TP108 – what is it to those in stormwater management? How was it developed? And what are its advantages and disadvantages?

Technical Publication 108 (TP108) was published by Auckland Regional Council in 1999 and has been used as the primary flow estimation tool in the Auckland region since then. At the 2017 Water New Zealand Stormwater Conference, I presented a technical paper on TP108, including commentary on its use and misuse over the years. Awarded Paper of the Year at the conference, my aim was to stimulate further discussion and action towards robust and standardised urban hydrological analysis in New Zealand, including the wider (and appropriate) use of the TP108 approach. I hope to continue the discussion on Ignite your Thinking.

The full paper sets out some underlying principles on which the US SCS / NRCS curve number method was adapted and validated to suit the particular requirements that Auckland Regional Council had defined, resulting in TP108. It discusses some examples where the method has been used inappropriately, or in new areas, resulting in poor estimation of runoff characteristics. From there, the paper provides some guidance on factors that should be addressed, particularly in relation to validation, when transferring the method to other parts of New Zealand. There is also commentary on appropriate contexts in which to use the method, and where other tools might be more appropriate.

Below are a few key points from of the full paper which you can read here. Feel free to share your thoughts in the comments area at the bottom of the page.

Overview of TP108

  • TP108 is based on the NRCS (Natural Resources Conservation Service of the US Department of Agriculture) curve number method which evolved in the US over a number of years.  It was one of several methods that underwent rigorous testing against Auckland Regional Council’s performance criteria, and it came out on top.
  • The method should not be seen as a “one size fits all” approach. While it is suitable for the purpose for which it was originally developed, there are situations where it should not be used, or should only be adapted and used with caution.
  • Being a moderately simple method to apply, TP108 and its relatives will generally give a consistency of outcome no matter who uses it. However, this does depend on full adherence to the methodology and not mixing inputs or method components from different sources.
  • TP108 is more clumsy that the rational formula, with little benefit when calculations are dominated by impervious surfaces and small catchments (such as highways or small subdivision drainage).
  • TP108 or related methods based on NRCS are being used more widely, but are not necessarily reliable or validated for the particular context or regions where they are being used.
  • The ideal would be a nationwide consistency of approach to rainfall runoff estimation that gave the industry and the public greater confidence in reliability
  • Consideration should be given to methods that allow for continuous simulation, to better reflect the importance of environmental effects and everyday flows, rather than just for flood estimation.


Thank you to Mark Pennington (T&T) and Nick Vigar (Auckland Council) for providing helpful comments that informed aspects of this paper. And Mike Law (Beca) for his review.

About the Author
Graham Levy

Senior Technical Director - Water Resources

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