Understanding Bridge Scour: A Guide to the Latest Federal Updates

In the U.S., bridge owners meticulously catalog highway bridges. Critical data is collected and submitted to the Federal Highway Administration (FHWA) according to the stringent National Bridge Inspection Standards (NBIS). These standards were significantly updated in 2022.

Part of the NBIS updates, the Specifications for the National Bridge Inventory (SNBI) document received a comprehensive overhaul, superseding its 1995 predecessor. Among the notable changes in the revised SNBI is the requirement to record scour vulnerability and scour condition ratings. The refreshed SNBI aims to heighten awareness about bridge scour, enhancing data quality, asset management, and reports to Congress and the public.

For a long time, bridge foundation design focused on the forces imposed by the superstructure, often overlooking potential impacts due to natural elements like water flow. This oversight changed dramatically after a tragic incident in 1987 when the Interstate 90 bridge over Schoharie Creek collapsed during a massive flood, claiming 10 lives.

The disaster was a wake-up call for engineers and policymakers, underscoring the critical need to address scour-related issues in bridge designs and maintenance. The Schoharie Creek incident illuminated the vulnerability of bridge foundations to erosive forces and prompted a significant shift in engineering practices.

More rigorous inspection protocols, advanced modeling techniques, and improved construction methods have been developed to help ensure that bridge foundations withstand the erosive effects of flowing water. Historical insight has driven the advancement of regulations and standards that prioritize the inclusion of scour considerations, ultimately enhancing the resilience and safety of bridge infrastructure worldwide.

Following the I-90 incident, the FHWA established rigorous guidelines for bridge foundation design and regular underwater inspections to identify and mitigate scour risks. Engineers use federal calculations, as noted in the Hydrologic Engineering Circular No. 18 (HEC-18) manual, to estimate scour depth and design appropriate countermeasures. The HEC-18 manual outlines various methods for estimating scour for different hydrologic and hydraulic conditions to help ensure bridges can withstand even extreme scouring events.

Evaluating scour at bridges involves a detailed hydraulic and hydrologic assessment to understand the water flow characteristics. The scour analysis considers multiple components, such as long-term scour, contraction scour, and local scour at piers or abutments.

Long-term scour refers to gradual bed erosion over an extended period, often due to natural water changes or human activities such as upstream construction. Contraction scour occurs when water flow is constricted, such as at bridge crossings where the channel width is reduced, leading to increased water velocity and subsequent streambed erosion. Local scour is the removal of sediment from around bridge piers, abutments, or other obstructions caused by the turbulence and vortices generated by water flow.

Each type of scour poses significant risks to bridge stability and safety, requiring specific calculations for estimating scour depths. These calculations inform both new bridge designs and the retrofit design of existing structures.

Engineers employ various countermeasures to combat the risks associated with bridge scour at existing bridges. These include placing riprap, a rock layer designed to stabilize the base and absorb water energy, and/or installing sheet piling to provide a barrier against scour. Articulated concrete block mat systems offer another solution, providing robust protection with a more aesthetic appeal than traditional methods.

Engineers can also consider jacks or reno mattresses, also known as gabions, which are wire mesh containers filled with rock or other suitable materials. These techniques are all considered temporary countermeasure solutions. New bridge foundations should be designed to withstand scour without the need for countermeasures.

Bridge scour is a significant concern for infrastructure safety, requiring ongoing research, monitoring, and improvement of design practices. By understanding the causes and effects of scour and employing effective countermeasures, engineers can enhance the resilience of bridges, safeguarding them against natural forces and protecting the communities they serve. Through a combination of historical lessons and modern engineering practices, the goal is to prevent future tragedies and ensure the longevity and safety of vital transportation networks.