Physical impacts
Codes PH1 to PH3. The first link in the POESE chain, and the most direct: what the hazard physically took out of service.
Physical asset-level disruption is reflected in reduced road network availability, which can be measured by the number and total length of road links affected.
The metrics
| Code | Metric | Applies to |
|---|---|---|
| PH1 | Number of road links affected, for example by flooding | Short and long-term |
| PH2 | Total length of road links affected (km) | Short and long-term |
| PH3 | Total length of critical roads, such as A-Roads in the UK, affected at varying depths (km) | Short and long-term |
Why three, not one
Each answers a different question, and the third is doing the most work.
PH1 counts links. Useful for scale, but a link is an arbitrary unit: a 50 m residential stub and a 3 km dual carriageway both count as one.
PH2 measures length, which corrects for that.
PH3 narrows to critical roads and, crucially, records the depth at which they are affected. This is what connects the physical dimension to transport thresholds, since 50 mm of standing water and 500 mm are not the same event for a road, even though both flood it.
Depth matters because closure is not binary
A shallow flood slows traffic. A deep one stops it, and may damage the asset.
Recording affected length at varying depths preserves that gradient, so the operational consequences downstream can be graded rather than treated as a simple open or closed state.
Required input data
Physical performance metrics are derived from GIS software, for example QGIS, by overlaying the road network with flood maps.
| Layer | Source |
|---|---|
| Road network | Transport supply |
| Flood maps | CityCAT, via hazard models |
The 20-minute time series is what makes this tractable
CityCAT produces one flood-depth map every 20 minutes rather than a single maximum extent. Overlaying each of those against the network gives affected length as it changes through the event, rather than a single worst-case figure.
That time-varying physical picture is what the operational metrics then intersect with traveller movements.
Where this leads
Reduced network availability is the input to everything else. The operational consequences are covered in operational impacts.