Rock Shape and Size Decide Which Barriers Save a Historic Village from Falling Boulders

In the fairy-chimney landscape of Cappadocia, Türkiye, the boulders that threaten villages are not abstract hazards but blocks of volcanic rock that have detached from cliffs for millennia. In the historic village of Belisirma, perched in the tourist-famous Ihlara Valley, rockfalls have repeatedly damaged houses and even caused casualties, prompting authorities to declare the settlement a disaster-prone area back in 1993. A new open-access study now shows that the choice of computer model used to predict these falling rocks can mean the difference between a barrier that stops nearly everything and one that lets dangerous boulders sail straight past it. The research, published in Discover Geoscience, demonstrates that treating rocks as simple points rather than real, irregular three-dimensional bodies can dangerously underestimate the threat to people living below a slope.

The study, conducted by Mehmet Sari of Aksaray University, focused on a slope above Belisirma where a rock mass composed of hard ignimbrite, soft ignimbrite, and soft tuff serves as a persistent source of falling blocks. The upper section of the source rock consists of hard ignimbrite cut by persistent joints, while the lower portion is softer ignimbrite with nonpersistent joints, and the base is joint-free tuff. Stereographic analysis of the rock mass revealed three major joint sets: two sub-vertical orthogonal sets created by thermal cracks during cooling of the ignimbrite, and a near-horizontal bedding plane formed by different outflow facies of the ignimbrite deposition. Because joint spacing largely controls the size of blocks that can detach, this geological architecture determines whether the slope releases small debris or massive boulders.

To quantify what the slope could actually produce, Sari used the RocSlope3 program to reconstruct the fractured rock mass in three dimensions. Synthetic traverses defined on the upper slope generated a probabilistic joint network in which joint planes were represented as circular discs with known spacing and radius. The program then discretized the rock mass into individual blocks, identifying 354 discrete blocks with volumes ranging from a mere 0.01 cubic meters to a substantial 18.16 cubic meters. This in-situ block size distribution, a cumulative frequency curve capturing the natural variability of joint spacing and orientation, became the statistical backbone for the subsequent trajectory simulations, ensuring that the modeled rocks reflected what the cliff face could genuinely deliver.

With the block inventory established, the study turned to the central question: how do different impact models change the predicted behavior of falling rocks? The research compared two fundamentally different approaches available in the RocFall3 software. The lumped-mass model treats every rock as a dimensionless point mass, an infinitesimal particle with no physical volume, shape, or true rotation. Energy loss during impacts is governed entirely by a preset coefficient of restitution, and rotation is reduced to a kind of pseudo-rotation derived from an equivalent radius. The rigid-body model, by contrast, uses non-smooth multi-contact dynamics that explicitly account for the rock’s exact three-dimensional geometry, volume, and mass distribution. Impacts are computed at the actual instantaneous contact points on the rock’s surface rather than at its center of mass, allowing the software to capture the eccentric, chaotic behavior of real boulders.

The contrast between the two models was stark. In the lumped-mass simulations, most particles stopped close to the slope toe, and not a single one reached the edge of the model where the endangered houses stand. The rigid-body model told a very different story: most blocks crossed the road, and some reached the downslope houses, successfully reproducing the rockfall paths observed in previous events. The blocks accumulated in three distinct zones along the slope, with rounded, more angular blocks traveling furthest and sharp-edged, less angular blocks coming to rest closer to the toe. Larger blocks consistently traveled farther than smaller ones, carrying greater kinetic energy throughout their descent while also possessing the mass to propel themselves onward after each impact.

Shape emerged as a surprisingly powerful variable. Spherical blocks traveled the longest distances, as expected, but were excluded from the main simulations because freshly detached blocks rarely remain perfectly round. Among the fifteen custom shapes tested across seven size groups spanning 5 to 46,000 kilograms, blocks with extruded octagonal geometry exhibited unexpected lateral spread compared with all other shapes. This finding echoes large-scale experiments by the WSL Institute for Snow and Avalanche Research and ETH Zurich, in which wheel-shaped concrete blocks diverged laterally and occupied far larger impact areas than cube-shaped blocks, which followed the steepest descent path in narrow corridors. Lateral dispersion matters enormously because it controls the width of hazard zones and determines how long a protective barrier must be.

