|The crossing at 8.00am on 2 May 2015. The water had been higher than this during the night.|
This rain fell over an extended period, and in the Lilydale Creek catchment some three kilometres away was not heavy enough to cause any significant erosion on a recently graded track (Lilydale Creek is a tributary of Redbank Creek, joining it above the crossing).
This was not an unusual level of rain in 24 hours in this area. Rainfall events of around 150mm in a few hours when the ground is already saturated can cause flash flooding of Lilydale Creek. This would be expected to also apply to Redbank Creek at the crossing, which receives input from its own upstream catchment and that of Donnybrook and Lilydale Creeks.
We have had the experience of crossing a small tributary of Lilydale Creek at Vinegar Hill which as flooded but not dangerous, driving about 600m to Lilydale Creek and finding it impassable, then returning to the tributary, which had risen sufficiently in around 10 minutes to make crossing it a risky venture. The rainfall which caused that particular flood had mostly fallen higher in the catchment, so that the flooding was unexpected.
The motocross application document says that:
acceptable management practices can be put in place to ensure patrons remain put in a place of safety (ie the subject site) for the duration of any flood events affecting the road. Given the small catchment area, flooding times within Adare Road are not prolonged [Planning Assessment Report p.8].
I'd say that such an approach could be regarded as a calculated gamble when the ground has been saturated by previous rain events, even if the creek is not flowing strongly.
Saying that Redbank Creek "has a small catchment" is somewhat subjective. It's catchment extends west to Seventeen Mile Road, south to the Sandy Creek catchment and north to the Buaraba Creek catchment.
Locals report that the Redbank Creek crossing on Adare Road can be closed for some days as a result of high rainfall events, and may not be open immediately after the water goes down because of log-jams across the culvert.