Sewers, drainage and stormwater

What is a rising main, and why does it fail differently?

• It runs full and pressurised. There is no air space and no free surface inside it.
• No manholes. Nothing along its length can be opened, looked into or rodded.
• It bursts, it does not block. The failure is sudden and usually at a joint or a weak point.
• It is the classic sulphide generator. Sealed, full and slow is close to ideal for going septic.

Why it exists at all

Gravity does most of the work in a sewer network, but it only works downhill. Where the ground rises, or where a low lying area has to discharge into a higher trunk sewer, gravity runs out and something has to lift the flow.

That job goes to a pumping station, and the pipe running from its pumps up to the point where gravity can take over again is the rising main. It is the only part of a foul network that is normally under positive pressure.

The consequence is that the pipe is designed to entirely different rules. It is sized for velocity rather than for a part full flow profile, it is sealed rather than vented, and it has no access chambers because there is no gravity flow to inspect or rod.

Why the failure mode is different, and worse

A gravity sewer usually gives warning before it fails. Debris accumulates, flow slows, levels rise upstream, and a chamber surcharges before anything reaches the surface. There is a window in which the problem is visible and still cheap.

A rising main gives no such warning. It is either intact and pumping, or it has burst. When it bursts it discharges pressurised foul water directly into the ground, which is why rising main failures turn into pollution incidents quickly and are disproportionately represented in serious incident statistics.

The other difficulty is locating it. A burst on a buried, pressurised main can surface a long way from the actual defect, or not surface at all in permeable ground. Nothing along the route can be opened to narrow the search.

What can actually be monitored

Because the pipe itself is inaccessible, the practical monitoring points are the ends and the behaviour of the pumps that feed it.

Pressure at the pumping station is the most direct signal. A main that has burst loses back pressure, so the pumps see an easier duty and the discharge pressure falls. A main that is silting or partially obstructed does the opposite, and pressure climbs over weeks.

Pump run time is the corroborating signal. If the station is running longer to clear the same wet well volume, something downstream has changed. If it clears the well unusually fast, that is consistent with flow escaping somewhere it should not.

Reading the two together separates the cases. Level in the wet well tells you what is arriving, pressure and run time tell you what is leaving and how hard it was to move.