Daniel Germain

Dendrogeomorphic reconstruction of snow avalanche regime and triggering weather conditions A classification tree model approach

While dendrogeomorphology has been recognized as a useful tool to identify past avalanche activity, there is only a handful of papers that focus on the assessment of weather or climatic triggers of tree-ring reconstructed avalanche events. This paper compares the potential of logistic regression and classification tree algorithms to highlight weather scenarios responsible for the occurrence of high-magnitude avalanche activity in the Presidential Range of the White Mountains, New Hampshire (USA). Our tree-ring procedure improves the modern GD-It threshold with the implementation of a second criteria based on the Moran index. 450 trees sampled in seven different avalanche paths allowed us to reconstruct 45 avalanches that occurred during 19 different years for the period 1936–2012. The results show that while statistically significant, the logistic regression models are less accurate than classification trees to assess avalanche activity based on annual and monthly weather variables. Moreover, even if snow related covariates are located at the root node of every classification tree model, the addition of temperature and wind predictors increases their robustness. This suggests that high-magnitude avalanches in the Presidential Range not only respond to snow, but also to atmospheric conditions responsible for the creation of weak layers within the snowpack.

Subaerial sediment-water flows on hillslopes Essential research questions and classification challenges

Subaerial sediment-water flows on hillslopes have received growing attention from the scientific community in recent years. Interest in this type of geomorphic process is twofold: they constitute a major risk factor and they have generally been poorly defined in the past. Major classification schemes are considered here, with a particular interest in criteria for discriminating between various subaerial flow type processes. Identification techniques from various fields (morphometry, rheology, geomorphology, sedimentology) are reviewed. Recent accounts of flow processes as part of a continuum of processes as opposed to belonging to distinct categories are discussed. It is proposed that process identification include information drawn from various methods in order to minimize identification error, given that correct identification is essential to appropriate hazard assessment and mitigation measures.

Cliff-Top Eolian Sedimentation Reflecting Mid- to Late-Holocene Environmental Changes at Anticosti Island, Gulf of St. Lawrence, Eastern Canada

Abstract The study site (Cape Sandtop) consists of a 15 m high marine terrace exposed to strong easterly winds, at the eastern end of Anticosti Island in the Gulf of St. Lawrence (Québec, Canada), where eolian sedimentation occurred. Sections at the terrace edge exposed thick, well-humified, buried organic deposits with many wood fragments. Sedimentological and plant-macrofossil analyses were conducted from four sections to provide a chronology of eolian activity and to evaluate the causal factors for the development of treeless plant communities. Plant remains indicate that Cape Sandtop was forested between 6520 and ca. 4740 cal YBP. After ca. 4740 cal YBP, the terrace experienced a rapid change from coastal conifer forests and treed fen to marshes. Sustained erosional activity by easterly winds along the upper limestone cliff and the terrace edge started 1560 cal YBP and caused peat burial. The key factors responsible for cliff-top eolian sedimentation were relative sea level changes, increased exposure to easterly winds associated with higher elevation (15 m asl) of the marine terrace and sediment availability. In spite of its limited extent, this coastal site appeared as a system that was sensitive to environmental changes during the Mid- to Late-Holocene.

Visibility analysis on a massively data-parallel computer

Visibility analysis algorithms use digital elevation models (DEMs), which represent terrain topography, to determine visibility at each point on the terrain from a given location in space. This analysis can be computationally very demanding, particularly when manipulating high resolution DEMs accurately at interactive response rates. Massively data-parallel computers offer high computing capabilities and are very well-suited to handling and processing large regular spatial data structures. In the paper, the authors present a new scanline-based data-parallel algorithm for visibility analysis. Results from an implementation onto a MasPar massively data-parallel SIMD computer are also presented.

A STATISTICAL FRAMEWORK FOR TREE‐RING RECONSTRUCTION OF HIGH‐MAGNITUDE MASS MOVEMENTS: CASE STUDY OF SNOW AVALANCHES IN EASTERN CANADA

Abstract This paper investigates the application of the peaks‐over‐threshold method in combination with fitting of the generalized Pareto distribution for estimating the frequency of high‐magnitude geomorphic events, on the basis of tree‐ring data. In particular, attention is focused on extreme value distribution of tree‐ring responses and in the minimum threshold or index number required to assure the past occurrence of high‐magnitude events. The objective is to set a threshold, adapted to the sample distribution, which will make the distinction between high‐magnitude years and remaining years, where the response of sampled trees is too weak to be attributed to a high‐magnitude geomorphic event. The statistical framework proposed is based on the statistics of extremes. Through the use of simple, strong and effective mathematics, this model should strengthen the position of dendrogeomorphology in the evaluation and management of natural hazards.

Fifty-year spatiotemporal analysis of landscape changes in the Mont Saint-Hilaire UNESCO Biosphere Reserve (Quebec, Canada)

Diachronic analysis with a GIS-based classification of land-use changes based on aerial photographs, orthophotos, topographic maps, geotechnical reports, urban plans, and using landscape metrics has permitted insight into the driving forces responsible for landscape fragmentation in the Mont Saint-Hilaire (MSH) Biosphere Reserve over the period 1958–2015. Although the occurrence of exogenous factors, such as extreme weather and fires, can have a significant influence on the fragmentation of the territory in time and space, the accelerated development of the built environment (+470%) is nevertheless found to be primarily responsible for landscape fragmentation and the loss of areas formerly occupied by orchards, agriculture, and woodlands. The landscape metrics used corroborate these results, with a simplification of the shape of polygons, and once again reveal the difficulties of harmonizing different land uses. MSH has become somewhat of a forest island in a sea of residential development and agriculture. To counter this isolation of fragmented habitat components, forest corridors have been proposed and developed for the Biosphere Reserve and particularly for the core area. Two corridors, to the north and south, are used to connect the protected area and other wooded areas at the regional scale, in order to promote genetic exchange between populations of various species. In that regard, the forest buffer zone around the hill continues to play a key role and has great ecological value for species and ecological preservation and conservation. However, appropriate management and landscape preservation actions should recognize and focus on landscape composition and the associated geographical configuration.