Hydrology - Proven Ways to Cheat

Contrary to the title, this presentation is not intended to provide a blueprint as to how to cheat. This paper is aimed at all stormwater professionals, on both sides of the consent application process.

Hydrologic applications in practice are still based on many assumptions and simplifications. Some of them are necessary and defendable, others are misleading and can be used to bias the model output in favour of the modeller.

This paper provides a collection of examples and errors that often occur in stormwater resource consent applications. Intentional or not, those errors can result in under or over sizing of stormwater infrastructure. In the interest, therefore of the receiving environment and the efficient use of (financial) resources, these errors should be avoided. This paper can only cover a small portion of the wide field of hydrology and concentrates on the use and abuse of the ARC guideline TP 108. The examples will show:

1. The significant influence of the catchment lag time on simulation results and how this is calculated correctly.
2. How changes to the proposed simulation approaches affect the results of runoff peak flow and volume calculations.