Factor | Apparent increased likelihood % | Is the result statistically significant? |
Combined pipes are more likely than foul pipes to block | 146 | Yes |
Pipes that surcharge during a 5-year return period design storm | 44 | No |
Pipes that flood during a 5-year return period design storm | 839 | Yes |
Pipes that convey flows influenced by a backwater effect | 55 | No |
Proximity to flow confluence: pipes just upstream of a junction | 38 | No |
Pipes that fail to achieve peak flow velocity of 1 m/s during a 2-year return period event | 81 | Yes |
Pipes laid at a gradient less than 1 pipe diameter | 102 | Yes |
Pipes with large direct inputs | 167 | Yes |
Pipe size―the majority of blocked pipes were ≤225 mm | 521 | n/a |