Yesterday 12th May 2015, Nairobi residents took practically the whole night to get home from work due to flooding caused by heavy rains.
There are several factors that contribute to flooding such as below.
1. design and maintenance of storm water drainage infrastructure.
Design and maintenance of storm water drainage infrastructure is the responsibility of Nairobi County Government. The Civil Engineering department at Nairobi County usually comes up with designs and maintenance solutions. Nairobi storm water and sewer drainage has a national grid, similar to the electricity grid, that drains towards Ruai area. All parts of the city and its suburbs are supposed to have their drainage designed to fit into the national grid for storm water to drain towards Ruai. For large land subdivisions in Nairobi, the County Government requires the developer to submit the storm water designs to County Engineers for approval to check if the design fits well. When this checking method fails, we have developers designing and directing their storm water drainage anyhowly, causing a grid lock, pun intended, hence flooding.Periodic maintenance of the drainage channels is also useful to prevent flooding.
Some storm water drainage channels are huge, eg the ones draining water from State House road through to Uhuru Park. Someone can stand tall and walk through the underground channels. The design of the storm water drainage determines the volume of the water to be transported which in turn determines the size of the drainage chanels.
2. Zoning controls to reduce surface water runoff through Ground Coverage control.
Nairobi County Government has the responsibility to zone Nairobi infrastructure developments to control the amount of surface left open to the sky. In Karen, for example, the Ground Coverage is 25% and in CBD, its 80%, meaning, one can only build 25% and 80% of the ground floor surface, leaving the rest of the percentage open to the sky.
This ground coverage control helps to soak in rain water into the ground, reducing the volume of the runnoff. With lower ground coverages, leaving more land space unbuilt,this will reduce the surface runoff. So as to make this practical since demand for land is high, the County Government can allow for high rise buildings to be constructed, over 5 storeys, so as to compensate for the land left open to the sky, hence mitigate the issue of excess land demand.
3. Height of building from ground level.
Buildings and roads in areas prone to flooding such as South C, South B and Ruai should be built at least 1 meter from ground level. The foundations should also be raft of pile types that do not get affected by soggy grounds. Perimeter walls should have weep holes to let water out to prevent build up of pressure. Walls without weep holes should be made using reinforced concrete strong enough to withstand water pressure. This will prevent walls collapsing from water pressure as was seen yesterday where 9 people lost their life in South B Mosque area.
4. Conservation of water through dams.
Nairobi County should work closely with Kiambu county to create and design storm water drainage that eases of into man made dams for collection to be used in the dry periods. Storm water is easily directed to a particular path then easily collected. Natural water storage situations such as dams , lakes, marshes have all been destroyed around Nairobi. The Southern Bypass near Kikuyu town cuts right through Ondiri swamp, releasing water that has been naturally stored in the swamp for thousands of years. This water then easily finds its ways into Nairobi City, creating flooding problems. Runda area also have natural swamps which have all been drained away to pave way for roads and human settlements, releasing water stored for centuries down stream.
Since human population expansion is inevitable, National Environment Management Authority should become strict to prevent encroachment into lakes, rivers and swamps around Nairobi. County Government should also in a way help nature by creating dams and water ways that can hold this excess water .
Francis Gichuhi Kamau, Architect.