The research focuses on supercritical flow, the regime that develops when inertial forces overwhelm gravity, expressed technically as a Froude number greater than one. In this state, water slamming into a cylinder behaves in ways that have no counterpart in slower, subcritical rivers. The flow climbs the upstream face as an upward wall jet, periodically collapses backward in a phenomenon called reverse spillage, and generates surface shock waves analogous to sonic booms in aerodynamics. Previous work had identified these structures, including the horseshoe vortex system that wraps around an obstacle’s base and drives scour, but almost all of it assumed a smooth bed. Natural mountain rivers, however, are floored with coarse gravel where the ratio of obstacle size to grain size can be as small as two to ten, meaning individual stones are large enough to matter.

To capture this reality, the team combined laboratory experiments with extraordinarily detailed computer simulations. In a flume at the Universidad de Concepción, they placed a four-centimeter cylinder at the centerline of a mobile bed of agate stones between six and ten millimeters in diameter, tilted the channel to a four percent slope, and ran water at a discharge of 4.92 liters per second. The resulting flow was just under two centimeters deep, moving at 0.70 meters per second, giving a Froude number of 1.68 and a Reynolds number near 49,000. Such shallow, fast, air-entraining flow defeated conventional measurement techniques like particle image velocimetry, so the experiments pinned down bulk quantities while the simulations resolved the full three-dimensional flow field.

The digital side of the study is where the technical ambition becomes remarkable. The researchers photographed the bed with a smartphone camera and used Structure-from-Motion photogrammetry to build digital elevation models with an average resolution of 0.2 millimeters, fine enough to capture every individual grain. These surfaces were converted into computational meshes of roughly 13.5 million cells using OpenFOAM, with refinement down to 0.1 millimeters near the bed. The simulations employed detached eddy simulation, a hybrid turbulence approach that resolves large eddies directly in the main flow while modeling near-wall regions, coupled with a volume-of-fluid method that tracks the churning air-water interface. Time steps averaged around 2.5 microseconds, dictated by the demands of tracking the free surface through violent aeration.

The team examined three morphodynamic states that mirror natural channel evolution: a perfectly smooth bed analogous to bedrock channels, a flat rough bed representing gravel deposited after a flood, and an equilibrium scoured bed produced by running the flow at sediment-moving velocities for eight hours until a symmetric scour hole 3.5 centimeters deep had developed around the cylinder. The model validated well against the experiments, reproducing the wall jet climbing the obstacle, the reverse spillage rolling backward at the base at frequencies between roughly five and seven hertz, the aerated lateral jets, and the measured upstream water depth of 1.76 centimeters.

The headline finding concerns what roughness does to flow organization. Over the smooth bed, the time-averaged flow displayed textbook coherence: a well-defined horseshoe vortex system with a primary necklace vortex located about one obstacle radius upstream, secondary vortices further out, and a nearly symmetric bed shear stress distribution whose mean and median differed by only 1.5 percent. The coefficient of variation of stress, a measure of spatial variability, was 37 percent. Over the flat rough bed, that organized picture collapsed. Individual grains acted as micro-obstacles, each generating its own wake and separation zone, fragmenting the horseshoe vortex until the main necklace vortex was barely recognizable and secondary structures vanished entirely. The stress distribution became skewed, its coefficient of variation jumping to 86 percent.

The scoured bed produced the most complex fields of all, because large-scale topographic modification and grain-scale disruption operated hierarchically, with the scour hole reshaping the approach flow within which individual grains continued to generate their own turbulence. Across the three configurations, the coefficient of variation of bed stress climbed from 37 percent to 86 percent to 115 percent, while the mean stress actually fell, from 0.00183 to 0.00133 to 0.00115 square meters per second squared. The paradox is the point: smoother statistics hide wilder extremes. Localized stress concentrations around individual grain crests exceeded smooth-bed conditions by factors of two to three, and the stress distributions grew progressively right-skewed, from a skewness of 0.41 over the smooth bed to 2.60 over the scoured bed.

