Flooding in Accra Research report
6/24/2009 10:33:44 PM -
The main drainage problems facing urban cities in Ghana may be categorised into 3 groups namely erosion, flooding, and pollution of surface waters.
In simple terms, urban flooding is a natural process in which drainage systems spill to their plains during storms. In Ghana the causes of urban flooding although diverse, are to some extent interrelated. This document seeks to investigate the extent to which municipal solid waste management systems and practices cause flooding during the rainy season in Accra.
In Accra low-lying areas are subject to severe perennial flooding, which is generally attributed to inadequately sized culverts, and blockage of the major drains by accumulated silt caused by years of neglect and lack of maintenance. There is also the effect of tidal variations. The capacities of most rivers have been greatly reduced by the deposition of silt and garbage and weed growth in and along the riverbanks. At the same time the amount of storm water runoff has increased several times in the last two decades as a result of increased residential, commercial, and infrastructure developments, which have produced more impervious surfaces in most drainage basins. In recent years, extensive road rehabilitation works have not only resulted in increased runoff but have also caused flooding, and erosion in some areas which had not known such problems in the past.
Inadequate capacities of some critical culverts, insufficient stream channel capacity and obstruction of flows by buildings across natural stream courses and deposition of garbage into the streams may also give rise to flooding.
In Accra as in most urban centres in Ghana, provision of infrastructure facilities has substantially lagged behind the rapid rate of housing development. Inadequate storm water drainage is one of the most serious problems facing Accra today. Flooding in low-lying areas, erosion of steep slope areas, and pollution of streams by waste discharges, have been identified as the major environmental problems facing the city. These problems are interrelated in that, flooding is caused by insufficient carrying capacities of the respective streams, which have been brought about by the accumulation of silt brought into the streams by erosion and by blockages caused by solid waste deposited in the streams.
Eight main drains frequently flood affecting over 1000 people at Mateheko, Nima , Tesano, Kaneshie, Mukose, Mampon Stream, Chemu Stream and Dzorwulu. Serious flooding also occurs in Sukura. The cost in human suffering is poor health, stress, worry and disruption of commercial activities.
While the causes of flooding are varied, this document seeks to investigate in particular the extent to which municipal solid waste management systems and practices cause flooding during the rainy season in the Greater Accra region of the country.
Accra is the capital of Ghana and lies in the dry savannah coastal zone of the southern part of country. It is approximately five and a half degrees north of the Equator and less than half a degree west of the Greenwich Meridian. Accra metropolitan area covers over 200 km².
The monthly distribution of rainfall and temperature are presented in Table 1.1 and Figures 1.1 and 1.2. There is a marked seasonality in the rainfall distribution, with June being the wettest month, on average, and August the driest. The seasonality in the precipitation patterns is brought about by the movement of the inter-tropical convergence zone (ITCZ). Its northerly advancement during the period from March to June results in pronounced instability, with frequent storms and intense precipitation. A secondary rainy period occurs in October and November as the ITCZ moves south across the region. The average annual rainfall at Accra airport is approximately 800mm occurring on less than 80 days. More than 50% of the total precipitation falls in the period from April through June. The dry periods result from the prevailing north-easterly winds blowing around and away from Saharan high-pressure system. These are known regionally as the Harmattan and lead to generally dry and hazy conditions.
Temperatures are high all around the year with daily variations higher than seasonal variations. The average monthly temperature has a range of only 4ºC throughout the year. Daily temperatures range from 19ºC to 32ºC from December to June. Between July and November the days are cooler and temperatures range from 18ºC to 29ºC. The winds are of low velocity prevailing from the south in the day and from the southwest in the night.
Accra extends from Malam Quarry to the northwest and Madina to the northeast with the catchment areas outfall to the sea. The land rises to 100m above sea level at Legon. The Odaw catchment lies between the two main ridges on the west and east.
In Accra the plains are mostly underlain by Precambrian rocks and to a less extent by Togo Quartzites and Tertiary Sediments. The Precambrian rocks consist of the Dahomeyan, which comprises alternating acidic and basic bands of massive crystalline gneisses with subordinate schists and migmatites. The acidic types decompose to slightly permeable calcareous clay and the basic type to impermeable clay (Junner & Bates; 1945). The clay is more than 150 feet (47m) thick at the foothills of the Akwapim Range while on the plains the thickness is generally less than 20 feet (6.2m) deep in most areas.
