Flooding during the rainy season has become a yearly phenomenon for the people of Accra, Ghana. Rapid urbanisation coupled with climate change, highlight the deficit in urban water infrastructure. Bertha Darteh and Marieke Adank look at the issue of stormwater management in the city and recommendations from a recent research programme.
Withn West-African, Ghana is one of the fastest growing economies. Its population of approximately 24 million people generate a per capita GDP of 24,187.30 million Ghana Cedis. Accra is the administrative, political and commercial capital of Ghana with a population of about two million. It is the largest and fastest growing metropolis in Ghana with an annual growth rate of 4.3 % (National Population Census, 2000). In addition to its residential population, Accra has large fluctuating migrant population who come to Accra to trade or work during part of the year. The city is growing rapidly, especially on its fringes, while slum areas have developed and are increasingly growing in number and density.
Many areas in Accra are prone to frequent floods. The city's inability to adequately deal with them has resulted in the fact that in recent times even with the lowest intensity of rainfall, there is flooding in the city. This is almost becoming a yearly ritual and some residents dread the onset of the rainy season. The challenges with stormwater management are complex and inter-related and can be categorised into infrastructural and institutional challenges. Some of the identifiable infrastructural challenges include:
- An increase in run off caused by large new developments (including roads) which have surfaces do not absorb water
- New development in areas that should be reserved for flood management
- Changes to drainage systems which reduce capacity to handle peak flows
- Poor maintenance of surface water drains.
Drainage systems in Accra
As a low lying area, when it rains water from the city and surrounding hilly areas drain through Accra into the sea. Drainage of stormwater from the city mostly takes place though natural drains and there are eight drainage basins in the city namely; Densu, Korle, Lafa, Chemu, Osu, Kpeshie, Sakumo and Songo Mokwe Basins.
None of the drains in the Densu (including Lafa) and Mokwe basin are lined and in the other basins only part of the drains are lined or otherwise improved [1]. Design capacity of major drains in the city is based on a 25-year return period (i.e. the maximum rainfall occurring once in 25 years). The average annual rainfall over the period 1970 – 2008 is 756 mm. With a total land area of 1,261 km2, the average total amount of rain that falls in GAMA (Greater Accra Metropolitan Area) within one year is about 0.954 km3/year. Variations in intensity of rainfall are considerable and rates of 203 mm/h may be reached and even exceeded for short periods (Ghana Metrological Services Department, 2002, in Lundgren and Åkerberg, (2006). In his assessment of run-off in the GAMA area, Nyarko (2002) considered a rainfall intensity of 140.2 mm/h, with highest rainfall values being recorded in the months of May-June.
Peak stormwater run-off in the basins in the Greater Accra Metropolitan Area can be calculated based on the size of the basins, the run-off coefficient, storage coefficient and the rainfall intensity. Run-off coefficient of urban areas ranges between 0.7 and 0.95.
With increased urbanisation of the drainage basins, the run-off coefficient will go up, as will the peak run-off. The peak run-off as determined by Nyarko (2002) is presented in the table below.
The challenge
Decades ago there was no extensive development. When it rained the water could make use of the natural water systems: streams and wetlands to drain gradually. Water could also infiltrate into the groundwater system through the soil. The city is developing and expanding, especially to the northern and western outskirts. This development has resulted in increasing 'sealing' of the soils in the upstream parts of the urban catchments.
The loss of permeability has resulted in a large increase in stormwater flows in the southern part of the city, which is likely to increase the flooding frequency and the subsequent damage to infrastructure, the economy and could endanger human lives. In other instances, construction of roads have not only increased run off but has also prevented water from one point being carried over to the other point.
Unfortunately there is a growing threat of losing natural drainage systems especially in areas where wetlands [2] are being reclaimed to put up residential houses. There has been an increase in construction of residential houses in areas just below the Akwapim hills such as Ashongman, which have all been developed when they could have been used as detention basins for stormwater management.
Secondly, the capacities of the constructed drains are limited; by their size and also by the fact that they are sometimes silted or choked with refuse.
According to Anomanyo (2004), about 60% to 75% of the solid waste generated in the city is collected. The solid waste that remains uncollected often finds its way into open drains, blocking them and creating a breeding ground for mosquitoes and flies but also for foul smells (Fobil, 2007).
In addition to stormwater, wastewater is also handled by the storm drains. The amount of grey and black water transported through the storm drains is however difficult to determine. The future urbanised areas in the northern part of the city are likely to also result in a large increase in grey and black wastewater flows through the storm drains.
Finding lasting solutions
A major challenge in addressing the challenges with stormwater management is the focus on ad-hoc solutions, after flood occurrences, rather than on long-term solutions. Seeing the need to have longer term strategic directions for managing stormwater various stakeholders from within the city have been involved in strategic planning process/research to identify some solutions for the city's management of stormwater. This was facilitated by a project known as SWITCH.
