Detection, characterization, and analysis of land subsidence in Nairobi using InSAR

This study examines the extent of land subsidence in Nairobi, Kenya using interferometric synthetic aperture radar (InSAR). Land subsidence is a geohazard that can affect both rural and urban areas. Human activities, such as water extraction, make it particularly intense in urban areas. In the past, Nairobi had been identified as a vulnerable area to such environmental hazards. Due to the lack of a functioning deformation-monitoring system in Kenya, the subsidence in Nairobi has yet to be empirically quantified. This paper reports the results of the first InSAR-based spatial assessment of land subsidence in Nairobi. InSAR’s ability to monitor large spatial extents makes it well suited for monitoring land subsidence. InSAR also has the advantage of providing other auxiliary information such as damage to structures, profling of aquifer systems, and the infuence of geological structures on land subsidence.

The study's analysis indicates both localized and regionalized subsidence in several locations in the west and north west of Nairobi. The largest deforming unit in Nairobi’s western part is subsiding at approximately 62 mm/yr. Land subsidence can be attributed to groundwater overexploitation because it coincides with regions with the highest decline in groundwater levels. However, subsidence can also be attributed to consolidation associated with rapid urbanization in other areas such as east of Nairobi. This evaluation corroborates previous hydrogeological investigations which indicated that Nairobi was at risk of subsidence, contributing to flooding in some residential areas. The findings will help guide future decision-making in several agencies as well as provide an effective tool for planning mitigation measures to prevent further subsidence. A few of these include regulating borehole drilling, planning of roads and buildings, and locating groundwater observation wells. In addition, the observed significant land subsidence stresses the need for an updated geodetic reference system. Since Nairobi plays a significant role in the economy of Kenya, the effects of subsidence may be devastating and it is imperative that steps are taken to minimize their impact.