The 2017 Rigopiano Avalanche—Dynamics Inferred from Field Observations
2. Comparison with a Topographic-Statistical Run-Out Model
3. Velocity Estimates
4. Estimate of the Mass Balance
- Along most of the track and probably also in the run-out zone, most of the snow cover was eroded, including the old snow.
- There is no observational information about erosion and deposition in the upper track above 1550 m a.s.l., but based on experience from other avalanche paths (Monte Pizzac, Vallée de la Sionne, Ryggfonn), it is likely that while the net entrainment rate probably was almost equal to the erosion rate so that increased rapidly in that path segment.
- In the middle and lower track, deposition appears to have equaled or exceeded erosion. The likely causes are the diminishing slope angle and increased dissipation due to still intact or already uprooted trees.
- The deposited mass in the run-out area below 1200 m a.s.l. is much larger than the mass of the snow cover before the avalanche. It is likely that the mass increase in the run-out zone also exceeds the release mass. This would mean that and that the running mass, , of the avalanche increased, at least in the steeper parts of the path down to perhaps 1400 or 1300 m a.s.l.
- Detailed measurements at Vallée de la Sionne as well as dynamical considerations  indicate that deposition occurs only in the tail of an avalanche, except right before it stops. This suggests that much of the fully entrained snow—possibly stemming from the upper layers of the snow cover—traveled over a large distance, whereas older snow from near the ground was dragged along a relatively short distance and subsequently deposited when the tail of the avalanche passed over it.
- On the one hand, substantial masses of tree debris were deposited near the distal end of the avalanche, where there had been no forest. This implies that some trees were dragged at least 200–300 m by the avalanche. On the other hand, uprooted or broken trees were found a short distance below the point where the avalanche entered the forest. This suggests that the tree destruction rate (the mass of trees per unit area that were broken and/or uprooted) in a dense forest exceeds the debris deposition rate at least in the steeper reaches of the path and that it also exceeds the debris entrainment rate along the forested part of the path.
5. Observational Limits on Impact Pressure
5.1. Limits Inferred From Forest Destruction
- Between 1550 and 1450 m a.s.l., we assume m, m s, W = 80 m, m, , and . This leads to GW.
- Between 1400 and 1200 m a.s.l., we set , m, m, , m and m s. From this, .
5.2. Limits from Damage to Hotel Rigopiano
6. On the Vulnerability of Buildings and Persons Hit by Snow Avalanches
- The main building can probably be classified as a large multi-story masonry building, whereas the newer spa complex was a one- or two-story reinforced-concrete construction. Neither of them was specifically dimensioned for avalanche impact. Note that these classifications need to be confirmed.
- The impact pressure of the avalanche can only be inferred from numerical simulations, which are fraught with large uncertainties because the initial conditions are poorly known and most models do not explicitly consider fluidization or the formation of a powder snow cloud.
- The damage to the main building was in Category 5, .
- Images in the media suggest that the spa complex collapsed only partially. Snow entered the building in large quantities and blocked the way for the children in the gaming room, but left them with enough space and air for surviving over an extended period. This points to damage in Category 3 and .
- There were a total of 38 persons inside the hotel complex, while two persons were in the parking lot outside the devastated area and were unharmed.
- Several children were playing in a room near the spa area, which was less exposed to the avalanche. The degree of damage was presumably in Category 3, i.e., between 0.4 and 0.7, as it took up to four days to rescue some of the children.
- Five of the adults were found alive. In at least two cases, the degree of damage seems to be in Category 4, yet the persons were only lightly injured.
- The death toll was 29 adults who were gathered in the hotel lobby at the time the avalanche struck. As the entire four-story main building was displaced some 10 m and completely collapsed, one can assume the degree of damage to be in Category 5.
- The data for damage in Categories 4 and 5 appear consistent between the Longyearbyen and Rigopiano events. Somewhat surprisingly, six persons out of 35 survived building damage of degree 5 in Rigopiano, despite the long time it took to find and free them.
- Within the statistical uncertainty, the data from the Rigopiano game room is also consistent with the findings from Longyearbyen, even though the three children in Rigopiano may have survived essentially unscathed in a quite severely damaged room. The degree of damage should, however, be reassessed once more detailed information is released.
- Among the four avalanches that occurred in the vicinity of Rigopiano on or around 18 January 2017, the avalanche that destroyed the hotel had a much longer run-out than the others relative to the prediction of the - model. The reasons for this remain uncertain.
