Lunchtalk by Prof. Dr. V.G. (Victor) Jetten
Title: “Integrated Flood Management in Kampala: the right mix of urban planning, hard and soft engineering to deal with flash floods.”
Abstract for the presentation:
Victor Jetten & Richard Sliuzas (ITC), Georg Petersen (HYDROC consultants), Shuaib Lwasa & John Wasige (Makerere University)
Kampala is one of the fastest growing cities in east Africa and situated on a series of low hills and former wetlands. The wetlands are now often occupied by informal settlements (slums). Heavy rainfall causes immediate runoff from the hills that have clayey soils and are also sealed because of a high housing density, causing flash floods in the slums. These are usually not life threatening because of relatively low water levels, but disrupt the daily life, economy and form a health hazard. The departments of Urban Planning and Management and Earth Systems Analysis at ITC have a project in the UN-HABITAT program Cities and Climate change, to analyze the current and future flood risk situation in the city and come up with an Integrated Flood Management plan. The counterpart is KCCA, the Kampala City Council Authorities, that combines planning and engineering in one government body. The main part of the project focused on the Lubigi catchment, a 28 km2 catchment inside the city. A drainage system of lined channels connect the hills to the central drains in the main valley, that further drains into the natural wetlands downstream. Three sets of data were collected: institutional aspects and stakeholder information, spatial data (DEM, and land use from a 0.5 m resolution Geoeye image) and field data (high res meteo data, channels dimensions and road network).
Using the open source spatial model openLISEM (http://blogs.itc.nl/lisem), which combines a 1D Kinematic Wave with a 2D st Venant shallow flood solution, a series of scenarios were simulated and evaluated for a 1:10 year rainstorm. These are: the current baseline situation, improvement of channels with “hard” engineering, improvement of channels with “soft” engineering (grassed water ways, rainwater harvesting) and a planning only solution (designating hazard zones and restricting housing). Flood dynamics, houses affected and flooding hotspots were compared. These scenarios were further done for a the future city in 2020 using a trend growth rate of 4.2% per year and a high growth rate of 6.5%. Furthermore the flood model SOBEK was used for a detailed analysis of one of the slums.
The results show that: 1) the “inconvenient truth” is that city growth and disregard for planning has a much higher effect than any possible climate change, 2) there will always be a flood risk so that hazard zonation combined with building regulations is imperative; 3) the best results will be achieved by a combination of hard engineering in the central valley to protect the highway, and green engineering on the hill slopes, with different strategies for planning and maintenance depending on the type of solution, and 4) Community efforts such as cleaning of drainage channels has a very clear positive effect.