Concrete Armor Units Advancing Scour Protection

Introduction

Water always wins if you give it enough time. It carves canyons, flattens coastlines and (more quietly but no less powerfully) erodes the very foundations of the bridges we depend on every day. This process, known as scour, is responsible for more bridge failures in the United States than any other hydraulic force1. It’s not dramatic like an earthquake or a hurricane. It doesn’t strike in an instant. Scour works in silence, grain by grain beneath the surface, until the supporting material is gradually removed and the structure above is left unstable. Ironically, the key to preventing scour might not lie in resisting the flow but rather reshaping how we engage with it. Concrete Armor Units (CAUs) can provide an alternative approach by transforming the way flowing water interacts with the foundations that support our bridges.

Scour Process

To truly understand why CAUs succeed where so many traditional scour countermeasures struggle, we must first understand how water moves. Flowing water is a persistent force of nature, continuously molding the landscape beneath us. It shaves grains of sand from the riverbeds, whirls them into turbulent eddies and deposits them downstream in patterns that evolve constantly through time. This ongoing process has no regard for the stability of the infrastructure we build in its path.

Bridge abutments and piers often stand at the front lines of hydraulic forces, where swift, turbulent flows put them at constant risk of scour-related failure. The velocity of water near these structures is anything but uniform—it fluctuates wildly. As flow approaches a solid object such as a pier or abutment, it separates and accelerates around the edges, forming zones of high velocity and swirling vortices, spinning like invisible tornadoes that dig deep into the riverbed (see Figure 1). These concentrated vortices amplify erosive forces, removing soil and threatening the stability of the foundation. The risk is further compounded by the surrounding geometry. Steep channel slopes elevate flow energy, while abrupt transitions or constrictions, often introduced by bridge features, intensify turbulence. The convergence of flow dynamics creates a perfect storm of destructive energy at the very locations where structural integrity is most critical.

Traditionally, designers rely on rock riprap as the primary defense against the restless energy of moving water. It’s a practical solution, born from a simple idea that piling enough rock in the right spot can hold the water at bay. Riprap, for all its weight and texture, lies atop the riverbed as a barrier, attempting to deflect flow but never quite absorbing it. Through time, currents weave through the loose arrangement of riprap, carving out channels, seeping through gaps and eroding the underlying soil. Without true interlock, even the heaviest stones are displaced as water follows the easiest path forward.

What Are Concrete Armor Units?

Structures that survive the test of time do so by adapting to their environment. CAUs are engineered, precast concrete units designed with increased interlock and roughness to provide an alternative to traditional rock riprap. One such CAU listed in the FHWA Hydraulic Engineering Circular No. 23 (HEC-23)2 is A-Jacks. A-Jacks are six-legged precast concrete units designed to interlock with adjacent units, forming a stable and permeable matrix capable of withstanding the persistent forces of flowing water (see Figure 2).

Figure 2. Concrete Armor Unit matrix.

The openings formed between the units in the A-Jacks matrix provide approximately 40 percent void space in a uniform placement pattern. These voids not only allow for water conveyance and sediment deposition but also create habitats that support fish and other aquatic life, contributing ecological value to the hydraulic function. These units are not just about form and function beneath the surface; they’re also engineered for practicality in the field. Even the smallest of these units carries weight. A 24-inch A-Jack unit weighs about 80 pounds, light enough to be handled by a crew of laborers without the aid of heavy machinery. But how do you move a unit like that efficiently? Each unit is shipped in two interlocking halves, neatly palletized for transport (see Figure 3). Once onsite, these halves are assembled into full units by hand and placed directly onto the streambed, even in conditions where water is already flowing.

Hydraulic Performance

One of the most important hydraulic characteristics of A-Jacks is the way their roughness adapts to flow depth and sediment transport. In shallow flow conditions, a greater surface area of each unit is exposed, increasing hydraulic roughness. As flow depth increases, the effect of this roughness on flow decreases, allowing the system to convey water more efficiently without inducing excessive resistance.

Sediment plays an integral part in this dynamic. During floods, sediment remains suspended, leaving A-Jacks more exposed to dissipate the highest energy flows. When flood waters recede, sediment naturally settles into the voids between the units, gradually embedding them and smoothing hydraulic roughness under normal conditions. Flume testing has confirmed this behavior, revealing how A-Jacks reduce near-bed velocities and shear stresses, especially in critical areas such as bridge piers and abutments (see Figure 4). This self-moderating behavior enables A-Jacks to remain effective during both low- and high-flow events.

From a design perspective, the variable roughness of A-Jacks presents unique challenges and opportunities. Manning’s n values derived from extensive flume testing cover a range of conditions, including clear water, sediment-laden flows, shallow flows and deeper depths. Engineers are encouraged to apply these values thoughtfully, bracketing them to reflect the natural variability of river systems.

Flume testing also has provided insight into the hydraulic thresholds of A-Jack units. Velocity, being one of the most critical parameters in unit sizing, plays a central role in determining stability under flow. Experiments identified a critical velocity based on varying bed slopes at which the units may begin to mobilize. The data from these experiments have been used to create a simple design chart (see Figure 5), which outlines critical velocities for various size units. This provides a practical starting point to select the right unit size for specific site conditions.

Design Considerations

For A-Jacks to perform their protective role effectively, engineers must carefully balance size, placement and site conditions. The cornerstone of design begins with determining the appropriate design flow velocity. Faster currents exert stronger forces, demanding larger and more-resilient protection to resist drag and uplift without shifting.

