Nairobi floods: What can be done to remedy the situation

Ainsworth School field submerged with flood water after recent light showers. About 900 students were sent home and missed classes as a result the school had to be closed indefinitely. [Robert Tomno,Standard] The recent floods in Nairobi have exposed serious drainage problems within the city centre and its suburbs. The floods, according to the chief executive officer of the National Water Harvesting and Storage Authority, Julius Mugun, highlight major structural weaknesses in Nairobi's stormwater management system. "First is ageing and undersized drainage infrastructure. Much of Nairobi's stormwater network was designed decades ago when the city had a much smaller population and far less built-up area," said Mugun during a recent interview with Real Estate. "Today, the city has expanded rapidly, and the drainage system simply cannot accommodate the increased runoff from dense urban development." Nairobi Governor Johnson Sakaja recently said the county needs Sh25 billion to overhaul the drainage system and build new waterways and canals. He noted that the facilities were built for 500,000 people but now serve a population of five million dwellers. There are also blocked or poorly maintained drainage channels. Solid waste, sediment, and illegal structures, he says, often obstruct drainage corridors, preventing water from flowing effectively during heavy rainfall. Third is the loss of natural drainage systems. Urban expansion has encroached on wetlands, rivers, and riparian corridors that historically absorbed and conveyed stormwater. "As a result, when intense rainfall occurs, as seen recently when Nairobi received more than a month's average rainfall within 24 hours, the city's drainage network is quickly overwhelmed," he said. Hydrological analysis Some city areas, such as Nairobi South near Wilson Airport, received up to 160mm of rainfall, about eight times the normal rainfall. Sakaja attributes flooding in residential estates to planning failures, infrastructure problems, and poor maintenance of drainage systems. "Planning failures occur when housing developments are approved without proper hydrological analysis of runoff patterns. In some cases, estates are built on natural waterways or floodplains," said Mugun. "Infrastructure limitations are also significant. When estates densify, but drainage capacity remains unchanged, the volume of runoff increases dramatically." Mugun said maintenance is critical as well-designed systems fail if drains are blocked or neglected. "What we are seeing in many parts of Nairobi is a cumulative effect of rapid urbanisation outpacing planning, infrastructure investment and maintenance regimes," he said. Mugun noted that stormwater management is key in property development, yet in some places, developers do not give it the attention it deserves when building new estates. He said stormwater management is key to responsible urban development. "Every building, parking lot, and paved surface increases the amount of rainwater runoff because it prevents water from naturally infiltrating the soil. Without proper storm water design, such as detention ponds, permeable surfaces, and drainage channels, water accumulates quickly," said Mugun. "Unfortunately, in some developments, stormwater management is treated as a secondary engineering requirement rather than a core planning component. In rapidly growing cities like Nairobi, developers must integrate stormwater management systems from the earliest design stage." Stormwater planning prevents floods and protects infrastructure, property values, and public safety. The chief executive said rainwater harvesting can play a vital role in reducing urban flooding, especially in rapidly growing cities like Nairobi. This is because when rainwater is captured and stored, whether in rooftop tanks, underground reservoirs, or retention ponds, it reduces the amount of water entering the drainage system at once. "If widely adopted across residential estates, commercial buildings, and institutions, rainwater harvesting could help slow down peak storm water flows, which is the primary cause of flash flooding in cities," said Mugun. In addition to flood mitigation, harvested rainwater can be used for domestic non-potable uses, landscaping, and urban agriculture, reducing pressure on municipal water supplies. At the National Water Harvesting and Storage Authority, Mugun said they strongly advocate integrating rainwater storage infrastructure into urban developments as part of climate-resilient city planning. In some estates, storm water mixes with sewage during heavy rains, revealing weaknesses in system design and in how urban utility systems are designed and managed. This situation reveals ageing infrastructure. "In well-designed cities, stormwater and sewage systems are separated. However, in older parts of Nairobi, the systems are often interconnected or overloaded. During heavy rainfall, sewer lines reach capacity, and storm water enters the sewer network, causing backflows and contamination," said Mugun. "This indicates the urgent need for modernised urban utility systems, including separate storm water and wastewater infrastructure, increased sewer capacity, and better maintenance of existing networks." Some of the most common mistakes that developers make when designing drainage and water management systems for housing projects, he said, are ignoring hydrological studies before development approval and underestimating runoff volumes from paved surfaces and dense housing. Rivers and wetlands Others are failing to incorporate water storage or detention systems, blocking natural drainage paths, building too close to rivers and wetlands, and designing drainage systems that handle only normal rainfall rather than extreme storm events. With climate change increasing rainfall intensity, designs must consider future climate scenarios and not just historical rainfall patterns. Mugun wants stricter regulations requiring developers to incorporate stormwater harvesting or storage systems in residential developments. "Yes, there is a strong case for clearer and stricter regulations. Globally, many cities now require developments to incorporate storm water retention, infiltration, or harvesting systems before building approvals are granted," he said. In Kenya, he said, planning approvals could include requirements such as minimum rainwater harvesting capacity per building, storm water detention ponds in large estates, permeable surfaces to increase groundwater recharge, and protection of natural drainage corridors Such policies, he said, would significantly reduce urban flood risk while promoting sustainable water management. As rainfall patterns become more extreme due to climate change, Mugun urges cities to rethink water management in housing and infrastructure planning. He said climate change is already altering rainfall patterns in East Africa, increasing the frequency of intense downpours. "Studies have shown such storms are becoming more likely due to global warming. Cities must therefore shift from reactive responses to climate-resilient planning," he said. "This means adopting an integrated urban water management approach, including large-scale storm water storage infrastructure, urban wetlands restoration, green infrastructure such as parks and infiltration zones, protection of riparian corridors and floodplains, and smart drainage systems designed for extreme rainfall events." He said urban planning must now treat water as a central design element, not an afterthought, noting that homeowners and property buyers should be aware of some warning signs that an estate may have poor drainage or flood risk. Homebuyers should be attentive to several red flags, including the fact that the estate is located near a river or riparian corridor. Visible signs of past flooding, such as water marks or erosion, poorly maintained or blocked drainage channels, roads, or parking areas that slope toward houses instead of drains and nearby developments that may alter natural water flow. "It is also wise to ask whether the developer conducted proper drainage and storm water studies before construction," he said. Mugun noted that Kenya should invest in long-term solutions to prevent urban flooding and make better use of rainwater as a resource. He says Kenya needs a multi-layered long-term strategy that combines infrastructure, planning and environmental restoration. Key investments should include modern stormwater drainage systems in Nairobi and other major cities, large-scale rainwater harvesting and storage infrastructure and restoration of rivers, wetlands, and riparian buffers that naturally manage floodwaters. Others include urban planning reforms that prevent construction in floodplains and community-level water-harvesting systems in estates and institutions. "If we manage rainfall as a resource rather than simply a hazard, we can simultaneously reduce floods, increase water security, and build climate-resilient cities," said Mugun.

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