Only last year Queensland authorities were considering lowering the flood prevention buffer at the Wivenhoe Dam — the buffer currently preventing a whole lot more water flowing into Brisbane.

Queensland’s Wivenhoe Dam — built on the Brisbane River, about 80km north of Brisbane — has quite literally been the only thing between a bad situation and a situation that’s a whole lot worse. Experts have praised the dam — which currently sits at 187.8% capacity — for reducing the amount of water surging into the Brisbane River. Due to the quantity of rain, authorities have been forced to release large amounts of water in order to maintain the dam’s flood capacity.

But in 2010, authorities seriously considered “raising operational levels” in the Wivenhoe Dam — that is, removing or reducing the flood buffer — according to the South East Queensland Water Strategy.

The dam has a total storage capacity of 2.6 million megalitres — but it is considered 100% full at 1.15 million megalitres, with any additional water allowed to flow through. This creates a flood buffer of 1.45 million megalitres — the dam holds the extra water and lets it out slowly, stopping or minimising any serious flooding. The Queensland Water Commission stopped short of advocating a reduction in flood mitigation levels, saying that more research would be needed because of Brisbane’s long history of floods.

Wivenhoe was built in 1984, as a direct response to the 1974 record flooding which inundated much of Brisbane. At the time, premier Joh Bjelke-Petersen said the dam would mean the floods of 1974 would never happen again.

In March 2010, the Liberal National Party were calling for the government to stop releasing water from the Wivenhoe, and to conduct an inquiry into whether such a large flood buffer was still needed.

Queensland’s metro consumption, including industry, is about 430,000 megalitres a year.  The plan estimates that if the Wivenhoe dam level falls below 40% they will have difficulty meeting this demand. The Wivenhoe spent nearly all of the period from 2005 to 2009 well below 40% capacity.

At the time opposition water spokesman Jeff Sweeney cited a 2005 state government report, which he said showed that 2m of extra storage (and 2m less of flood buffer) would add 228,000 megalitres of drinking water. With the Wivenhoe running above 90% capacity since March, it’s likely that much or all of this increase would have been added to the flood.

Queensland Water Commission acting executive director Dan Spiller responded by saying that more research would be needed and that authorities were preparing for “events much worse than anything on record”. “Flooding and rainfall remain very unpredictable and that is the reason why the dam sensibly contains a buffer for flood mitigation,” he said.

Calls to flood and engineering experts in Queensland were difficult this morning, with many calls failing to get through because of the congested telecommunications network.