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Scour Repairs

Bridge Scour Mitigation Program

Bridge Scour Program
Bridge Scour is defined as the removal of soil from around bridge foundations.  Flowing water transports soils from around a bridge foundation and moves it down stream, leaving the foundations exposed and in some cases undermined. Depending on the severity of the scour a bridge’s integrity could be at risk and in some cases lead to a partial or total collapse.

Scour is the leading cause of bridge failures in Washington State and nationwide. Of the 70 documented bridge failures in Washington State history, 43 were due to scour.

In 1979, a Department of Transportation employee, Russell Barker, was killed when he drove a dump truck off the US101 Bogachiel River bridge. Scour caused an approach span to collapse during a period of high water flow in the river.

WSDOT has approximately 1,583 vehicular bridges and culverts over 20-feet in length that span over water. 262 of these bridges are considered “scour critical” which means there is potential for the bridge to be damaged by scour. WSDOT’s efforts to ensure that these bridges are safe and haven’t been damage by scour fall in to two categories, monitoring and response.

Monitoring for Bridge Scour

Monitoring for Bridge Scour
WSDOT performs routine inspections of its bridges at least once every two years. In some cases where there are specific concerns, bridge inspections are more frequent. During these inspections, the bridge inspectors observe conditions at the bridge piers and abutments as well as the waterway upstream and downstream of the bridge. Bridge scour conditions are recorded in the bridge inspection report. If significant scour is observed, the bridge inspector will call for a repair.

Where conditions are difficult to evaluate due to water depth or other conditions, the WSDOT Dive Team will perform an underwater inspection of bridge elements. Divers performing the underwater inspection are able to gather detailed information about scour conditions. Once again, if significant scour is observed, the bridge inspector will call for a repair.

All WSDOT scour critical bridges are monitored by WSDOT staff during flood events. The monitoring is conducted in accordance with a “Scour Plan of Action”, which is a documented plan that has been prepared ahead of time. The plan of action includes a summary of the scour vulnerabilities and guidance for actions to be taken during flood events. Actions include monitoring bridges during and after flood events as well as closing bridges if conditions exceed a pre-determined threshold or if scour damage is suspected.

Response to Bridge Scour Damage

Nolan Creek Bridge
US 101 Nolan Creek Bridge
If scour damage has occurred that compromises the integrity of a bridge, the bridge is closed to traffic until repairs are made.

If scour has occurred but the damage does not compromise the integrity of the bridge, WSDOT will take action to repair the damage. Some repairs are completed by WSDOT Maintenance crews while others are completed under contract. Who completes the repair depends on several factors including what part of the State the bridge is located in, the nature of the repair, and the anticipated cost of the repair.

In years past, the majority of bridge scour repairs were completed by WSDOT maintenance crews. Repairs were completed quickly and efficiently at a relatively low cost. WSDOT maintenance crews were able to install and maintain scour countermeasures that significantly reduced the risk of scour damage to WSDOT bridge.

Today, the efforts required to obtain permits for bridge scour repairs preclude WSDOT maintenance crews from completing bridge scour repairs in many areas of the State. WSDOT maintenance crews in these areas don’t have the staff or funding that would be required to obtain the necessary permits. So, most bridge scour repair projects are completed through the Bridge Scour Mitigation Program.

Bridge Scour Mitigation Program

The WSDOT Bridge Scour Mitigation Program addresses the top priority bridge scour deficiencies by designing, permitting, and constructing bridge scour repairs under contract.

There are several steps necessary to complete bridge scour repairs in today’s world. The steps are:

  • Identify Needs – Bridge inspection data is reviewed to identify bridges that have a bridge scour repair need.
  • Prioritize Needs – Once the list of needs is determined, the details of each case are reviewed and prioritized against each other on a statewide basis.
  • Program and Fund Projects – The funds that are available for bridge scour repairs are assigned to the top bridge scour needs. Usually only three or four bridge scour repair projects are programmed each biennium.
  • Design Repair – WSDOT staff including bridge engineers, hydraulic engineers, and environmental staff work together to design the repair and appropriate mitigation.
  • Obtain Permits – WSDOT staff work to obtain permits from the appropriate entities to construct the repair. This process usually takes about 2-years.
  • Construction – The project is advertised and awarded to the Contractor with the lowest bid. Construction of most scour repairs typically takes less than a month.

Humptulips River Bridge
US 101 Humptulips River Bridge
Completing bridge scour mitigation projects is challenging. One of the major challenges is in identifying and addressing scour needs before they jeopardize a bridge. Bridge scour is dynamic and conditions can change rapidly. There are many variables that affect the rate at which bridge scour happens.

