Guidelines for stormwater runoff modelling in the Auckland region

Author:

Auckland Regional Council, Beca Carter Hollings and Ferner Ltd

Source:

Auckland Council, Auckland Regional Council

Publication date:

1999

Topics:

Environment

TP108 updated August 2026, for consultation.

Draft TP108 and supporting information available on the Auckland Design Manual website.

Submissions close, 7 September 2026.

Introduction

These guidelines present a recommended method for the application of the U.S. Soil Conservation Service¹rainfall-runoff model to catchments in the Auckland Region. They are based largely on Technical Release No. 55 (TR55) prepared by the U.S. Soil Conservation Service (SCS, 1986).

The Soil Conservation Service (SCS) model has been selected for stormwater management design in the Auckland Region on the basis of an evaluation against gauged catchments (BCHF, 1999a, 1999b, 1999c). Those reports provide the background to the selection, calibration and validation of the method. Streamflow data from the gauged catchments have been used to select input parameters for the model (soil classifications, times of concentration, etc.) and to validate the model.

The model is recommended for use in stormwater management design in the Auckland Region. It has been designed as a standard tool that will provide consistent results from different users. It is suitable for:

assessing the effects of landuse change,
modelling both frequent and extreme events,
applying to distributed (a network of sub-catchments) or lumped catchments
and simulating natural systems as well as engineered systems (such as pipe networks).

The model can be applied using a number of available software packages to predict runoff volumes, flow rates, and the timing of peak flows. Peak flow rates can also be estimated using an alternative graphical method.

1.1 Model Overview

Key features of the stormwater runoff model are illustrated in Figure 1.1 and described below:

Design 24 hour rainfall depths are provided in the form of rainfall maps covering the Auckland Region.
A standard 24 hour temporal rainfall pattern, having peak rainfall intensity at midduration.
Shorter duration rainfall bursts with a range of durations from 10 minutes to 24 hours are nested within the 24 hour temporal pattern.
Runoff depth is calculated using SCS rainfall-runoff curves, with curve numbers determined from the SCS guidelines according to classifications assigned to Auckland soil types.
Runoff hydrograph is calculated using the standard SCS synthetic unit hydrograph. Time of concentration is estimated using an empirical lag equation derived from a regression analysis of data from the Auckland Region.
Separate analysis of pervious and impervious components of urban catchments.
Effects of development on runoff depth are predicted using the standard SCS guidelines. Effects on catchment time response are allowed for using a channelisation factor and runoff parameter in the time of concentration relationship. ...

1. now known as the Natural Resources Conservation Service, part of the U.S. Department of Agriculture.

Auckland Regional Council, Beca Carter Hollings and Ferner Ltd, April 1999