Flood Risk Management in a Changing Climate

• Rising global temperatures increase moisture in the air, leading to more intense precipitation and exacerbated flooding due to urban expansion and inadequate sewer systems.

• In colder climates, rapid warming periods lead to quick snowmelts and potential rainstorms, causing rivers to swell quickly and increase the risk of major floods.

• Advanced flood damage and loss models use past data, climate projections, and infrastructure changes to help asset managers understand potential losses and prioritize adaptations.

As the climate changes and average global temperatures rise, the warmer air holds more moisture, which can result in more intense precipitation when it rains.

This heightened rainfall intensity can exacerbate both riverine and surface flooding, although these flooding events are also influenced by various other factors.

A huge impact on assets suffering inundation is land use patterns.

As cities expand, urbanisation paves over fields and forests which used to absorb a lot of rainfall and snowmelt. Instead, the water now immediately runs off into low-lying areas. There, it pools for surface flooding or flows into waterways leading to river flooding.

Sewer systems are not often built for the volume of increased runoff, from both increased rainfall intensity and land use changes.

The drainage backs up, streets and sidewalks flood, and water with dirt flows into buildings. Leaves and litter wash toward the drains and sewers, blocking them for even worse flooding.

In cold climates, thaws have traditionally posed risks of surface and river flooding. Additionally, a warmer climate is leading to reduced snow cover.

Where a rapid warming period leads to a quick snowmelt, possibly in tandem with a rainstorm, then rivers can swiftly swell producing major floods.

In the meantime, a warming climate means that rivers are frozen for less time and typically with less ice. When the ice melts, it leads to milder flooding, both from the thawing ice itself and from ice jams that obstruct the rivers' flow.

For surface flooding and river flooding, asset managers and loss modelling need to connect all these influences from climate change, ageing infrastructure, and land use.

This is especially important when the demand due to increased rainfall or worsened flooding highlights levees, dikes, and flood walls. Build something big to separate people and water and suddenly flooding is assumed to no longer be a concern - yet, two problems remain.

First, any structure can fail.

Floods can overtop, undermine, seep through, or lead to the collapse of flood defences. There might be little warning. In addition to damage from contacting flood water and its contaminants, assets are harmed by the water’s speed and from the debris of the flood defence.

Second, people think that they are protected by the flood defence structure, so they build much more behind the wall.

Retrofitting and adaptation to river flooding would be assumed to be a pointless cost, since the assumption is that flooding cannot occur.

Loss modelling results from defence failure are dismissed. Risk increases over the long-term as property managers shift into a mode with few floods, forgetting that a bigger and rarer flood will happen at some point.

Flood damage and loss models can have a high level of sophistication, which can account for all these interconnected factors.

Applying a combination of past flood, future climate, and infrastructure data to the models offers asset managers a deep understanding of their possible losses from floods and where to prioritise adaptation and retrofitting measures.

People’s behavioural changes can even be included, if useful, or measures can be suggested which minimise the impact of individuals’ flood-related choices.

High resolution infrastructure maps are available for many countries, which are overlaid on flood scenarios to show which properties will be flooded and how. Flood depth, flood velocity, and flood duration can be modelled at the resolution of specific properties, such as a house or a bridge, for different flood and climate circumstances.

Then, scenarios can be added to examine contaminants, waves, salinity, and other aspects specific to a particular flood. Highly localised decisions can augment or decrease damage.

Rescuers and sightseers driving through flood water create wakes and waves which can exacerbate damage. If a flood wall has to be intentionally breached in order to drain an area, then the forces from the water draining can bring down weakened structures or wash out possessions.

Databases exist of property types, indicating the assets which might be damaged or lost as well as the types of damage and losses.

Most houses need to be dried out for weeks or months after a flood, meaning alternative accommodation and significant stress for the inhabitants. The likelihood is high of needing to rip out and then reinstate the finishings of any storey flooded.

Electrics might need to be replaced and plumbing thoroughly cleaned. If doors or window frames are wood without any water resistance measures, then they will likely be replaced, as will most insulation. All these costs are estimable, leading to profiles of property types which can then be aggregated according to the maps of how many properties of each type are in a particular location.

Flood scenarios for each location can be built on long-term datasets of floods indicating likely depths, velocities, durations, waves, contaminants, debris, and other flood properties.

Then, climate change projections are layered on other flood trends from changes to land use and infrastructure (such as drainage), to show flood damage expectations for the future. Decisions about and locations to prioritise for retrofitting, adaptation, and asset investment or divestment are readily identified. This could be replacing all doors, raising all electrics, changing finishes to being water-resistant and easily disinfected, or other measures.

Using data and models offers cost-benefit analyses, for different timeframes and different trends in floods.