River vs surface water
flooding: what's the difference?

In our recent blog, we discussed the different terms used for the major flood types, providing an introduction to flood terminology. 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.

What is surface water (pluvial) flooding?

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.

The rain hits the ground quicker than it can drain or flow away, water builds up and develops the potential to flood properties. In some places, it forms isolated puddles in ground depressions and in others it accumulates in valleys and flows downhill towards rivers. 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.

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. 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. 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).

What is river flooding?

Rainfall over a longer period of time will gradually make its way to a river via surface runoff, infiltration and drainage channels. The speed of this journey is dependent on catchment characteristics including topography, land cover, geology and soil type . River (fluvial) flooding occurs when the river level exceeds the height of the bank and spills out onto the floodplain. 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.

How can we define the two flood types?

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.

However, it isn’t always that simple. 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.

To help clients better understand their potential exposure to both types of flooding, we model river and surface water flooding separately in our flood maps, using different modelling techniques, to account for different behaviours of each flood type. This enables users to identify which areas might be susceptible to river flooding, surface water flooding or both to better manage the risk and accumulations.

Figure 1: An extract from JBA’s 5m France Flood Map, showing river flood (blue) and surface water flood (purple). Our river maps show flood water that has overtopped the river banks, with flooding tending to follow the main river channels, while our surface water maps tend to identify smaller streams and places where water has pooled after heavy rainfall.

Different modelling approaches

In JBA’s flood maps, river flooding and surface water flooding are differentiated by the way in which they are modelled.

For river flooding, we simulate what happens when the flow exceeds the capacity of the river channel and the overflowing water is routed over the floodplain as illustrated in figure 2. Modelled river flooding therefore mostly occurs near to our defined river network, unless the area being modelled is flat, when the topography can allow the floodwaters to spread many kilometres away from the river channel.

Figure 2: River flooding is modelled by allowing a volume of water to escape from the channel at points along the river network (e.g. at the purple dot). The model then allows the water to flow based on the terrain (blue arrows)*.

In contrast, surface water flooding in JBA’s maps is modelled by simulating what happens when rain falls directly onto the land. The direction and movement of the water across the land surface over a defined time period is simulated, as illustrated in figure 3. Allowances are made to account for natural infiltration and man-made drainage systems which adjusts the volume of water available to cause surface water flooding.

Figure 3: Surface water flooding is modelled by inputting rainfall directly onto the surface of the terrain (purple dots) and allowing it to flow under gravity and pool in natural depressions and valleys*.

In both types of flooding, topography is one of the main drivers of where the flooding occurs. Therefore, there are some locations where both river and surface water flooding can (and do) occur, at different times or simultaneously.

Definitions aside, surface water flooding has been responsible for large flood losses in recent years. In June 2007, heavy rainfall in the north of England overwhelmed drainage ditches and sewers and caused significant surface water flooding. Two thirds of the properties affected by this event are thought to have been affected by the associated surface water flooding. Similarly, surface water flooding is believed to have been a major cause of insurance loss during Hurricane Harvey in Texas, when existing flood maps failed to identify surface water hazard leading many people to believe they were not at risk from flooding.

It’s therefore paramount to use the most up-to-date scientific techniques and modelling tools to ensure that the risk from surface water flooding is well represented. JBA offers river and surface water flood maps at 30m resolution globally, with the UK, Europe and US available at 5m resolution. Get in touch for more information.

References

Gesamtverband der Deutschen Versicherungswirtschaft e. V. 2018. Naturgefahrenbilanz 2018. [online] Available at: https://www.gdv.de/de/medien/aktuell/versicherer-leisten-2-7-milliarden-euro-fuer-stuerme-und-starkregen-42732 [28 May 2019]


*Image for illustrative purposes only, JBA models use a regular grid

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