pinecone/core-2020-05-10-deduplication huggingface dataset:
Turbidity current with a roof: Success and failure of RANS modeling for turbidity currents under strongly stratified conditions

Density underflows in general and turbidity currents in particular differ from rivers in that their governing equations do not allow a steady, streamwise uniform “normal” solution. This is due to the fact that density underflows entrain ambient fluid, thus creating a tendency for underflow discharge to increase downstream. Recently, however, a simplified configuration known as the “turbidity current with a roof” (TCR) has been proposed. The artifice of a roof allows for steady, uniform solutions for flows driven solely by gravity acting on suspended sediment. A recent application of direct numerical simulation (DNS) of the Navier-Stokes equations by Cantero et al. (2009) has revealed that increasing dimensionless sediment fall velocity increases flow stratification, resulting in a damping of the turbulence. When the dimensionless fall velocity is increased beyond a threshold value, near-bed turbulence collapses. Here we use the DNS results as a means of testing the ability of three Reynolds-averaged Navier-Stokes (RANS) models of turbulent flow to capture stratification effects in the TCR. Results showed that the Mellor-Yamada and quasi-equilibrium k-ε models are able to adequately capture the characteristics of the flow under conditions of relatively modest stratification, whereas the standard k-ε model is a relatively poor predictor of turbulence characteristics. As stratification strengthens, however, the deviation of all RANS models from the DNS results increases. All are incapable of predicting the collapse of near-bed turbulence predicted by DNS under conditions of strong stratification. This deficiency is likely due to the inability of RANS models to replace viscous dissipation of turbulent energy with transfer to internal waves under conditions of strong stratification. Within the limits of modest stratification, the quasi-equilibrium k-ε model is used to derive predictors of flow which can be incorporated into simpler, layer-averaged models of turbidity currents.Fil: Yeh, Tzu Hao. University Of Illinois At Urbana; Estados Unidos. Shell International Exploration and Production; Estados UnidosFil: Cantero, Mariano Ignacio. Comisión Nacional de Energía Atómica. Gerencia del Area de Energía Nuclear. Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Cantelli, Alessandro. Shell International Exploration and Production; Estados UnidosFil: Pirmez, Carlos. Shell Nigeria Exploration and Production; NigeriaFil: Parker, Gary. University Of Illinois At Urbana; Estados Unido
Polydisperse suspensions : erosion, deposition, and flow capacity

Deposition from particle-laden flows is often described in terms of the capacity and competence of the flow, but robust definitions of these terms have proved elusive. In this paper we provide a mathematical modelling framework within which erosion and deposition of polydisperse sediment, and thus flow capacity and competence, can be rigorously defined. This framework explicitly captures the coupling between the suspension and an active layer of sediment at the top of the bed, and is capable of describing both depositional and erosional flows over both erodible and non-erodible beds. Crucially, the capacity of a flow is shown to depend on the erosional and depositional history because these processes determine the composition of the active layer. This dependence is explored within models of bidisperse and polydisperse suspensions. It is further demonstrated that monodisperse representations of suspended sediment transport may severely underpredict actual flow capacity. The polydisperse model is validated against recent experimental studies of the evolution of suspended material in waning turbulent flows, and is used to demonstrate that loss of capacity is the principal driver of sediment deposition
Numerical simulation of bar and island morphodynamics in anabranching mega-rivers

Bar and island morphodynamics in the world’s largest anabranching rivers are investigated using a new numerical model of hydrodynamics, sediment transport, bank erosion and floodplain development,operating over periods of several hundred years. Simulated channel morphology is compared to that of natural rivers and shown to be realistic, both in terms of the statistical characteristics of channel width, depth and bar shape distributions, and mechanisms of unit bar, compound bar and island evolution. Results demonstrate that bar and island stability may be sensitive to hydrologic regime, because greater variability in flood magnitude encourages the formation of emergent bars that can be stabilized by vegetation colonization. Simulations illustrate a range of mechanisms of unit bar generation that are linked to local bed and/or bank instabilities. This relationship may explain the reduced frequency of unit bars observed in some large anabranching rivers that are characterized by stable vegetated islands and slow rates of channel change. Model results suggest that the degree to which sand-sized bed material is carried in suspension likely represents an important control on bar morphodynamics and channel network evolution, because of its influence on sand transport direction. Consequently, differences in the partitioning of the total sand load between bedload and suspension may provide a partial explanation for contrasting styles of anabranching in the world’s largest sand-bed rivers. These results highlight a need for spatially distibuted flow and sediment transport datasets from large rivers, in order to support improved parameterizations of sand transport mechanics in morphodynamic models
Numerical simulation of bar and island morphodynamics in anabranching mega-rivers