- The estimated front speeds in the range of 35 to 60 m s have a wide margin of uncertainty, but the most likely values in the range of 40 to 45 m s are of the same order as observations, measurements and numerical simulations of avalanches of similar size. The slow-moving, dense parts of the avalanche appear to have moved at about half the speed of the front. This agrees with observations and analyses in small-to-medium size avalanche paths in the Swiss Alps .
- Estimates of the mass balance are also fraught with large uncertainties, but a consistent picture emerges. It is probable that a large part of the snow cover was eroded, but much of this mass was only dragged along for some distance without becoming properly entrained into the flow. Tree debris contributed much to the mass growth of the avalanche.
- The observed damage patterns are consistent with the pressures derived from the velocity estimates, both with respect to the destroyed forest and the obliterated and displaced hotel. Breaking or uprooting of the forest was not a dominant factor in the energy balance of the entire avalanche, but must have had a significant braking effect on the fluidized flow in the forest destruction zone.
- When expressed as a function of the degree of building damage, the lethality curve resulting from the Rigopiano event is consistent with the one found in the 2015 Longyearbyen avalanche within the large uncertainties due to the small statistical base. This supports the usefulness of the degree of damage as the proper variable for obtaining a universal relation that does not depend on the building type.
Conflicts of Interest
Appendix A. Derivation of Flow Velocity from Run-Up
Appendix B. Derivation of Flow Velocity from Super-Elevation
Appendix B.1. Characterization of the Topography
Appendix B.2. Time Dependence
Appendix B.3. Can Inertial Effects Be Neglected?
Appendix B.4. McClung’s Model for Super-Elevation in Granular Avalanches
Appendix B.5. Alternative Treatment of Granular Effects
Appendix B.6. Remarks on the Pudasaini–Jaboyedoff Super-Elevation Model
Appendix C. Forces, Moments and Energetics of Tree Breaking and Uprooting
Appendix D. Estimate of the Strength of Hotel Rigopiano against Avalanche Impact
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|Avalanche Path||Date||Drop||Run-Out||Return||Path Steep-||-Angle (°)||-Angle (°)|
|(m)||Length (m)||Period (year)||Ness (°)||Observed||Predicted|
|Grava di Valle Savina||2017||675||1200||10–50||29||29||27|
|Grava di Valle Cupa||(often)||500||890||∼10||26||29||23|
|Grava di Costa||(often)||380||580||∼10||28||33||26|
|Grava dei Bruciati||(often)||425||700||∼10||22||31||20|
|Monte San Vito||1963||1165||2470||30–100||26||25||24|
|(m a.s.l.)||(m)||(m)||(m)||(t m)||(t m)||(kt)||(kt)||(kt)|
|Exterior walls long||2||25||0.3||3||2000||90|
|Exterior walls short||2||12||0.3||3||2000||40|
|Interior walls long||7||20||0.2||2.5||1200||80|
|Interior walls short||20||10||0.2||2.5||1200||120|
|Snow on roof||1||25||12||2||250||150|
|Degree of Damage||Damage Description|
|Category 1: 0.0–0.1||All spaces intact to slightly skewed. Big voids and structure are stable.|
|Category 2: 0.1–0.4||Impact side partly pushed in or skewed, limited voids at impact side, big voids at lee side, partly skewed/damaged internal walls. Snow/avalanche debris in 10–20% of the building.|
|Category 3: 0.4–0.7||Impact side pushed in/collapsed, big voids approx. 50%, small voids due to snow avalanche debris approx. 20%. Snow/avalanche debris in at least 50% of the building.|
|Category 4: 0.7–0.9||Impact side pushed in/collapsed, internal walls collapsed, no big voids, small voids due to snow avalanche debris approx. 20%. Snow/avalanche debris in at least 90% of the building.|
|Category 5: 0.9–1.0||All spaces destroyed, (almost) no voids remain, large part of building scattered, most walls destroyed.|
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Issler, D. The 2017 Rigopiano Avalanche—Dynamics Inferred from Field Observations. Geosciences 2020, 10, 466. https://doi.org/10.3390/geosciences10110466
Issler D. The 2017 Rigopiano Avalanche—Dynamics Inferred from Field Observations. Geosciences. 2020; 10(11):466. https://doi.org/10.3390/geosciences10110466Chicago/Turabian Style
Issler, Dieter. 2020. "The 2017 Rigopiano Avalanche—Dynamics Inferred from Field Observations" Geosciences 10, no. 11: 466. https://doi.org/10.3390/geosciences10110466