What Is a Flood? Causes, Types and Flash Floods

What is a flood?

According to the EU Floods Directive, signed in Strasbourg on 23 October 2007, a flood is the temporary covering by water of land that is normally dry. The water may come from a river, a mountain torrent, one of the Mediterranean streams that flow only at certain times, or the sea along the coast. Flooding from sewer systems can be left out of the definition. ISPRA, Italy’s national institute for environmental research, works from the same idea and also names canals and lakes.

The directive starts from the premise that floods are natural events and that nobody can prevent them. European law therefore deals with “flood risk”, which combines two things: how likely an event is and how much harm it could do to people, the environment, cultural heritage and the economy.

Italy’s Civil Protection Department describes a flood from the river’s point of view. Once the banks can no longer contain the water, it spreads out all around and reaches homes, factories, roads and farmland.

Flood definition: the river overflows, the land goes under

English uses one word, flood, for the whole event. The Io non rischio campaign, run by Italy’s Civil Protection Department, splits it into two moments. First the river, swollen by rain, spills over its banks or breaks through them; then the area around it is flooded, and that flooding is the flood proper.

Sea water belongs in the picture too. ISPRA describes coastal flooding when the sea moves inland, as it did along the coast of the Marche and Emilia-Romagna in May 2023, and the European definition of a flood already covers water arriving from the sea. In these sources, overflow means only the river topping or breaking its banks.

Term	What it describes	An example from the sources
River overflow	The swollen river spills over its banks or breaks through them	23 rivers and streams overflowed in Emilia-Romagna, May 2023
Flood	Temporary covering by water of land that is normally dry	45 municipalities hit in 3 provinces in the same event
Coastal flooding	Land along the coast under water from the sea	The coastal flooding in the Marche and Emilia-Romagna, 2023

From rain to river: how a flood forms

According to Io non rischio, most floods start with heavy or prolonged rain, which can sharply increase the flow of rivers, streams, canals and sewers. The USGS calls that flow discharge: the volume of water passing a point on a river in a given time.

Every river collects the water of its drainage basin, which English also calls a watershed or a catchment. The EU Water Framework Directive of 2000 defines it as all the land whose surface water flows, through streams, rivers and sometimes lakes, to a single mouth at the sea. Rain that lands on a distant hillside ends up in the same channel.

How much of that water actually reaches the river depends on the ground. The USGS, the United States Geological Survey, explains that dry soil lets more rain soak in, while soil already wet from earlier rain absorbs less and sends more to the stream. In the largest floods the ground is saturated, and nearly all the water runs over the surface, on pavement and soaked soil alike.

Human causes come on top. According to the Civil Protection Department, floods have become more frequent partly because so much of the land is built over and sealed under impermeable surfaces, so rain barely gets into the ground and more of it reaches the rivers, faster. The department also points to poorly cleared rivers, where debris and vegetation in the channel obstruct the current. The 2007 directive adds homes and businesses built on the plains next to rivers, land use that holds back less water, and climate change, all of which help make floods more likely and their damage worse.

Practical example: in May 2023 Emilia-Romagna was hit by two waves of rain less than twenty days apart, and in several places the total for the month passed 450 millimetres. Across the two events of 2-3 and 16-17 May, the regional government reports, 23 rivers and streams overflowed and 45 municipalities in 3 provinces were hit. The basins on the ridge and in the hills around Forlì had received up to 300 millimetres in 48 hours, and ISPRA’s 2024 report counts more than 80,000 landslides in the region that month.

River floods: large rivers and small streams

Floods do not all rise at the same speed. Io non rischio notes that floods on large rivers build slowly and can be tracked and forecast, while those on small rivers and mountain streams climb very fast and leave little time to act. The USGS ties this to basin size: rivers with larger basins need longer spells of rain before they rise significantly.

Italy, the Civil Protection Department points out, generally has small drainage basins that react to rain very quickly. Little time may pass between the first drops and the arrival of the flood peak. During summer downpours in particular, streams and minor watercourses can go from dry to full flood in moments, even after a thunderstorm that broke further upstream.

Some watercourses in southern Italy only turn into real rivers after heavy rain. Then there are culverted streams, which run for long stretches beneath roads or buildings; like sewers, they cause flooding when they cannot carry away the excess rain.

Flash floods: when the water arrives within hours

NOAA, the US National Oceanic and Atmospheric Administration, defines a flash flood as one caused by very heavy rain over a short time, generally less than six hours. It can arrive anywhere from minutes to a few hours after the storm.

In Italy, according to the Civil Protection Department, floods in small basins are common, especially in Liguria and Calabria, where a flood can form within hours of intense, localised rain. That kind of rain is hard to forecast, and it can also set off mudflows. Io non rischio adds that weather forecasts give the probability of rain over a wide area, not a promise that it will rain on one particular spot.

A flash flood can even strike without any rain at all. NOAA lists a failed levee or dam and the sudden release of water held back by debris or ice. Burned land is highly exposed as well: after a wildfire, rain that soil and plants would normally have taken up runs off almost at once, and streams rise sooner and higher.

Type	What happens	Causes listed by NOAA
River flood	The river level rises over its banks	Excessive rain from tropical storms, thunderstorms that keep hitting the same area, rain on melting snow, ice jams
Flash flood	The water arrives minutes to a few hours after the rain	Very heavy rain over a short time, generally less than six hours
Coastal flood	Land along the coast goes under water	A high tide above the usual level, made worse by heavy rain and wind blowing in from the sea

The return period and the “100-year flood”

Hazard is measured with the return period, also called the recurrence interval: the average number of years between two floods of a given size. The USGS warns that real intervals vary widely and that an estimate needs at least ten years of records, becoming more reliable as the years add up.

