Examples of design sewer flooding criteria for standing floodwater, according to the recent standard (EN 752, 2017).
| No. . | Impact . | Example locations . | Return period (years) . | Probability of exceedance in any year (%) . |
|---|---|---|---|---|
| 1 | Very small | Roads or open spaces away from buildings | 1 | 100 |
| 2 | Low | Agricultural land (depending on land use, e.g. pasture, arable) | 2 | 50 |
| 3 | Low to medium | Open spaces used for public amenity | 3 | 30 |
| 4 | Medium | Roads or open spaces adjacent to buildings | 5 | 20 |
| 5 | Medium to high | Flooding in occupied buildings, excluding basements | 10 | 10 |
| 6 | High | Deep flooding in occupied basements or road underpasses | 30 | 3 |
| 7 | Very high | Critical infrastructure | 50 | 2 |
| No. . | Impact . | Example locations . | Return period (years) . | Probability of exceedance in any year (%) . |
|---|---|---|---|---|
| 1 | Very small | Roads or open spaces away from buildings | 1 | 100 |
| 2 | Low | Agricultural land (depending on land use, e.g. pasture, arable) | 2 | 50 |
| 3 | Low to medium | Open spaces used for public amenity | 3 | 30 |
| 4 | Medium | Roads or open spaces adjacent to buildings | 5 | 20 |
| 5 | Medium to high | Flooding in occupied buildings, excluding basements | 10 | 10 |
| 6 | High | Deep flooding in occupied basements or road underpasses | 30 | 3 |
| 7 | Very high | Critical infrastructure | 50 | 2 |
The return period should be increased (probabilities reduced) where the floodwater is moving faster. When undertaking rehabilitation of existing systems and where achieving the same design criteria for a new system would entail excessive cost, a lower value may be considered.