Flooding is a complex and varied peril, with flood characteristics differing across the different sources of flooding. At JBA Risk Management, we create datasets that provide our clients with an understanding of flooding and the associated flood risk. Within our datasets, we use various terms to refer to the different types of flooding and hydrological processes associated with each, while other and different terms can also be used globally. This blog provides an overview of flooding terms to help clarify some of these different definitions.
The main terms for flooding
The words used to describe flooding can vary from organisation to organisation, industry to industry and country to country. At JBA, we use three main categories: river flooding (when rivers overflow), surface water flooding (when rain-runoff becomes excessive) and coastal flooding (when sea levels are high). In North America, these three terms are known as fluvial, pluvial and storm surge flooding, respectively.
Note: Flash flood is a term commonly used in the media and by the insurance industry but it is ambiguous. It refers to the sudden speed at which flood waters appear and recede and does not tell us what the source of flooding is. A flash flood is often a combination of river and surface water flooding.
River flooding
River flooding occurs when the capacity of the river channel to hold water is exceeded. The excess water overtops the bank and inundates the surrounding land, causing flooding.
Alternative and related terms for river flooding are below:
- Fluvial flooding is the most common alternative term used in the industry and academia, especially in North America, where JBA’s maps are called fluvial flood maps.
- Riverine flooding is a less common term, with “riverine” referring more broadly to the area alongside rivers.
- Overbank flow is more commonly used in hydraulic modelling and research settings. As the name suggests, it represents all flow that is out of the bank.
- Snowmelt flooding is a type of river flooding that results from an, often rapid, melting of snow. In many places, a snowmelt flood happens every year but might not exceed the bank or cause damage because the channel, naturally or artificially, has sufficient capacity to convey all the meltwater.
- On-floodplain flooding is a less common alternative used in the industry and academia, referring to flooding confined to the river floodplain.
Bursting banks may be mentioned colloquially, especially by the media, but this is misleading as river banks don’t burst or break; instead, they overtop. Levees and embankments that are engineered can burst but the term “breach” is more widely used.
Pictured: main sources of inland flooding, including river and surface water flood
Surface water flooding
Water starts to accumulate on the surface of the ground (terrain) when the rate of rain falling is greater than the ability of the land to absorb it. This can happen for a number of reasons, for example if the ground is already saturated from previous rainfall or if the urban drainage system capacity is exceeded, reducing the ability of drains and sewers to carry the water away. This water flows across the ground and ponds (collects) in low lying areas or dips in the terrain. This flowing water and/or ponding is called surface water and, if the conditions remain the same for a prolonged period, flooding can occur.
Where the water will go and how quickly is dependent on many complex and interlinked factors such as the surface material (e.g. soil or concrete), the slope of the land, the air temperature, the capacity of the urban drainage network and the catchment’s conditions prior to the rainfall, including previous ground saturation.
Alternative and related terms for surface water flooding are below:
- Pluvial flooding is used in many countries, especially in North America where JBA’s maps are called pluvial flood maps. The term is also used in the modelling and engineering sectors when discussing the science of hydraulics.
- Saturation excess is a hydrological term for water runoff from land that is already saturated, with no further infiltration possible.
- Overland flow is more commonly used in hydraulic modelling and research settings. It refers to the movement of water across the land surface.
- Urban flooding occurs in built-up areas of infrastructure where the volume of surface water exceeds (surcharges) the drainage and sewer network capacity.
- Disconnected flooding means that the area of flooding does not connect to a river channel.
- Off-floodplain flooding is a less common alternative used in the industry and academia and refers to any flooding beyond the river floodplain.
Coastal flooding
Weather-related coastal flooding occurs when the air pressure is low and the mean sea level rises. If the air pressure is exceptionally low, this rise can be significant, particularly when being pushed along by a storm or when coinciding with a high tide. Predicted tides can also be higher than expected if there are high winds, if water levels change or if the shape of the coast has changed due to erosion.
Alternative and related terms for coastal flooding are below:
- Sea/storm surge is a widely used alternative, especially in North America, and refers to the physical process that leads to coastal flooding.
- Hurricanes, tropical cyclones, extra-tropical cyclones and windstorms generate storm surges and wave action in coastal areas, resulting in flooding.
- Tsunami floods are coastal in nature, but they are not weather-related or something that we include in our standard modelling methods.
Pictured: the main sources of coastal flooding
At JBA Risk Management, we produce global flood maps covering surface water and river flooding, along with coastal flooding in some countries, for a comprehensive overview of flood risk. Please get in touch to find out how our data could help you.