These results carry immediate consequences for engineering. Bridge piers and bank protection in steep channels are routinely designed using smooth-bed assumptions inherited from subcritical flow research, yet the study shows that natural rough beds subject structures and sediments to frequent low-stress periods punctuated by intense local peaks, a loading pattern that standard methods do not capture. The findings also reshape understanding of sediment transport, because entrainment of any given grain depends on the force balance on that single particle, not the reach-average stress. Meanwhile, the patchwork of high- and low-stress zones may sustain the microhabitat diversity that benthic organisms depend on, and the pressure gradients generated around individual grains may help drive the exchange of water between the river and its subsurface.

The authors are careful about the limits of their approach. The bed was fixed and impermeable, so the extreme stress heterogeneity they observe represents an instantaneous upper bound that grain mobility would erode over time as mobile stones respond to the flow. Validation relied on bulk parameters and visual comparison rather than spatially resolved velocity measurements, and grain-scale stress values at individual particle crests should be read as indicative patterns rather than precise predictions. Even so, the demonstration that supercritical flow over rough beds abandons the organized physics of subcritical flows, and instead becomes grain-dominated chaos, marks a turning point. Predicting how mountain rivers carve their channels, and how the structures we build within them will fare, may now require models that resolve, or at least honestly parameterize, every last stone on the bed.

Subject of Research: Grain-scale bed roughness effects on velocity and bed stress fields around obstacles in supercritical mountain river flow

Article Title: Grain roughness controls on velocity and bed stress fields around a fully protruding obstacle in supercritical flow

Article References: Monsalve, A., & Link, O. (2026). Grain roughness controls on velocity and bed stress fields around a fully protruding obstacle in supercritical flow. Earth Surface Dynamics, 14(3), 469-491. https://doi.org/10.5194/esurf-14-469-2026

Image Credits: AI Generated

DOI: 10.5194/esurf-14-469-2026

Keywords: supercritical flow, grain roughness, horseshoe vortex, bed shear stress, scour, mountain rivers, detached eddy simulation, Structure-from-Motion, sediment transport, bridge piers, turbulence, geomorphology

The team focused on a type of structure that is deceptively common: a bridge pier founded on a group of piles, embedded in a slope affected by a slow-moving landslide. Unlike catastrophic, fast-moving slope failures that destroy bridges in seconds, slow-moving landslides creep at rates of millimeters to centimeters per year, often going unnoticed until cumulative deformation begins to distort the superstructure or crack the foundations. Yet these quiescent masses of soil are not necessarily benign. When an earthquake arrives, the weakened, sheared, and sometimes saturated landslide material can be reactivated, subjecting the piles to a combination of inertial forces from the shaking structure and kinematic forces from the moving ground. The study’s central question was deceptively simple: does the state of the slope before the earthquake matter for the pier’s seismic performance?

To answer it, the researchers built a nonlinear finite element model of the coupled soil-foundation-structure system, using a reference case study chosen precisely because it is paradigmatic of the numerous Italian bridges located in areas exposed to both seismic and hydrogeological hazards. Bridge piers, after all, may sit on the crest of a slope, along its face, or downstream of it, and the relative position of the bridge and the slope strongly influences the type and severity of the actions the structure must resist. The numerical model captured the essential mechanics of the problem: the nonlinear, strain-dependent behavior of the soil under cyclic loading, the interaction between the piles and the surrounding ground, and the dynamic response of the pier itself.

The analysis compared three distinct scenarios. In the first, the pier stands in a stable slope, representing the conventional design assumption in which the ground is competent and the only seismic demand comes from the shaking itself. In the second, the slope hosts a quiescent landslide that the earthquake reactivates, so the ground begins to move as the shaking progresses. In the third, the landslide is already active before the earthquake, meaning the piles have already accumulated deformation and stress from the creeping soil. This progression allowed the researchers to isolate the influence of pre-seismic slope conditions on the dynamic response of the pier, something that standard seismic assessment procedures, which typically assume stable ground, simply do not address.

