Flood Risk Assessment for Tanzania Posts Corporation (TPC).
The Tanganyika Plantation Company Ltd. (TPC), is one of the largest sugar estates in Tanzania, located approximately 50km south of Mount Kilimanjaro. The western boundary of the estate is bounded by the Kikuletwa River, which feeds into the Nyumba ya Mungu Dam, a critical water reservoir in the region.
TPC’s proximity to the Kikuletwa River and its major tributaries means that the estate resides in a large floodplain. To protect the estate from regular flooding, an earthen dyke was previously constructed. The dyke was designed to force river flows and flood waters onto the western flood plain, away from the estate. However, the dyke had begun failing, with several breaches opening up.
GroundTruth was appointed by TPC to assess the estate’s flood protection dyke. To do so, GroundTruth carried out an unmanned aerial vehicle (UAV)-based Light Detection and Ranging (LiDAR) survey of the length of river, extending 33km down the length of the dyke, covering 5320 ha. In addition, GroundTruth conducted a site inspection of the length of river, western flood plain, the dyke, and other hydraulic infrastructure on the river. General observations were also recorded concerning the type and density of vegetation and landcover present in the flood plain, focussing on aspects related to hydraulic roughness and flow impedance.
Once the site visit and LiDAR survey were completed, a high-resolution imagery dataset was extracted from the UAV survey data and used, in conjunction with information from the site visit, to inform creation of a Digital Terrain Model. The Digital Terrain Model was used to setup a calibrated Hydrologic Engineering Center River Analysis System (HEC-RAS) 2D hydrodynamic model of the study area.
The hydraulic simulation, using real flow and flood magnitude data from gauging stations in the river, indicated that the dyke was able to withstand a generic flood magnitude of ~230m³/s. However, the model identified that flood magnitudes exceeding 240m³/s would result in several simultaneous breaches.
Consequently, various proposed potential interventions were detailed to accommodate larger flood volumes and ensure safety of the estate, including raising and armouring the dyke, monitoring the river and maintaining its integrity, developing a flood and stormwater management plan for the estate to optimise flow and reduce flood risk, and engaging in broader catchment management activities. Additionally, several peripheral observations were made based on the outputs of the hydraulic modelling, including the risk of the river avulsing (cutting a new channel) caused by river aggradation (rising bed levels), sections of the river above TPC lands which were potentially causing drainage problems on the estate, and the continued degradation of the catchments feeding the river. Finally, GroundTruth also provided advice on dyke maintenance best practices to minimise further risk.