Vegetation: the future of bridge defense

Plants are the oldest organisms on the planet. They have survived extinction-level events, adapted to an evolving world, clean the air of carbon dioxide, and are found almost anywhere on earth.

There are figuratively countless varieties – from trees and flowers to fruits and vegetables – with unique characteristics and interesting growth habits. People have used plants for shelter, food, clothing, and transportation needs along with aesthetics.

However, with progress has come drawbacks. Rubber, metal, plastic, and concrete have reduced our utilization of vegetation because they have proven themselves invaluable resources for economic growth. Bridges, in particular, were once built with wood and stone. Now they are resilient behemoths built out of metal and concrete, crossing over air and water simply so people can get to work.

Bridges are supported by beams driven into streambed, rives, and ocean floors where they are punished by a multitude of factors. The weakening of their supportive ability is called bridge scour, and engineers have struggled to find a modern solution for it – until they rediscovered vegetation.

Engineers have recently been researching and implementing biotechnical engineering for a multitude of projects including slope and scour protection. Where vegetation was once considered an obstruction and removed, engineers are re-implementing it as a defence against bridge scour.

Bridges by the numbers

The golden gate bridge, which is considered among the American Society of Civil Engineers’ Historic Landmarks, built in 1937 measures in at 1.7 miles long. It is one of the longest suspension bridges in the world, and one of 484,500 bridges in the U.S. Even though it’s one of the most well-known bridges, it’s as susceptible to bridge scour as every other bridge. Considering the golden gate’s daily traffic alone, bridge failure due to scour can lead to a major, lethal disaster.

Scour has caused an average of 30 bridge failures per year since 1966. Not only is that a high number of failures, but also a relatively short amount of time for them to occur in. It attacks bridges in multiple areas that are hard to test and can work inconsistently which makes it more difficult to measure.

What is bridge scour?

The short definition of scour is the weakening of bridge supports and erosion of surrounding land due to water movement. From this simple definition, it can be further separated into three types of scour:

• Local scour: Sediment is removed from piers and abutments by water which creates complex, changing water patterns.
• Degradation scour: Water channels are lowered due to the removal of sediment from the bottom. As water flows, it eats into the bottom floor and generally lowers the channel, exposing weaker structure points.
• Contraction scour: The narrowing of water channels – such as at bridge supports – increases water flow which, in turn, removes sediment surrounding the supports.

It’s difficult to protect against bridge scour because it affects protective methods at the same fundamental level as the supports themselves. Bridge supports are dug deep into the ground, are wide to ensure stability, and given protection from the constant water movement around them – but that doesn’t stop water from affecting the surrounding sediment which supports the abutments and protection. To effectively hind scour, the sediment and materials supporting the bridges have to remain in one place instead of being swept away.

How vegetation protects against bridge scour

By growing certain plants on the shore, waterway bottom, and supports themselves, engineers are discovering they can inhibit the effects of bridge scour. Here’s how:

• Using vegetation is eco-friendly, and reduces our reliance on materials that hurt the environment.
• Root systems are complex and strong. They dig into the sediment and soil, stabilizing themselves against the constant water current. The root systems fortify the sediment and soil like rebar fortifies concrete.
• The plants reduce water flow and runoff
• The plants improve water retention
• Using vegetation also promises to be less expensive than previous methods

Using vegetation as a bridge defence sounds great, but still has a few obstacles to overcome. For one, it’s difficult to measure the long-term performance. Factors such as global warming are influencing water currents and levels which affect the rate of bridge scour. Second, if growing conditions change and the plants don’t mature, they won’t effectively do their job.

Finally, engineers want to create a biotechnical defence that needs minimal maintenance. It is unclear whether or not current attempts will last long enough to meet those long-term goals.

 

Photo credit: Pixabay