The results, expressed in terms of maximum and residual displacements and rotations of the pier, show that the starting condition of the slope is far from a secondary detail. When the landslide is active before the earthquake, the piles enter the seismic event already stressed and deformed, and the ground movement during shaking adds kinematic demands on top of the inertial ones. The interaction between the two loading mechanisms can amplify displacements and, critically, leave the pier with larger residual deformations after the shaking stops. Residual displacement matters enormously for bridges, because permanent tilting or lateral shifting of a pier changes the geometry of the deck, alters load paths, and can compromise serviceability or trigger collapse even when the structure survives the earthquake itself.

Underpinning these findings is the physics of soil-structure interaction, a phenomenon that simplified design methods often reduce to a set of springs or, worse, ignore altogether. In reality, the piles, the surrounding soil and the pier form a single dynamic system whose response depends on the stiffness and damping of every component. Decades of research, from classical work on dynamic pile stiffness to modern studies of kinematic pile bending in layered soils, have shown that the ground is not a passive support but an active participant in the seismic response. The new study extends this understanding to the multi-hazard setting, demonstrating that the dynamic properties of a slope containing a landslide mass differ from those of a stable slope, and that these differences propagate directly into the demands on the foundation.

The Italian context gives the work particular urgency. The national landslide inventory compiled by the Institute for Environmental Protection and Research documents hundreds of thousands of landslide phenomena across the country, and recent research has catalogued extensive interactions between landslides and bridge foundations in Italian case studies. At the same time, Italy has endured repeated bridge failures, some linked to the degradation of structures subjected to unanticipated environmental actions. The national guidelines for classifying and managing the risk of existing bridges, introduced in 2020 and operationalized in subsequent years, pushed owners to assess their assets systematically, and recent technical guidance has begun to address landslide interaction explicitly. The new study provides a quantitative basis for that effort, showing what is at stake when a bridge pier’s foundation crosses a slowly moving mass of soil.

What makes the study especially valuable for practitioners is its methodological clarity. By using a nonlinear finite element approach within a single consistent model, the researchers could track the full path from static slope condition to dynamic seismic response, capturing the state of stress and strain in the soil at the moment the earthquake begins. This matters because the initial stress state governs how the soil stiffens or softens under cyclic loading, how pore pressures evolve, and how much permanent deformation accumulates. The approach also reflects the reality of earthquake-triggered landslide reactivation, a phenomenon documented in devastating detail after events such as the 2008 Wenchuan earthquake in China and the 2016 Kumamoto earthquakes in Japan, where seismically induced slope failures destroyed bridges that had performed adequately under shaking alone.

The broader message is a call for multi-hazard thinking in the assessment and retrofit of existing infrastructure. A bridge evaluated only for seismic loads, assuming stable ground, may be far more vulnerable than its safety factor suggests if it stands in the path of a slow-moving landslide. Conversely, retrofit strategies designed for landslide stabilization alone may leave the structure exposed to seismic demands that the modified soil-foundation system now transmits differently. The authors argue that accounting for soil-foundation-structure interaction is essential when assessing the seismic response of bridge piers in the presence of slow-moving landslides, and that a multi-hazard approach is needed to enhance both the assessment of existing bridges and the sustainability of retrofit strategies. As climate change alters rainfall patterns and slope hydrology, and as seismic hazard remains a permanent feature of the Mediterranean landscape, the quiet creep of a hillside may prove to be the hidden variable that determines whether the next earthquake is survivable for the bridges that carry our lifelines.

Subject of Research: Seismic response of pile-supported bridge piers founded in slopes affected by slow-moving landslides

Article Title: Seismic response of a pile-supported bridge pier under multi-hazard conditions in slow-moving landslides: a numerical investigation

Article References: Ambrosino, A., Servodio, M., Sica, S., Losanno, D., Parisi, F., & Santolo, A. S. D. (2026). Seismic response of a pile-supported bridge pier under multi-hazard conditions in slow-moving landslides: a numerical investigation. Bulletin of Earthquake Engineering. https://doi.org/10.1007/s10518-026-02678-4

Image Credits: AI Generated

DOI: 10.1007/s10518-026-02678-4

Keywords: soil-structure interaction, landslides, earthquake engineering, multi-hazard assessment, bridge piers, pile foundations, finite element analysis, slope stability, seismic retrofit, geotechnical engineering, infrastructure resilience, residual displacement