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