The geology of Accra gives rise to generally lateritic soil groups, which are readily erodible, and provide a significant source of sediment for the drains. The constraints imposed by the geology and rainfall restrict the vegetation to savannah scrubland over much of Accra. Around most villages and communities even this limited vegetation is not easily maintained due to grazing by animals and movement of persons and vehicles.
The problem of the Accra Plains is lack of water for both domestic and agricultural purposes. Thus even though the areal coverage of the present exploration programme is only about one-eighth of the total area of the Plains, the establishment of the existence of groundwater in this portion would make an impact on the socio-economic development of the Plains as a whole. Proven aquifer zones may serve as potential areas for groundwater development for watering of crops and animals.
In general, groundwater use in farming and in particular irrigation, depends upon its quantity, quality, soil type of the area, proposed crops and their water requirements and irrigation method envisaged. Considering only quantity, production test results show that groundwater is available in the plains for small-scale irrigation.
The city has a standing population of 2,000,000 inhabitants, and a daily influx of about 300,000 visitors. The current growth rate is 3.3% per annum. With the present growth rate the population is expected to double by the year 2017.
The existing drainage system is based on gravity flow with most of the drainage basins being open. Accra metropolitan area comprises 3 principal catchment basins for the main streams of rivers flowing into various lagoons and the sea. These are:
Sukumo catchment to the west
Odaw catchment in the centre; and Kpeshie
In between these basins there are several minor drainage basins which outfall to the sea, the most important two being Chemu sub-catchment to the west and Osu Klottey sub-catchment in the centre. The Odaw stream is the main stream, with major tributaries of the Dzorwulu and Nima streams flowing from the east. The Onyasia River (Dzorwulu) joins the Odaw immediately upstream of the Nsawam Road crossing at Caprice while the Nima Stream, joins just south of Kwame Nkrumah Circle at Ring Road. Sections of the main Odaw and Nima channels have been desilted, while the remaining sections are yet to be lined. The primary drains, namely the Kaneshie, Korle-Gonno, Awudome, Kpehe and others have been lined. The Odaw River catchment is approximately 30km long with its headwaters near Aburi in the north. It discharges into Korle lagoon, the outlet to the Gulf of Guinea in Accra in the south. The widest part of the catchment is just over 10km wide just north of Accra. Small tributaries also flow to the Odaw River in Accra. The low-lying plains of the Odaw stretch from Korle Lagoon to the confluence of the Dzorwulu and the Odaw near Abuofo in the north. Korle Lagoon occupies a large portion of the lowest-lying area of the basin and stretches northwards about 1.5km.
The lower Odaw area is the centre of all commercial and industrial activities of the city, resulting in the construction of commercial houses, financial institutions, industries, roads and other infrastructure including drains. Major drains have been constructed to carry storm water and dry weather flows, from industrial and commercial operating areas as well as residential areas. Secondary and tertiary drains are provided alongside roads to carry runoff and sullage water from houses along these roads.
Unfortunately, most of the drains are used as garbage dumping receptacles, especially in crowded and low-income areas where garbage-collecting basins are placed very close to open areas near drains. Others are badly choked with weeds and bushes, and serious erosion has damaged drains and culverts in places. Stagnant foul waters are found in drains in the central part of the city where there are stores, markets and restaurants, as well as lorry parks. No maintenance is carried out to remove the garbage and silt in the industrial areas, causing flooding.
The drains are in deplorable conditions as a result of poor and, in some cases, lack of maintenance; dumping of refuse and human excreta in the drains; flow obstructions caused by service and utility lines; undersized culverts crossing roads and unauthorized structures located within the flow paths of some of the drains. Some portions of the drains need lining to check scouring and to improve their carrying capacities.
The land use structural plan derived from the framework of the Strategic Plan for GAMA represents the distribution of the various zones in the metropolitan area and the hierarchy of the road network connecting them.
The categories of land use include the following:
Residential land use forms about 58% of the total land use in the metropolitan area and can be categorised into three distinct income zones – the low income zones, the middle income zone and the high income zone. One of the most notable features residential areas in all three categories is that development is predominantly single storey (low rise).
