Grey infrastructure
Grey infrastructure measures are engineered physical interventions designed either to reduce hazard intensity or to protect infrastructure from its impacts.
That "either" is the defining feature of this category, and it determines how each measure is modelled.
What they look like
For flood adaptation, these may include:
- Drainage upgrades
- Storage facilities
- Flood barriers
- Other engineered flood-management infrastructure
The two modelling routes
Grey adaptation impact is modelled in two ways, depending on the mechanism.
| Mechanism | Represented by | Acts at |
|---|---|---|
| Measures that alter the hazard itself | Modifying the hazard-model inputs | The flood model |
| Measures that protect assets without changing the hazard | Changing the threshold at which a network link becomes disrupted | Transport thresholds |
Why the distinction is real, not bookkeeping
A storage facility holds water back, so less of it arrives. The flood itself is smaller, and everywhere downstream benefits. That is a hazard change.
A raised road does nothing about the rain. The same flood arrives at the same depth; that particular link simply keeps working through it. That is a threshold change, and the benefit is local to the protected asset.
Modelling the second as though it were the first would credit a measure with a catchment-wide benefit it does not deliver.
How protection is implemented
For asset-protection measures, grey infrastructure is implemented during the coupling between flood-model outputs and the transport model.
The procedure works as follows:
- Flood depths are assigned to road links.
- Links requiring speed reduction or closure are identified, forming an affected-road list.
- Road links protected by grey infrastructure are removed from that list, representing their enhanced resilience to flooding.
This is a clean intervention point
Because the protection is applied to the affected-road list rather than to the flood model or the transport model, alternative protection schemes can be tested without re-running the hydraulic simulation.
That makes it comparatively cheap to explore which links are worth protecting.
Required input data
| Input | Why it is needed |
|---|---|
| Location of road links protected by grey infrastructure | Identifies which links leave the affected-road list |
| Storage, drainage or protection capacity | Quantifies how much the measure can handle |
| Relevant engineering and hydraulic parameters | Describes how the structure behaves under load |
Capacity implies a limit
The second and third inputs are what prevent a protected link being treated as invulnerable. A barrier rated to a given depth is overtopped beyond it, and a drainage upgrade sized for one return period will not hold at a rarer one.
Recording capacity explicitly is what allows protection to fail realistically under severe events rather than simply switching off.
Generated output data
For grey infrastructure, the coupling procedure generates a modified set of flood-affected road links.
Links protected by grey infrastructure are:
- Excluded from flood-induced speed reductions or closures
- Kept operational within the transport simulation
The transport model then runs against this modified network state, and the difference against the baseline is what quantifies the measure's benefit.
Where the benefit shows up
Because grey protection keeps links open, its effect appears first in the physical metrics, PH1 to PH3, as reduced affected length, and then propagates through delay, cancellation and cost exactly as the unprotected case would have done.