Understanding and Assessing Groundwater Flood Risk

Groundwater flooding is the emergence of groundwater at the ground surface or into subsurface voids arising as a result of:

• abnormally high groundwater heads or flows;

• the introduction of an obstruction to groundwater flow; or

• the rebound of previously depressed groundwater levels.

  
  

Groundwater flooding usually occurs following a prolonged period of low intensity rainfall. As groundwater flow is much slower than surface flow, the flooding may not recede for long periods of time, typically weeks or even months. In addition, groundwater flooding can often occur after other sources of flooding have receded (such as surface or fluvial flooding) as groundwater flooding can take time to reach the surface.

It is important to recognise the risk of groundwater flooding is typically highly variable and heavily dependent upon local geology, topography and weather conditions, as well as local abstraction regimes. Groundwater flooding is hard to predict and challenging to mitigate.

Groundwater susceptibility maps produced by local councils through their SFRA are typically designed for planners and not a great tool to identifying groundwater flood risk at a site specific level, and so only effective at identifying areas where groundwater flooding might be an issue. The computer models used to classify areas produces maps with a pixel size of approximately 50 metres. Because of this coarse resolution it is possible that the actual susceptibility at your property may be different due to small-scale variations in rock and soil type and local landscape and topgraphy. Its important to note that Susceptibility is not the same as risk. These maps show where there is the potential for groundwater flooding but do not give any indication of the probability that it will occur.

So, although information from the maps can be used to highlight an area at risk, it is not designed to be used as a sole indicator of groundwater flooding susceptibility and risk at individual properties. The BGS and SFRAs recommend the following for assessing site specific risk:

1. Review BGS geological maps or and BGS (or site specific) borehole records for information on water strikes and groundwater levels.  Where underlying geology of is sufficiently permeable, this may allow groundwater to rise more freely and frequently

2. Review the Risk of Flooding from Surface Water mapping (refer to the EAs Long Term Flood Risk maps found through the Flood Maps for Planning). These maps can be used to estimate (that should groundwater emerge at surface upstream of a site), where the surface flows may be directed or pond. However, it should be noted that groundwater flows at surface are a lot lower than surface water runoff events, however groundwater flows may occur over a longer period.  Therefore, the risk event assessed for the DFE for groundwater flooding is typically considered to be the high-risk surface water event (or the 1 in 30 year event).

3. Review historical records and SFRA mapping, for example, if a specific property has experienced groundwater related flood risk in the past.  This is often the best indicator of site-specific risk for groundwater flooding and can carry the most weighting in assess risk.

   
   

It is a combination of the above factors that can be used to determine how at risk a site may be from groundwater flooding (at surface).

Groundwater flooding to basements are considered slightly differently, where further assessment on how a basement structure could affect groundwater flows (often called a Groundwater Impact Assessment) may be required.

If need us to undertake a more detailed assessment, get in touch and we will help where we can.