![]() Although the different flood types behave in different ways, in many flood events they can occur simultaneously, making it hard to identify which flood type may have impacted a property. The two flood types can be loosely differentiated as follows: surface water flooding is caused by water that is on a journey towards the river channel and river flooding is caused by water spilling out of a river channel. As some river catchments are very large, such as the Mississippi and Mekong systems, river flood events can occur hundreds of kilometres from where the rain falls and typically have a longer-lasting impact. River (fluvial) flooding occurs when the river level exceeds the height of the bank and spills out onto the floodplain. The speed of this journey is dependent on catchment characteristics including topography, land cover, geology and soil type. Rainfall over a longer period of time will gradually make its way to a river via surface runoff, infiltration and drainage channels. For example, analysis using JBA’s flood maps has estimated that nearly five million UK properties are at risk of flooding from surface water, and annual reports from the German Insurance Association indicate that surface water flooding dominated flood damages in Germany in 2018 for the third year in a row (GDV, 2018). Water can build up in local depressions almost anywhere, thereby potentially affecting a far larger proportion of the land's surface and many more properties than river flooding. However, some surface water flooding can be geographically extensive and in many parts of the world can remain in-situ for a long period of time. Typically, surface water flood events have localised effects, impacting properties in close proximity to where the rain fell and for a short amount of time. In some places, it forms isolated puddles in ground depressions and in others it accumulates in valleys and flows downhill towards rivers. The rain hits the ground quicker than it can drain or flow away, water builds up and develops the potential to flood properties. ![]() Surface water flooding, pluvial flooding, flash flooding, cloudburst and storm runoff are all used synonymously to describe flooding that can occur after a heavy downpour. What is surface water (pluvial) flooding? Here, we consider surface water (pluvial) flooding and its global impacts in more detail, as well as how we model this flood type differently to river flooding. In our recent blog, we discussed the different terms used for the major flood types, providing an introduction to flood terminology.
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