Determination and Analysis of Land Deformation in Nairobi City County Based ALOS/Palsar and Sentinel-1 InSAR Processing

Abstract Nairobi city is one of the fastest-growing cities in Africa. The rapid development has put pressure on the existing resources such as water resulting in overexploitation of groundwater. Nairobi had been reported to be susceptible to environmental hazards such as subsidence associated with the overexploitation of groundwater due to depletion of its aquifer. However, the subsidence has not been empirically quantified because Kenya lacks a functioning deformation monitoring framework. Through the use of InSAR, land subsidence in Nairobi was spatially quantified for the first time that confirms fears of subsidence as suggested by previous hydrogeological studies. We observed an increase in the spatial extends and the rates of subsidence, the highest rate of subsidence being approx. 62mm/yr located in the largest deforming area West of Nairobi. The deformation can majorly be linked to overexploitation of groundwater as it matches with regions with the highest rate of decline of groundwater levels. However, there exist also subsidence linked to rapid development. Given the significance of Nairobi to the Kenyan economy, the ripple effects of subsidence could be devastating and measures need to be taken to mitigate them. The deformation map provides an important tool for planning mitigation measures to prevent further deformation such as regulating the drilling of boreholes, planning of construction of buildings and transport networks, and locations for the observation wells for monitoring groundwater levels. It also reinforces the need for updating the geodetic network that has been rendered unreliable due to land subsidence.