Imagine standing on the beach with the waves washing back and forth around your feet. The force of the water washes away the sand, creating a hole around and under them. The faster the water, the bigger the hole. What you experience is scour, caused by the most powerful force in nature - moving water.

Bridge scour is the number one reason for bridge failures. Other failures are from ships, truck, or train impacts, weight overloads, and earthquakes.

The United States has 575,000 bridges in its inventory. Of these, 84% span water and 25,000 are scour critical, meaning their foundations could fail because of erosion. If you own a bridge over water, you have good reason to be concerned about scour.

In 1987, the I-90 bridge over Schoharie Creek in NYS collapsed when a flood scoured the bridge foundations. Soon after, the Federal Highway Administration (FHWA) required states to identify bridges that are likely to have scour problems. Knowing where scour is a potential problem allows officials to monitor and improve conditions ahead of time—before bridges become dangerous to travel on.

How to Identify Scour: If you own a bridge, the FHWA requires that it be inspected at least every 2 years. A component of the inspection is to look for evidence of active scour or the potential for such. To do this, your inspector will perform visual inspec-tions, measure channel cross-sections, and compare the data against sections taken at prior inspections. Visual inspections consist of walking the upstream and downstream channels, noting:

• Evidence of footing exposure, undermining, or settlement


• Undermined or moving bank protection


• Changes in channel alignment


• Loss of road embankment


• Evidence of high water or recent flood events

You will know if you have active scour or a scour-critical bridge based on the Hydraulic Vulnerability Assessment in your biennial inspection report. If you do, you may need more frequent inspections. These should include an inspection during and/or after every flood event. Flooding increases water velocity, which increases the potential for scour and bridge failure.

If you have active scour, you need to identify the magnitude of the problem. To find out if your bridge is scour critical, compare the depth of scour under the base flood (or tide) to the depth of the foundation. (A base flood is often called the 100-year flood.) Your engineer will run these calculations using FHWA HEC-18 process, Evaluating Scour at Bridges. He or she will compare the scour depth to known foundation depths (from record plans), apply a factor of safety, and determine if the foundation is able to support design loads under the base flood conditions.

Unfortunately, more than 85,000 bridges have no record plans. This means that once the scour depth has been computed, it cannot be compared to the foundation depth to determine if your bridge is scour critical. If you own one of these bridges, it’s best to assume that the bridge is subject to scour unless proven otherwise.

If you have a scour-critical bridge, you are required by the FHWA to have a plan of action to address it. Counter measures include:

• Hydraulic - to alter river flow or reduce erosion. One example is to change the skew of the river by building weirs or bulkheads. Another is to protect piers, abutments, and banks with ‘armour’ – something heavy to keep the soil around piers and abutments in place. Riprap, gabions, and grout or sand-filled bags are often used.
• Monitoring - to provide early scour warnings. Tools are fathometers (to measure changes in river bed elevation); tilt and vibration sensors (to measure pier and abutment movement); velocity meters; sounding rods (to locate scour holes); underwater inspections; and/or rain gauges (to provide high rainfall alerts.
• Structural - to reduce or divert the impact of scour. Examples include foundation reinforcing, foundation extensions, and changing the shape of a pier or abutment.

Be diligent during flood events. Don’t wait for the storm to subside; head out to your scour critical bridges and check for scour holes, cracks in bridge decks, stream banks erosion, and movement in piers and abutments. Close your bridges if there is any question of its safety. Once the storm has subsided, you can undertake a detailed study to see if the bridge is safe to drive on.

The best way to prevent scour is to design against it. Conduct a hydraulic study for each new or reconstructed bridge. Identify the optimal bridge site, span length, size of opening, shape, and skew. Eliminate piers if possible or align piers with the direction of flood flows. Provide extra freeboard for streams that carry debris and ice, and eliminate overlap of local scour holes at piers and abutments. Also use riprap, sheet piling or other measures to protect from localized scour.

Be Prepared: Knowing whether or not you have scour is a good first step, the next step is ensuring that the inspection findings are promptly communicated to others. Your plan of action should include a definition of ‘prompt’, so that your inspection staff and maintenance crews know what is expected of them when a problem does come up.

Scour is a complex issue to address. If you have questions or concerns about scour, contact Structures Manager Joe Logan, P.E. at 585-334-1310 or via e-mail at JLogan@fisherassoc.com