pipe inclination in roman aqueducts

Pipe inclination – What goes up must come down.

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 aqueduct

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:

  1. 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.
  2. 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.

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Cloacina

Ancient Roman Sewer – Cloaca Maxima

Cloaca Maxima

This ancient roman sewer is protected by a goddess!

Tiber river

Tiber River

Strolling along the Tiber River in Rome, you could be forgiven for missing this wastewater wonder of the ancient world. 

While there were many strollers and joggers along the neatly paved banks of the ancient river, my husband and I were bee-lining for our next stop on our Rome itinerary: Cloaca Maxima. 

What is Cloaca Maxima?

Cloaca Maxima is a large diameter drain built in Ancient Roman times in the 6th century BC.  The drainage structure was named after Cloacina, an ancient Roman deity that represented purification. A shrine to this deity was said to be near the original stream, which is said to be the inspiration for the name. The literal translation in Latin is ‘Greatest Sewer’. 

The design and construction

It was initially a stream that was lined with stone, aimed at draining the local marshes and removing waste from the city. By the 3rd century BC, it was enclosed with a stone barrel (semi-circular) vault so that the swampy land around the Roman sewers could be filled in.  

Cloaca Maxima Catchment area in Ancient Augustan Rome

Figure 1 Catchment area map created in 1886 of Cloaca Maxima in Ancient Augustan Rome (source: https://www.wikiwand.com/en/Cloaca_Maxima#Media/File:Roma_Plan.jpg) 

The sewer was said to be dimensions that allowed inspection boats and boats fully laden with hay to pass through. 

This post goes into more detail about the construction and history of the famous sewer for those that are keen to learn more: https://www.wikiwand.com/en/Cloaca_Maxima 

Cloaca Maxima today

Today, the sewer is still in use, though nothing more than a trickle comes out of the once ‘Greatest Sewer’, most likely groundwater, as the city’s sewage has been long diverted to more modern pipe systems.

Cloaca Maxima today

If you are looking to replicate this enviable photo opportunity, you can find Cloaca Maxima still visible where it joins the Tiber River near the Ponte Rotto and Ponte Palatino bridges. 

Lessons from the Ancient Romans

Whilst the sewer is still technically in operation today, its much-diminished role is reportedly due to the misuse of the sewer (people putting all sorts of solid waste down there, even some political execution victims!) and backflow from the Tiber. Over time the once ‘greatest sewer’ has been re-routed to more the modern sewer system that sends sewage to treatment plants for treatment before discharge.

It’s certainly interesting to see and learn about the challenges of legacy wastewater infrastructure. Design challenges such as back flow design can be accounted for with careful planning. Social challenges with sewer misuse can be accounted for with meaningful community awareness campaigns. Either way, engineers today are working to deal with similar challenges today, just manifested with a modern twist. 

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