Pipe inclination - What goes up must come down?
What is pipe inclination?
Most of the world’s sewers and storm pipes operated as gravity pipe network. What that means is that the water in the pipes drain to the outlet point under gravity. For this to happen, the entire pipe network needs to be laid on a slight incline in the direction of the downstream outlet.
This is the most efficient method of transporting water from one point to another and has been a method used since ancient times. The Roman aqueducts brought water from the various springs in the Anio valley and its uplands to Rome, over 92000 metres or 301837.27 ft away, entirely using this method; slowly declining the aqueduct over the length of it so that water would fall in the direction of Rome.
Roman aqueducts crossing a valley
That is why when aqueducts needed to cross valleys, the Romans made every effort to keep the level of the structure constant, as dropping the level to match the valley would eventually require some sort of pumping action.
Why is inclination in pipes so important?
Pipes have inclination for the same reason as the Roman aqueducts do, water transport efficiency. The degree of inclination in pipes is a subject of much debate and is an ongoing engineering design and maintenance challenge. The more inclined the pipe, the faster the water will flow, the slighter the incline, the slower the water will flow. An interesting thing happens when water moves fast; it starts to pick up debris in the flow. Conversely, when the same flow starts to slow down, the debris that was once swept up in the flow begins to settle out.
For this reason, design engineers try to maximise inclination of the pipe network to encourage ‘self-cleaning’ velocities in the pipe network as a low-cost way to prevent blockages in the network. Anecdotally, a reasonable self-cleaning inclination would be between 1-2% grade depending on the diameter. A grade of 2.00% is the same as a ratio of 1 in 50. This means over a distance of 50 m the trench or pipe will slope down or fall 1 m from the horizontal.
However, in a flat catchment, there may not be a big enough difference in height between the top of the catchment and the bottom. When this happens, there is an increased risk of blockages and flooding because the water is not able to move itself (or debris) to the outlet as efficiently.
How can I test for inclination?
Testing for inclination can be done a couple of ways, but the most common are:
- Using pipe lengths and elevations between access points – Measuring the reduced level of the pipe from an upstream node and subtracting it from the reduced level of the downstream node would give you the elevation drop. Dividing that elevation drop by the length of the pipe would give you the indicative inclination of that pipe. The reason this method is only indicative is that there may be other dips and changes in direction of the pipe between the two access points that would affect the exact calculation.
- Using dynamic inclination measurements during CCTV inspections – Most standard CCTV camera crawler systems come with inclinometers as part of the equipment and the inclination is logged as part of the camera telemetry data. For water authorities to access this information, they will need to request and have the ability to decode this telemetry data. Or asking your CCTV contractor to display inclination on the on-screen display (OSD) as part of the inspection will also provide you with the information you need.
Conclusion
For gravity pipe networks, the inclination is very important for the efficiency of the network performance. Introducing additional debris, such as wet wipes or any other items that should not be flushed, into the sewer or storm pipe network only adds to the challenges that gravity networks have, which has the unfortunate impact of increases operational and maintenance costs for water authorities.
Effective public awareness campaigns to reduce sewer misuse such as #binthewipe and #weirdthingsinpipes can help to raise awareness about sewer misuse in the community to ensure we keep our pipes running as efficiently as possible.
