The Accra Metropolitan drainage catchment area extends from the eastern boundary of the Nyanyanu catchment on the west of greater Accra regional boundary to Laloi east of Tema.
Densu River Catchment and Sakumo Lagoon
This is the largest of all the four coastal basins within the study area. The total drainage area is about 2500 sq. km. It is divided into two sections above and below the Weija dam.
The northern section of the basin, which extends inland along the Densu River and its tributaries 100 km, is hilly with the highest point reaching 230m above mean sea level.
The southern section of the basin is low lying land comprising the Sakumo I lagoon and Panbros salt pans. The Lafa steam flows into the lagoon and drains much of the western area of Accra including Dansoman, Kwashieman, McCarthy Hill and Awoshie. Much of this catchment is now urbanised.
Flooding is common along the 8km of the Densu River below the Weija dam whenever there is overtopping or deliberate release of water over the spillway.
Flooding is also prevalent in Dansoman area and along the Lafa stream where it crosses the Winneba and motorway extension roads. Few of the drainage channels in the catchment are constructed.
As a result, there is heavy erosion of drainage channels - many of which flow down existing tracks and roads. Access to this area is often cut off and roads become impassable during heavy rains.
This catchment covers an area of 250 km2. It is bounded on the west by a line running roughly south-north from Gbegbeyise, Dansoman, Kwashieman, New Achimota; on the north by the Achimota -Lagon axis; and to the east by East Legon, the Airport and Ridge. The highest spot on the basin in about 308m above sea level.
The principal streams that drain the catchment are the Odaw River and its tributaries, the Nima, Onyasia, Dakobi and Ado. The principal outlet for water in this catchment is the Korle Lagoon. There is a minor outlet at Chemu Lagoon.
The Korle - Chemu catchment contains the major urbanised areas of Accra. Many of the drainage channels are poorly developed and maintained.
Erosion and siltation of drains is a serious problem. In low-lying areas flooding is a serious problem, with many houses being inundated by floodwater during and after heavy rains.
Following the Netherlands Engineering consultants (NEDECO) report in 1963, a considerable amount of work was done, particularly in connection with the Korle Lagoon and the Odaw River, but work was eventually stopped. The drainage channels, until recently, had not been maintained resulting in the silting of the major watercourses and the lagoon.
In low lying areas near the Accra Academy in Kaneshie, rapid run off from Asoredanho overflows into the Bank of Ghana flats because inlet to the Kaneshie drains is inadequately designed. Similar problems occur in the industrial land and cemetery area around the Obetsebi Lamptey Circle where the interceptor drain and gullies are inadequate.
There are many other areas, like Nima, Dzorwulu, Darkuman and Alajo which have localised flooding problems caused by inadequate drainage channels and the flat nature of the terrain.
Most roads in the catchment are not surfaced and side drains are mainly uncovered. The channels become convenient places for the dumping of refuse. This, combined with erosion, results in choked and silted drain age channels.
These problems must be tackled by regular maintenance and the construction of new channels and culverts where existing ones are shown to be inadequate. Until good access roads with properly constructed side drains are provided in all urban areas, old and developing, drainage and flooding will continue to be a problem every rainy season.
The Kpeshie drainage basin covers a relatively small catchment area of 110 km2. It is bounded on the east by the Military Academy at Teshie, on the north by a line south of Madina and Ajirignano.
It covers the eastern part of Accra, Ridge, Cantonments, Osu, Labadi and Burma camp areas. Streams in the catchment generally flow north to south, emptying directly into the principal outlet to the sea at Kpeshie Lagoon or the small Korle Lagoon at Osu.
Improvements have been made to straighten some of the watercourses in Christianborg and South La. Drainage in the La Township is inadequate and many waterlogged areas become flooded with light rains. In heavy rains, fence walls collapse and foundations are undermined.
Southern Teshie is subject to severe flooding in the rainy season. Large parts of Teshie are without proper drainage with only the lower channel sections leading to the sea outfall. Most channels are heavily silted and choked with refuse in the middle reaches.
This is the smallest drainage basin in the metropolitan area. It covers about 50km2, draining the area of Teshie to the ridgeline with the Sakumo II catchment. Two main streams drain the area flowing into the Mokwe and Songo Lagoons. Much of this catchment is undergoing residential development.
Drainage channels in the western part of the catchment are not adequate resulting in serious flooding in the Teshie/Nungua estates and the cutting of the main coastal road to Tema.
The storm-water channels constructed alongside and under this road are completely inadequate. In central Nungua, the market cannot operate regularly because of waterlogged ground and poor drainage.
The valley before the police barrier at Nungua also has inadequately sized culverts. Poor maintenance of the earth drains along the area of the Maritime Academy also causes flooding of the coastal road.