The depositional processes and gas pore pressure in pyroclastic flows are investigated through scaled experiments on transient, initially fluidized granular flows. The flow structure consists of a ...sliding head whose basal velocity decreases backwards from the front velocity (
U
f
) until onset of deposition occurs, which marks transition to the flow body where the basal deposit grows continuously. The flows propagate in a fluid-inertial regime despite formation of the deposit. Their head generates underpressure proportional to
U
f
2
whereas their body generates overpressure whose values suggest that pore pressure diffuses during emplacement. Complementary experiments on defluidizing static columns prove that the concept of pore pressure diffusion is relevant for gas-particle mixtures and allow characterization of the diffusion timescale (
t
d
) as a function of the material properties. Initial material expansion increases the diffusion time compared with the nonexpanded state, suggesting that pore pressure is self-generated during compaction. Application to pyroclastic flows gives minimum diffusion timescales of seconds to tens of minutes, depending principally on the flow height and permeability. This study also helps to reconcile the concepts of
en masse
and
progressive
deposition of pyroclastic flow units or discrete pulses. Onset of deposition, whose causes deserve further investigation, is the most critical parameter for determining the structure of the deposits. Even if sedimentation is fundamentally continuous, it is proposed that late onset of deposition and rapid aggradation in relatively thin flows can generate deposits that are almost snapshots of the flow structure. In this context, deposition can be considered as occurring
en masse
, though not strictly instantaneously.
Entrainment of underlying debris by geophysical flows can significantly increase the flow deposit extent. To study this phenomenon, analog laboratory experiments have been conducted on granular ...column collapse over an inclined channel with and without an erodible bed made of similar granular material. Results show that for slope angles below a critical value θc, between 10° and 16°, the run out distance rf depends only on the initial column height h0 and is unaffected by the presence of an erodible bed. On steeper slopes, the flow dynamics change fundamentally, with a slow propagation phase developing after flow front deceleration, significantly extending the flow duration. This phase has characteristics similar to those of steady uniform flows. Its duration increases with increasing slope angle, column volume, column inclination with respect to the slope and channel width, decreasing column aspect ratio (height over length), and in the presence of an erodible bed. It is independent, however, of the maximum front velocity. The increase in the duration of the slow propagation phase has a crucial effect on flow dynamics and deposition. Over a rigid bed, the development of this phase leads to run out distances rf that depend on both the initial column height h0 and length r0. Over an erodible bed, as the duration of the slow propagation phase increases, the duration of bed excavation increases, leading to a greater increase in the run out distance compared with that over a rigid bed (up to 50%). This effect is even more pronounced as bed compaction decreases.
Key Points
There is a critical angle over which the flow dynamics change
Above the critical angle a slow propagation phase appear after deceleration
Increasing the duration of the slow propagation phase changes the dynamics
•The eruptive discharge rate controls runout of pyroclastic density currents (PDCs).•The discharge rate of PDCs is 13.6 times greater than that of the plinian phase.•PDC runout increases with the ...discharge rate according to power law relationships.•The runout of dilute turbulent PDCs is in agreement with theory.•Dilute and concentrated currents have distinct values of the deposit aspect ratio.
We address, through quantitative analysis of results from independent studies, the control of the discharge rate of explosive volcanic eruptions on the runout distance of highly hazardous pyroclastic density currents. We analyze with statistical methods data from 47 well-documented currents with runouts of ∼3–185 km and generated by minor eruptions to super-eruptions with discharge rates Q∼107–1012 kg/s. Our analysis shows first that the discharge rate during the phase of pyroclastic density currents is on average 13.6 times greater than the rate during the preceding plinian phase. We further find that the runout of both dilute turbulent currents and of two-layer flows with a concentrated base correlates remarkably well with the discharge rate. By applying the power law relationships we infer, we next model the as yet unknown discharge rates of over 53 events, including 27 super-eruptions. At a given rate, dilute currents travel on land generally farther than their concentrated counterparts, and they are even more mobile when propagating over water. We further demonstrate that the runout of dilute current scales with (Q/w)0.5, with w the particle settling velocity, in agreement with theory. Assuming concentrated PDCs obey the same principle we infer particle settling velocities of ∼1–10 m/s for these currents. We show also that the classical deposit aspect ratio, AR, allows to discriminate between emplacement from dilute (AR<∼5×10−5) and concentrated (AR>∼5×10−5) current, which permits us to discuss the dynamics of PDCs produced by the Ito (29 ka) and Taupo (AD 232) eruptions.
