Learn more about the Flooding Risk Category in EarthScan™
After reading this article, you will learn:
- How to define coastal and riverine flooding
- How climate change will impact flooding
- How flooding events can impact built assets and business operations
- Which metrics inform the Flooding Risk Category
- About our flooding modeling approach
- The data sources we incorporate in the Flooding Risk Category
- How we increase the resolution of our flooding data
What is flooding?
Flooding is the most common and widespread hazard event, when water overflows onto land that is normally dry. Floodwater can inundate built assets and damage critical infrastructure, creating significant financial impacts for businesses. The probability of flooding happening in a given location is described using return periods, or reoccurrence levels. Return periods statistically estimate the probability of a flood event of a specific intensity (flood depth) happening over a specific period of time.
Coastal flooding
Coastal flooding occurs when local sea-level rises, and can happen anywhere along the coastline. Coastal flooding events are driven by a combination of physical processes, including changing local mean sea-level, wind-driven waves, storm surges and tidal processes. The impacts of coastal flooding can be amplified by local topography and changing wind conditions. In some coastal locations, where river basins flow out to sea, riverine discharge can also be a factor in coastal flooding.
Storm surges are the driver of the most extreme coastal flooding events. Storm surges are short-term increases in local sea-level. They are driven by low atmospheric pressure and strong winds associated with storms, mediated by local coastal topography and existing flood defenses. Storm surges create the greatest risk of coastal flooding when peak storm surges coincide with high tides. Tidal processes are related to the gravitational pull of the moon, and occur on regular and predictable timescales.
Coastal Flooding signal overview
The table below shows a summary of the coastal flooding signal.
Riverine flooding
Riverine flooding occurs when excessive precipitation or snowmelt causes a river, lake or stream to exceed its natural capacity and overflow into surrounding, typically dry land. Damage from riverine flooding can be widespread, as overflow impacts the river system downstream, causing smaller rivers to also overflow. Riverine flood events can happen quickly, or can build up gradually over time. Riverine flooding can cause significant damage and disruption to built assets and critical infrastructure. The impacts of riverine flooding events are amplified by existing factors, such as soil saturation and local topography.
Riverine Flooding signal overview
The table below shows a summary of the riverine flooding signal.
(10) Bankfull discharge describes events where the river channel fills up to its limit
without overflowing onto the surrounding floodplains (WMO).
Pluvial flooding
A pluvial flood occurs when an extreme rainfall event creates a flood, independent of an overflowing water body. Urban areas, with concrete/asphalt surfaces and poor drainage, are susceptible to this hazard.
Pluvial flooding is a complicated hazard to represent at a global scale, requiring projections of expected trends in extreme precipitation combined with a sophisticated high-resolution digital representation of the topography and detailed runoff modelling. Given the additional challenges required to generate these data, there is no pluvial flooding signal available on EarthScan at present.
However, we can provide crucial building blocks to this complex signal; for example, 1-day maximum precipitation data can illustrate how climate change is expected to affect heavy precipitation in future. These intense precipitation events are the primary driver for pluvial flooding.
Our 1-day maximum precipitation data are the next generation of the precipitation risk data that are currently available on EarthScan. Data deliveries for this hazard can be performed by your Customer Success Manager outside of the platform environment prior to its formal release on EarthScan.
How will climate change impact flooding?
Coastal flooding
Sea level rise due to climate change is likely to increase coastal flooding in many areas (1). When increases in mean sea-level combine with storm surges and high tides, extreme sea levels will occur more often. In some locations, this will mean more frequent coastal flooding events that reach higher water levels, last longer and impact further inland. However, untangling details on future coastal flooding exposure is complex; storm surges are tightly tied to the strength of the storms which drive them, and global climate models carry large uncertainties in the future trends of such extreme events.
Riverine flooding
As climate change intensifies, global hydrological models indicate that the extent of land areas affected by riverine floods will increase (2). Seasonal riverine flooding events in cold regions are shifting to earlier in the year due to earlier snowmelt and its contribution to streamflow.
Compound flooding events
Where high precipitation and extreme sea-levels happen at the same time, this can compound flooding in low-lying areas. For example, in November 2019, Venice suffered its worst flooding since 1966, whereby the tide rose to 194cm, submerging over 80% of the city (3).
How can flooding events impact built assets and business operations?
Examples of the impacts that could potentially arise as a result of a flooding event include:
- When flood water inundates built assets, it can damage building materials and structure. In extreme situations, flood events can lead to total structural failure.
- Flood water can also damage the buildings contents such as stock, furnishings, equipment and technology.
- Buildings may be forced to close during post-flood maintenance.
- The overflow of sewers can affect sanitation and drinking water can become contaminated.
- Access to sites can be disrupted by damage to the surrounding infrastructure.
- Clean-up and repair post-flood can be extremely costly and time consuming.
Flooding Metrics
The Flooding signal is based on two key physical metrics:
- Maximum water inundation height (cm) from coastal flooding
- Maximum water inundation height (cm) from riverine flooding
The Coastal flooding metric refers to the inundation of land connected to oceans, seas and estuaries. The coastal flooding signal shows the maximum water depth (in cm) of coastal flooding at a given geographic location based on undefended land. The signal is derived from relative sea level, local mean sea level, terrain elevation, historical vertical land movement and extreme sea levels. The greater the coastal flooding value (in cm), the greater the predicted water depth of a flooding event.
The Riverine flooding metric refers to the inundation of land connected to inland waters, such as lakes, reservoirs, wetlands, rivers and streams. The riverine flooding signal shows the maximum water depth (in cm) of riverine flooding at a given geographic location based on undefended land. The signal is derived from riverine discharge data and river and flood plain geometries. The greater the riverine flooding value (in cm), the greater the predicted water depth of a flooding event.
Modeling
Our modeling approach integrates:
- global flooding models - these allow for a consistent comparison of exposure across different locations.
- regional flood modeling - this increases the spatial resolution of our data.
- hydrological models - these account for different processes, boundary conditions and catchment properties across river networks.
- digital elevation model - terrain elevation (m) is derived from a Digital Surface Model. This model provides a measure of elevation against which we assess whether dry land could be flooded by the inundation of water.
Learn more about the EarthScan Flood Risk Methodology.
Data Sources
We incorporate multiple data sources in our Flooding Risk Category including coastal inundation, riverine inundation and global hydrography datasets, as well as high-resolution, validated regional and local data. We have recently added the latest extreme sea level datasets that also include the effects of tropical storms. Other flooding data sources include weather station observational data, topographical data and digital elevation maps.
Both our coastal and riverine data do not take into account special flood defences. However, we do take into account flood defences that are big enough to show up in the terrain model we use. These are permanent features that you could consider part of the landscape.
Learn more about EarthScan's data.
Resolution
To simulate realistic local and regional scale flooding patterns, we use a process called ‘downscaling’, whereby high resolution information is derived from a coarser data source. By incorporating topographical data from digital elevation maps, we are able to bring the spatial resolution down to asset-level. Furthermore, the use of hydrological models simulates the way water flows over the topography.
Coastlines are complex, so we use the highest resolution data possible in our coastal modeling processes which take into account geophysical features of coastlines, and how they change over time.
Our strategy is to continuously build our flood modeling capacity over time by developing our global models, increasing spatial resolution and working with expert data providers to add ever-increasing detail.