Landslides and flash floods are geomorphic hazards (GHs) that often co-occur and interact. They generally occur very quickly, leading to catastrophic socioeconomic impacts. Understanding the temporal patterns of occurrence of GH events is essential for hazard assessment, early warning, and disaster risk reduction strategies. However, temporal information is often poorly constrained, especially in frequently cloud-covered tropical regions, where optical-based satellite data are insufficient. Here we present a regionally applicable methodology to accurately estimate GH event timing that requires no prior knowledge of the GH event timing, using synthetic aperture radar (SAR) remote sensing. SAR can penetrate through clouds and therefore provides an ideal tool for constraining GH event timing. We use the open-access Copernicus Sentinel-1 (S1) SAR satellite that provides global coverage, high spatial resolution (∼10–15 m), and a high repeat time (6–12 d) from 2016 to 2020. We investigate the amplitude, detrended amplitude, spatial amplitude correlation, coherence, and detrended coherence time series in their suitability to constrain GH event timing. We apply the methodology on four recent large GH events located in Uganda, Rwanda, Burundi, and the Democratic Republic of the Congo (DRC) containing a total of about 2500 manually mapped landslides and flash flood features located in several contrasting landscape types. The amplitude and detrended amplitude time series in our methodology do not prove to be effective in accurate GH event timing estimation, with estimated timing accuracies ranging from a 13 to 1000 d difference. A clear increase in accuracy is obtained from spatial amplitude correlation (SAC) with estimated timing accuracies ranging from a 1 to 85 d difference. However, the most accurate results are achieved with coherence and detrended coherence with estimated timing accuracies ranging from a 1 to 47 d difference. The amplitude time series reflect the influence of seasonal dynamics, which cause the timing estimations to be further away from the actual GH event occurrence compared to the other data products. Timing estimations are generally closer to the actual GH event occurrence for GH events within homogenous densely vegetated landscape and further for GH events within complex cultivated heterogenous landscapes. We believe that the complexity of the different contrasting landscapes we study is an added value for the transferability of the methodology, and together with the open-access and global coverage of S1 data it has the potential to be widely applicable.

Landslides can lead to high impacts in less developed countries, particularly in tropical environments where a combination of intense rainfall, active tectonics, steep topography, and high population density can be found. However, the processes controlling landslide initiation and their evolution through time remains poorly understood. Here we show the relevance of the use of the multi-temporal differential radar interferometric (DInSAR) technique to characterise ground deformations associated with landslides in the rapidly-expanding city of Bukavu (DR Congo). We use 70 COSMO-SkyMed synthetic aperture radar images acquired between March 2015 and April 2016 with a mean revisiting time of eight days to produce ground deformation rate maps and displacement time series using the small baseline subset approach. We find that various landslide processes of different ages, mechanisms, and states of activity can be identified. Ground deformations revealed by DInSAR are found consistent with field observations and differential GPS measurements. Our analysis highlights the ability of DInSAR to grasp landslide deformation patterns affecting the complex tropical-urban environment of the city of Bukavu. However, longer time series will be needed to infer landside responses to climate, seismic, and anthropogenic drivers.

Risk perception is an essential element to consider for effective risk management at the time of eruption, especially in densely populated cities closeto volcanoes like Goma in the east of the Democratic Republic of the Congo, which ishighly exposed to volcanic hazards from Nyiragongo. The perception ofvolcanic risk involves the processes of collecting, selecting andinterpreting signals about uncertain impacts of volcanic hazards. Using aquestionnaire survey, this study describes the spatial differences andfactors influencing the individual volcanic risk perception of 2224 adultsfrom eight representative neighbourhoods of Goma before the May 2021Nyiragongo eruption. A composite risk perception indicator was built fromthe perceived severity and perceived vulnerability. Statistical analysis of thesurvey's results shows that the risk perception was high (mean = 3.7 ona five-point Likert scale) and varies less with demographic and contextualfactors than with cognitive and psychological factors. Volcanic hazards were perceived to be more threatening the city and its functioning than theindividuals themselves. The spatial analysis shows that respondents from the eastern neighbourhoods, affected by the 2002 eruption, demonstrated asignificantly higher level of risk perception than participants living inthe western neighbourhoods. This study will help to improve volcanic riskawareness raising in Goma.

Rapid urbanization, demographic pressure, and sprawl of cities are key factors in the vulnerability and damage related to geo-hydrological hazards. Dysfunctional urban services that favor informal settlements are at the forefront of elements that increase vulnerability. Cases of cities that suffer from geo-hydrological hazards are increasingly reported in many regions, especially in tropical countries in the Global South. Yet, studies on such examples are rare and commonly overlook the human and societal components of hazard risks. Here, we focus on Bujumbura, a city in Africa that has experienced rapid unplanned growth and sprawl into unserviced areas because of the non-application or a lack of a valid urban planning law. After filling in the gap in data collected using high-resolution field surveys and focus group discussions, this study highlights various factors of vulnerability to geo-hydrological hazards in the urban area. Indeed, 108 events of flood and flash floods and 81 gullies were inventoried in Bujumbura between 1997 and 2021. These geo-hydrological hazards have had a significant impact, particularly on housing, and have caused increasing displacement of the population. This vulnerability is exacerbated by the inefficiency of the rainwater drainage system in the urban space. Our result demonstrates how the failure of the institutions responsible for urban management is at the top of all the causes of the vulnerability of the sprawling city. We anticipate that our empirical approach is an effective way to obtain concrete information to develop practical strategies to prevent and mitigate vulnerability to geo-hydrological hazards in urban sprawling contexts.