We present the results of analogue laboratory experiments on pyroclastic fountain collapse. Mixtures of air and glass beads ranging in size around 75 ± 15 μm, with Stokes number of ~10−3−101 and ...representative of 0.1–1 mm sized particles in nature, were released from a hopper at heights of 0.45–2.95 m above the base of a horizontal channel. Free fall caused continuous dilation of the granular material and led to mean particle concentrations of ~9–36 vol%, with concentration inversely proportional to drop height, before the particles impacted the channel base. Decoupling between the particles and the ambient air upon impact caused deflation of the mixture, which then propagated laterally as a dense granular flow overridden by a dilute suspension. Measurements at the impingement surface revealed that pore fluid pressure, generated through high air-particle relative velocity during deflation, counterbalanced up to ~50% of the weight of the emerging granular flow. The runout distance of the dense flow increased linearly with the fall height, similar to published results on unidirectional flows generated from collapse of packed granular columns. This suggests that the runout of flows resulting from release of granular material is controlled essentially by conversion of potential to kinetic energy and that the initial particle concentration is a second order parameter. We conclude that fountaining of pyroclastic material containing large amounts of particles with Stokes numbers of the order 10−3−101 can generate dense pyroclastic flows with some degree of pore fluid pressure.
•We present analogue experiments on pyroclastic fountain collapse•We release glass beads from a hopper at different heights above a horizontal channel•Stokes number ~10−3–101 corresponds to 0.1–1 mm sized particles in nature.•Pore fluid pressure measured at impact is high enough to cause partial fluidization.•The runout distance of the dense flow formed upon impact scales with the fall height.
This study investigates the influence of the substrate surface roughness on the emplacement mechanisms of pyroclastic flows. We carried out laboratory experiments on gravitational flows generated ...from the release of initially fluidized or nonfluidized columns of fine particles (diameter d = 0.08 mm) in a horizontal channel. The roughness of the channel base was uniform in each experiment, created by gluing particles of diameter d0 = 0.08 to 6 mm to the base. Other things being equal, the flow runout distance increased with the channel base roughness (d0) to a maximum of about twice that of flows on a smooth substrate when d0 = 1.5–3 mm, before decreasing moderately at higher roughness values of d0 = 6 mm. Long runout originated mainly during the late stages of emplacement as flow deceleration was strongly reduced at high substrate roughness. This was caused by (partial) autofluidization due to an upward air flux escaping from the substrate interstices in which flow particles settled. Autofluidization was evidenced by high pore fluid pressure measurements at the base of initially nonfluidized flows and also by reduced flow runout when the interstices were initially partially filled so that less air was available. Furthermore, the runout distance of flows of large particles (d = 0.35 mm), which could not be fluidized by the ascending air flux, was independent of the substrate roughness. This study suggests that autofluidization caused by air escape from the interstices of a rough substrate is one important mechanism to explain the common long runout distance of pyroclastic flows even on subhorizontal topographies.
Key Points
Flows of fine particles on horizontal rough substrate can be autofluidized
Autofluidization is due to air escape from the substrate interstices
Autofluidization is expected to cause long runout of pyroclastic flows
Background In a retrospective multicenter study, we evaluated the efficiency and outcomes of the different therapeutic options for infection after reversed shoulder arthroplasty. Methods Thirty-two ...patients were reoperated on for infection after reversed shoulder arthroplasty between 1996 and 2011. The mean age was 71 (55-83) years. The involved implants were primary prostheses in 23 cases and revision prostheses in 9 cases. The average preoperative Constant score was 34 (11-69). Six of these patients needed 2 successive procedures. A total of 38 procedures were performed: débridement (13), 1-stage (5) or 2-stage revision (14), or implant removal (6). At last follow-up (mean, 36 months; range, 12-137 months), every patient had clinical, biologic, and radiographic evaluation. Results Infections were largely caused by coagulase-negative staphylococci (56%) and Propionibacterium acnes (59%). The complication rate was 26%. At last follow-up, 26 patients were free of infection (81%). The final Constant score was 46 (12-75). After débridement with implant retention, the mean Constant score was 51 (29-75), but the healing rate was only 54%. Implant revision (1 or 2 stage) led to better functional results than implant removal (46 vs. 25; P = .001), with similar healing rates (73% and 67%, respectively). Patients with low initial impairment (Constant score > 30) were not significantly improved by surgical treatment. Conclusion Débridement is the less aggressive option but exposes patients to healing failure. It should be proposed as a first treatment attempt. Revision of the implant is technically challenging but preserves shoulder function, with no higher rate of residual infection compared with implant removal.
