Research shows that global warming is increasing flood risks rapidly across northwest Europe. Jon Herbert asks how the urban environment can cope with larger and more unpredictable volumes of water.

Key flood water management advice for site operators and property-owners is often summarised as preparation, preparation and preparation. The goal is to minimise immediate damage while planning carefully for safe emergency evacuation and long-term business recovery and continuity.

However, climate change is making more strategic approaches essential. New research suggests that northern Europe and the UK are facing growing flooding risks, plus a pincer-movement threat from torrential rains and powerful sea surges generated by intense low-pressure weather systems.

European storm clouds

Climate researchers forecast increasingly erratic, heavy and slow-moving future downpours made worse by the “atmospheric rivers” phenomena that can carry moisture for thousands of miles before dropping massive volumes of water quickly onto tarmac, concrete and saturated soils.

A Viennese research team has been studying the problem and what it means for flood protection given that globally river flooding damage is estimated to average $100 billion (£78 billion) annually.

Measurement limitations have historically made collecting flood pattern changes evidence difficult. The team, which has now isolated flood discharge trends across Europe over five decades, says they are linked clearly to changing climate.

Autumn and winter rainfall will increase flooding in northern and western Europe while southern and eastern Europe become drier. And the fact that flood risks have already changed, and will almost certainly continue to change, means that resilience must be flexible and progressive.

The study tracked more than 3,700 European measurement points from 1960 to 2010 and found that in some northern hotspots – Scotland, plus coastal France and Norway – flows increased by nearly 18% every decade. It predicts that more atmospheric moisture will make storms wetter, move them northward, and slow them down to drop more rain over river catchments.

Another finding is that benchmark 100-year floods — with a circa 1% chance of happening in any given year — are likely to become 50-to 80-year floods.

Flood pincer movement

Meanwhile, another study warns that coastal areas including Devon, Cornwall and the Bristol Channel will face additional “compound flooding” dangers as low pressure areas unleash both rainstorms on land and storm surges out at sea — particularly with high tides — at an increasing rate.

A quoted example is the Avon flooding in Bristol in 2014. Computer models show that with current climate change, some 3% of coastal areas are vulnerable to compound-flooding more often than once every six years. But that figure could rise to 11% by 2100.

Risks versus probabilities

Risks to individual properties are often assessed as a balance between the probabilities of specific flood levels occurring versus anticipated damage. Lower levels tend to cause less damage; lower defences usually cost less.

Similarly, higher levels are less likely but linked to greater damage and more expensive defences. The calculation for property-owners is then what damage they can live with versus overall costs.

However, intense damage is becoming more frequent, partly due to the random nature of where atmospheric rivers make landfall. This means that, firstly, more flexible and cost-effective tools are needed, and secondly, the concept of resilience becomes more relevant in terms of property design — ground floors as storage space, water-resistant materials, electrical layout, plus sumps and pumps.

From tactics to strategy and resistance to resilience

Planners and hydrologists are turning to more collaborative solutions to try to address the difficulty in protecting individual businesses and homes from local surface and sub-surface water flows.

One such solution is catchment-wide management systems that consider water flows in natural basins holistically. Another is upland interventions that hold back and allow potentially damaging water to dissipate naturally rather than racing down restricted channels to swollen rivers and flood plains.

In the urban environment the sophisticated use of Sustainable Drainage Systems (SUDS) is also providing not only flood management benefits but also additional habitat — for plants, invertebrates and even migrating birds – plus community amenity advantages and clean water sources for drought periods.

And as future climate and weather become harder to predict, there is a gradual but crucial transition from resistance — building ever larger flood defences — to resilience — learning to live with and recover quickly from regular soakings.

Catchment-wide

The catchment flood management plan concept was introduced a decade ago to improve the understanding of flooding risks and formulate practical management policies. The aim is to design distributed flood defences as part of a broader model that accounts for all land and infrastructure areas within natural water flow systems, such as river catchments.

The Environment Agency uses the data in sustainable Catchment Flood Management Plans (CFMPs); England and Wales have 77 CFMPs for individual river catchments.

Upland intervention

With upland intervention, again, rather than hard barriers, the goal is to encourage the formation of marshes, sphagnum moss bogs, vegetation, woodlands — and perhaps even future beaver dams – to hold water back in a myriad “small places” as part of “source control”.

Both will run in combination with smaller site-based flood management solutions that can include SUDS (Sustainable Drainage Systems) which replicate natural processes.

Landscape profiling

SUDS works best where land can be profiled into shallow swales, reed beds, filter trenches, retention ponds and basins that clean and store storm water temporarily until it can infiltrate into the ground, or discharge slowly into the wider catchment. No artificial energy is needed beyond that used for maintenance and SUDS are intrinsically robust, attractive and environmentally sound.

“Leaky” SUDS solutions work best recharging local ground water and sub-surface aquifers slowly , creating a more consistent water table and base line supply for intervening drought periods. They also mean that rather than coping with, say, 1-30 year major flooding events, resilience can potentially be extended to 1-100 year events.

Rooftops of London and Manchester

Future cities will be much greener, with green roofs, strategic tree planting and specialist arboriculture helping to stop, store, clean-up and lessen the immediate impacts of flood water, absorb airborne pollution, offer shade when temperatures soar, generate important new habitat, plus biodiversity, and create attractive amenity spaces for residents. Green roofs are rapidly becoming storm water management’s first line of defence on high-density urban developments.

London is leading the way on SUDS — and green roofs — in the UK with its Urban Greening Factor initiative. Manchester is following suite with its City of Trees project and structural tree pits; the conurbation’s green infrastructure is an important part of the ambitious Northern Forest project. Across the rest of the UK, green roofs can now be seen from Southampton to Hull and Abelour in Scotland.

Legal requirement

By law, a SUDS option should be included in development planning applications. The revised National Planning Policy Framework (NPPF) requirements expect a biodiversity net gain to be made, which on dense urban/industrial sites can be via SUDS.

The Welsh Government has taken the lead. Since 7 January 2019, all new developments of more than one dwelling house, or where the construction area is 100m² or more, have needed SUDS for surface water.

Far too often, green infrastructure-driven SUDS is omitted on the grounds of adverse site conditions, costs and barriers to adoption.

Summary

Standard approaches to flood water management are changing as research and practical experience show that flood risks are increasing across north-western Europe. Parts of the UK coast are also becoming increasingly susceptible to “compound-flooding” as intense low-pressure areas create both heavy rainfall on land and powerful tidal sea surges that hammer coastlines and estuaries.

The traditional approach of building ever-stronger “hard defences” is being replaced by “resilience” where infrastructure is designed from the outset to recover quickly from frequent soakings.

Meanwhile, catchment-wide and upland intervention strategies are being developed to provide greater protection for both individual buildings and wider communities. The creative use of Sustainable Drainage Systems (SUDS) is also becoming a more widely used feature of “green” cities.