Denis Mercier

Focusing on the spatial non-stationarity of landslide predisposing factors in northern Iceland Do paraglacial factors vary over space?

Most studies focusing on landslide spatial analysis have considered the relationships between predictors and landslide occurrence as fixed effects. Yet spatially varying relationships, i.e. non-stationarity, often occur in any spatial data set and should be theoretically considered in statistical models for a better fit. In Skagafjörður, a landslide-rich north–south oriented area located in northern Iceland, we investigated whether spatial non-stationarity in the relationships between paraglacial variables (glacio-isostatic rebound and post-glacial debuttressing, both captured in this area by latitude) and landslide locations is detectable. To explore the non-stationarity of factors that predispose landslide occurrence, we performed two logistic regression models, one global (GLR) and the other enabling the regression parameters to vary locally (geographically weighted logistic regression, GWLR). Each model was computed with two types of outcome, one based on the entire masses of landslides and the other only on the scarps of landslides. GLR results reveal that increasing latitude is associated with increasing probability of landslide occurrence, confirming that post-glacial rebound is of prime importance at the regional scale. Nevertheless, GWLR indicates that this relationship is absent or reversed at some locations, meaning that the influence of paraglacial and other predisposing factors of landsliding (slope, valley depth and curvature) vary at the local scale. This result sheds light on the spatial clustering of three subzones where landsliding drivers are homogeneous. We conclude that a GWR-based approach provides some significant inputs for spatial analysis of mass movement processes, by identifying multi-scale process control zones and by highlighting local drivers, indecipherable in global models.

POST-LITTLE ICE AGE PATTERNED GROUND DEVELOPMENT ON TWO PYRENEAN PROGLACIAL AREAS: FROM DEGLACIATION TO PERIGLACIATION

Abstract This study aims to observe post‐Little Ice Age glacier retreat and the constitutive patterned ground development at two French Pyrenean glacier forelands (Taillon Glacier and Pays Baché Glacier). Periglacial feature observations are associated with periods of deglaciation using aerial photos and archive files. Four conclusions are drawn. (1) The two glaciers have lost respectively 68% and 92% of their surface since 1850, which corroborates observations on other Pyrenean glaciers. (2) Patterned ground can develop very rapidly, sometimes only 10 years after deglaciation. (3) Patterned ground size does not systematically increase as a function of the time elapsed since deglaciation. (4) All the forms, even those developing near the Little Ice Age moraines, are active. We propose that the location, activity and size of patterned ground are more probably linked to drift characteristics and local wetness conditions than to the time elapsed since deglaciation.

The impact of storm Xynthia in 2010 on coastal flood prevention policy in France

Storm Xynthia occurred on 28 February 2010. It was one of the most destructive climatic events to hit metropolitan France for several decades, causing 47 victims in France, among whom more than half died in the department of Vendée. The occurence of various natural phenomena (atmospheric pressure, strength and orientation of the wind, tidal range) at the same time caused a major coastal flood in several urban areas from Vendée and Charente-Maritime (central Atlantic region of France). The evaluation of the disaster highlighted a number of problems, especially delays in approving the PPR-L (Plan de Prévention des Risques Littoraux) that allowed urbanization in the coastal flood plain that exposed population to major natural risks, such as sea level rise and floods. Since 28th February, 2010, the French government has focused on these issues and tried to define in a more accurate way the criteria used for the flood zonings of the PPR-L. A number of circulars and recommendations have been produced and they all specified rules of protection for properties against the risk of flood and the necessary measures for adapting or building current or future houses (e.g. height of the upper floor, presence of an upper floor refuge) in accordance with the level of risk. The objective of this paper is twofold: we wish to highlight the impact of the disaster Xynthia on the production of PPR-L in the short and medium terms while focusing on the example of the town of La-Faute-sur-Mer. We will also analyze the evolution of the regulations since 28 February 2010, with special attention paid to the proposed criteria to define the hazard.

The use of a micro-scale index to identify potential death risk areas due to coastal flood surges: lessons from Storm Xynthia on the French Atlantic coast

Storm Xynthia (February 2010) was responsible for a large sea surge along the French Atlantic coast. It resulted in the flooding of low-lying coastal areas during the night. Urbanized areas were impacted and 41 people died by drowning in their homes. The location and type of construction of the houses in the affected area contributed to the death toll. The fact that the inadequacy of construction with regard to coastal flood hazard could lead to death was one of the most important lessons of the storm. The French government decided to buy back and scrap the most dangerous buildings hit by Xynthia. In order to prevent future deaths by drowning, we have developed a tool (the V.I.E. index) to identify houses where a risk of death due to a coastal flood cannot be excluded. The goal is to propose individual-based solutions for risk mitigation of residential houses. This tool uses a micro-scale-level analysis along with four criteria: (1) the potential water depth per house, (2) the distance between the dike and the house, (3) the architectural typology and (4) the closeness to rescue point. The methodological background and the first results for three towns are presented in this paper. Limitations and further developments are also discussed.

