Bridge Scour Countermeasures and Monitoring per FHWA HEC 23

The primary technical reference for calculating scour depths at bridge piers, abutments, and contraction zones. HEC-18 provides the pier scour equation, contraction scour methodology, and abutment scour prediction procedures used by hydraulic engineers nationwide. Scour depth estimates from HEC-18 directly inform the design of countermeasures selected through HEC-23 and determine whether a bridge is classified as scour critical under the SNBI coding framework. The most recent equation revisions for coarse-bed pier scour are incorporated into the FHWA Hydraulic Toolbox for efficient analysis.

Provides the methodology for assessing stream channel stability and identifying channel instability mechanisms that contribute to scour. HEC-20 covers riparian vegetation assessment, channel evolution models, and stream classification systems that help engineers determine whether a scour problem is caused by local pier scour, contraction scour, or broader channel degradation. This distinction determines which category of countermeasure — pier protection, channel armour, or hydraulic modification — is appropriate for the specific site conditions.

Consolidates countermeasure experience, selection methodology, and design guidance into 14 Design Guidelines covering physical armour systems, hydraulic countermeasures, and monitoring instrumentation. HEC-23 Volume 1 covers countermeasure selection and management strategy; Volume 2 provides the detailed design guidelines for each specific countermeasure type. The document also outlines the Plan of Action framework for scour critical bridges and provides guidance on fixed and portable scour monitoring instrumentation.

HEC-18 (Evaluating Scour at Bridges) provides the equations and methodology for calculating anticipated scour depths at piers, abutments, and channel contractions. HEC-23 (Bridge Scour and Stream Instability Countermeasures) provides the design guidance for countermeasures that protect against those calculated scour depths. In practice, you use HEC-18 first to determine how deep scour is expected to be at each bridge element, then use HEC-23 to select and design the appropriate countermeasure to resist that scour depth. The two documents are designed as companion references and are used sequentially in the scour evaluation and countermeasure design process. HEC-20 (Stream Stability at Highway Structures) is used before both to assess the stream channel context. Talk to an expert about integrating the three HEC documents into your scour programme workflow.

Real-time monitoring is appropriate as a countermeasure when the bridge presents moderate risk, when physical countermeasures are not feasible due to site constraints such as limited access or environmental permitting restrictions, or as a temporary measure while permanent physical countermeasures are designed and constructed. FHWA guidance recognises monitoring as one of three countermeasure categories, with the understanding that monitoring does not prevent scour but enables timely action when scour reaches critical depths. Bridges with monitoring as the sole countermeasure remain classified as scour critical and require a POA that specifies scour alert elevations, closure triggers, and notification procedures. For high-risk bridges with deep foundations subject to rapid scour progression during flood events, a physical armour countermeasure or foundation retrofit is typically more appropriate than monitoring alone. Book a Demo to see how iFactory supports monitoring-based POA documentation.

Selection depends on hydraulic conditions, material availability, environmental constraints, and lifecycle cost. Riprap is the most common and generally the most cost-effective option where suitable rock is available within economic haul distance. Articulating concrete block systems are preferable where the channel bed is expected to degrade over time, because the articulated blocks maintain coverage while a rigid mattress would become unsupported at its edge. Grout-filled mattresses provide an impermeable armour surface suitable for sites where seepage forces or high-velocity flow could remove material through riprap openings. Partially grouted riprap offers a middle option — smaller rock with increased stability — appropriate where D50 requirements for loose riprap exceed economically available rock sizes. HEC-23 provides a countermeasure selection matrix that guides engineers through the decision process based on flow velocity, scour depth, bed material, construction access, and environmental factors. Talk to an expert about a countermeasure selection review for your scour critical bridges.

The SNBI requires three scour-related data items for every bridge: B.AP.03 Scour Vulnerability (a single-digit code indicating the current scour risk status), B.C.11 Scour Condition Rating (the observed scour condition at inspection), and B.AP.04 Scour Plan of Action (whether a POA exists and its status). A bridge is classified as scour critical when B.AP.03 codes indicate that the foundation is unstable due to observed or calculated scour. All scour critical bridges must have a POA documented. The POA does not change the classification — a bridge with a POA remains scour critical until physical countermeasures or foundation improvements are completed and the foundation is recoded based on re-evaluation. Monitoring countermeasures documented in a POA do not change the classification either. Recoding from scour critical to stable requires a scour re-evaluation demonstrating that the calculated scour depth no longer threatens foundation stability, supported by field observations or real-time monitoring data where available. Book a Demo to see how iFactory integrates SNBI scour coding with POA tracking.

Real-time sonar monitoring data provides direct field measurement of scour depth during flood events, which can be compared against the HEC-18 predicted scour depths that originally determined the scour critical classification. If multiple flood events with discharges approaching the design flood produce observed scour depths consistently less than the predicted depths, the data may support a re-evaluation that recodes the bridge from scour critical to stable. USGS studies in Idaho demonstrated this application — real-time pier scour monitoring data showed that the coarse-bed pier scour equation was conservative for the monitored sites, and the data provided the engineering basis for recoding. The key requirement is that the monitoring data must capture bed elevation during a flood event of sufficient magnitude to test the scour prediction. Agencies should document the monitoring period, the flood events captured, the comparison between observed and predicted scour, and the engineering analysis supporting the recode recommendation. Talk to an expert about developing a sonar monitoring plan for scour critical recode evaluation.

HEC-23 requires a filter layer under all riprap scour countermeasures to prevent soil loss through the armour openings. The filter can be either a granular filter (one or more layers of graded aggregate meeting specified permeability and retention criteria) or a geotextile filter fabric. The filter must satisfy two primary criteria: retention (the filter openings must be small enough to prevent the underlying soil from washing through) and permeability (the filter must be permeable enough to prevent hydrostatic pressure buildup beneath the armour layer). For geotextile filters, HEC-23 references AASHTO M 288 specifications. For granular filters, the filter-to-base soil ratio criteria are provided in the design guidelines as a function of D15 filter / D85 base soil and D15 filter / D15 base soil ratios. Geotextile sand containers have been demonstrated as a proven technique for placing a filter under water for both riprap and partially grouted riprap installations. Improper filter design is a common cause of riprap countermeasure failure. Book a Demo to see how iFactory documents filter design specifications for HEC-23 compliance.