Deposits of pyroclastic density currents (PDCs) often contain accidental lithic clasts of typical size of 0.1–1 m captured from an underlying substrate by the parent flows at distances up to several ...tens of kilometers from the eruptive vent. In order to gain insights into the nature of PDCs, this study investigates the conditions required for entrainment of particles from a granular substrate by a gas–particle density current, with special emphasis to ignimbrite-forming currents whose dynamics are controversial. The two types of physics of emplacement of PDCs proposed in literature are considered. The first model deals with a hydraulically rough, dilute turbulent PDC of bulk density of ∼1–10 kg/m3 and considers that entrainment through both traction and saltation is controlled by a Shield criterion at high (>104) particle Reynolds number. The second model considers entrainment by a PDC consisting of a dense basal flow of bulk density of the order of 103 kg/m3 and with high interstitial gas pore pressure. This model involves uplift of substrate particles, caused by an upward pressure gradient at the flow-substrate interface, and then transport and deposition on the aggrading basal deposit of the flow as demonstrated by recent laboratory experiments. Results show that a dilute PDC can entrain blocks of maximum size of ∼10–15 cm (for a block density of 2000–3000 kg/m3) if maximum current velocities up to ∼100 m/s are taken into account. This, in turn, suggests that larger (heavier) blocks found in deposits were captured by PDCs if these had a dense basal flow. The dense flow model predicts that PDCs have the potential to entrain metric blocks, whose maximum size (up to ∼2–5 m) decreases with decreasing atmospheric pressure (i.e. increasing altitude). Application of the model considering published data on the characteristics of accidental blocks in several well-studied ignimbrites indicates that the velocity of the parent dense PDCs was up to ∼25–30 m/s.
•Entrainment of substrate particles by pyroclastic density currents is investigated.•Dilute turbulent currents can entrain blocks of size up to ∼10–15 cm.•Blocks larger than ∼10–15 cm are captured by PDCs if these have a dense basal flow.•Size of blocks in ignimbrites give velocities of some dense flows up to ∼25–30 m/s.
Congenital tufting enteropathy (CTE) is a rare and severe enteropathy recently ascribed to mutations in the epcam gene. Here we establish
SPINT2
, previously ascribed to congenital sodium diarrhea, ...as a second gene associated with CTE and report molecular and immunohistochemistry data in 57 CTE patients. Inclusion criteria were early onset diarrhea and intestinal insufficiency with the typical histological CTE abnormalities. The clinical phenotype was registered, the entire coding regions of epcam and
SPINT2
sequenced, and immunostaining of EpCAM and SPINT2 performed on intestinal biopsies. An epcam mutation was involved in 41 patients (73 %) who mainly displayed isolated digestive symptoms. Mutations severely affected gene expression since the EpCAM signal on intestinal tissues was either undetectable or low and irregular. Twelve other patients (21 %) carried mutations in
SPINT2
, and were phenotypically characterized by systematic association with keratitis (
p
< 10
−4
) and, for half of them, with choanal atresia (
p
< 10
−4
). Dependency on parenteral nutrition (PN) was comparable in patients with epcam or
SPINT2
mutations, but the frequent epcam mutation c.556-14A>G (abnormal splicing) was significantly associated with a better outcome (
p
= 0.032) with milder PN dependency to weaning in some cases. Finally, four patients (7 %) with isolated digestive symptoms had no detectable epcam or
SPINT2
mutation. Two candidate genes,
Elf3
and
Claudin7,
were excluded from this population. Our study allows us to separate CTE patients into at least three genetic classes, each with specific phenotypes. The genetics approach raises the question of the distinction between two congenital enteropathies. Our findings should help improve the diagnosis of CTE, guide toward strategies of long-term PN management, and limit indications for intestinal transplantation to life-threatening PN complications.
The diversity of femoral morphology renders femoral component sizing in total hip arthroplasty (THA) challenging. We aimed to determine whether femoral morphology and femoral component filling ...influence early clinical and radiological outcomes following THA using fully hydroxyapatite (HA)-coated femoral components.
We retrospectively reviewed records of 183 primary uncemented THAs. Femoral morphology, including Dorr classification, canal bone ratio (CBR), canal flare index (CFI), and canal-calcar ratio (CCR), were calculated on preoperative radiographs. The canal fill ratio (CFR) was calculated at different levels relative to the lesser trochanter (LT) using immediate postoperative radiographs: P1, 2 cm above LT; P2, at LT; P3, 2 cm below LT; and D1, 7 cm below LT. At two years, radiological femoral component osseointegration was evaluated using the Engh score, and hip function using the Postel Merle d'Aubigné (PMA) and Oxford Hip Score (OHS).
CFR was moderately correlated with CCR at P1 (r = 0.44; p < 0.001), P2 (r = 0.53; p < 0.001), and CFI at P1 (r = - 0.56; p < 0.001). Absence of spot welds (n = 3, 2%) was associated with lower CCR (p = 0.049), greater CFI (p = 0.017), and lower CFR at P3 (p = 0.015). Migration (n = 9, 7%) was associated with lower CFR at P2 (p = 0.028) and P3 (p = 0.007). Varus malalignment (n = 7, 5%), predominantly in Dorr A femurs (p = 0.028), was associated with lower CFR at all levels (p < 0.05). Absence of spot welds was associated with lower PMA gait (p = 0.012) and migration with worse OHS (p = 0.032).
This study revealed that femurs with insufficient proximal filling tend to have less favourable radiological outcomes following uncemented THA using a fully HA-coated double-tapered femoral component.Cite this article:
2020;9(4):182-191.
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