other floodplains serving as sinks for excess runoff are been depleted by businessmen and other encroachers to the admiration of the Lands Commission and the Ministry of Local Government, Chieftaincy and Religious Affairs (MLGCRA). I suggest the Flood Managers at the Ministry of Works, Housing and Water Resources (MWHR), NADMO, CSO’s and other stakeholders must engage these two institutions to halt the degradation and restore the wetlands. The sustainable way is to plant resilient tree species to serve as sinks for water and carbon, reduce greenhouse gases and thereby mitigating climate impacts.

In our urban settings, there is a high degree of paved areas in and around households thereby reducing water penetration in case of heavy downpour. This phenomenon creates increased runoff and eventually cause flooding so the narrative of excessive pavements must change.

Our drainage systems are really not sustainable since the hydrology of the urban settlements has changed over the years. Urban catchments have been altered due to human developments and most of the drainage systems are inadequate to contain the urban flood. Additionally, most of the drainage systems have been silted especially with plastic wastes. The MMDAs should encourage and motivate industry players to make recycling and composting a major part of solid waste management and communities along the drainage systems should be given education as well

In addition, Flood Managers at the MWHR must ensure the continuous desilting of all major and minor drains in communities.

One area that a critical look must be taken at are stationary vehicles especially small cars and tri cycles and their impacts on flooding. In the recent floods in Valencia, Spain, most of the flow pathways were obstructed by vehicles and hindered the flow of water which over 300 people died.

All flood-prone areas must be mapped and vehicles, tricycles and containers which would impede the flow of the runoff should be removed and taken to safer grounds. City authorities must address structures along waterways as well.

Flood Evacuation

Flood evacuation is one critical step in the non-structural measures that needs to be taken. People would continue to live in flood -prone areas along major streams, downstream of dams, valleys etc. and as indicated earlier flooding cannot be stopped but when it occurs, we must minimize the fatalities.

Flooding particularly can be very destructive in dense urban areas. Hazard mapping is the fundamental of identifying safe shelters, safe routes, and saving for trapped populations, and in turn is crucial for flood evacuation. Evacuation planning can be initiated after the flood hazard mapping. Hazard mapping requires flood information from either the hydrologic or hydrodynamic model from a vehicle or pedestrian perspective.

The level of hazard is different based on the receptors, e.g. adults, children, or vehicles. For example, adults can withstand deeper water and higher flow velocities than elderlies and children or that instability can be linked to individuals Body Mass Index (BMI). Vehicles are susceptible to stalling in deep water areas with water depth thresholds lower than people. Flood evacuation is generally carried out by walking or driving and therefore hazard mapping for people and vehicles is a fundamental part of evacuation planning. The flood hazard should be assessed from two hazard perspectives: hazard to people and vehicles.

Flood Evacuation Planning process

Flood evacuation should purposeful, organized, and planned action to save more people. The planning process starts with the identification of evacuation shelters. These places so identified (sports centres, churches etc.)areexpected to protect people from disasters and provide food, medical care, and accommodation for evacuees.  In the June 3, 2015 flood in Accra, the narrative was that most of the people that perished had gone to the petrol filling station to seek shelter and there was the explosion that caused the fatalities, so we need to plan well.

The next step is to identify the possible evacuation routes. The evacuation routes are the possible pathways from the point of evacuation to the evacuation shelters. The rule of thumb will be to identify routes with the shortest distance with minimal risks associated with that route. AI may be used for purposes like this.

The third step is to look at evacuee movement. Efficient and organised movement of evacuees can help improve the evacuation process and reduce mortality rates during flood events. Three types of movement have been identified: Self-Evacuation, Assisted Evacuation, and Supported Evacuation.

In Self-Evacuation individuals can use transport or walk to a safe place whereas with Assisted Evacuation, individuals can move but need the public authorities or community to provide information on safe places, routes, and even transportation.

Supported Evacuation where individuals cannot move and require more help from the public authorities or community.

During the hazard mapping, people with vulnerabilities like children, women and the elderly should be identified and categorized into these three groups for safe evacuation. When evacuees are moving, more people will choose routes that are shorter and less hazardous. This unplanning evacuation can cause crowd jams which will lead to herd effect and panic.

One other key factor is the effect of overhead powerlines on rescue operations. As flood event is unfolding and the water level is rising, the available distance between the water surface and overhead power lines reduces, exacerbating the risk of electric shock during evacuation, minimising rescue efficiency, and may increase the number of fatalities. This issue is particularly pronounced in areas with low-hanging or aging overhead power lines, where evacuation and rescue operations become extremely challenging or even isolated. All these must be identified in the flood-prone areas during the hazard mapping.

Conclusion

Flood risk management requires a multi-faceted approach to mitigate effects and eventually minimise the fatalities. Issues raised and discussed in this article can bring a lot of discussions on several fronts. Government agencies alone cannot address all challenges associated with hazards of flooding and private sector players, CSOs and other stakeholders must come together and confront the perennial flooding issues. In the short to medium term, there should be sustained capacity building of staff at HAS and the private sector especially in the area of operational and urban hydrology. In the medium to long term, flood forecasting and warning using hydrologic and hydrodynamic models should be a priority.

By Dr. Anthony Osei-Twumasi, (Hydrologist/ Environmental Engineer), Executive Director, Gant Institute of Hydrology and Environmental Management and Senior Lecturer (PT), Kumasi Technical University.

Email: a.oseitwumasi@gmail.com

Tel: 0549606660

Address: UP 249, KNUST