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Flood water no match for Brazil's porous pavement

posted 10 Apr 2011, 07:19 by Sam Mbale   [ updated 10 Apr 2011, 07:22 ]

Brazilian researchers have developed a new type of permeable pavement that can store rain water and ease the impacts of floods. They say the pavement could save lives and money in flood-prone countries around the world.

With rainy seasons gaining strength every year, researchers at Brazil's University of Sao Paulo have developed an innovative permeable pavement which they say may be key to preventing floods.

Watching water levels rise to waist-height in minutes is not an uncommon sight in many cities in and outside of Brazil. But according to hydraulics expert Afonso Luis Virgiliis and his team, the porous pavement being tested at the university's parking lot could be the solution to this problem.

Pervious roads, which are basically composed of crushed stones, sand and asphalt binder, are commonly used to prevent water accumulation and runoff. But the secret to this new pavement lies underneath its top layer. Stones of different sizes are placed side by side in a 35-centimeter deep (13.8 inches) area allowing large volumes of water to be stored for longer periods during heavy showers. A plastic sheet beneath this layer prevents the rainwater from infiltrating into the soil below like other pervious paving systems.

Virgiliis says the new pavement works like a drainage system, preventing streets and sewers from overflowing during storms.

"In reality this is a drainage system. It is composed of a permeable coating and by a series of granular layers that have stones of different sizes. What happens is that when the rain falls over this coating, this coating allows the water to flow down through it and the water is stored in the empty spaces that exist between these stones. This storage is done in such way that when the rain stops, once it goes away, then the water stored in the grainy reserve is slowly driven out to the city's conventional drainage systems," he said.

One of the downsides of this new system is its cost. Like other porous asphalts, it costs around 30 percent more than regular paving and has a lower load-bearing capacity. Because the asphalts are more absorbent, they are also less dense and are thus not ideal for areas of heavy traffic. Virgiliis says heavy trucks could quickly deform the permeable pavement, which has a lifespan of about eight years.

"What we always recommend is for this type of pavement, due to its characteristics, should not take a lot of heavy traffic, in fact, no heavy traffic at all. Only light traffic can pass over it -- that is, cars, small trucks and large trucks on rare occasions, because it is a pavement designed especially to absorb water," he said.

Virgiliis says the layer below is capable of storing rainwater for hours before it drains into the city's sewers. After one of the rainiest summers in history, results so far have been positive at the 1,600 square meter (17,222 sq. ft.) parking lot where the new technology is being tested. It is being monitored for storage capacity and time, durability and pollution absorption.

Virgiliis says their main goal is to come up with more resistant combinations of porous asphalt.

"We have been seeing very good results in terms of slowing down the time water takes to flow down to the drainage system. We have also seen positive results in terms of improving the general conditions of the parking lot itself -- the noise levels were lowered, because this pavement also absorbs noise. Our main expectation for the future is to develop a type of pavement that can endure heavier traffic and this depends, of course, on a bigger technological investment and more research," he said.

Sao Paulo's government plans to start testing the new pavement this year in areas where traffic is light.