T1 and T2 location classes

These urban location classes are fairly straightforward.  T1 is for ordinary suburban areas and they are allowed to include small patches of higher density (up to 10%).  It also includes other land uses with similar population density.  T2 is for multi-storey areas or large commercial centres where there are normally many people present.  I don’t think there is much difficulty with any of that.

One point worth considering is the borderline between R2 and T1.  As discussed in the previous post on R2 areas, the number of people affected by a pipeline rupture depends not only on population density but also on the measurement length and hence pipeline size.  So an area of 2 ha blocks around a very large pipeline might present a risk to a larger number of people than a small pipeline in a T1 area, even though strictly speaking it would fit the R2 definition (blocks of 1 – 5 ha).  For this reason in some low-density urban fringe areas it might be worth extending the T1 area beyond the extent required by a black-letter interpretation of the Standard.

Another grey area concerns places that can accumulate large crowds but only infrequently or for short periods.  Playing fields are a classic example.  I don’t have any definitive suggestions for these cases.  It can be worth considering whether the activities that might cause a pipeline failure could occur at the same time that the people are present – eg. construction work on weekdays, local footie game on the weekend, no overlap between threat and crowd.

A variation on this temporary crowd issue is busy roads in urban areas.  If traffic congestion means that roads become virtual parking lots at peak hour then there is the potential to have a very large number of people concentrated along the road.  Think about a three lane road with a car every 7 m.  Even with only single occupants that’s a couple of hundred people who might be within the measurement length of a DN 300 pipeline.

As I’ve said before, I think actual location classification is less important than recognition of the number of people who might be affected.  This is particularly important if a threat needs a full risk evaluation.  Risk evaluation provides the tool to deal with the intermittent crowds because the likelihood of the failure occurring at the same time as the crowd can be included in the frequency estimate – failure with a couple of people present might have frequency F, failure next to the traffic jam might have frequency 0.25 F, failure with the crowd at the footie field might have frequency only 0.01 F (examples only, real cases of course depending on the exact circumstances).

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