GEO-RIO built Alerta Rio after the 1996 Tijuca massif landslides: 33 telemetric stations report rainfall every five minutes, backed by two weather radars, and their readings set the city's alert stage. Since 2011, 164 sirens in 103 high-risk communities can order residents out to support points — but no reading sounds them by itself. When a gauge passes 55 mm in an hour, municipal Civil Defence staff confirm the protocol and trigger the sirens remotely from the city operations centre, where Alerta Rio's meteorologists sit. The siren network has not been extended since it was installed.
An independent evaluation of alerts issued 2010–2013 found the rainfall thresholds kept missed alerts low but generated a large number of false alarms, one zone's threshold probably being set too low (Calvello et al., International Journal of Disaster Risk Reduction, 2015).
🎓 Lesson
A warning system is only as wide as the map it was built on. Rio's sirens still match the 2011 risk survey, and two documented landslides — Morro da Babilônia in 2019, where two people died, and Rocinha in 2023 — happened below the trigger threshold.
Evidence🔬 Peer-reviewed
Scale🏙️ City-wide
Sources
↗ Sistema Alerta Rio — Quem somos (GEO-RIO)Operator. Created 25 September 1996 by Decree 15142, managed by Fundação GEO-RIO; network of 33 telemetric stations (25 rainfall-only, 5 with temperature/humidity, 2 full weather stations) transmitting every 5 minutes; radars at Sumaré and Mendanha.
↗ Centro de Operações e Resiliência — Rio em Estágio 2, balanço de 21/02 (2026)Currency and mechanism, 21 February 2026: eight sirens sounded in four North Zone communities (Borel, Jamelão, Arrelia, Morro do Andaraí) after a gauge passed 55 mm in one hour, 'characterising the protocol'; all switched off at 20:30 as rain eased. Confirms a human stand-up and stand-down, not automatic actuation.
↗ Centro de Operações e Resiliência — 'Protocolo de acionamento em 24 horas não foi atingido' (2023)The city's own account of a landslide in Rocinha, 27–28 August 2023, with no sirens sounded: 104.6 mm fell in 24 hours against a stated criterion of 150 mm in 24 hours. Note that sources differ on the 24-hour figure — RioOnWatch reports the landslide threshold was cut from 180 mm to 125 mm/24 h after a 1997 study by D'Orsi et al. — so no 24-hour number is asserted in the entry.
↗ RioOnWatch — Rio de Janeiro's landslide mitigation tactics, part 1: a technical historyOrigin in the 1996 Tijuca massif landslides (350 mm in 48 hours; 44 dead, 222 houses destroyed); first 30 rain gauges; radar and satellite added 1999; the community alert and alarm system (A2C2) as two levels — SMS to trained community leaders, then sirens; 194 support points and 165 sirens across 103 communities by March 2019; risk mapping not updated since 2013.
↗ RioOnWatch — Rio de Janeiro's landslide mitigation tactics, part 2The decision step, quoted: 'a preliminary consultation between Civil Defence and Geo-Rio leadership determines whether risk is sufficient before sounding an actual alarm'; a target of roughly two hours' warning; the 2019 Morro da Babilônia landslide that killed two with sirens not activated; and residents who stay put for fear of losing homes and possessions.
↗ Agência Brasil — 'Rio precisa ampliar sistema de alerta de deslizamentos' (April 2025), republished by Eu, Rio!Carries the attributed audit figure: a 2022 Tribunal de Contas do Município report found roughly 400 Rio communities with neither sirens nor geological-geotechnical risk studies, mainly around the Pedra Branca massif in the west zone; and Rio 60ºC coordinator Miguel Vilela stating the system 'was launched in 2011, but they never installed sirens again'. The original agenciabrasil.ebc.com.br page currently serves an electoral-law placeholder to automated fetches, so the text was read through this republication; the TCM audit itself was not opened.
↗ Alerta Rio — download of rainfall dataPer-station rainfall series are published for download (records go back to 1997) and station metadata is mirrored on the city's data.rio portal. No licence or reuse terms are stated at the download point — downloadable, unlicensed, the same posture as Shanghai and Bangkok in this atlas.
📎 Cite this project
1001 Smart Cities (2026). “Alerta Rio: rain gauges, thresholds & favela sirens” — Rio de Janeiro, Brazil. The Smart City Atlas. https://1001smartcities.org/projects/rio-alerta-rio/ (last verified 2026-07-26). Data: CC BY 4.0.
The underlying data is free to reuse with attribution — see the open data page.
