Flood Types and Understanding a Building's Risk
In the past, there was a good understanding of where to position buildings to minimise the risk of flooding and water penetration. Buildings constructed next to rivers and streams for transportation purposes were expected to flood. Damage was mitigated by using materials that could withstand intermittent flooding and recover quickly.
Types of flooding
Many people assume that a building is only at risk of flooding if it is near a water source. However, risks also exist from surface water flooding and groundwater flooding, particularly where areas of hard surfacing are extensive and severe drought is preventing the natural drainage of rainfall.
Due to the changing climate, it is critical to consider a building's exposure to repeated flooding, especially where flood events occur in quick succession and building materials cannot reach equilibrium. This is of particular concern when the materials impacted are organic, as these are more likely to rot and increase mould within a building.
There are several types of flooding, and each will impact a building and landscape differently. All types of flooding have associated hazards, some of which are identified below.
River flooding is usually the result of heavy rainfall, which causes water levels to rise. Rivers vary in the way they react depending on their catchment characteristics, capacity, connection to the flood plain and land management practices around them.
Flood plains allow excess water to be stored, which reduces water volume and slows down the river flow. Development on flood plains can reduce their capacity to store water, which, in turn, may create flooding further down the river where it had not previously occurred.
Coastal flooding is caused by a combination of high tides, wave action and tidal surge. High tides usually occur in spring and autumn. A build-up of low pressure can coincide with high tides and lead to a tidal surge.
Coastal flooding may occur rapidly during storm events, but the impacts, including those on coastal structures (both natural and manmade), can continue over a prolonged period of time and across successive events. By increasing the frequency of storm events, climate change is fuelling this pattern. This leads to accelerated deterioration rates of coastal structures, thus increasing erosion and the probability of failure.
Coastal barriers may be breached permanently, and sites that were formerly protected by beaches, shingle spits or dunes may become subject to marine erosion.
This type of flooding is usually caused by sudden torrential rain, particularly in urban areas, and it can be highly unpredictable. Drainage systems may be unable to cope with large volumes of excess water and overflow. Impermeable surfaces and hard landscaping, such as driveways, concrete and paving, increase the risk and likelihood of this type of flooding.
Flash flooding is exacerbated when the ground is either dry and compacted during summer months or saturated during winter months. In these conditions, the capacity of the ground to absorb water decreases and run-off increases as absorption rates are low.
With higher levels of run-off, culverts are susceptible to overflowing. This is a particular problem if channels are not properly maintained and become blocked by rubbish or structural debris.
Flooding from groundwater can happen when the level of water within the rock or soil underground – known as the water table – rises. When the water table rises and reaches ground level, water starts to seep through to the surface and flooding can happen. This means that water may rise up through floors or underground rooms such as cellars or basements.
Groundwater flooding occurs far more slowly than river flooding: it will usually happen days, weeks or even months after heavy or prolonged rainfall. It may last weeks or months. Flooding from groundwater is most common in areas where the underlying bed rock is chalk, but it can also happen in locations with sand and gravel such as in river valleys.
Compound flooding occurs when coastal, surface or river flooding happen simultaneously. Although compound flooding occurs less frequently than other types of flooding, it has a much broader impact, affecting a larger area and remaining present for longer.
Flooding can create associated hazards, such as downed electrical lines. Dead animals or faecal matter may be present in the floodwater and then left on land after it recedes, causing contamination.
Where flooding occurs near farmland, there is an increased risk of phosphates, salt deposits (efflorescence), nitrates and sulphates being present. All of these products can cause damage to buildings and surrounding land.
Depending on its height, speed and direction, the flow of water may cause structural damage to nearby buildings and bridges, as may any large debris in its current.
Coastal flooding can cause efflorescence in the building fabric, which may take several attempts to clear. Other hazards include impacts from debris, high waves and strong winds during storms.
In the longer term, subsidence and heave are hazards associated with groundwater flooding, because the change in groundwater levels will cause movement in the soil and minerals under and around a building. This will lower the bearing capacity of granular soils (but not clays). In granular soils, the position of the water table is important, and the bearing capacity in a saturated sand can be as much as 50 per cent lower than in dry sand.
Groundwater can also cause supersaturation of soils, meaning that external soils can take longer to dry out at the base of a building. This may delay the drying of internal floors where the ground levels are a similar height.
Understanding the flood risk
Establishing the flood risk of a building is an essential first step when considering potential alterations or change of use. It is important to understand the surrounding landscape and geology of the site, and to identify any potential sources of water that may impact the building. This knowledge will inform appropriate design choices.
Answering the following questions will provide insight into a particular building's potential risk of flooding.
- Is the building in an area of flood risk? Near the coast, a river, stream, ditch, lake or reservoir? In, or near, a flood plain?
- How high is the building above the flood risk level?
- Are the building and its drainage in good condition?
- Does the landscape slope towards the building or away?
- Does the building have a basement or semi-basement?
- Does any water rise on the site or near to the building, for example a well or spring?
- Is any part of the building constructed below drainage routes or culverts?
- Are there any active measures in place, such as defences, bunds, or barriers?
The extent and degree of flood damage will depend on the following factors:
- advanced warning is available
- predictability and speed of flooding
- where water enters the building
- depth water reaches and length of time it remains
- contaminates and debris the water brings with it
- whether the water is salt or fresh water
- construction of the building and type of materials used
- occupancy type and ability to deploy resistance measures if present
Flood risk assessment
A risk assessment is the process of identifying which hazards exist and deciding how likely they are to cause harm – and to what extent. To allow for establishing metrics to mitigate, eliminate or control the hazard, a flood risk assessment considers the risk of flooding to a building from all potential sources.
The building occupants and how they use the building should be considered alongside the impact of flooding on the building fabric. For example, a highly flood resilient building will still need to assess how occupants will evacuate during a flood event. If an adaptation design fails to consider occupants and building use, then it is ultimately inappropriate. It is important to create, communicate and practice evacuation plans.
Flood risk assessments should be undertaken by an experienced professional who can demonstrate their understanding of flooding and traditional construction. There are many specialists that can carry out a flood survey to identify a building's flood risk. They will be able to recommend how to adapt a building to flooding, according to its construction type and occupants. CIWEM (Chartered Institution of Water and Environmental Management) is in the process of developing the CIWEM Special Register for certified Property Flood Resilience (PFR) professionals, which will provide an accreditation and qualification. Historic England also offers advice on How to Find the Right Professional Help.
Where to find information
Checking local flood maps to identify flood zones, defences and storage areas is an essential first step, and it will help you understand the existing risk to the building. Talking to the occupants or management company will confirm how accurate and up to date the local flood maps are. Researching historic flood events in the area to understand frequency, cause and extent is also a good idea.
To assess the future flood risk potential, consult long term flood risk and what management plans the Environment Agency have in place. This service identifies whether the flood risk from rivers, the sea or surface water is likely to be high or low in the future.
It is important to note, however, that surface water flooding is harder to predict and local flood maps do not consider the management of external or internal drainage by the building owner, council or highway agency. Obstructions in drainage channels, culverts, pipes, ditches and other pathways where water might otherwise flow will affect the level of surface water and how long it remains.
Check shoreline management plans for places at risk of coastal erosion.
Looking at climate change impacts more broadly, and the interconnected nature of climate hazards, are things that the flood risk maps are unable to consider.
Drought conditions and saturation from long periods of wet weather can increase flood risk, and this is not reflected in the mapping. We are experiencing drier, warmer summers, and increased temperatures with prolonged periods of drought are forecast. Drought conditions, in which landscapes become unable to absorb sudden bursts of rainfall, can result in localised flooding.