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UAV LiDAR and Bathymetric Survey for Flood Modeling

Solution Overview

A comprehensive bathymetric and topographic surveying project was conducted on the Msimbazi River in Dar es Salaam for the World Bank, comparing UAV Photogrammetry and UAV LiDAR technologies for generation of Digital Terrain Models for flood modelling purposes.[1] This World Bank funded survey project served two purposes: generation of reliable, accurate and fit for purpose bathymetric and topographic data of the Msimbazi River and its floodplains for riverine flooding related detailed studies, and rigorous comparison of Digital Terrain Model generation by Unmanned Aerial Vehicle Photogrammetry and UAV LiDAR.[3] The Tanzania Urban Resilience Program utilized an innovative combination of Light Detection and Range (LiDAR) UAV surveying and bathometric surveying to support flood modelling within the flood-prone Msimbazi river basin.[5]

Technical Components

CDR International in association with Shore Monitoring and Research executed a full survey campaign for the lower basin of the Msimbazi River in February 2019.[3] The UAV photogrammetry achieved ground point densities of approximately 70 points per square meter across the project area, compared to 57 points per square meter for LiDAR.[2] The project area covered the lower basin of the Msimbazi River valley at the outfall into the Indian Ocean, where two tributaries (The Sinza River and the Kibangu River) merge with the Msimbazi River.[2] The data comparison methodology included selection of typical terrain types, performance assessment, point cloud statistics, land use analysis on a basin scale level, and validation with GNSS data for typical terrain types.[1] The German International Cooperation Agency (GIZ) worked with OpenMap Development Tanzania (OMDTZ) and Dutch knowledge institute Deltares to collect cross-section surveys, extending an existing flood model for the Lower Msimbazi to include a much larger upstream portion of the river.[6] OMDTZ collected accurate and precise river cross-section surveys of the Msimbazi River using an innovative, low-cost technique developed by the Resilience Academy and OMDTZ, in collaboration with Deltares and with funding from GIZ.[6] TU Delft student Kirsten van Dongen co-developed a method with OMDTZ to use low-cost, dual-frequency GNSS receiver hardware, and demonstrated that it could deliver similar accuracy and precision as professional-grade survey equipment.[6] OMDTZ deployed two units that use the real-time kinematic (RTK) mode of the new, low-cost hardware that are able to provide centimeter precise surveys.[6]

Implementation Details

The bathymetric and topographic surveying of flood-prone rivers in Dar es Salaam was conducted on the Msimbazi River from February 14th until 20th, 2019.[2] The Msimbazi Basin project required a new survey of river cross-sections to be carried out as soon as possible, as the 2010 data with limited sections was insufficient for detailed hydrodynamic modeling.[9] OMDTZ rapidly collected over 80 cross-sections of the Msimbazi River after refining methods and safety precautions during a first field survey with Deltares associates Bobby Russell and Hessel Winsemius.[6] More than 300 students and locals were trained to use drones, GPS, and mobile apps to collect data in the Msimbazi Basin.[8] At the time of the report, UAV LiDAR surveys cost approximately 2 to 3 times more than UAV photogrammetry surveys.[2] A professional-grade set of survey equipment with dual-frequency Global Navigation Satellite System (GNSS) units or acoustic profiling equipment costs tens of thousands of dollars.[6] The study recommended on when to use which surveying methods with respect to purpose, project area size, landuse/vegetation cover and challenges like import of equipment for the Sub-Saharan African context.[3] Preliminary insights of the Msimbazi River UAV survey analysis were presented at the Land and Poverty Conference 2019 in Washington, DC on March 25-29, 2019 and at a workshop for Dar es Salaam stakeholders at World Bank Office in Dar es Salaam on April 11th, 2019.[3]

Benefits and Impacts

The OMDTZ survey of river cross-sections and hydraulic structures along the Msimbazi River supported the creation of a comprehensive hydraulic model of the area that led to the development of a more realistic flood hazard map of the most vulnerable areas.[6] The product of this assignment was the development of a wider flood model that assisted in providing the evidentiary basis for investments in the basin, complementary to the ongoing Tanzania Urban Resilience Program (TURP), implemented by the World Bank.[6] Outputs were used to guide river upgrades that improved Dar es Salaam's resilience to flooding.[5] Community-generated data from drones, GPS, and mobile apps now guides the Msimbazi Basin Development Project's design, informing the layout of a new flood-resilient city park, the placement of green infrastructure like terracing and mangrove buffers, and the alignment of upgraded transport corridors.[8] The usefulness of a model built this way was demonstrated by using the model to investigate the impact of drains blocked by solid waste on flooding.[11] Based on this study, for monitoring of morphologically dynamic river systems recurrent UAV Photogrammetry surveys are considered sufficiently accurate.[2][8] The Government of Tanzania established the Msimbazi Basin Development Project (2022-2028) with a $200 million credit from the World Bank Group's International Development Association (IDA) to strengthen flood resilience and integrate urban development efforts in Dar es Salaam.[7] The Msimbazi Basin Development Project will help reduce the flood exposure of over 300,000 people, including many low-income communities, while providing access to better infrastructure and services.[7]

Climate Adaptation Relevance

The Msimbazi Basin is home to an estimated 27 percent of Dar es Salaam's population and has historically been a fertile floodplain and the city's main water source.[6] Over the past decade, the middle and lower part of the Msimbazi Basin has been experiencing frequent, severe flooding at least twice a year during every rainy season, with nine major flood events taking place in 2019 alone.[7] Each year, Msimbazi flooding leads to fatalities and destroys critical infrastructure.[6]

Business Analysis

The bathymetric and topographic surveying project was funded by the World Bank and served as part of the Tanzania Urban Resilience Program.[3][4] The German International Cooperation Agency (GIZ) provided funding for the collaborative work with OMDTZ and Deltares to collect cross-section surveys.[6] The Government of Tanzania established the Msimbazi Basin Development Project (2022-2028) with a $200 million credit from the World Bank Group's International Development Association (IDA).[7] The innovative low-cost surveying technique developed by the Resilience Academy and OMDTZ provided a cost-effective alternative to professional-grade survey equipment that costs tens of thousands of dollars.[6] At the time of the report, UAV LiDAR surveys cost approximately 2 to 3 times more than UAV photogrammetry surveys, making photogrammetry a more economical option for monitoring morphologically dynamic river systems.[2]

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