Velocity governs the magnitude of hydraulic loading, but the surrounding terrain shapes how that energy interacts with the system and, in turn, influences unit sizing. Factors such as channel slope, side slopes and local topography determine not only how flow concentrates or disperses but also how stable the placed units will be under those forces. On steeper gradients or embankments, maintaining interlock and resisting downslope movement may require tiered placements to preserve structural cohesion. By accounting for these geometric complexities, designers can better match unit size and configuration to site-specific demands.

Selecting the right size and understanding the site conditions are essential first steps in the design process. Equally important is how the units are arranged across the riverbed to counteract the ongoing forces of erosion. To provide effective protection for bridge piers and abutments, A-Jacks must extend beyond the anticipated scour limits. Guidance from engineering standards such as HEC-23 for bridge piers and the National Cooperative Highway Research Program Report 587 (NCHRP-587)3 for bridge abutments help establish the necessary coverage area. Additionally, vertical embedment of the units is critical, particularly along the edges of the protected limits, where sediment may be most vulnerable to erosion. This embedment acts as a safeguard against undermining by anchoring the system in place, even if surrounding soils are washed away during high-flow events.

Bridge Protection

Two key documents stand out to design effective scour countermeasures: HEC-23 and NCHRP-587. The design methodology outlined in HEC-23 approaches the A-Jacks systems as a collection of modular units, often referred to as modules or bundles, to protect bridge piers. These modules are formed by banding together individual A-Jack units into a tightly interlocked matrix, creating a larger structural element that behaves as a unified system (see Figure 6). By adjusting the number of units within a module, engineers can customize both the length (L) and width (B) of the module to accommodate specific hydraulic and geometric conditions.

The hydraulic stability of an A-Jacks module is evaluated by weighing the forces trying to move it against the forces anchoring it in place. Specifically, the overturning moment caused by flow-induced drag (Fd) is set equal to the resisting moment, which depends on the module’s submerged weight, Ws, and specific gravity, SG, (see Figure 7 and Equation 1). Larger module sizes, by virtue of their increased weight and broader footprint, naturally provide greater resistance to hydraulic forces, making them more stable in high-energy environments. Additionally, embedment of the units will also provide greater stability since the exposed height (Hexposed) of the unit (as defined in Figure 7) is smaller, further reducing the overturning moment. The drag coefficient, Cd, of 1.05 has been validated through rigorous physical model testing, providing a solid foundation for design calculations.

FdHd = Ws(L/2)

where:

Fd = 0.5CdρAfv2

Af = B x Hexposed

Ws = W x ((SG-1)/SG)

Equation 1

While HEC-23 lays out the design framework for protecting bridge piers, abutments face their own hydraulic challenges. NCHRP-587 offers design guidance tailored specifically to abutment scour and the unique flow conditions they encounter. Layout considerations vary depending on whether the abutment is a spill-through or wing-wall configuration, and typical schematics for both layout arrangements are illustrated in Figures 8 and 9.

The A-Jacks sizing chart offers a practical method for selecting the appropriate A-Jack size for abutment protection based on approach velocity at the bridge opening. From there, the recommended coverage limit (W) extends laterally 1.25 times the predicted scour depth, providing a conservative buffer against erosion. Where deeper embedment is required, the layout may be excavated down at a 1:1 (H:V) slope to match the anticipated scour depth. For wing-wall abutments, coverage should extend at least 1.5W upstream and 1.0W downstream of the abutment wall, helping intercept erosive forces before they can concentrate around the structure. As always, engineering judgment is key to adapting these recommendations to the unique geometry and site-specific hydraulics of each project.

Constructability Considerations

The best designs are only as good as their execution in the real world. And when it comes to A-Jacks, that execution starts at ground level with how the site is prepared and the units are placed. The performance of these interlocking concrete armor units depends heavily on proper installation.

Installation begins with preparing the site to support the interlocking units. Bedding design for A-Jacks installations calls for careful attention to what’s happening beneath the surface. Whether it’s a layer of stone, a geotextile fabric or a combination of both, the goal is the same: to keep native soils in place while allowing the system above to perform as intended. When stone is used as a filter layer, it must be sized and graded to trap underlying sediment, relieve excess pore water pressure and remain stable within the open legs of the A-Jack units. Sometimes, this balance requires multiple layers of stone to meet all the design criteria. Alternatively, a properly selected geotextile fabric can be laid directly on the channel bed, allowing the A-Jack modules to rest securely above without the need for bedding stone. In fast-moving currents, a practical construction technique involves attaching the geotextile directly to the bottom of the modules, streamlining installation and keeping the fabric in place. Design guidance for choosing appropriate geotextile materials is outlined in HEC-23, specifically in Design Guideline 16.

Conclusion

The effectiveness of any scour countermeasure relies not only on the material used but also on how well it’s designed and installed. A-Jacks provide a durable and adaptable solution. When properly implemented, they disrupt erosive flow patterns and help distribute hydraulic forces more evenly across the channel bed. With thoughtful application, they extend the service life of bridges and reduce long-term maintenance needs.

References

1. FHWA Hydraulic Engineering Circular No. 18 (HEC-18), “Evaluating Scour at Bridges,” Fifth Edition (2012).

2. FHWA Hydraulic Engineering Circular No. 23 (HEC-23), “Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Design Guidance,” Third Edition, Volume 2 (2009).

3. National Cooperative Highway Research Program Report 587 (NCHRP-587), “Countermeasures to Protect Bridge Abutments from Scour” (2007).