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 northeasterly 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.
Topography: 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.
Geology: In Accra the plains are
mostly underlain by Precambrian rocks and to a less extent by Togo Quartzite
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.
Groundwater Development and Utilization: The problem of the Accra Plains is
lack of water for both domestic and agricultural purposes. Thus even though the area 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.
Population: 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.
Storm Water Management: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:
i.
Sukumo catchment to the west
ii.
Odaw catchment in the centre;
and
iii.
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.
Land use: The land use structural plan
derived from the framework of the Strategic Plan for GAMA (Figure 3) below)
represents the distribution of the various zones in the metropolitan area and
the hierarchy of the road network connecting them.
Municipal Solid Waste Management
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 centres 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.
Urban Run-off
in GAMA
GAMA urban run-off contains varying quantities of all the following,
depending on the location where the runoff is generated:
·
Floatable and visual
contaminants
·
Degradable organics
·
Suspended solids
·
Nutrients
·
Bacteria, virus
·
Toxicants
·
Dissolved solids
In the industrial areas, chemicals comprising both organic and
inorganic substances are often wasted away from the premises of factories and
warehouses. The volumes of theses
substances are usually small except perhaps in cases of spillage. Some factories also dispose of effluent in
open drains, a practice that clearly constitutes an environmental hazard. Industrial effluent may contain toxicants
such as heavy metals.
Floatable and
visual contaminants are usually a result of improper solid waste disposal. Plastic bags and bottles are the commonest
items to be found in urban runoff.
By far the most worrying contaminant is sanitary waste. Indiscriminate defecation in drains, open
spaces water courses and dump sites is common, giving rise to excreta – related
diseases, and generally posing a health hazard to the public. During storm events, liquid waste is wasted
along by run off into areas of human settlements and water sources. At public toilets, holding septic tanks often
overflow during rains to compound the problem further.
Flooding In GAMA
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 five (5) hours. 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.
Characterizing Flooding Types
One of the principal flooding
types in GAMA is due to the rate and dynamics of urbanization. Floods in this regard can be attributed to
the increase of the impermeable areas and inadequate drainage systems such as
conduits and channels. The land use surface in small urban pockets within GAMA
are made of aluminium roofs, untarred streets and others impervious surfaces.
Runoff flows through these surfaces to the storm sewers and low laying areas.
It changes the hydrologic cycle, increasing the overland flow and decreasing
the groundwater flow (White and Hass, 1975). Under these circumstances the peak
discharge increases together with the flood frequency. In addition, the washed
urban surfaces during rainy days increase the pollution load in urban
environment and to downstream rivers(Tucci, et al., 1999)
In GAMA, flood control, storm water runoff and
urban drainage control has been managed through the institution of detention
and retention ponds, permeable surfaces, infiltration trenches and others
source indigenous control measures such as channelling stormwater in a
haphazard manner to downstream drainage.
The other principal attribute
to flooding is illegal settlements and construction of housing structures
within flood plain. (Tucci, 1999) These are natural floods, which mainly occur
in medium and large size rivers. When no reliable urban plan and regulation
exists, the population occupies the flood plain after a sequence of low flood
years, because these areas have a flat topography and are near to valuable
public land
Causes of Flooding
Urbanization: Rapid Urbanization and its attendant activities have impacted
negatively on drainage systems in Ghana especially in GAMA and other major
metropolitan areas. 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.
Reduced Capacity of Drainage Channels:
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.
Incapacity of Drains and Culverts: 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.
Maintenance: Although there exist 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.
Public
Attitude: 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 agencies 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.
Erosion and Sediment Delivery: 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.
Obstructions: 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.
Closed Outfalls: 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.
Damming of Water Bodies: 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.
High Rainfall Intensities: 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/communities 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.
Tidal Effects of the Korle Lagoon: 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.
Specific Impact of Waste Management Practices: 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.
Direct
Dumping:
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.
Indirect
Dumping:
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 thought 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.
Technical Recommendations and Engineering Solutions
Recommendations
for containing the situation are outlined as follows:
1.
Engineering Solutions
2.
Land Use Controls
3.
Improved Solid Waste Collection
and Disposal
4.
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.
Land-Use
Controls: 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.
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