Highway upgrade model underestimates flood risk: residents
THE Coldstream River will exceed predicted flood heights on the stretch of the Pacific Hwy upgrade in the Pillar Valley, says a residents group.
The Pillar Valley Flood Focus Group has been in talks with the highway upgrade overseer Pacific Complete, over its fears modelling of the one in 100 years flood heights for the region are out of date.
Group member George Oxenbridge said photographs of flooding during the 2013 flood event showed water reaching a depth of 4.5m above sea level (Australian Height Datum), well in excess of the flood modelling height of 3.009m.
He said local knowledge of potential rainfall and the catchment area of the Coldstream River revealed how these models were inaccurate.
"The modelling predicts rainfall of 250mm in 48 hours over a 70sq/km catchment would create the 1-100 year flood," Mr Oxenbridge said.
"Local information from our own rainfall expert Roy Bowling has records of 425mm falling in a 12-hour period.
"We asked if Pacific Complete had a flood model that showed 400mm falling in 12 hours. They said that would be a 1 in 1000 year flood event."
Mr Oxenbridge said during the latest meeting with Pacific Complete a hydrologist claimed the 1-100 flood would be just 300mm above AHD.
"He had to correct himself and change it to 3000 millimetres above AHD," Mr Oxenbridge said.
"That's three metres difference, not 30 centimetres. That's a huge amount of water."
Mr Oxenbridge was also worried the modelling Pacific Complete relied on did not account for climate change sea rise and extreme weather events.
Residents are also worried the earth banks the highway upgrade will use to cross the Pillar Valley floodplain will act as levee banks, increasing flood heights and slowing the dispersal of floods.
Mr Oxenbridge said the stretch of highway might need to include a 2.5km bridge in place of a section of the raised highway to lessen this effect.
"There's already three bridges in that stretch with about 700m combined length, so they'll have to nearly triple the amount of bridge they have already."
The Daily Examiner has contacted the RMS for a comment in response to the residents' views.