In line with the infrastructural challenges, a number of solutions were identified. To improve the discharge of stormwater in existing drains and waterways, which are currently often blocked by solid waste and silt, the following interventions could be considered:
• Preventing the inflow of sand and gravel of roads into drains
• Improving solid waste management and preventing solid waste from going into drains
• Improving maintenance of storm drains
• Enforcement of by-laws to prevent the construction of real estate in water courses.
Besides improving existing drains, there is a need to expand the existing drainage system, especially in the long-term. Even though there is a proposed drainage master plan for the city, there are challenges with funding for the implementation of these plans (Personal communication, Mr Wise Ametefe (2011) and Ametefe, forthcoming).
Extending the stormwater drainage system is expensive. Concrete lined drainage channels on both sides of residential streets are often more expensive to construct than the road itself. There is the need to review the current design standards to ensure all drainage systems have adequate capacity but also that standards adopted are not excessively expensive (AMA, 2006b).
In a system where maintenance is a challenge, there is a strong case for the use of natural systems for stormwater management. A range of sustainable urban drainage measures could be adopted to reduce the run-off which leads to flooding. A green belt approach could be applied around the 2010-city and develop the new city to the north and north-west of the green belt. The green belt will accommodate a number of functions:
• It will provide space for stormwater retention, storage and/or infiltration, thus reducing the stormwater flows that pass through the 2010-city
• It will provide space for urban agriculture and for reuse of stormwater, treated wastewater and/or composted sludge. For that purpose the wastewater and sludge will have to be treated in treatment plants located just north of the green belt.
In order to realise this, there will be a need for good planning and strict enforcement which may be a big challenge for a city like Accra. Alternative options for stormwater management in the city were also modelled with the City Water Balance (CWB) package, which has been developed by SWITCH to assess the impacts of future water management options the urban water cycle.
The model allows for the assessment of different water and wastewater management options to find more appropriate and sustainable water use improvements on various spatial and temporal scales. A model was developed and calibrated for the Accra Metropolitan Area using different population scenarios for Accra in 2030, tested with different water management options for present day city. The use of sustainable urban drainage schemes showed a potential for reducing stormwater run off. The model showed that implementing swales to prevent flooding reduced stormwater run-off by up to 30%. Large scale stormwater tanks and wastewater tanks showed a decrease of imported water by 29% (Simister et al, 2011).
Harvesting stormwater and using it for other purposes was also identified. Stormwater serves as a resource for the production of agricultural products in several areas in Accra, when urban agriculture is practiced. Practically any open space is used for farming vegetables and other crops because of the high demand from the city. Based on the irrigated areas, the annual volume of wastewater that is used in Accra in urban and peri-urban agriculture is estimated to be 4.4 million m3. It was recognised that infrastructural/physical interventions should not be decoupled from institutional interventions. "Master plans" should not just include the structural interventions and budgets/costing but should also have a clear financing plan. Since financing is a critical aspect but is difficult to attract, investments should be phased out, starting with low cost high impact interventions.
Secondly, the lag between the drawing of master plans and implementation needs to be taken into account. Drainage plans need to be updated consistently with new developments that have taken place since the plans were drawn out. Modern technology such as GIS and satellite imagery can be employed for this purpose as well as to map out the extent of floods.
These interventions will also require the involvement of different institutions and good coordination between these institutions. All key stakeholders in the city agree that the suggested strategic directions provide an opportunity to better manage stormwater. Yet there remains a guarded optimism about the ability of the city to implement the plans. Despite these misgivings the stakeholders expect that with public support and a strong political will, the city could begin to take steps towards achieving a water sensitive city that has the ability to deal with stormwater management.
References
1. SWITCH Accra City Story (2008)
2. Wise Ametefe, Personal Communication (July 2011)
3. Ametefe, W. (forth coming) Using reservoir storage effects for urban flood management, case study of Mamahuma Basin of Tema, Mphil thesis KNUST.
4. Adank, M., Darteh, B., Moriarty, P., Osei-Tutu, H., Assan, D., Daan van Rooijen (2011) Towards Integrated Urban Water Management in the Greater Accra Metropolitan Area:Current status and strategic directions for the future
5. Darteh (2010); Flooding in the City, the blame game. Article published in the Ghanaian Times Newspaper on 13th April, 2010.
6. Simister,R., Darteh, B., Sharp, P., and Mackay, R., Urban water management: Using The city water balance model to model urban water systems in Accra, Ghana, Paper represented at SWITCH Scientific meeting in Paris, 2011
7. Nyarko, B. K. (2002) Application of rational model in GIS for flood risk assessment in Accra, Ghana, J. Of Spatial Hydrology, vol.2, No 1.
8. Yankson, P.,Kofie, W.K., Richard, Y., Moller-Jensen, Lasse (2004) Monitoring urban growth: urbanisation of the fringe areas of Accra, Working paper.
Author's note:Bertha Darteh and Marieke Adank are from SWITCH, a consortium of 33 partners from around the world to find solutions to increase efficiency of urban water systems. This article was produced from the report 'Towards integrated urban water management in the Greater Accra Metropolitan Area', produced in association with the Accra Learning Alliance. For more information, please email Peter van der Steen: [email protected].
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