Abstract
The core principles of collecting good quality data from site for creating CCTV inspection reports including some explanations of how to avoid common mistakes.
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The PLR Naming Convention
Most global drainage inspection standards include a system for naming pipes based on either the upstream manhole ID or a combination of both the upstream and downstream manhole IDs. The Manual of Sewer Condition Classification (MSCC) defines the naming convention for WRc pipe and sewer inspection using the Pipe Length Reference (PLR) system at all times:
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Because the name of the pipe (the PLR) is defined by the upstream node ID and the PLR Suffix, this allows a manhole or other node to have up to 3 outgoing pipes. This may seem a strange concept to many CCTV inspectors, particularly at a domestic level because they have very likely never seen a manhole with 2 or more outgoing pipes. However, this is extremely common in highways drainage and is constructed this way by design because on highways (more than any other place) it is essential to get the water off the road at all costs and as quickly as possible during heavy rainfall events so as to avoid traffic accidents. Those inspecting highways drainage will commonly come across drainage designs like this as an example:
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Multiple outlets.
Now the manhole MH1 has 3 out going pipes, so this is where we start to adjust the PLR Suffix value. By default it is always X, and in cases like this, the X PLR should always be the ‘most significant’ outgoing pipe (i.e. most likely the deepest and the largest outgoing pipe).
Then, we use Y (maybe Z as well) to name the other outgoing pipes as MH1Y and MH1Z (remember. the PLR only considers the upstream node and the PLR Suffix).
There is no hard and fast prescribed way of deciding how to use which PLR Suffix for each pipe except that the X pipe should always be the primary outgoing pipe. After this, you can select to name the PLR Suffix for the other outgoing pipes by:
Rising up through the manhole from the bottom (will be explained why this makes sense in the next paragraph) as shown in the diagram above, so the Y PLR is the next highest one from the X, and the Z PLR is the highest one.
Going clockwise around the manhole from the X position (in the example above, the Y and Z PLRs would be swapped over).
Random selection. This is not recommended, because all that matters is that every pipe in the project has a unique PLR, and contractors should be consistent with how they manage and deliver their data across all projects and customers.
Why is the highways drainage system shown above been constructed this way?
We never really know how hard it is going to rain when it rains and highways are impervious so they create an awful lot of runoff water which we need to catch and get away from the highway surface as quickly as possible to protect driver’s lives.
Under a normal light to medium rainfall event, the water will come into the system and run away down the X pipe MH1X without any drama.
But, what if it is raining extremely hard or pipe MH1X is blocked with silt (common on highways drains). Ok, no drama, the manhole MH1 will fill up a bit with water and then the additional rainwater will start to flow down pipe MH1Y.
Now what if it is either raining harder than ever before in a ‘once in a hundred years rainfall event’, or both the MH1X and MH1Y pipes are blocked or the CP1X pipe is blocked. The highways engineers still want to get the water off the road, so now the chamber MH1 will fill up until the water starts to flow down the MH1Z pipe, which as you can see goes off in a completely different direction to a a different part of the drainage network because the chances of 2 different parts of the network failing at the same time are very slim, but not impossible.
This is great idea which includes built in resiliency into the drainage network and usually works well, except that during normal operational conditions, we are only alerted to a problem when all three pipes are blocked or failed, so the remedial works will be much more significant than if there is only 1 outgoing pipe.
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Good Site Drawings
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Understanding STC25 Manhole References
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