Simulation of Turbulent Flow in An Urban Forested Park Damaged by a Windstorm

The damage caused by windstorms to forest ecosystems is often very heterogeneous. In order to improve the stability of forested landscapes, it is of great importance to identify the factors responsible for this spatial variability. The structure of the landscape itself may play a role, through possible influences of canopy heterogeneities on the development of turbulence. For the purpose of investigating the role of landscape fragmentation on turbulence development, we used a numerical flow model with a k–ε turbulence scheme model, previously validated in simple cases with well-defined surface changes (roughness change and forest edge flow). A series of two- and three-dimensional simulations were performed over a heterogeneous urban forested park in Europe, which was severely damaged in various places by the Lothar windstorm in December 1999. The model shows the development of a region of strong turbulence, resulting from the generation of large wind shear at the top of the canopy. A sensitivity study shows how the location, extension and intensity of the region depend on canopy characteristics such as the leaf density, the nature of the edge or the presence of gaps and clearings. Simulations performed in conditions representative of the windstorm show that the location of the damaged areas corresponds very closely to the regions where the turbulent kinetic energy was above a certain threshold.     

Fungal weathering of basaltic rocks in a cold oceanic environment (Iceland): comparison between experimental and field observations

This paper presents evidence for strong biochemical weathering of basaltic outcrops induced by fungal communities in a cold environment. Weathering rind formation is considered to be a consequence of the biological activity. Comparisons between in vitro experiments and in situ observations allow a characterization of fungal effects on rocks and help to define the place of these micro‐organisms in the cold environment weathering chain. It is concluded that biological weathering is chronologically the first process of weathering, probably leading to the subsequent expression of cryogenic processes. Information presented here suggests the need for reconsideration of the traditional frost‐driven morphogenetic system normally considered for subpolar areas. Copyright © 2002 John Wiley & Sons, Ltd.     

Ethnic groups’ response to the 26 December 2004 earthquake and tsunami in Aceh, Indonesia

The 26 December 2004 earthquake and tsunami unfairly hit the different ethnic groups of Aceh, Indonesia. About 170,000 Acehnese and Minangkabau people died in the Northern tip of Sumatra while only 44 Simeulue people passed away in the neighbouring Simeulue island located near the earthquake epicentre. Such a difference in the death toll does not lie in the nature of the hazard but in different human behaviours and ethnic contexts. The present study draws on a contextual framework of analysis where people’s behaviour in the face of natural hazards is deeply influenced by the cultural, social, economic and political context. Questionnaire-based surveys among affected communities, key informant interviews and literature reviews show that the people of Simeulue detected the tsunami very early and then escaped to the mountains. On the other hand, Acehnese and Minangkabau people, respectively in the cities of Banda Aceh and Meulaboh, did not anticipate the phenomenon and were thus caught by the waves. The different behaviours of the victims have been commanded by the existence or the absence of a disaster subculture among affected communities as well as by their capacity to protect themselves in facing the tsunami. People’s behaviours and the capacity to protect oneself can be further tracked down to a deep tangle of intricate factors which include the armed conflict that has been affecting the province since the 1970s, the historical and cultural heritage and the national political economy system. This paper finally argues that the uneven impact of the 26 December 2004 earthquake and tsunami in Aceh lies in the different daily life conditions of the ethnic groups struck by the disaster.     

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Erosion balance in the watersheds of the western Paris Basin by high-frequency monitoring of discharge and suspended sediment in surface water

An accurate quantification of erosion, based on high-frequency monitoring of river discharge and suspended sediment fluxes is proposed for two watersheds in the western Paris Basin, a sensitive area with respect to erosion phenomena. This continuous monitoring makes it possible to include flood events of short duration, but significant erosion potential. The obtained erosion rate (16 and 21 t?km^?2?yr^?1) is among the weakest of the planet (3.5 to 18?000 t?km^?2?yr^?1). However, this annual balance does not reflect the behaviour of these rivers which can be torrential in certain cases. To cite this article: B. Laignel et al., C. R. Geoscience 338 (2006).     

Detection of preferential infiltration pathways in sinkholes using joint inversion of self-potential and EM-34 conductivity data

The percolation of water in the ground is responsible for measurable electric potentials called self‐potentials. These potentials are influenced by the distribution of the electrical conductivity of the ground. Because sinkholes are associated both with self‐potential and electrical conductivity anomalies, a joint inversion of EM‐34 conductivity and self‐potential data is proposed as a way of delineating the location of these features. Self‐potential and EM conductivity data were obtained at a test site in Normandy (France) where sinkholes and crypto‐sinkholes are present over a karstic area in a chalk substratum overlain by clay‐with‐flint and loess covers. The presence of sinkholes and crypto‐sinkholes is associated with negative self‐potential anomalies with respect to a reference electrode located outside the area where the sinkholes are clustered. The sinkholes also have a conductivity signature identified by the EM‐34 conductivity data. We used the simulated‐annealing method, which is a global optimization technique, to invert jointly EM‐34 conductivity and self‐potential data. Self‐potential and electrical conductivity provide clear complementary information to determine the interface between the loess and clay‐with‐flint formations. The sinkholes and crypto‐sinkholes are marked by depressions in this interface, focusing the groundwater flow towards the aquifer contained in the chalk substratum.     

Detecting communities with the multi-scale Louvain method: robustness test on the metropolitan area of Brussels

Detecting communities in large networks has become a common practice in socio-spatial analyses and has led to the development of numerous dedicated mathematical algorithms. Nowadays, however, researchers face a deluge of data and algorithms, and great care must be taken regarding methodological questions such as the values of the parameters and the geographical characteristics of the data. We aim here at testing the sensitivity of multi-scale modularity optimized by the Louvain method to the value of the resolution parameter (introduced by Reichardt and Bornholdt (Phys Rev Lett 93(21):218701, 2004.  https://doi.org/10.1103/PhysRevLett.93.218701) and controlling the size of the communities) and to a number of spatial issues such as the inclusion of internal loops and the delineation of the study area. We compare the community structures with those found by another well-known community detection algorithm (Infomap), and we further interpret the final results in terms of urban geography. Sensitivity analyses are conducted for commuting movements in and around Brussels. Results reveal slight effects of spatial issues (inclusion of the internal loops, definition of the study area) on the partition into job basins, while the resolution parameter plays a major role in the final results and their interpretation in terms of urban geography. Community detection methods seem to reveal a surprisingly strong spatial effect of commuting patterns: Similar partitions are obtained with different methods. This paper highlights the advantages and sensitivities of the multi-scale Louvain method and more particularly of defining communities of places. Despite these sensitivities, the method proves to be a valuable tool for geographers and planners.