A few areas in the high and middle class areas have some two-storey housing, but multi storey residential housing is a rarity found only in some institutional developments such as SSNIT flats
The low-income residential areas include Sukura, Teshie, Chorkor, and Darkuman while middle-income residential areas include Dansoman, Adabraka, Kaneshie, Asylum Down, and Achimota. High-income residential areas include Airport residential area, Labone, Cantonments and East Legon.
Also in these residential areas are schools, small-scale commercial and industrial activities, minor roads and drains.
b. Institutional and Special Uses
These land uses include health facilities (Korle Bu Teaching Hospital, Ridge Hospital, 37 military Hospital and Police Hospital), and educational facilities (University of Ghana, Achimota School, GIMPA and all the large secondary schools). The rest are emergency services like the Fire Service and the Police Station.
c. Civic and cultural uses
These comprise the ministerial area around Victoriaborg, libraries and museums.
d. Commercial land use
This occurs mainly in the Central Business District (CBD). This area contains the main commercial centre and some of the largest traditional markets in the Metropolitan area.
e. Industrial land use
Industrial activity occupies the established industrial areas at the Ring Road West/North Kaneshie, the Ring Road South, northern outskirts along the Nsawam Road and the South Motorway Industrial Zone.
f. Open Space
Open spaces in the Accra Metropolitan Area include wooded green areas like the Achimota Forest, lagoons like the Korle, Kpeshie in the Trade Fair area, Chemu in Dansoman and Sakumo lagoon in Mpoase west of Accra. Also included are the Ridge Park, the Castle Dive Park and the Kpeshie Basin.
Waste management in Ghana is a complex issue that has been a major feature on the priority list of successive governments, local authorities, and international donors in recent years. Generally existing public facilities including sanitary facilities are inadequate to serve the user population, and the sheer volume of municipal solid waste generated in the country's urban centers is overwhelming. Problems are encountered at all levels of waste management namely, collection, transportation and disposal. In several cases waste collection vehicles, compactors and other heavy equipment required for effective waste management are too few and so existing resources have to be stretched to cover wider catchment areas than is desirable. Existing final disposal sites for municipal solid waste are not engineered and may be described as crude dumpsites. There is no waste separation at the sources of generation, and hazardous and clinical wastes are often handled together with municipal solid waste. The situation creates a suitable environment for breeding of disease vectors such as mosquitoes and cockroaches and the proliferation of rodents such as rats and mice.
Clearly the existing systems cannot cope with the ever-increasing volume of solid waste being generated in Ghana. Therefore the public disposes of rubbish indiscriminately especially in watercourses and drainage channels and often through burning. Huge piles of refuse and overflowing refuse containers are seen throughout the urban centres particularly near markets and squatter settlements.
While existing solid waste disposal facilities are inadequate to deal with the quality and quantity of waste generated, sophisticated systems such as incineration and biogas production are currently not in use as these entail a high level of technology. Besides, maintenance requirements are high.
Flooding in Accra has become a perennial phenomenon. Experts are grappling with ways and means of containing the floods in order to save lives and property. Over the past decade beginning in 1995, floods have claimed several lives, and destroyed public infrastructure and property. The rainfall of 4th July 1995 was the highest recorded in one event since 1936. The Meteorological Services Department (MSD) recorded 258mm of rain with the intensity of 64mm in 12minutes and lasted for approximately 5hrs. The last major rainfall recorded in Accra was 192.8mm. This occurred in 1959.
The 1995 floods caused damage to lives and property, disrupted infrastructure services like water supply, telephone, electricity, roads and railways. Seventeen (17) lives were lost in this flood while commercial and industrial activity was disrupted. The most affected areas were those located within the flood plain of the Odaw and the Onyasia rivers.
Newspaper excerpts and photographs on the 1995 floods are attached.
The causes of flooding are summarised below:
Rapid urbanisation and its attendant activities have impacted negatively on drainage systems in Ghana. There is massive peripheral development of houses and estates as far as the outlying areas of the major cities, particularly Accra and Kumasi. Out of ignorance and sheer disregard of building regulations many people build in the green belt zones of the country depleting these areas of vegetation and leaving them prone to erosion and flooding. Development of residential buildings and paved roads increases the imperviousness of the catchment areas considerably, giving rise to quicker catchment response to rainfall and therefore increased runoff.
