We propose a simple and effective heuristic that allows fast generation of a large set of shortest path alternatives in weighted directed graphs. The heuristic is based on existing deviation path algorithms for exact k shortest paths. It precalculates a backward shortest path tree and thus avoids doing many shortest path computations, but as a result it does not necessarily find the exact set of k shortest paths. Computational results on real-world road networks are reported. Our tests show that the quality of the paths produced by the heuristic is most satisfactory: typically, the kth path found by the heuristic is less than 1% longer than the exact kth shortest path, for values of k up to 10,000. Moreover, the heuristic runs very fast. We also show how the heuristic can be enhanced to an exact k shortest paths algorithm, which performs well in comparison with the existing exact k shortest path algorithms. 相似文献
A large spatial variability in sediment yield was observed from small streams in the Ecuadorian Andes. The objective of this study was to analyze the environmental factors controlling these variations in sediment yield in the Paute basin, Ecuador. Sediment yield data were calculated based on sediment volumes accumulated behind checkdams for 37 small catchments. Mean annual specific sediment yield (SSY) shows a large spatial variability and ranges between 26 and 15,100 Mg km− 2 year− 1. Mean vegetation cover (C, fraction) in the catchment, i.e. the plant cover at or near the surface, exerts a first order control on sediment yield. The fractional vegetation cover alone explains 57% of the observed variance in ln(SSY). The negative exponential relation (SSY = a × e−b C) which was found between vegetation cover and sediment yield at the catchment scale (103–109 m2), is very similar to the equations derived from splash, interrill and rill erosion experiments at the plot scale (1–103 m2). This affirms the general character of an exponential decrease of sediment yield with increasing vegetation cover at a wide range of spatial scales, provided the distribution of cover can be considered to be essentially random. Lithology also significantly affects the sediment yield, and explains an additional 23% of the observed variance in ln(SSY). Based on these two catchment parameters, a multiple regression model was built. This empirical regression model already explains more than 75% of the total variance in the mean annual sediment yield. These results highlight the large potential of revegetation programs for controlling sediment yield. They show that a slight increase in the overall fractional vegetation cover of degraded land is likely to have a large effect on sediment production and delivery. Moreover, they point to the importance of detailed surface vegetation data for predicting and modeling sediment production rates. 相似文献
This paper studies the effect of interfacial areas (air–water interfaces and solid–water interfaces) on material strength of unsaturated granular materials. High-resolution X-ray computed tomography technique is employed to measure the interfacial areas in wet glass bead samples. The scanned 3D images are trinarized into three phases and meshed into representative volume elements (RVEs). An appropriate RVE size is selected to represent adequate local information. Due to the local heterogeneity of the material, the discretized RVEs of the scanned samples actually cover a very large range of degree of saturation and porosity. The data of RVEs present the relationship between the specific interfacial areas and degree of saturation and gives boundaries where the interfacial area of a whole sample should fall in. In parallel, suction-controlled direct shear tests have been carried out on glass beads and the material strength has been corroborated with two effective stress definitions related to the specific air–water interfacial areas and fraction of wetted solid surface, respectively. The comparisons show that the specific air–water interfacial area reaches the peak at about 25% of saturation and contributes significantly to the material strength (up to 60% of the total capillary strength). The wetted solid surface obtained from X-ray CT is also used to estimate Bishop’s coefficient χ based on the second type of effective stress definition, which shows a good agreement with the measured value. This work emphasizes the importance to include interface terms in effective stress formulations of unsaturated soils. It also suggests that the X-ray CT technique and RVE-based multiscale analysis are very valuable in the studies of multiphase geomaterials.
The Upper Devonian carbonate reefs in West‐central Alberta are important petroleum reservoirs that are well‐known for their extensive secondary porosity. An outcrop analogue study indicates that an early matrix‐selective dolomitization event occurred which is characterized by a major Late Devonian sea water component with increased salinity because of evaporation. It is interpreted that the matrix (replacive) dolomite formed during the Famennian as the result of a combination of both seepage and latent reflux dolomitization, although an additional type or overprinting of later intermediate burial dolomitization cannot be excluded. Formation of the moulds is attributed mainly to the dissolution of undolomitized fossil cores, most typically stromatoporoids. Geochemical modelling indicates that carboxylic acid fluids have the highest potential for dissolving residual calcite in this case. Geochemical models consistent with this analysis and interpretations can reproduce the secondary porosity and suggest a viable dolomitization process for the localities studied. 相似文献
This paper aims to review the relation between cold-water coral bank development and submarine landslides. Both are common
features on continental margins, but so far it has not been reviewed which effect—if at all—they may have upon each other.