The physics behind these differences is rooted in rotational dynamics. When an irregular rock strikes the slope, the contact point is usually offset from its center of mass, producing an impulse torque that converts translational kinetic energy into rotational energy and back again. Flat edges can trigger sudden sliding or dampening, while sharp vertices cause chaotic, high-velocity angular deviations. Size also determines the moment of inertia, the resistance of a body to changes in its rotation. Larger, more elongated blocks have higher moments of inertia, meaning that once they begin to roll or spin, they store substantial rotational energy that stabilizes their trajectory and drives longer runout distances and higher bounce paths. None of these effects can be represented in a model where the rock has no shape at all.

Armed with the rigid-body results, the study proceeded to design an actual protection system. The optimal fence location was chosen at the end of the first accumulation zone, where block energies and bounce heights are relatively low, minimizing the size of the structure needed. Barrier length was calculated from the geometry of lateral spread using a similarity ratio between the distances from the fence to the source and to the houses, yielding a design length of 20 meters that was doubled for safety. Simulations of a 1400-throw scenario then tested barriers of increasing energy capacity. Without a barrier, 595 of the 1400 blocks, or 42.5 percent, passed the proposed fence location. A 100-kilojoule barrier captured only small blocks, letting 19.6 percent through, while 500-kilojoule and 1000-kilojoule barriers still allowed a handful of large blocks to reach the model edge where the houses stand.

The decisive insight came when specific block geometries were excluded from the analysis. Large, rounded, wheel-like blocks proved to be the highest-risk shapes for bypassing barriers, driven either by excessive energy or by eccentric motion arising from their shape. When these round and large blocks were systematically removed from the simulations, both 1000-kilojoule and 2000-kilojoule barriers achieved complete capture. A sensitivity analysis across 175 to 1400 throws confirmed that barrier performance was independent of sample size. The final recommendation was clear: a 2000-kilojoule fence barrier achieves a 99 percent catching ratio under the full range of realistic block shapes and sizes, while a 1000-kilojoule barrier suffices only if large, rounded blocks are excluded from the hazard scenario.

The implications reach well beyond one Turkish village. The study acknowledges limitations, including input parameters derived from back analysis of only two observed rockfall events, and the simplification of barriers as flat, rigid, undeformable bodies when real fences are flexible structures that deform under repeated impacts. Even so, the message for engineers and heritage managers is unambiguous: nominal barrier energy ratings alone may be insufficient, and design protocols must account for block geometry and rotation beyond standard vertical drop tests. For historic sites like Belisirma, where tourism, cultural heritage, and human lives converge beneath unstable cliffs, the difference between a point mass and a real boulder is not a technical footnote. It is the difference between a barrier that looks adequate on paper and one that actually stops the rock.

Subject of Research: Three-dimensional rockfall trajectory modeling and protection barrier design for a slope above the historic Belisirma village in Cappadocia, Türkiye

Article Title: Application of 3D rockfall impact models for a slope in the historical Belisirma village (Cappadocia) considering block shape and size effects on the design of protection barriers

Article References: Sari, M. (2026). Application of 3D rockfall impact models for a slope in the historical Belisirma village (Cappadocia) considering block shape and size effects on the design of protection barriers. Discover Geoscience, 4(1), Article 324. https://doi.org/10.1007/s44288-026-00699-9

Image Credits: AI Generated

DOI: 10.1007/s44288-026-00699-9

Keywords: rockfall, 3D modeling, rigid-body dynamics, lumped-mass model, Cappadocia, Belisirma village, Ihlara Valley, protection barriers, block shape, block size, hazard assessment, geotechnical engineering