Quantitative potentials for rainwater handling using the “Three Points Approach” (3PA)

Rainwater in cities is both a resource for e.g. recreational and amenity purposes and a potential problem due to e.g. pluvial flooding. This study provides a new tool to communicate the potentials for alternative rainwater handling taking into account the properties of rainfall making it both a problem and a resource. The quantitative potentials of alternative rainwater handling are calculated in relation to the total precipitation volume, the total supplied potable water volume and the total volume of wastewater treated. Two different cases are investigated: one where rainwater is infiltrated and one where it is harvested and used for toilet flushing. The potential is calculated for the two largest municipalities of Denmark: Copenhagen and Aarhus. The Three Points Approach is used to distinguish between different rain domains, and to subdivide the quantitative potentials accordingly. This analysis shows that designing alternative rainwater handling systems for larger return periods than 5 to 10 years result in a very marginal increase in rainwater volume handled by these systems compared to systems designed for less severe events.