Civil engineer develops method to stop sandbar movement, stem floods

by Neal Singer

In a step toward creation of a sophisticated method to prevent inland
flooding, A UI researcher has been able to stabiliae the position of
scale-model and computer-image sandbars that direct water flow.

"Water is like a drunk person moving through a hallway," said Marcelo
Garcia, a professor of civil engineering.  "It tends to collide with
its containing walls.  So, instead of building extensive dikes or
straightening streams and rivers to control it, a cheaper and more
lasting effect could be achieved by controlling the migration of
the sandbars that direct it."

Sandbars direct flowing water against a river's banks - like a pinball
machine directs balls - cusing erosion through a scouring action.  "If
these sediment bars could be made to stay in one place, only the river-
bank sections receiving the water's thrust would need to be fortified,"
Garcia said.

Also, where the straightening of a stream is desired, Garcia's method
could determine the proper bank sculpting needed to stop sandbar shift.
A straight stream stirs up silt, increases water flow and becomes
environmentally inhospitable to animals that live in it.  Such streams
also tend to return to uncontrolled meandering because water "bounces
off the banks, continually creating new debris and new geometries to
pass through."

Garcia used a laboratory flume, a kind of artificial stream, to
demonstrate the behavior of liquid and sediment over a curved bed at a
variety of flow speeds and volumes.

He was able to stop movement of the forming sandbars by using computer
and physical modeling to adjust stream-bank curvature to match the
velocity and amount of water flow, a condition called resonance.

"I looked for a condition of sinuosity where the bars would stop
moving," Garcia said.  "I wanted the right amount of sinuosity to allow
for formation of sandbars but stop them in place."

Under ordinary conditions, sandbars constantly creep - at speeds about
one one-hundredth of the river's flow.  They shift and reform due to
deposit and removal of sediment by river water.

To find which banks in a natural stream are being eroded, aerial photos
taken over a period of years can be compared, Garcia said.

Some engineering plans "have a tendency to cut through meandering rivers
to produce channels with high banks," he said.  "But instead of
chammelizing all the river, why not just control the sandbar migration
and protect only those areas where you have scour?"

A report on Garcia's research will appear in October in the journal
Hydraulic Research.