What can be done to mitigate flood risk and protect properties?

© Martin Applegate

Mike Piotrowski, principal hydrologist at GeoSmart Information, explores how flooding will affect construction and property development in the future and looks at what can be done to mitigate flood risk

Flood risk is an increasing concern for developers, mortgage lenders, insurers and property owners. As the world grapples with the devastating impact that flooding and climate change is having on communities, the way that both commercial and residential developments are assessed has had to change.

Whether asset managers or homeowners seek to mitigate flood risk to existing developments, or developers look to construct new buildings on brownfield or greenfield sites – water management and risk has to be at the forefront of any design, with consideration of the lifespan of the development.

It’s not just because a flood event can be devastating for any business, resulting in extended shutdown, economic losses, redundancies or even closing down altogether, but because flood risk also has implications for insurance: premiums may soar or insurance companies may refuse cover.

The same applies to homeowners, who are often left in temporary accommodation for many months while their properties are repaired following a devastating flood, adding to an already stressful period.

Where risks are identified, flood mitigation measures should be included in both the design of new development and retrofitted to existing property to reduce the impacts of flooding from all sources – not just coastal, rivers, lakes and streams, but also groundwater, surface water and sewers – over the lifespan of the property.

Current projections of global warming and recently observed weather events indicate the frequency and severity of storm events are set to increase over the next 60-100 years. The inclusion of flood resistance and resilience measures should therefore be included as standard to protect property and its occupants, but also, additional mitigation may be required to ensure risks are not displaced to neighboring sites or buildings.

Flood risk assessment

An assessment can be undertaken through the collection of catchment and site-specific information, such as modeled data, topographical surveys of the ground including building thresholds (air bricks and door steps) and anything else that might affect the flow of flood water. Rainfall, river level and flow datasets can also be used to calibrate and validate modeled data, which is key to understanding the source, likelihood and severity of flooding in the greatest reality possible.

The assessment can then be used to confirm the likely impacts and assign a risk to an organization or local authority as part of a planning application or to confirm the potential risks to the operational side of a development.

Every site is unique, of course, and has its own risks and constraints that must be considered, including national and local planning regulation, as well as an attitude to risk and budget of the site owner.

This means the design of a building will also need to include consideration of the specific measures most beneficial to mitigate flood risk.

It is essential to understand flooding risks prior to purchasing a site. If risks are found to be too high, development may be constrained to the point where it does not generate enough of a return on investment. Similarly, if flooding risks are not prioritized in the initial stages of scheme design, this could later result in expensive redrawing of designs or even the removal of a significant number of developable units from a scheme.

When is building on a floodplain acceptable?

There are occasions when building in the floodplain may be considered acceptable. These are where:

  • constructing in the floodplain could potentially enhance the floodplain or reduce flooding risks to lower than previous;
  • where a local council’s wider aims and objectives require such development to contribute to its economic growth;
  • when a site doesn’t have enough space to allow development to be located outside of the “at risk” areas.

For the vulnerability of the development and the flood zoning to be acceptable, it would need to be in alignment with national and local policies. If the development is in line with national and local policies and the development can be made safe for its lifetime, there are a number of interventions that can be taken to mitigate flood risk.

The first, and most obvious, mitigation option is to not build within the designated floodplain, with consideration of flooding from all sources. If a development masterplan can be adapted to move buildings and essential infrastructure outside the floodplain, to minimize risks and to ensure operational areas are affected for as short a time as possible, that would provide some reassurance to developers, potential tenants and to insurance companies.

The second mitigation option for new developments is to raise the property threshold to prevent flood water entering it. The Environment Agency, Scottish Environmental Protection Agency and Natural Resources Wales typically require the finished floor level to be raised above the 1 in 100-year (fluvial) and 1 in 200-year (tidal) flood levels, including freeboard, to allow for climate change, wave action and modeling inaccuracies

However, when thresholds cannot be raised for other planning reasons, alternative flood resistant measures could be considered to prevent flood waters entering property through doors, air bricks, walls and via sewer pipes.

The most up-to-date, kitemarked flood barrier systems are extremely effective at holding flood water at bay, but only for short periods if the flood depths are less than 30cm and even then there is a “trickle rate” behind the barrier that may require secondary systems such as pumps, depending on the duration of the flood event.

These systems also only work properly when trained personnel have prior warning of an event and can install them quickly, before a storm event. They also need to be properly stored and maintained, to make sure they are in good working order when needed, over the lifetime of the development.

There are additional passive measures that can be used, such as installing airbrick covers and those that automatically shut off to prevent water entering a building, while non-return valve systems can be used to prevent water entering via the foul pipe systems. These can be retrofitted, too.

Treating flood risk with Sustainable Drainage Schemes

Treating flood risk as an opportunity enables some creative options such as the use of Sustainable Drainage Schemes (SuDS), which slow water down (attenuate) before being discharged from the site (ie enters a watercourse or a sewer).

SuDS are valuable resources that provide areas for storage, allowing water to soak into the ground, evaporate from the surface or transpire from vegetation. A wetland, swale or basin can also help to increase biodiversity and can be an attractive environmental feature, reinforcing or helping to meet the criteria of the all-important Environmental Social Governance (ESG) initiatives.

Reducing the density of a development can also make a significant difference when it comes to reducing flood risk on site and reducing displacement elsewhere.

If an existing property has been flooded, a combination of measures could be introduced, such as raising electrical sockets, waterproofing floors and walls or raising white goods off the floor.

It is important to note that flood risk is not static – as advancements in computational hardware and modeling technology bring updates to modeling methods and maps, there is a chance a previously low-risk site is reassessed as being higher risk. If this happens, insurance companies might well decide it will no longer cover a building or development.

This can be challenged, though. An environmental consultancy, such as GeoSmart Information, can examine a wider range of datasets and maps to see if the reported risk is overstated. Quite often, this is caused by limitations in modeling methodologies or the resolution of national or catchment scale datasets produced by the Risk Management Authorities such as the Environment Agency (EA), the Natural Resources Wales (NRW) and the Scottish Environmental Protection Agency (SEPA). .

However, through analysis of a combination of more detailed area and site-specific datasets, a more realistic risk profile can be drawn up, which could potentially save a business or homeowner thousands of pounds.

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