The modern cities has impermeable soils, and trust in the drain system to relieve water.
According to FEMA "Around 20-25 percent of all economic losses resulting from flooding occur in areas not designated as being in a “floodplain,” but as a consequence of urban drainage".
Rainfall can be estimated by satellites data sets and measured by rain gauges, but this say nothing about flooding by itself. The amount of water over and under floor tell us much more about flooding or about clogged drainage sewers.
Water level, flow sensors on drainage sewers help to understand, detect and monitor problems on the system.
RFID sensors, LIDAR scanners on sewer cleaning trucks and drainage monitoring equipment help acquire a better vision of the drainage system.
Chemical sensors on drainage pipes help to understand chemical pollution caused by flooding like high level of ammonia and nitrates.