Safer levees and bridges thanks to new erosion and scour detector
By Grant Banks
November 22, 2010
Erosion through water flow (called scour) causes the majority of bridge collapses in the U.S and was responsible for the levee failures in the aftermath of Hurricane Katrina. It has been difficult to assess the erosive potential of a soil profile without extensive digging on site followed up by hours of off site testing, but researchers at North Carolina State University (NCSU) have developed a device that significantly improves the process by measuring the scour and erosion potential of soils without the need to excavate and remotely test samples. Having tested the sensor in the lab the team are ready to conduct their first field tests.
“The In-situ Scour Evaluation Probe (ISEP) is the first technology that allows technicians in the field to measure the scour potential of soils without the need for excavation,” said project leader Dr. Mohammed Gabr. “Previous technologies required engineers to take samples and process them in a lab. The ISEP’s ability to measure scour potential at different depths helps us predict how the soil will behave in the future as a support media, as various layers of soil are eroded or scoured.”
The ISEP uses a water jet to burrow a hole into the soil with measurements of the rate at which the water displaces the soil taken. This reading determines the erosion and scour rate of the soil. Researchers can also manipulate the velocity and flow rate of the water to simulate various natural events from normal stream flow to hurricane-induced surges.
Several laboratory tests in a chamber that is 9 ft in diameter and 20 ft deep have been undertaken at NCSU.
“Recently, we have received a permit from the National Park Service to deploy it, on a trial basis, to test some of the barrier islands breached during hurricane Isabel on the North Carolina Outer Banks. This work will happen before the end of December,” Dr Gabr said.
It has taken two years and around US $120,000 in research funding to get to this point and Dr. Gabr told Gizmag another year's funding has recently been secured.
The results of the field testing should be available by mid-2011. While the ISEP is currently still only in testing phase Dr. Gabr expects the technology to gain American Society for Testing and Materials (ASTM) Standards approval within three years – a normal length of time for such certification. The device could be available sooner with draft approval from the ASTM.
An ISEP unit able to test down to 20 ft is expected to cost around US$12 000.
Understanding scour potential is important for helping authorities prepare for and minimize the impact of events such levee and bridge failures. Scour has been linked to 60 percent of the bridge failures in the U.S according to the Federal Highway Administration.
End-users of the ISEP such as federal and state agencies and private consultants will be able to perform scour assessment more frequently, since they will not have to take physical samples back to a lab for analysis. More test data means researchers will have a larger data set to work with helping them to more accurately predict scouring rates and behavior. It is hoped that this will reduce the number of bridge and levee failures in the future.