Onlineopen article ©2013 American Geophysical Union.Bar and island morphodynamics in the world's largest anabranching rivers are investigated using a new numerical model of hydrodynamics, sediment transport, bank erosion, and floodplain development, operating over periods of several hundred years. Simulated channel morphology is compared to that of natural rivers and shown to be realistic, both in terms of the statistical characteristics of channel width, depth, and bar shape distributions, and mechanisms of unit bar, compound bar, and island evolution. Results demonstrate that bar and island stability may be sensitive to hydrologic regime, because greater variability in flood magnitude encourages the formation of emergent bars that can be stabilized by vegetation colonization. Simulations illustrate a range of mechanisms of unit bar generation that are linked to local bed or bank instabilities. This link may explain the reduced frequency of unit bars observed in some large anabranching rivers that are characterized by stable vegetated islands and slow rates of channel change. Model results suggest that the degree to which sand-sized bed material is carried in suspension likely represents an important control on bar morphodynamics and channel network evolution, because of its influence on sand transport direction. Consequently, differences in the partitioning of the total sand load between bed load and suspension may provide a partial explanation for contrasting styles of anabranching in the world's largest sand-bed rivers. These results highlight a need for spatially-distributed flow and sediment transport data sets from large rivers, in order to support improved parameterizations of sand transport mechanics in morphodynamic models.Natural Environment Research Council (NERC). Grant Numbers: NE/I023228/1, NE/E016022/
Differing source water inputs, moderated by evaporative enrichment, determine the contrasting δ18OCELLULOSE signals in maritime Antarctic moss peat banks

Oxygen isotope palaeoclimate records, preserved in moss tissue cellulose, are complicated by environmental influences on the relationships between source water inputs and evaporative conditions. We carried out stable isotope analyses of precipitation collected from the maritime Antarctic and cellulose extracted from co-located Chorisodontium aciphyllum dominated moss peat bank deposits accumulated since 1870 A.D. Analyses of stable oxygen and hydrogen isotope composition of summer precipitation on Signy Island (60.7°S, 45.6°W) established a local meteoric water line (LMWL) similar to both the global MWL and other LMWLs, and almost identical to the HadAM3 isotope-enabled global circulation model output. The oxygen isotopic composition of cellulose (δ18OC) revealed little temporal variation between four moss peat banks on Signy Island since 1870. However, δ18OC followed two patterns with Sites A and D consistently 3‰ enriched relative to δ18OC values from Sites B and C. The growing moss surfaces at Sites A and D are likely to have been hydrated by isotopically heavier summer precipitation, whilst at Sites B and C, the moss banks are regularly saturated by the isotopically depleted snow melt streams. Laboratory experiments revealed that evaporative enrichment of C. aciphyllum moss leaf water by 5‰ occurred rapidly following saturation (ecologically equivalent to post-rainfall or snow melt periods). In addition to the recognized source water-cellulose fractionation extent of 27 ± 3‰, such a shift would account for the 32‰ difference measured between δ18O of Signy Island precipitation and cellulose
Tracing the origin of Arctic driftwood