Indirect and direct relations between coral banks and landslides are evaluated here, based on four case studies: the Magellan
Mound Province in the Porcupine Seabight, where fossil coral banks appear partly on top of a buried slide deposit; the Sula
Ridge Reef Complex and the Storegga landslide both off mid-Norway; and the Mauritania coral bank province, associated with
the Mauritanian Slide Complex. For each of these locations, positive and negative relationships between both features are
discussed, based on available datasets. Locally submarine landslides might directly favour coral bank development by creating
substratum where corals can settle on, enhancing turbulence due to abrupt seabed morphological variations and, in some cases,
causing fluid seepage. In turn, some of these processes may contribute to increased food availability and lower sedimentation
rates. Landslides can also affect coral bank development by direct erosion of the coral banks, and by the instantaneous increase
of turbidity, which may smother the corals. On the other hand, coral banks might have a stabilising function and delay or
stop the headwall retrogradation of submarine landslides. Although local relationships can be deduced from these case studies,
no general and direct relationship exists between submarine landslides and cold-water coral banks. 相似文献
Some natural substrates record environmental information and, as such, provide a means to reconstruct the environmental conditions
from the period these substrates were formed. Samples from environmental archives are not always equally spaced in distance.
When a periodic time series model is estimated from these unequally spaced proxy records, the search for reasonable starting
values is the main difficulty. In this work, a non-parametric method based on the regressive Fourier series is first presented,
which reduces averaging errors starting from unequally spaced records. The method is applied to synthetic data and generally
performs well in all circumstances. Secondly, a parametric method for the construction of a time base and the elimination
of averaging errors from unequally spaced records is presented. This parametric method uses the non-parametric method to produce
starting values for the parameters. The method is compared with the time series construction method with the averaging effect
taken into account and it is observed that only the current method produces acceptable results. The statistical performance
of the method is verified with a Monte Carlo simulation and the estimator is proven to be an efficient estimator. The applicability
of the method is demonstrated on the vessel density measurement in a mangrove tree, Rizophora mucronata, which is a proxy for the rainfall in tropical coastal regions. 相似文献
Identifying a periodic time-series model from environmental records, without imposing the positivity of the growth rate, does
not necessarily respect the time order of the data observations. Consequently, subsequent observations, sampled in the environmental
archive, can be inversed on the time axis, resulting in a non-physical signal model. In this paper an optimization technique
with linear constraints on the signal model parameters is proposed that prevents time inversions. The activation conditions
for this constrained optimization are based upon the physical constraint of the growth rate, namely, that it cannot take values
smaller than zero. The actual constraints are defined for polynomials and first-order splines as basis functions for the nonlinear
contribution in the distance-time relationship. The method is compared with an existing method that eliminates the time inversions,
and its noise sensitivity is tested by means of Monte Carlo simulations. Finally, the usefulness of the method is demonstrated
on the measurements of the vessel density, in a mangrove tree, Rhizophora mucronata, and the measurement of Mg/Ca ratios, in a bivalve, Mytilus trossulus. 相似文献
Tropical mountain regions are prone to landslide hazards. Given the current land pressure with increasing occupation of steep uplands, landslide hazards are expected to increase in the near future. Understanding the factors that control landslide hazards is therefore essential. Rare event logistic regression allows us to perform a robust detection of landslide controlling factors. This technique is here applied to the tropical Andes to evaluate the impact of dynamic land cover changes on landslide occurrences. Land cover change trajectories (i.e. dynamic evolution of land cover through time) were specifically included in the probabilistic landslide analysis. While natural physical processes such as slope undercutting by rivers and failure of oversteepened slopes are important in this tropical mountainous site, landslides are increasingly associated with human activities. The data show that land cover trajectories are associated with landslide patterns. In this humid mountainous site, forest degradation does not lead to a measurable increase in landslide occurrence. However, few years after forests are converted to pastures, a rapid decline of slope stability is observed. Land cover conversion from forest to pasture permanently reduces slope stability. It is assumed that major changes in soil properties and hydrology induced by the vegetation conversion play a role in accelerating landslide hazards. 相似文献
The St Bavo's Cathedral tower in Ghent, Belgium, is currently under restoration. This tower was built during the fifteenth and sixteenth centuries over a period of 73 years, using Eocene white stone. Restorations over the last two centuries introduced different Jurassic limestones from the Paris Basin. Documentation of these interventions, however, is very poor for the period before the twentieth century, and documentation on the original construction is non‐existing. Nevertheless, the use of the different stones in their historical context forms a typical example of the historical mindset on the use of white stone, representative for Ghent and by extension northwestern Belgium. 相似文献