A “100-year flood” does not come once a century. The USGS defines it as a flood with a 1% chance of happening in any given year, and over a million years an event like that would be expected about 10,000 times, at irregular intervals. Nor does a “100-year” rainstorm necessarily produce a “100-year” flood. What also matters is how much of the basin the rain covers, how wet the ground already is, and how the length of the storm compares with the size of the basin.

Italy’s flood maps follow three scenarios set by Legislative Decree 49 of 2010, the law that brought the EU directive into Italian law. High hazard means frequent floods, with a return period of 20 to 50 years; medium hazard means less frequent floods, every 100 to 200 years; low hazard covers rare or extreme events. In ISPRA’s 2020 national compilation of these maps, high flood hazard areas cover 16,224 square kilometres, 5.4% of the country. Medium hazard areas cover about 30,200 square kilometres, 10%, and low hazard areas 42,376 square kilometres, 14%.

Flood examples from Italy

Among Italy’s worst floods for deaths and damage, the Civil Protection Department lists the Po in the Polesine in 1951, the Arno in 1966 and the Po across northern Italy in 1994 and 2000.

Practical example: ten days of heavy rain came first. On the night of 3 to 4 November 1966 the Arno left its bed upstream, in the Casentino and the upper Valdarno, before it had even reached Florence. In the city, the Civil Protection Department writes, the flood wave stood three metres high and moved through the streets at 60 kilometres an hour. No monitoring network existed along the river, so nobody announced the overflow and Florentines were caught unprepared. A plaque in Via dei Neri, in the Santa Croce district, marks the highest water level, 4.92 metres. The department reports 35 deaths in Tuscany; on 6 November, when the river withdrew, Florence lay under 600,000 tonnes of mud. Law 996 of 1970 drew up an overall framework for civil protection in Italy and, for the first time, gave formal recognition to volunteers.

ISPRA data processed in 2020, and carried in its 2024 report on hydrogeological instability in Italy, put 6.8 million people, 11.5% of residents, inside the zones that would flood in the medium hazard scenario, and count 2.9 million households exposed to flood risk. More than 1.5 million buildings stand in flood-prone areas in the medium scenario, 10.7% of all buildings in Italy, and nearly 34,000 cultural heritage sites are exposed in the same scenario. The regions with the most residents exposed to landslides and floods, adding the two risks together, are Emilia-Romagna, Tuscany, Veneto, Campania, Lombardy and Liguria. Io non rischio says the worst consequences come in cities, which hold more people and buildings and where changes made to the land often make floods more dangerous. The campaign adds that places that have flooded before are likely to flood again.

Italy’s Civil Protection Department counts floods among hydrogeological hazards, together with landslides, avalanches, coastal erosion and ground subsidence. In 2024, according to ISPRA, 94.5% of Italian municipalities had at least one area at risk from one of these hazards, ground subsidence excepted.

Climate and cities: what the IPCC says

The IPCC, the UN body that assesses climate science, states with high confidence in its 2021 report that heavy rainfall has become more frequent and more intense since the 1950s over most land areas with enough data. It judges it likely that human-caused climate change is the main driver. In its projections, extreme daily rainfall worldwide grows about 7% more intense for each extra degree of warming (high confidence).

At 1.5 °C of global warming, the IPCC projects heavier and more frequent intense rainfall, with the floods that come with it, in most regions of Africa and Asia, with high confidence. For Europe the same projection carries medium confidence. Along coasts, rising sea level makes flooding in low-lying areas more frequent and more severe (high confidence). Cities count here as well: urbanisation increases rainfall over and downwind of built-up areas (medium confidence) and the intensity of surface runoff (high confidence).

Flood defences, alerts and safety advice

The Civil Protection Department sorts defences into two groups. One is engineering works such as embankments, basins that hold water back, channels that divert excess floodwater and straightened riverbeds; the other is non-structural measures, meaning land management, warning systems and emergency plans. The EU directive required each member state to complete flood hazard and risk maps by 22 December 2013 and to publish flood risk management plans by 22 December 2015.

In Italy, alerts are issued by the regions and autonomous provinces, coordinated by the Civil Protection Department, and mayors then activate municipal plans. The national table, set on 10 February 2016, gives each level of criticality a colour: green for none, yellow for ordinary, orange for moderate and red for high. For thunderstorms the top level is orange, because red-level conditions already fall under the red alert for hydrogeological risk.

During a flood, Io non rischio explains, water can rise suddenly by one or two metres within a few minutes. Indoors the danger is greatest below street level and on the ground floor. Outdoors, the campaign singles out underpasses, stretches near embankments and bridges, steep roads and the lowest ground around. The campaign advises against going down to a cellar or garage to rescue belongings and against heading out to move the car. In its guidance, people on the ground floor go up to higher floors by the stairs, since a lift can get stuck. Tap water could also be contaminated.

The US National Weather Service puts figures on the force of moving water. About 6 inches of fast water can knock an adult over, about 12 inches can carry away most cars and about 2 feet can sweep off SUVs and pickup trucks: roughly 15, 30 and 60 centimetres.