Flood Rainfall Accumulation Calculator (mm)

Flood Time and Rainfall Accumulation (mm)

Rainfall accumulation is the total depth of precipitation that piles up over a given stretch of time, measured in millimetres (1 mm = 1 L/m²). Cities usually put out an urban flood alert once the total goes past 100 mm/24h, with the severe ones running above 150 mm/24h. The accumulation time is how long it takes from the first real downpour until the runoff outpaces what the drainage can handle.

A common way to estimate urban runoff is the Rational Method, Q = C · i · A / 360. Here Q is the flow in m³/s, C the runoff coefficient (0.10–0.95), i the rainfall intensity in mm/h, and A the contributing area in hectares. In Brazil, CEMADEN (National Centre for Monitoring and Early Warning of Natural Disasters) runs more than 4,700 pluviometers and puts out flood and landslide alerts in real time.

Applications

It shows up in storm-water drainage design under the Brazilian standard NBR 10844, in the flood risk maps that civil defence agencies put together, in agriculture as a way to set soil-saturation thresholds, and in insurance underwriting for climate-related claims.

FAQ

How much rain causes urban flooding? In Brazilian cities, totals of ≥ 50 mm in 1 h or ≥ 100 mm in 24 h are usually enough to swamp the drainage and trigger localised flooding.

What does 1 mm of rain mean? Picture 1 litre of water spread evenly across 1 m² of ground. That's 1 mm, and it works out to 10,000 L per hectare.

Where can I check official rainfall data? For Brazil, both real-time and historical pluviometric data are published by CEMADEN (cemaden.gov.br), INMET (portal.inmet.gov.br) and the municipal Civil Defence agencies.