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What the previous answers often underestimate is that it's not just about how much rain falls in a short time, but also that the soil simply can't absorb any more water after prolonged wet periods. Once the ground is already saturated, even normal rainfall can't seep through and runs off on the surface instead - which massively intensifies the flooding. On top of that, you've got factors like sealed surfaces in cities or the topography that channels the water into specific valley areas. So with 2024, it wasn't just "a lot of rain," but a pretty unfortunate combination of several factors all happening at once.
Are you asking because you're wondering how that much water can even come together in the first place?
The water in floods like the 2024 one comes mainly from extreme rainfall - so heavy rain that falls over hours or days. When the soil is already saturated (which happens after longer periods of rain), it can't absorb any more water. Everything runs off on the surface, collects in streams and rivers, and leads to massive flooding. Where exactly it rains also plays a role: in river catchment areas, rainfall from a huge region can flow together, so the amount of water multiplies.
On top of that, impervious surfaces (roads, parking lots, buildings) make the problem worse - water can't seep into the ground and just runs off. In some regions, snowmelt also adds extra water when rain falls at the same time. The rivers and streams then simply get overwhelmed and overflow their banks. It's less of a mystery and more a matter of physical law: if you pour more water into a riverbed than it can hold, it's going to spill over.
In our area, it rained massively in 2024, and what surprised me was that the ground was already saturated from all the rain in spring, so the water just didn't seep in - it ran straight into the streams and then into larger bodies of water. Heavy rain alone might have been manageable, but the combination of prolonged rainfall beforehand + extreme precipitation in a short time + saturated soil is what made it critical.
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