Increasing risk in a city of millions

Accra, the capital of Ghana, is a city of over 4 million people. Urbanisation and the influx of refugees from neighbouring countries cause the city to grow rapidly. At the same time, climate change makes Accra increasingly more vulnerable to floods. And with the growth of the population, this risk is only increased. How can Accra better cope with this threat, and at the same time enhance quality of housing, work and life?

The question the World Bank posed was how to develop Accra in an adaptive and attractive way. The city is situated in a narrow coastal zone, between the inland hills of Ghana and the Golf of Guinee. An abundance of natural creek systems veins the city fabric, transporting water from the hinterland through the city to the sea. Precipitation is irregular: long periods of drought are interspersed with shorter periods of extreme rainfall. This makes for high peak loads. To prevent flooding, Accra has invested heavily in improvement of its draining capabilities in the past. Natural creeks were straightened out and transformed into wide concrete drains. This also included the finer capillary systems, which have literally been cast in concrete.

Despite all of this, lack of water security remains a serious issue. Each year, floods take lives and disrupt the capital’s economy. Climate change adds to the urgency to develop a more structural solution.

A solution that spells trouble

Accra’s one-sided focus on improved drainage has turned into a disadvantage, for two reasons. Faster drainage increases pressure on the system downstream, which generally is also the most heavily populated and capital-intensive area.

Furthermore, the concrete drains only carry water for part of the year. Most of the time, the drains are dry, and turn into an unclean, unsavoury open gutter that is not beneficial to living conditions in the city.

System change

A mind shift is necessary, from ‘drainage as fast as possible’ to as much collection and retention of water as possible, and drainage as only a last resort. This requires a differentiated approach for upstream, midstream and downstream parts of the system.

Upstream, the emphasis is on retention. Ideally this includes the development of a more ‘spongy urban area’ where water is retained in ponds, and as much permeable surface as possible, to allow the infiltration of water into the earth. Retention decreases transport of water downriver, and reduces pressure on the entire system as a result.

Midstream, the emphasis is on the expansion of retention capacity, giving excess water more room.

Downstream, possibilities for fast drainage are expanded, to relieve the system when the situation becomes dire. This also allows the freeing up of extra retention capacity, to anticipate extreme rainfall.

4 types of intervention for structural improvement

Water retention

Accra developed as a city of remarkably low densities. Using outdoor space within the urban fabric for water retention effortlessly expands the retention surface from less than 1% to over 5 %. Depending on possibilities of incidental relocation of residents, this percentage could rise to 10% or more.

Wadi development

Almost all of the drainage system is cast in concrete. Replacing concrete gutters with green ditches retains water for a longer stretch of time, allowing it to seep into the earth to replenish natural aquifers.

Expansion of drains

In Accra, peak loads are too extreme for the capacity of its drainage system. Drains would need a width of one hundred meter, instead of the current thirty-five. This would insert excessively wide reservoirs to the urban fabric, without adding any real quality. Of course, their size can be optically reduced by adding elevated buildings on top of drains, but this would not prevent the presence of open and mostly dry sewers. The alternative would be to cover drains on strategic locations. Examples are the sections of the drain running along the railroad track, where drain cover coincides with development of a railway/business centre.

Korle Lagoon

The Korle Lagoon is the only location in Accra that always carries water. This provides opportunities for the development of the Accra central business district. In the Lagoon, two areas are developed, called ‘boat’ and ‘fish’, with well-developed, public watersides. Here people meet, looking out across the water towards the city centre, experiencing the coolness that water brings.

Part of the proceeds is used to widen the opening towards the ocean. This way, in times of peak loads the Lagoon provides increased retention capacity, while simultaneously improving the exchange between fresh and salt water, sediments and nutrients.