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LiDAR and Bathymetric UAV Surveying for Flood Basin Modeling

Solution Overview

The Tanzania Urban Resilience Program deployed an innovative combination of Light Detection and Range (LiDAR) UAV surveying and bathymetric surveying to support flood modelling within the flood-prone Msimbazi river basin.[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 (DTM) generation by UAV photogrammetry and UAV LiDAR.[2] The outputs are being used to guide river upgrades that will improve Dar es Salaam's resilience to flooding.[1]

Technical Components

The implementation utilized both a photo camera and laser scanner mounted to the same drone, with data obtained through identical flights for both methods.[4] The LiDAR scanner used was a Velodyne HDL 32e with 16 out of 32 lasers used to construct the point cloud, providing 2 returns per measurement.[4] The study demonstrated that both UAV photogrammetry and UAV LiDAR are capable of producing high-resolution and high-accuracy DTMs for flood modelling purposes.[3] The generation of a high-resolution DTM served as a key input for the flood modeling and engineering designs under the Dar es Salaam Metropolitan Development Project.[3] The UAV LiDAR data was able to penetrate the water surface to some extent, which provided a more accurate representation of the river channel for the Msimbazi River.[3] In collaboration with Deltares and with funding from GIZ, OMDTZ collected accurate and precise river cross-section surveys using an innovative, low-cost technique developed by the Resilience Academy and OMDTZ.[5] The organization 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.[5] TU Delft student Kirsten van Dongen co-developed a method with OMDTZ to use the hardware, and demonstrated that it could deliver similar accuracy and precision as professional-grade survey equipment.[5]

Implementation Details

CDR International in association with Shore Monitoring and Research defined an integral full survey campaign for the lower basin of the Msimbazi River, which was executed in February 2019.[2] The project area was 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.[4] The survey was conducted for The World Bank and financed by UK aid from the British people, with CDR International BV and Shore Monitoring and Research BV as contractors.[4] The performance to measure ground level elevation with UAV photogrammetry and UAV LiDAR in a Sub-Saharan African context was investigated for fourteen different types of vegetated areas and three types of urbanised areas in the Msimbazi Basin.[4] 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.[5] After refining their methods and safety precautions, OMDTZ was able to scale up and rapidly collect over 80 cross-sections of the river.[5] Preliminary insights of the analysis were shared and presented at a master class at the Land and Poverty Conference 2019 in Washington, DC on March 25-29, 2019, and at a workshop on DTM drone applications for Dar es Salaam stakeholders at World Bank Office in Dar es Salaam on April 11th, 2019.[2] The UAV LiDAR survey conducted for this study was more expensive at the date of the report than UAV photogrammetry.[4] Compared with a UAV photogrammetry survey, a UAV LiDAR survey costs about 2 to 3 times more.[4]

Benefits and Impacts

The comparative analysis demonstrated that urbanized areas render approximately the same results for both UAV photogrammetry and UAV LiDAR techniques.[4] UAV photogrammetry is a cost-effective and fast method for generating DTMs in areas with sparse vegetation, though it is not suitable for areas with dense vegetation, as the photogrammetric process cannot penetrate the vegetation to capture the ground surface.[3] For tall reed and grass areas, photogrammetry errors are in the order of 1m, covering more than 30% of the vegetated area of the basin.[4] For areas with trees and mangroves, photogrammetry leads to mean errors of approximately 3 m, with maximum errors in DTMs between 5 to 10 m, covering approximately 33% of the vegetated area of the basin.[4] Based on the results of this comparative analysis it is considered sufficiently accurate to conduct recurrent surveys for aforementioned purpose with UAV Photogrammetry.[4][7] LiDAR UAV is considered a more suitable method for DTM generation for flood modelling purposes in Msimbazi type of basins with relatively large coverage of vegetation with variation in type of vegetation, density and heights.[4] The product of this assignment is the development of a wider flood model that will assist in providing the evidentiary basis for investments in the basin, complementary to the ongoing Tanzania Urban Resilience Program (TURP), implemented by the World Bank.[5]

Climate Adaptation Relevance

The Msimbazi River, which flows through the heart of Dar es Salaam, is particularly prone to flooding, causing widespread damage to infrastructure, property, and livelihoods, and posing a significant risk to human life.[3] The Msimbazi Basin, home to an estimated 27 percent of Dar es Salaam's population, has historically been a fertile floodplain and the city's main water source.[5] Flooding has become increasingly severe over the past decade in Msimbazi Basin, with major flood events experienced in seven out of the last 10 years.[8] 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, and in 2019 alone, nine major flood events took place.[8] The Msimbazi River is one of the major rivers in Dar es Salaam, originating from the Kisarawe highlands, passing through the city's industrial area and the densely populated area of Jangwani, and discharging into the Indian Ocean through the Selander Bridge.[3]

Business Analysis

The study will recommend on when to use which surveying methods with respect to purpose, project area size, landuse/vegetation cover and challenges like import of equipment.[2] The comparative analysis between UAV photogrammetry and UAV LiDAR provides decision-making guidance for future flood modelling investments, with photogrammetry offering cost advantages in sparse vegetation contexts while LiDAR delivers superior accuracy in densely vegetated basins despite higher costs.[4] The World Bank funding structure, supported by UK aid from the British people, demonstrates the viability of international development finance mechanisms for technology-enabled climate adaptation infrastructure.[4] The successful deployment of low-cost RTK survey equipment by OMDTZ, validated to deliver similar accuracy as professional-grade systems, suggests potential for scaling cost-effective surveying capabilities across similar Sub-Saharan African contexts.[5]

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