Arctic environments, where surface temperatures increase and sea ice cover and permafrost depth decrease, are very sensitive to even slight climatic variations. Placing recent environmental change of the high-northern latitudes in a long-term context is, however, complicated by too short meteorological observations and too few proxy records. Driftwood may represent a unique cross-disciplinary archive at the interface of marine and terrestrial processes. Here, we introduce 1445 driftwood remains from coastal East Greenland and Svalbard. Macroscopy and microscopy were applied for wood anatomical classification; a multi-species subset was used for detecting fungi; and information on boreal vegetation patterns, circumpolar river systems, and ocean current dynamics was reviewed and evaluated. Four conifer (Pinus, Larix, Picea, and Abies) and three deciduous (Populus, Salix, and Betula) genera were differentiated. Species-specific identification also separated Pinus sylvestris and Pinus sibirica, which account for ~40% of all driftwood and predominantly originate from western and central Siberia. Larch and spruce from Siberia or North America represents ~26% and ~18% of all materials, respectively. Fungal colonization caused different levels of driftwood staining and/or decay. Our results demonstrate the importance of combining wood anatomical knowledge with insight on boreal forest composition for successfully tracing the origin of Arctic driftwood. To ultimately reconstruct spatiotemporal variations in ocean currents, and to better quantify postglacial uplift rates, we recommend consideration of dendrochronologically dated material from many more circumpolar sites
Isoscapes of tree-ring carbon-13 perform like meteorological networks in predicting regional precipitation patterns

Stable isotopes in tree rings provide climatic information with annual resolution dating
back for centuries or even millennia. However, deriving spatially explicit climate models
from isotope networks remains challenging. Here we propose a methodology to model
regional precipitation from carbon isotope discrimination (Δ13C) in tree rings by (1)
building regional spatial models of Δ13C (isoscapes) and (2) deriving precipitation
maps from Δ13C-isoscapes, taking advantage of the response of Δ13C to precipitation in
seasonally dry climates. As a case study, we modeled the spatial distribution of mean
annual precipitation (MAP) in the northeastern Iberian Peninsula, a region with complex
topography and climate (MAP = 303–1086 mm). We compiled wood Δ13C data for two
Mediterranean species that exhibit complementary responses to seasonal precipitation
(Pinus halepensis Mill., N = 38; Quercus ilex L.; N= 44; pooling period: 1975–2008). By
combining multiple regression and geostatistical interpolation, we generated one
Δ13 C-isoscape for each species. A spatial model of MAP was then built as the sum of two
complementary maps of seasonal precipitation, each one derived from the corresponding
Δ13C-isoscape (September–November from Q. ilex; December–August from P.
halepensis). Our approach showed a predictive power for MAP (RMSE = 84 mm) nearly
identical to that obtained by interpolating data directly from a similarly dense network of
meteorological stations (RMSE = 80–83 mm, N= 65), being only outperformed when
using a much denser meteorological network (RMSE=56–57 mm, N=340). This method offers
new avenues for modeling spatial variability of past precipitation, exploiting the large
amount of information currently available from tree-ring networks.This work was funded by the Marie Curie project SMARTREES (MC-ERG-246725, European Union, FP7), and Spanish project PALEOISOTREE (CGL2009-13079-C02-01). J.d.C. is supported by a FPI fellowship (MCINN, Spain). JPF is supported by the Ramón y Cajal programme (RYC-2008-02050, MCINN, Spain)
Contrasting CO2 concentration discharge dynamics in headwater streams: a multi-catchment comparison\ud

Aquatic CO2 concentrations are highly variable and strongly linked to discharge but until recently measurements have been largely restricted to low-frequency manual sampling. Using new in-situ CO2 sensors we present concurrent, high-frequency (<30-min resolution) CO2 concentration and discharge data collected from five catchments across Canada, UK and Fennoscandinavia to explore concentration-discharge dynamics; we also consider the relative importance of high flows to lateral aquatic CO2 export. The catchments encompassed a wide range of mean CO2 concentrations (0.73 – 3.05 mg C L-1) and hydrological flow regimes from flashy peatland streams to muted outflows within a Finnish lake-system. In three of the catchments CO2 concentrations displayed clear bimodal distributions indicating distinct CO2 sources. Concentration-discharge relationships were not consistent across sites with three of the catchments displaying a negative relationship and two catchments displaying a positive relationship. When individual high flow events were considered, we found a strong correlation between both the average magnitude of the hydrological and CO2 response peaks, and the average response lag times. An analysis of lateral CO2 export showed that in three of the catchments the top 30% of flow (i.e. flow that was exceeded only 30% of the time) had the greatest influence on total annual load. This indicates that an increase in precipitation extremes (greater high-flow contributions) may have a greater influence on the flushing of CO2 from soils to surface waters than a long-term increase in mean annual precipitation, assuming source limitation does not occur
Lidar-derived estimate and uncertainty of carbon sink in successional phases of woody encroachment