Housing development has in some cases come into close proximity to streams and other primary drainage facilities and/or led to the almost total devegetation of hill slopes. Such stream channels have been rendered incapable of coping with the high volume of runoff generated during storms, which invariably carries large amounts of silt.
Existing drains are often choked with refuse or silted up, as are rivers and streams near the urban centres. This results in reduced capacity of the river and stream channels, and flooding may occur. Most rivers and their important tributaries are major receptacles of wastewater including faecal matter, and in some cases solid waste. In the densely built up areas of urban cities, these rivers act as open sewers, posing a serious threat to public health.
Growth of grass and weeds are a common sight in many sections of various river channels. This naturally results in retardation of flow and consequent flooding of the banks of the rivers during heavy storms. During dry weather the grass and weeds cause ponding in several sections, which provides breeding grounds for mosquitoes.
Sometimes water mains are made to cross existing drains, thus further reducing the capacity of the drains.
Absence of drainage facilities in some areas and inadequate capacities of the existing drainage facilities also contribute to the problem of flooding. Inspections and hydraulic calculations indicate that some drains and culverts in the Mataheko and Kaneshie catchments are of inadequate capacity.
Although there exists a schedule for periodic and routine maintenance of existing drains in Accra, this is implemented rather haphazardly due to financial and other constraints. Majority of the existing drains are poorly maintained and are in various states of disrepair. Accumulations of sediments, vegetation and refuse are in evidence both along drainage channels and adjacent to culverts. The integrity of the system can clearly be greatly improved by the regular inspection and cleaning of channels, pipes and culverts.
A major setback to the maintenance of existing facilities in Ghana is the prevalent notion that maintenance of public facilities is the sole responsibility of Government. Such facilities are used carelessly without regard for good user practices or minimum maintenance requirements. Vandalising and stealing of any movable appurtenances is common. It is necessary that the culture of apathy towards correct operation and maintenance of existing infrastructure facilities be changed. It is however a difficult and slow process to change social behaviour. An important tool towards achieving this aim is education of communities regarding the necessity of developing a good maintenance culture. The public needs more information campaigns regarding drainage systems in the country. The campaigns should include:
Information on the negative consequences for the entire community of building close to drains and disposing of rubbish improperly;
Announcement of sanctions for such behaviour
Demonstration in the information campaigns of how the community itself can impose sanctions of public disapproval of such behaviour.
It should be recognized that information campaigns, while needed, are not sufficient to solve the human problems related to drainage. Other government offices must fulfil their responsibilities. It must be made easier for people to do the socially responsible thing such as, dispose of rubbish in a bin. Furthermore, the people need to be aware that there will be rapidly imposed sanctions if they engage in undesirable practices. Therefore, sanctions need to be set by the government, clearly communicated through information campaigns, and enforced consistently and promptly.
In areas where roads are not surface-dressed, gullying and erosion are evident on both road surfaces and the soil between buildings. This leads to undermining of both roads and buildings, and prevents floodwater from entering roadside drains where they are provided. This occurs due to the lip of the drain being above the general level of the road.
The erosion problems described above, result in a high sediment delivery from the urbanised portions of the catchments. Once the floodwaters are able to enter either roadside drains or main drains, much of the sediment is deposited, leading to problems of reduced capacity. Other drains outfall to the Korle Lagoon, where the relatively tranquil conditions encourage further deposition of sediment.
Services, such as water, telephone and power crossing the drains at midway via pipes act as obstruction to the free flow and the vegetation carried by the flowing water is thus trapped which further reduces the flow capacity of the drains
Property in flood plains
Some buildings are located dangerously close to rivers and drains, and are consequently at a high risk from flooding. These buildings are built in flood-prone areas and flood plains while some buildings are built only a few metres from the stream channel or even across natural watercourses. Some residents, particularly in the industrial areas have even constructed low walls across gateways and doorways, and ramps across driveways to avoid flooding.
