New Zealand Rainfall and Runoff

Preamble

Modern analysis for flood protection and stormwater system design frequently involves the use of hydrological and hydraulic models. Hydrological models are used to derive runoff time series in response to input design rainfall data, and these runoff time series are often used in hydraulic models for determination of flood-related effects.

A key input parameter is design rainfall, and this is generally derived from frequency analysis of appropriate rainfall records. The results of such an analysis are generally given as depth-duration-frequency data (e.g. 10mm in 30 minutes is a 5-year Average Recurrence Interval event).

These depth-duration-frequency data are typically available throughout New Zealand by make use of NIWA’s HIRDS, although in some places more locally-specific data may be more applicable.

In order to run design rainfall through a hydrological model to produce a runoff time series, the distribution of the specific rainfall depth over the duration is required, and is not given by the typical analysis approaches. This is also not given by HIRDS. Consequently there is a proliferation of methods that have been developed to do just this. The distribution of rainfall with time is termed a “rainfall hyetograph”, and some regions exhibit “typical” rainfall hyetographs during flood events (especially if a “typical” flood event is generated by a frequently occurring weather pattern). However this is not always the case. Furthermore, the hyetograph applicable to one region is unlikely to necessarily be applicable to another.

There is considerable uncertainty in application of an appropriate rainfall-runoff method to various regions within New Zealand. In some bigger centres, area-specific methods have been developed that are specified for use within these areas. In many smaller centres, there are no locally-specific guidelines and as a result, the methods applied in bigger centres are frequently used – sometimes erroneously.

With the methods that have been developed for certain larger regions in New Zealand, there is little consistency in results across these methods even with the same input data. This presents significant uncertainty, particularly for those regions where no locally specific data exist.

Industry Needs

In response to member requests, the Rivers Group and the Water New Zealand Modelling Specialist Interest Group have begun consultation with the industry on rainfall-runoff analyses. There has been widespread support for the initiative of collation of appropriate methods for use in New Zealand, with guidance on where and when each may be applicable. This would result in significant savings to the industry in

  • Consent applications where analyses are undertaken using approved methods, saving the need for extensive justification of method employed and for peer review;
  • No need for development of separate guidelines across all regions – one set of guidelines could apply nationally;
  • Consistency in results across different methods would lend greater credibility to analyses, resulting in higher confidence in outputs.

International

The initiative of development of a set of rainfall-runoff guidelines for New Zealand has been influenced by the knowledge that the Australian Rainfall and Runoff Guide (ARR), which is frequently referred to in NZ, is currently undergoing an updating process with extensive research and development being undertaken. As this ARR document is in wide use around NZ, even without locally specific data being available, it is likely that a NZ-specific document of a similar nature would be even more widely used.

The next version of ARR will comprise 39 Chapters over a total of 9 books. Currently NZ has no national guideline on this topic.

The Vision

There is a desire within the NZ technical community to see the development of a comprehensive guideline, similar to the updated ARR, for use nationally around New Zealand. This would be a significant undertaking, but would ultimately deliver value to practitioners nationwide.

As a first start, the proposed guideline has been referred to as the New Zealand Rainfall and Runoff Guide (NZRR).

The Process

It is recognised that from the current position of there being no nationally applicable or recognised rainfall-runoff guideline, getting to the position of having a definitive document for the country is a large task that needs to be split into smaller sub-tasks. It seems apparent that there is broad technical community support for the proposed initiative of development of NZRR.

The process whereby this may be achieved should begin with an assessment and a collation of all that is currently known or frequently applied in NZ. Some accuracy testing of various methods will be required to determine the most applicable methods for use within NZ, based on actual data rather than on overseas performance. Reference to other documents and guidelines will ensure minimal re-work (e.g. use ARR where applicable, together with other international guidelines and best practice).

In order to guide the process the idea of setting up a Steering Group Committee (SGC) has been put forward. The role of this committee would be to guide the process towards the ultimate objective of a nationally applicable rainfall and runoff guideline. Several organisations and individuals have expressed interest in being involved in some sort of a steering group committee, with representation from the following seen as desirable:

  • Regional Councils
  • District or City Councils
  • Ministry for the Environment
  • Engineering New Zealand (possibly the Rivers Group)
  • WaterNZ (possibly the modelling special interest group)
  • Academic institutions (University of Auckland)
  • Representative(s) from the ARR steering committee
  • CRI’s (NIWA)

Currently the initiative is being taken by volunteer-based committees (Rivers Group and Modelling Special Interest Group) to establish some momentum. Clearly the overall task is larger than what could be expected of such volunteer-based organisations, and some external funding will be required.

At this stage no formal Steering Group Committee has been established, although widespread interest has been expressed in this.

Recent Consultation

During the recent SW conference in Wellington a workshop was held on this topic to help gauge the industry needs.  A summary of that workshop will be available on this website soon.