Woody encroachment is a globally occurring phenomenon that contributes to the global carbon sink. The magnitude of this contribution needs to be estimated at regional and local scales to address uncertainties present in the global- and continental-scale estimates, and guide regional policy and management in balancing restoration activities, including removal of woody plants, with greenhouse gas mitigation goals. The objective of this study was to estimate carbon stored in various successional phases of woody encroachment. Using lidar measurements of individual trees, we present high-resolution estimates of aboveground carbon storage in juniper woodlands. Segmentation analysis of lidar point cloud data identified a total of 60,628 juniper tree crowns across four watersheds. Tree heights, canopy cover, and density derived from lidar were strongly correlated with field measurements of 2613 juniper stems measured in 85 plots (30×30 m). Aboveground total biomass of individual trees was estimated using a regression model with lidar-derived height and crown area as predictors (Adj. R2=0.76, p<0.001, RMSE=0.58 kg). The predicted mean aboveground woody carbon storage for the study area was 677 g/m2. Uncertainty in carbon storage estimates was examined with a Monte Carlo approach that addressed major error sources. Ranges predicted with uncertainty analysis in the mean, individual tree, aboveground woody C, and associated standard deviation were 0.35-143.6 kg and 0.5-1.25 kg, respectively. Later successional phases of woody encroachment had, on average, twice the aboveground carbon relative to earlier phases. Woody encroachment might be more successfully managed and balanced with carbon storage goals by identifying priority areas in earlier phases of encroachment where intensive treatments are most effective
Physical ecology of hypolithic communities in the central Namib desert : the role of fog, rain, rock habitat, and light

[1] Hypolithic microbial communities are productive niches in deserts worldwide, but many facets of their basic
ecology remain unknown. The Namib Desert is an important site for hypolith study because it has abundant
quartz rocks suitable for colonization and extends west to east across a transition from fog- to rain-dominated
moisture sources. We show that fog sustains and impacts hypolithic ecology in several ways, as follows: (1) fog
effectively replaces rainfall in the western zone of the central Namib to enable high (≥95%) hypolithic abundance
at landscape (1–10 km) and larger scales; and (2) high water availability, through fog (western zone) and/or
rainfall (eastern zone), results in smaller size-class rocks being colonized (mean 6.3 ± 1.2 cm) at higher
proportions (e.g., 98% versus approximately 3%) than in previously studied hyperarid deserts. We measured
0.1% of incident sunlight as the lower limit for hypolithic growth on quartz rocks in the Namib and found that
uncolonized ventral rock surfaces were limited by light rather than moisture. In situ monitoring showed that
although rainfall supplied more liquid water (36 h) per event than fog (mean 4 h), on an equivalent annual basis,
fog provided nearly twice as much liquid water as rainfall to the hypolithic zone. Hypolithic abundance reaches
100% at a mean annual precipitation (MAP) of approximately 40–60 mm, but at a much lower MAP
(approximately 25 mm) when moisture from fog is available.This work was partially supported through NASA’s ASTEP Programhttp://agupubs.onlinelibrary.wiley.com/agu/jgr/journal/10.1002/(ISSN)2169-8961hb201
Straightening out planar poly-line drawings

We show that any $y$-monotone poly-line drawing can be straightened out while
maintaining $y$-coordinates and height. The width may increase much, but we
also show that on some graphs exponential width is required if we do not want
to increase the height. Likewise $y$-monotonicity is required: there are
poly-line drawings (not $y$-monotone) that cannot be straightened out while
maintaining the height. We give some applications of our result.Comment: The main result turns out to be known (Pach & Toth, J. Graph Theory
  2004, http://onlinelibrary.wiley.com/doi/10.1002/jgt.10168/pdf
On the number of quasi-kernels in digraphs

A vertex set X of a digraph D = (V, A) is a kernel if X is independent (i.e., all pairs of distinct vertices of X are non-adjacent) and for every v  V-X there exists x  X such that vx  A. A vertex set X of a digraph D = (V, A), is a quasi-kernel if X is independent and for every v  V-X there exist w  V-X, x  X such that either vx  A or vw, wx  A. In 1974, Chvátal and Lovász proved that every digraph has a quasi-kernel. In 1996, Jacob and Meyniel proved that if a digraph D has no kernel, then D contains at least three quasi-kernels. We characterize digraphs with exactly one and two quasi-kernels, and, thus, provide necessary and sufficient conditions for a digraph to have at least three quasi-kernels. In particular, we prove that every strong digraph of order at least three, which is not a 4-cycle, has at least three quasi-kernels
Simultaneous Diagonal Flips in Plane Triangulations

