When rivers and streams ‘break their banks’, the water will flow out onto adjacent low-lying areas (known as floodplains), and there are a number of reasons as to why (and how) this happens.

For example, if the runoff from heavy rain exceeds the natural capacity of the river channel, you will experience fluvial flooding. Also, if a channel is blocked or constrained – or in estuarine areas where high tide levels impede the flow of the river out to sea – your site may be vulnerable to river flooding.

Your Flood Risk Assessment report will identify the likelihood of your site being subjected to fluvial flooding, as well as other flood risk sources, including:

  • Pluvial (Surface Water/Overland flow)
  • Reservoirs
  • Groundwater
  • Foul and surface water sewers

This post sets out to give a high-level overview of what fluvial flooding is and how we report on it, so that you’re not left ‘drowning’ in our jargon.

Read on to find out more…

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What is fluvial flooding?

Fluvial flooding happens when the water level in either a river, ditch or stream overflows. The most common source of flooding occurs when river levels rise and spill over their banks ('fluvial' flooding). Fluvial flooding can also occur along coasts as a result of high tides and stormy conditions.

What causes a fluvial flood?

Fluvial (River) flooding is caused by excessive rainfall over an extensive period of time, which then makes rivers or streams exceed their capacity. Rarely, this can also be caused by snowmelt following heavy snowfall. 

How is the Probability of River Flooding Determined?

The probability of Fluvial/River flooding is determined by the Environment Agency (England and Wales) and Scottish Environment Protection Agency/SEPA (Scotland).

These agencies allow you to check the flood risk in your area via their flood risk maps and modelled flood data can be obtained from them in order to ascertain the depths of flooding for various storm events.

On some occasions, this data is provided with climate change added, and in some cases, we have to use various methods to apply this ourselves.

The Types of Flooding in the UK

Fluvial flooding is one of four main types of flooding that we experience in the UK. Tidal flooding, pluvial flooding, groundwater flooding and other sources of flooding* will all be taken into consideration during your Flood Risk Assessment.

Coastal flooding – When sea levels along the coast or estuaries exceed neighbouring land levels.

Flash floodingFlash flooding is categorised as flooding that occurs within 6 hours of heavy rainfall beginning. Flash flooding is most often caused by severe thunderstorms, hurricanes, tropical storms or intense weather patterns.

Groundwater flooding – When the level of water stored in the rock or soil underground ground rises. The level of water within the ground is known as the groundwater table, and this can rise slowly over a period of weeks or months after prolonged heavy rainfall.

Sewage flooding – This occurs when the existing drainage infrastructure cannot cope with the inflows due to heavy rainfall, or when the existing infrastructure has become blocked or there have been pipe collapses.

Get In Touch

Would you like to find out more about our approach to Flood Risk Assessments? Don’t hesitate to speak to an expert today.

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Sadia Luckett

Written by Sadia Luckett

Sadia Luckett is the Director at Nimbus Engineering, specialising in foul and surface water drainage & SuDS.