In Australia, guidelines have been developed for the validation of engineered treatment processes. However, these guidelines do not prescriptively address validation of passive water sensitive urban design (WSUD) treatment systems. The first step in developing validation guidelines for WSUD systems is pre-validation, which includes defining operational and challenge conditions for the systems. This study focused on defining two aspects of operational and challenge conditions: dry weather periods and volumes of water treated per event. This was undertaken using MUSIC to model the behaviour of catchments and WSUD systems over a 10 year period in Melbourne. From the MUSIC outputs, the lengths of continuous dry weather periods and volumes of water treated per rain event were evaluated to choose the most suitable threshold for validation of WSUD systems. The 5th/95th percentile dry weather periods were recommended as challenge conditions, together with the 95th percentile of volume of water treated. The challenge condition found for Melbourne was 1.2 (5th percentile) and 19.0 (95th percentile) days for dry periods. The volume per event considered as challenge for biofilters and wetlands varied with design specifications. For biofilters, the challenging volume of water treated in a single event per surface area of filter ranged from 0.33-1.98 m3/m2. For wetlands, the challenge volume of water treated in a single event per surface area ranged from 0.19-1.68 m3/m2.
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