TP108 Spreadsheet

What is TP108 New Zealand Rainfall?

TP108 refers to Auckland Council’s Technical Publication 108, a foundational document that provides rainfall depth–duration–frequency (DDF) data for use in stormwater and flood‑management design across much of New Zealand. It supplies statistically modelled rainfall intensities for a range of storm durations and return periods, allowing engineers to size pipes, culverts, detention basins, and overland flow paths with a consistent, evidence‑based approach.

The rainfall data in TP108 is derived from long‑term historical records and is presented in a way that supports both simple and advanced hydrological modelling. Designers use it to determine how much rain is expected during a 2‑year, 10‑year, 50‑year, or 100‑year storm, and how quickly that rain is likely to fall. This ensures stormwater systems are built to withstand realistic peak flows and protect communities from flooding.

Key features of TP108 rainfall data include:

• Depth‑duration‑frequency tables for multiple storm return periods
• Region‑specific calibration for New Zealand conditions
• Compatibility with common hydrological models (e.g., Rational Method, SCS, HEC‑HMS)
• Guidance for climate‑change allowances and sensitivity testing

Common TP108 New Zealand Rainfall Design Methods

Stormwater designers apply TP108 rainfall data through several established methods, each suited to different project scales and levels of complexity.

• Rational Method — A widely used approach for small catchments. Designers combine TP108 rainfall intensity with catchment area and runoff coefficients to estimate peak flow. It is simple, fast, and ideal for pipes, channels, and small urban drainage networks.
• Hydrograph‑Based Modelling — Tools such as HEC‑HMS, MIKE, or DRAINS use TP108 rainfall hyetographs to simulate how runoff accumulates and flows over time. This method is preferred for detention basins, wetlands, and larger catchments where timing and storage effects matter.
• SCS Curve Number Method — Often used for rural or mixed‑use catchments. TP108 rainfall depths feed into the SCS equations to generate runoff volumes and peak flows, especially where soil type and land cover strongly influence infiltration.
• Design Storm Hyetographs — TP108 provides the rainfall depths; engineers then apply standard hyetograph shapes (e.g., Auckland Council’s recommended temporal patterns) to model how rainfall intensity varies during the storm.
• Climate‑Adjusted Design — Many councils require designers to apply climate‑change multipliers to TP108 rainfall depths. This ensures infrastructure remains resilient under future rainfall intensification scenarios.

Each method requires careful selection of storm duration, return period, catchment characteristics, and safety factors to ensure the final design meets regulatory and performance requirements.

Why Use a TP108 New Zealand Rainfall Design Spreadsheet?

A TP108 design spreadsheet streamlines the process of converting rainfall data into usable design inputs. Because TP108 includes multiple tables, durations, and return periods, a spreadsheet helps avoid errors and speeds up repetitive calculations.

Benefits include:

• Automated lookup of rainfall depths and intensities for any duration or return period
• Consistent application of formulas for peak flow, hydrographs, and storage sizing
• Built‑in climate‑change adjustments to test future scenarios
• Scenario comparison tools for different catchment assumptions or design storms
• Clear documentation that supports council submissions and peer review

For consultants, councils, and developers, a well‑structured spreadsheet becomes a reliable tool that improves accuracy, reduces design time, and ensures compliance with New Zealand stormwater standards.

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