The Chemu and Klottey drains outfall into lagoons, as does the Odaw. Whereas protective embankments keep the outfall for the latter permanently open, the remaining lagoons are separated from the sea by beach material. Relief from flooding at the margins of the lagoon is often only achieved by the excavation of a breach in the embankments to the sea. Once initiated, the breach expands readily with a commensurate reduction in water levels. The design and construction of a suitable outfall, by protective embankments would reduce the dependence on manual breaching of the retaining wall.
Blocking of drains by locals to form water ponds for irrigation is sometimes the cause of flooding in certain areas. In the lower reaches of the Odaw River for example, vegetable farmers and horticulturists rely on the river water for watering their crops.
During the Accra flood of July 1995 assumed to have a return period of 50 years, there were several fatalities and damage to property and infrastructure. The most affected areas were Alajo, Achimota, Adabraka, Nima, Asylum Down, Labadadi, Laterbiorkoshie Chorkor, Kaneshie, South Industrial Area, Agbogbloshie and Abelemkpe. Excerpts of the relevant articles reported in the daily newspapers are provided in the following sections.
While Analysts confirm that the rainfall patterns have not shown increasing trends in recent times the effect of high tides on flooding needs to be investigated (findings of workshop on flooding in Accra, organised by GHIE. The 1995 flood event occurred during the high tide, during which the Korle Lagoon was not able to discharge the storm. The resulting backwater effect led to the severe flooding of low-lying areas. This is further worsened by the fact that the invert level of the Odaw drain at Nkrumah Circle is 1.0m below sea level. Therefore, storm discharges to Sea at high tide is not technically possible.
Clearly waste management practices in Accra and the incidence of flooding in the city are interrelated. Since the existing systems cannot cope with the ever-increasing volume of solid waste being generated the public disposes of rubbish indiscriminately. About 60% of solid waste collection is achieved by the existing waste management system while the remaining 40% is burnt or dumped at any point convenient to the public.
The public dump solid waste directly into watercourses, drains, culverts, and other drainage structures thus blocking them. During storms the drainage structures are unable to carry the storm water due to constricted flow and reduced capacity, and therefore overflow thus causing floods.
In an attempt to prevent flooding in most of the low-lying areas, refuse is piled near streams by residents, presumably to create levees with the result that storm water runoff carries refuse and other solid wastes into the streams reducing their capacity and posing serious health and environmental hazards to residents downstream.
Sometimes the wind aids this process by blowing litter into drainage channels thus causing blockages. The impact of solid waste has assumed a new dimension because of the increasing use of plastics as packaging material, cans, bottles etc. These material and other debris accumulate in drainage channels and cause flooding. One school of thaught argues that, though garbage may block channels and cause overflows, they serve as retention dams in the higher water basin and allow the drainage of the lower basin.
Recommendations for containing the situation are outlined as follows:
- Engineering Solutions
- Landuse controls
- Improved Solid Waste Collection and Disposal
- Improved and Coordinated Maintenance of drains and drainage structures
This will involve planning design and construction of new drains in Accra to remove the hydraulic deficiencies of the drainage channels. Newly developing areas have to be integrated into the drainage master plan. With regard to design, design floods need to be appropriately estimated based on updated rainfall intensity and duration data. Other options for flood control such as storm water delays and retention need to considered because of the problem of discharging to sea at high tide. Other options should include the pumping of storm water into the sea.
This should focus on preventing the development of flood plains to provide adequate storage volumes for floods and reducing the flood damage. This will also ensure that drainage channels are not blocked.
Improved Solid Waste Collection and Disposal
An improvement of the solid waste collection and disposal in Accra will bring about will alleviate the flooding problems in Accra. The capacities of channels that are frequently taken up by solid waste will be released for additional storage. The absence of solid waste in drains will ensure the hydraulic performance of the drains and increase carrying capacities.
Improved and Coordinated Maintenance of Drains and Drainage Structures
In order to achieve a coordinated maintenance of drains in the city of Accra, the institutional responsibilities need to be clearly defined and the appropriate structures put in place.
1. Accra Drainage Improvement Study, SNC LAVALLIN inc, 1995
2. Master Plan for Storm Water Drainage in Kumasi, WATERTECH, 1991
3. Preliminary Report on Ground Water Assessment in Accra Plains, L.G. Quist