Simultaneous diagonal flips in plane triangulations are investigated. It is
proved that every $n$-vertex triangulation with at least six vertices has a
simultaneous flip into a 4-connected triangulation, and that it can be computed
in O(n) time. It follows that every triangulation has a simultaneous flip into
a Hamiltonian triangulation. This result is used to prove that for any two
$n$-vertex triangulations, there exists a sequence of $O(\log n)$ simultaneous
flips to transform one into the other. The total number of edges flipped in
this sequence is O(n). The maximum size of a simultaneous flip is then studied.
It is proved that every triangulation has a simultaneous flip of at least
${1/3}(n-2)$ edges. On the other hand, every simultaneous flip has at most
$n-2$ edges, and there exist triangulations with a maximum simultaneous flip of
${6/7}(n-2)$ edges.Comment: A short version of this paper will be presented at SODA 200
Pathwidth of outerplanar graphs

International audienceWe are interested in the relation between the pathwidth of a biconnected outerplanar graph and the pathwidth of its (geometric) dual. Bodlaender and Fomin [3], after having proved that the pathwidth of every biconnected outerplanar graph is always at most twice the pathwidth of its (geometric) dual plus two, conjectured that there exists a constant c such that the pathwidth of every biconnected outerplanar graph is at most c plus the pathwidth of its dual. They also conjectured that this was actually true with c being one for every biconnected planar graph. Fomin [10] proved that the second conjecture is true for all planar triangulations. First, we construct for each p 1, a biconnected outerplanar graph of pathwidth 2p + 1 whose (geometric) dual has pathwidth p + 1, thereby disproving both conjectures. Next, we also disprove two other conjectures (one of Bodlaender and Fomin [3], implied by one of Fomin [10]. Finally we prove, in an algorithmic way, that the pathwidth of every biconnected outerplanar graph is at most twice the pathwidth of its (geometric) dual minus one. A tight interval for the studied relation is therefore obtained, and we show that all cases in the interval happen
List-coloring the Square of a Subcubic Graph

The {\em square} $G^2$ of a graph $G$ is the graph with the same vertex set
as $G$ and with two vertices adjacent if their distance in $G$ is at most 2.
Thomassen showed that every planar graph $G$ with maximum degree $\Delta(G)=3$
satisfies $\chi(G^2)\leq 7$. Kostochka and Woodall conjectured that for every
graph, the list-chromatic number of $G^2$ equals the chromatic number of $G^2$,
that is $\chi_l(G^2)=\chi(G^2)$ for all $G$. If true, this conjecture (together
with Thomassen's result) implies that every planar graph $G$ with $\Delta(G)=3$
satisfies $\chi_l(G^2)\leq 7$. We prove that every connected graph (not
necessarily planar) with $\Delta(G)=3$ other than the Petersen graph satisfies
$\chi_l(G^2)\leq 8$ (and this is best possible). In addition, we show that if
$G$ is a planar graph with $\Delta(G)=3$ and girth $g(G)\geq 7$, then
$\chi_l(G^2)\leq 7$. Dvo\v{r}\'ak, \v{S}krekovski, and Tancer showed that if
$G$ is a planar graph with $\Delta(G) = 3$ and girth $g(G) \geq 10$, then
$\chi_l(G^2)\leq 6$. We improve the girth bound to show that if $G$ is a planar
graph with $\Delta(G)=3$ and $g(G) \geq 9$, then $\chi_l(G^2) \leq 6$. All of
our proofs can be easily translated into linear-time coloring algorithms.Comment: This is the accepted version of the journal paper referenced below,
  which has been published in final form at
  http://onlinelibrary.wiley.com/doi/10.1002/jgt.20273/abstract. The abstract
  incorrectly stated that Thomassen solved Wegner's Conjecture for
  $\Delta(G)=3$; however, all of our results are correc