Satellite-derived longitudinal evidence of flood effects on community water and sanitation resilience in coastal Kenya

Background: Flooding can disrupt water, sanitation and hygiene services in low-income rural and peri-urban settings, yet there is limited quantitative evidence on how repeated flood exposure affects safe water and sanitation over time at the level of Community Health Units (CHUs), which are the operational units for primary health care in Kenya. Methods: We analysed repeated CHU-level data from ten Community Health Units in Kilifi County, Kenya, across eleven surveillance rounds from 2017 to 2024. For each round, we calculated the percentage of households with safe drinking water and with access to a functional latrine, and linked these outcomes to flood exposure derived from Sentinel-1 radar imagery at 10 m resolution during the relevant CHU-specific survey window. We fitted linear mixed-effects models with CHU random intercepts, adjusting for elevation and land cover composition, and constructed comparative flood-adjusted resilience scores. Results: We analysed 220 CHU-round observations. The flood-by-outcome interaction was negative and statistically significant: a one standard deviation increase in flood frequency was associated with a 5.6% point greater decline in functional latrine coverage than in safe water coverage (flood-by-outcome interaction coefficient = -5.63; 95% CI: -9.33 to -1.93; p = 0.003). The rural versus peri-urban contrast also differed by outcome. Rural CHUs had substantially lower safe water coverage than peri-urban CHUs, whereas the rural versus peri-urban difference in latrine coverage was much smaller. Comparative resilience scores showed a clear gradient, with peri-urban CHUs generally ranking highest and several rural CHUs clustering at the lower end of the distribution, particularly for safe water resilience. Conclusions: Repeated flooding was associated with greater disruption of functional latrine access than safe water access, and rural disadvantage was concentrated mainly in safe water coverage. Although the resilience scores are comparative CHU-level planning tools rather than absolute measures of system resilience, they can help identify low-performing units and guide targeted investments in protected water systems and flood-resistant sanitation in coastal Kenya.