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Quantifying the first flush phenomenon

D. Brett Martinson and Terry Thomas

12th International Rainwater Catchment Systems Conference, New Delhi, India, 2005

Removing the first millimetres before rainwater is diverted to a store is a much-used method for ensuring high water quality in rainwater harvesting systems. Such first-flush systems rely on the initial rain to partly wash the roof before runoff water is allowed in the store. While there is almost universal acceptance that this is beneficial, there is no agreement on just how much water should be diverted or indeed whether the diversion should be based on volume, depth or rainfall intensity.

This paper presents an analysis of first-flush based on theory used for calculating dirt loads in urban drainage. The first flush-phenomenon is found to match that theory, which results in the rule-of-thumb “contamination is halved for each mm of rainfall flushed away”.


Full paper available from the conference site

Full paper as a final draft submitted for publication (PDF Document)