Flood Seminar: Dr Maxine Zaidman
Our first speaker of the day was Dr Maxine Zaidman from JBA Consulting talking all about the hidden perils of groundwater. Maxine is a hydrologist specialising in flood and drought risk management and her role as Technical Director for JBA’s consulting arm involves providing engineering and science consultancy to the water and environmental protection sectors.
Within Maxine’s presentation she covered the following:
What is flooding?
A natural but infrequent problem which manifests
- through a range of mechanisms, and
- at a range of scales
Has a range of impacts, depending on
- event duration and magnitude
- event source/mechanisms
- nature of receptor
- existing resilience measures such as defences
Different flood types: Fluvial, pluvial/surface water, coastal, groundwater
Why groundwater flood is a hidden peril?
- groundwater is normally “not seen”
- manifestation only in extreme events
- more regionally constrained than fluvial and surface water flooding
- mechanisms and drivers can be complex, and vary locally
- often undifferentiated from other types of flood
- damage costs not as well understood/quantified
In the UK, Chalk is the main aquifer associated with groundwater flooding. The chalk crops out across the “down land” areas of eastern and southern England e.g. North Downs, Chilterns, Salisbury Plain. The high level of risk is associated with the wide variation in groundwater levels that are not normally seen in the chalk aquifer (which is locally 100s of metres thick). If the autumn and winter season is persistently wet, a much greater amount of water can percolate down into the aquifer causing groundwater levels to rise to levels higher than normally seen and, potentially, to levels that will cause flooding. There is often regional coincidence of groundwater flood events, for example almost the entirety of the chalk aquifer was affected in winter 2000/01 and winter 2013/14, two of the most recent major groundwater flooding events.
If levels become high enough, groundwater can start to ingress into subsurface infrastructure such as basements, cellars, tunnels and sewer networks. There is also a possibility for groundwater to emerge at the ground surface, particularly in valleys or low lying areas which could develop into flow routes and pose a risk to infrastructure.
Groundwater flooding has the potential to cause damage to subsurface assets, create transport network issues and cause surcharging of foul water networks. Groundwater flooding can often be prolonged, leading to the protracted inundation of properties and infrastructure and thus, this can increase the degree and cost of damages.
As it stands, there is currently no standard method for mapping flood hazard from groundwater sources. However, there are a number of commercial products on the market, including the JBA mapping that is offered by Future Climate Info (FCI). As with outputs of any flood hazard modelling methods, there remain uncertainties and local complexities that may impact on the reliability of mapping outputs at property level. There are a few things that conveyancers should look out for when considering the possible vulnerability of a property to groundwater flood risk. These include:
- If the property has suffered protracted past flooding or been affected in known groundwater flood events e.g. 2000/01.
- If the property is located on permeable unconfined aquifers, alluvial floodplains or karst areas.sv
- Low-lying areas e.g. valley bottoms particularly if near a watercourse.
- If there are local features representative of a high local groundwater table e.g. local spring-fed ponds, or groundwater wells with groundwater levels that are near the surface.
- Pumps employed in cellars or wells in cellars.
- Whether the property has basements.
Whilst the above can be indicators of an increased risk from groundwater flooding, they do not imply that groundwater flooding will necessarily occur. A more detailed desk or site assessment is often required to establish the likely scale of hazards at a property-scale.
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