A second challenge is in permitting bridge scour projects. In order to construct a scour repair, WSDOT must obtain permits from many entities including, The Washington State Department of Fish and Wildlife, Washington State Department of Ecology, The US Corps of Engineers, Native American Tribes, Local Jurisdictions, and land owners. Meeting the requirements of each entity is often very challenging, time consuming, and often results in added costs in construction, due to added constraints and required mitigation.

A third challenge is in addressing all of the high priority scour needs with available funds. Because the process to complete scour repairs is expensive and lengthy (2-years minimum), WSDOT can only address a few scour repairs each biennium. This means WSDOT often reacts to only the worst needs through emergency scour repair projects.

Completed Bridge Scour Repairs

In the past 10-years, WSDOT has completed 13 bridge scour repair projects (17-bridges) by contract, at a total cost of $12 million.

Bridge Scour Repair Projects Completed in the Past 10-Years
Project Bridge No.(s) Construction Year   Total Cost
SR 522 / Snohomish River Bridge Scour Repair 522/138    2005 $850,043
I-5 / Steamboat Slough Bridges Scour Repair 5/648E and 5/648W 2005 $280,521
SR 8 / Middle Fork Wildcat Creek Scour Repair 8/13 2005 $578,508
US 101 / Sol Duc River Bridge Scour Repair 101/314 2006 $335,498
US 101 / Humptulips River Bridge Scour Repair 101/150 2006 to 2008 $1,395,823
SR 9 / Pilchuck Creek Bridge Scour Repair 9/134 2008 $349,374
SR 9 & SR20 / Thunder and Coal Creek Bridges Scour Repair 9/222 and 20/232 2008 $739,367
US 12 / Touchet River Bridge at Touchet Scour Repair 12/624 2009 $465,415
SR 508 / Tilton River Bridge Scour Repair (Emergency) 508/32 2009 $664,301
SR 202 / Tokul Creek Bridge Scour Repair (Emergency) 202/58 2010 $4,767,998
SR 410 / White River Bridge Scour Repair 410/101 2011 $492,241
US 97 Et Al / Yakima & Benton County Bridges Scour Repairs 97/120, 224/10, and 823/3E 2011 $661,951
SR 108 / Wildcat Creek Bridge Scour Repair 108/4 2014 $490,209


Highest Priority Bridge Scour Repairs

Given the dynamic nature of scour, bridge scour needs and priorities can change rapidly. Currently the top three bridge scour repair needs are:

Pier 14
Chehalis River Bridge (Bridge No. 101/115)
Bridge No. 101/115 carries US 101 traffic over the Chehalis River Bridge in Aberdeen Washington. Scour action has removed soils from around and below the Pier 14 pile cap. The void is approximately 7.5-feet high and has exposed (16) untreated timber piles. Based on recent experience with the nearby Simpson Avenue Bridge, WSDOT is concerned that the exposed timber piles could be damaged by marine borers, which are small bugs that bore in to and eat wood. Repair of this bridge scour need will likely consist of filling the void under the pile cap with concrete and placing riprap around the pier. This repair is estimated to cost approximately $3.5 million.

Union Slough Bridge (Bridge No. 529/15E)
Bridge No. 529/15E carries SR 529 traffic over Union Slough near Everett Washington. Scour action has removed soils from around several of the bridge’s pile cap foundations. The scour has caused voids to form under the pile caps at Piers 8 and 9. The voids under these piers leave many untreated timber piles exposed. Similar to the Chehalis River Bridge, WSDOT is concerned that the exposed timber piles could be damaged by marine borers. Repair of this bridge scour need will likely consist of filling the voids under the pile caps with concrete and placing riprap over exposed pile caps. This repair is estimated to cost approximately $2.5 million.

South Fork Skykomish River Bridge (Bridge No. 2/119)
Bridge No. 2/119 carries US 2 traffic over the South Fork of the Skykomish River near the town of Skykomish Washington. Scour action has removed soils from around the pile cap at Pier 3. The scour has caused a void under the pile cap that is approximately 3.5-feet tall and exposes (18) untreated timber piles. Because the piles are in fresh water, marine borers are not a concern. However, being exposed, the timber piles are vulnerable to damage by debris moving down the river and decay. Repair of this bridge scour need will likely consist of filling the void under the pile cap with concrete. This repair is estimated to cost approximately $1.1 million.

 Skykomish River Bridge
 US2 Skykomish River Bridge - Pier 3

A summary of WSDOT’s scour mitigation program is available in a Scour Folio.