Flash floods and the other types of flood
PAGASA defines a flash flood as a flood that develops no more than six hours after intense rain, and often sooner, in hilly and mountainous places where rivers run steep. Other floods usually take from 12 to 24 hours or even longer to develop.
What to do before, during and after a flood, from PAGASA's Flood Safety Rules, is on the flood guide. This page explains what the words mean.
Written for the Philippines. Baha ang tawag dito sa Filipino. In an emergency here, the number is 911.
What PAGASA calls a flood
The definition PAGASA's flood forecasters work to is "an abnormal progressive rise in the water level of a stream that may result in the overflowing by the water of the normal confines of the stream with the subsequent inundation of areas which are not normally submerged".
In plain terms: a river rising above normal, sometimes spilling over its banks and covering ground that is usually dry. PAGASA adds that what most people mean by a flood is that covering of dry ground, which can come from water overflowing a river or other body of water, or from rainwater collecting where it cannot drain away.
Source: PAGASA, Floods: definition, causes and types. Read 23 October 2026.
The three types PAGASA describes
Minor flooding
- The river may or may not overflow its banks. Where it does not, the water is excess surface runoff collecting on low, flat ground.
- The water usually stays on the floodplain along the channel and in low spots and dips in the ground.
- It is usually shallow, and there may be no current you can see.
Major flooding
- Rivers and lakes overflow, dikes, levees or dams break, water held in a reservoir is released beyond control, or runoff builds up.
- The water covers a wide area and spreads fast to lower ground next to it.
- It is fairly deep across most of the area, with a current you can clearly see.
Flash flood
- It develops no more than six hours after intense rain, and often sooner.
- It happens in hilly and mountainous terrain, where the river runs down a steep slope.
- Rain falling anywhere in the catchment reaches the river in a short time, so the water rises fast until the channel overflows.
Source: PAGASA, Floods: definition, causes and types. Read 23 October 2026.
Why a flash flood comes so fast
PAGASA puts it down to the catchment. In steep, hilly country the rain that falls even at the point farthest from the river takes only a short time to reach the channel and join the flow. So the amount of water in the river climbs quickly, the level rises with it, and when the channel cannot carry any more it overflows.
Source: PAGASA, Floods: definition, causes and types. Read 23 October 2026.
Two other pages on this site mention flash floods. On the flood guide, PAGASA's General Flood Advisory warns of possible flash floods and landslides at its Alert level. On the earthquake hazards page, PHIVOLCS says a landslide set off by an earthquake can dam a river, which may then break through and send a flash flood downstream.
Source: PAGASA, Weather Icons, the flood warning legend. Read 23 October 2026. These are the non-telemetered rows of the General Flood Advisory, which PAGASA issues for river basins without automatic gauges.
Source: PHIVOLCS, Earthquake Hazards. Read 23 October 2026.
What causes flooding
PAGASA sorts the causes into natural and man-made. Behind the weather causes, it says, is the wet season the monsoons bring, along with tropical cyclones, thunderstorms, the Intertropical Convergence Zone and passing fronts, which together with the monsoon can bring intense rain.
Natural
- Weather
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- Intense and prolonged rain
- Unusually high water on the coast and in estuaries, from storm surges and seiches
- Earthquakes
-
- Tsunamis
- Land sinking in an earthquake, which lowers it
- Lake and river beds pushed up, which sometimes makes the water spill over
- Tides
-
- High tide arriving at the same time as heavy rain
Man-made
- Blasting that sets off landslides, which can dam rivers and streams
- Temporary dams that block a river or stream until it overflows
- Dikes and other control structures failing
- Dams and other structures managed badly, which may force a sudden release of excess water
- Forests and watersheds stripped bare
Source: PAGASA, Floods: definition, causes and types. Read 23 October 2026.
What comes after the water
PAGASA lists what it calls the cascading effects of a flood. Utilities go down, and food and safe drinking water run short. Sanitation breaks down, which in PAGASA's words "almost always results in the contamination of the water supply", and outbreaks of stomach and intestinal illness often follow. Respiratory illness from exposure is common too.
The disease the DOH warns about after wading in floodwater is on the leptospirosis page.
Source: PAGASA, Floods: definition, causes and types. Read 23 October 2026.
Is your address prone to flooding?
HazardHunterPH checks a point on the map against the flood and rain-induced landslide maps of the Mines and Geosciences Bureau, among other hazards. How to use HazardHunterPH.
Source: GeoRiskPH, DOST-PHIVOLCS, HazardHunterPH Frequently Asked Questions. Read 23 October 2026.
When the rain starts
PAGASA's rainfall warnings come in colors, and its flood advisories in levels. Both are worth knowing before the first one is issued.