flooded bridge with structural damage

Protecting Bridges from Flood Damage

Hurricanes and other extreme weather events have topped the news lately. More and more bridges are being damaged or wiped out by floods from record-breaking storms.

In this article, we’ll provide insights and ideas about what can be done to protect bridges from flood-related damage and destruction.

How bridges are compromised by water

Let’s start by looking at how bridges can be damaged by floods:

  • Large items, such trees, vehicles, appliances, or even whole structures, get picked up by flood water and slammed into a support pier, road bed, or other part of a bridge, causing structural damage.
  • Smaller pieces of debris accumulate upstream from a bridge, forming a large mass. The bundle moves downstream, lodges on the side of the bridge, and applies pressure to it, compromising its structural integrity.
  • An accumulation of debris limits the flow of water around a bridge. Concentrating water flow raises its height and increases the pressure it applies to the structure.
  • Water rises above the roadbed, tearing out the asphalt and other vulnerable parts of the structure.
  • Water levels rise on the roads leading onto a bridge, damaging the connectors that anchor it in place.

Did you know? The main reason many bridges are destroyed by floods is because of a phenomenon known as scour.

When a bridge over a river or stream is built on a bed of gravel, racing floodwater “scours away” the bed downstream from its piers, weakening their ability to hold up the structure. At the same time, floodwater and the elements carried in it put pressure on the bridge.

This effect is magnified on a bridge with multiple supports because each one causes scour to happen, weakening many sections of the structure. Multiple supports can also increase the “damming effect” on a bridge. Objects are more likely to get caught on narrowly spaced piers, becoming larger masses that place more pressure on the structure.

In the end, the most common reasons bridges ultimately fail during floods are:

  • Water, salt, or debris damages critical parts of the structure.
  • Pressure from water or debris breaks apart the bracing system.
  • Water lifts the structure off its supports.
  • Piers or abutments are knocked out by large debris, such as boats or vehicles that get caught in rapidly flowing water.
  • Extreme scour compromises the foundation.
  • Approach roads are cut, weakening structural supports.

How to protect bridges from water damage

So, what can be done to prevent damage to bridges during floods?

  • Engineer (or renovate) bridges with an eye to surviving 200-year flood levels, then add a margin of safety above that. Remember: There is a 0.5% chance that 200-year flood levels could be reached or exceeded in any given year. And with today’s more extreme weather, records seem to be broken more often. It’s less expensive to build a stronger structure than to be forced to replace a weaker one destroyed by a storm.
  • Design structures that have decks that rise high above flood levels. This can help avoid having the most dense and impenetrable part of a bridge hit by debris.
  • Develop bridges that have limited structural density below the roadway. Through-truss and stayed bridges allow water to easily pass through and around the foundation and don’t have elements that large physical masses can get caught on.
  • Build clear span bridges, when possible. This eliminates the possibility that large objects or masses will tear away critical foundational elements. It also eliminates the risks associated with scour.
  • Reduce the depth of the roadway so water can more easily flow around it. If possible, add structural elements that will let water pass through the roadbed.
  • Replace or rehabilitate bridges that have wood or metal decks. Wood bridges tend to be weaker than other types. Metal decks can be lifted by powerful aquatic forces. Concrete decks supported by steel provide both strength and flexibility and resist the shearing forces that can twist a bridge off its foundation.
  • Protect abutments against erosion using not just current flood guidelines but potential future events as well. With more and more weather records being broken every single day, it’s important to plan for record-breaking water levels. Consider using matting, artificial grass, or large boulders for this purpose.
  • Replace older bridges sooner rather than later. Many aging structures were built with piers not set deeply into the riverbed. Newer ones benefit from being built with modern pile drivers that place piers deeper and provide a more solid foundation.
  • Place boulders around the base of bridge piers to keep the riverbed in place and prevent the effects of scour.

Taking steps to prevent flood damage may cost more money today but could save a lot in repair and replacement costs for decades to come.