TP108 – Where to from Here?

Technical Publication 108 (TP108) was published by Auckland Regional Council (ARC) in 1999 and has been used as the primary flow estimation tool in the Auckland region since then. For the past ten years there has been consideration by ARC and then Auckland Council (AC) of updating the guideline, but a new version is yet to be adopted.

Similarly, there have been moves within the stormwater industry, supported and promoted by Water NZ, to develop a national standard similar to Australian Rainfall and Runoff to establish consistent and reliable flow estimation methods across NZ. In the absence of that guidance, many different flow estimation methods are in use, with variable reliability and suitability.

In the absence of national guidance, TP108 (or the NRCS (Natural Resources Conservation Service of the US Department of Agriculture) method on which it is based) has been adopted and adapted in part or in full for urban runoff estimation in other parts of New Zealand, and is widely used, but also in some respects misused. This misuse arises in part from a lack of understanding of the basis on which TP108 was adapted to the Auckland Region, and sometimes a lack of robust (or in some cases any) validation of the method to local conditions in new areas.

This paper sets out some underlying principles on which the application of the NRCS method to Auckland were based, and how it was adapted and validated to suit the particular requirements that ARC had defined. 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.