HIGHER silt levels in the Tamar estuary will mean bigger floods for Launceston?

No - it's a myth not founded on scientific evidence, says Launceston Flood Authority general manager Andrew Fullard.

In fact, dredging the Tamar has the smallest of impacts on flood levels, Mr Fullard said.

"And the impact tends to get less as the flood level increases towards the one in 200-year-flood," he said.

Mr Fullard and authority chairman Martin Renilson believe that it's time to dispel the myth and the associated community talk in favour of increased dredging, particularly around the time of substantial floods.

Dredging the silt won't lower the level of floods, Mr Fullard said.

In fact, left to her own devices, Mother Nature will have a far more dramatic effect on moving the silt through the Tamar estuary than dredging, says Professor Renilson.

The two have the drawings and graphs to prove it.

Professor Renilson and Mr Fullard are not speaking out because they are against dredging.

"We're not saying that no one should dredge - that's not our business and is for others to decide," Professor Renilson said.

"Keeping boating channels navigable is different story," Mr Fullard said.

"We're about flood protection for a one-in-200 flood."

Mr Fullard's diagram of the flood plain surrounding Launceston shows that dredging the river channels and shoal areas to remove 335,000 cubic metres of silt at a cost of more than $6.7 million would only reduce the flood level at the Charles Street bridge by about 130 millimetres.

"This is because the dredged cross-sectional area, when extended to the width of the flood plain, is reduced in height so that, in the case of Invermay, it only achieves a 130-millimetre reduction in flood height," he said.

What does have an impact on flood levels is development on the flood plain, Mr Fullard said.

"That narrows the available flood plain which in turn increases the depth of the flood," he said.

Mr Fullard's diagram of the flood plain with development encroaching from both sides shows that uncontrolled vegetation or construction of an unplanned levee as well as buildings will restrict the water course.

He said that strict planning controls had been placed over the flood plain in the Inveresk precinct to prevent inappropriate development.

The authority also plans to initiate talks with the West Tamar Council to get its co-operation on restricting development on its Riverside end of the flood plain.

Mr Fullard has another myth to refute.

He believes that the Tamar River is not choked by silt, as many in the community would have.

"It is attempting to reach its equilibrium and, as such, is behaving naturally," he said.

He refers to a survey of the river completed in 1882 by colonial surveyor Napier Bell before the major effects of European settlement.

"It shows a river in equilibrium with striking similarity to the siltation we see in the Tamar today," Mr Fullard said.

"All silty rivers and tidal flows in estuaries have an equilibrium - a state of equilibrium is when the tidal or river flow has a channel with a matching cross- sectional area."

When in equilibrium, the cross-sectional area of the channel causes the water to flow at a rate where the erosion and deposit of sediment is equal, or in a state of equilibrium, Mr Fullard said.

"Dredging causes the river to be out of equilibrium so that the more we dredge, the more the river wants to silt itself up," he said.

Tamar River surveys during the past three years show that the river silts up and returns quickly to its equilibrium state after a flood.

"Within six months of the August 2009 flood, the river had returned to its pre-flood state," Mr Fullard said.

"Dredging is not a quick fix to the siltation issue.

"It does maintain the navigability of the river so could be justified on that basis.

"But only if someone is willing to provide the funds."