Green infrastructure
Green infrastructure measures are nature-based interventions that use vegetation, soils and natural processes to manage climate-related hazards.
They are the only category that changes the hazard at source, before it reaches the transport network at all.
What they look like
The measures differ substantially depending on which hazard is being adapted to, because they work through different physical mechanisms.
For flood adaptation
Examples include:
- Rain gardens
- Green roofs
- Other Sustainable Drainage Systems (SuDS)
These reduce or delay surface runoff, which is what drives pluvial flooding.
Reduce or delay, and delay matters
Not all of the benefit is about absorbing water. Delaying runoff spreads the peak, which can be enough to keep a drainage system within capacity even when the total volume is unchanged.
Because CityCAT produces a flood map every 20 minutes rather than a single maximum extent, a measure that shifts the timing of a flood peak shows up in the results rather than being averaged away.
For heat adaptation
The focus lies more on:
- Shade-producing plants, such as trees or large-scale vegetation
- Vegetation supporting evapotranspiration, for example green roofs or walls
Two separate cooling mechanisms
Shade and evapotranspiration are not the same thing, and the IMP resolves them with different models.
Shade is geometric, and is what HiREx computes at metre scale from building and tree crown shadows. Evapotranspiration is a surface-energy process, handled in UrbClim's land-surface scheme through the low vegetation surface class.
A tree does both. A green wall mostly does the second.
How they are modelled
Green infrastructure is implemented directly within the flood model, where changes in surface characteristics, infiltration and storage affect the simulated flood hazard.
More broadly, green adaptation impact is modelled by altering vegetation and the corresponding infiltration and shading inputs across three models:
| Model | Receives |
|---|---|
| Hazard models | Altered vegetation, infiltration and shading |
| Behavioural models | Altered conditions affecting travel decisions |
| Transport models | The downstream consequences |
Required input data
| Input | Why it is needed |
|---|---|
| Location and spatial extent of the intervention | Where the measure sits, and how much ground it covers |
| Type of green infrastructure | Determines which mechanism applies |
| Surface and soil characteristics | Controls how water behaves at the surface |
| Infiltration and storage parameters | Quantifies how much water is absorbed and held |
| Baseline land-use and surface characteristics | The starting state the measure is a change from |
The baseline is an input, not an afterthought
The last row matters more than it looks. A rain garden replacing paving delivers a very different benefit from one replacing lawn.
The measure's effect is always the difference between the adapted surface and what was there before, so the baseline has to be specified with the same care as the intervention.
Generated output data
Because green infrastructure acts on the hazard, its output is a modified flood-hazard result from the flood model:
| Output | Description |
|---|---|
| Flood depth | How deep the water gets |
| Flood extent | How far it spreads |
| Flood duration | How long it persists |
| Spatial and temporal changes | How all of the above differ relative to the baseline scenario |
That modified hazard then flows through the rest of the framework normally, producing changed physical, operational, environmental, social and economic results via performance metrics.