Temporal scales and deglaciation rhytms in a polar glacier margin, baronbreen, Svalbard

Polar regions are very sensitive to climate variability. Glacial environments, such as Spitsbergen, respond drastically to climate warming by the disintegration of ice masses, release of huge amounts of free water and rapid evolution of terrestrial landforms. Paraglacial studies have shown that sediment yield decreases with time following a relaxation curve. The study examines the effects of scaling on rhythms of deglaciation and sediment fluxes in a small catchment. It appears that the paraglacial sequence, known as a morphogenic crisis, could be affected by internal small threshold events which are able to mobilize large quantities of sediment. It means that, at small scale, the curve profile is more irregular than suggested by large-scale models.

Actual paraglacial progradation of the coastal zone in the Kongsfjorden area, western Spitsbergen (Svalbard)

Abstract This research was carried out on the Brøgger Peninsula, northwest Spitsbergen, Svalbard (79°N 12°E). In the western part of Spitsbergen, cold-based valley glaciers have retreated more than 1 km from their Little Ice Age limits, and glacial meltwater has extensively reworked glacigenic sediments on exposed glacier forelands. In such areas, a paraglacial sediment transport regime has become predominant, with runoff as the dominant process. A combination of GIS, DEM, aerial photographic and field data was employed to estimate shoreline progradation at sandur outflows. Average shoreline progradation is estimated to amount to 3 m/annum over the last 30 years, a period of uninterrupted sediment provision from the glacial runoff system.

An early Holocene age for the Vatn landslide (Skagafjörður, central northern Iceland): Insights into the role of postglacial landsliding on slope development

Recent research in northern Iceland has highlighted a significant period of rock slope instability during the early Holocene due to the combined effects of postglacial rebound, relative sea-level fall, and glacially oversteepened mountain slopes. Using the Vatn landslide (Skagafjörður, central northern Iceland) as an example, this paper focuses on this period and describes the sequence of events that led to landsliding. Geomorphic mapping, stratigraphical evidence, and both radiocarbon and tephra dating were applied. Collectively, the data acquired indicate that the landslide occurred between 11,400 and 10,790 cal. yr BP. However, while rock slope failure represents a significant disintegration of mountain slopes, this study suggests that large postglacial landslides might also play a role in arresting sediment transport from other hillslope processes rather than contributing large volumes of sediment.

Denudation rates during a postglacial sequence in Northern Iceland: example of Laxárdalur valley in the Skagafjörður area

ABSTRACT For several decades, geomorphologists have focused on the functioning of geomorphic systems after deglaciation. The relative importance of paraglacial vs. periglacial processes has been highly debated. At present, the development of dating techniques allows to contribute to this debate. We reconstruct in this paper the geomorphic evolution of Tindastóll mountain slopes in Laxárdalur valley (Skagafjörður area, central northern Iceland), where a chronological framework can be established through tephrochronology and an assemblage of dated raised beaches. Volumetric calculations of constructed and excavated landforms were created from field data and from DEM and geographical information system techniques. Collectively, our data exhibit a first stage of paraglacial landsliding during the first half of the Holocene, followed by a stage of scree and rockglacier development (during the second half of the Holocene, but before 1100u2005AD). Our estimations indicate that more than 85% of the total sediment production were due to rock slope failure, and the rate of bedrock denudation due to periglacial processes was about one half of the rate of paraglacial processes. Nevertheless, paraglacial and periglacial processes cannot be seen here as antagonistic processes: they are organized in a sequence during which periglacial processes are conditioned (enhancement of bedrock denudation rates) by fracturing and consequent mass wasting. Screes and concomitant rockglaciers were indeed preconditioned by the landslide, while areas non-affected by landslides have remained mostly intact, characterized by a very low rate of accumulation due to geomorphic (periglacial) activity.

Vulnerability to coastal flood hazard of residential buildings on Noirmoutier Island (France)

Storm Xynthia was responsible for the death of 41 people by drowning after the flooding by the sea of low-lying areas along French Atlantic coast in February 2010. This toll was partly explained by inadequacy of houses to flood hazard: 78% of the deaths occur in single-storey constructions. In this paper, we propose a map linking architectural typology of constructions with potential water depth in case of coastal flood event. The goal is to identify residential houses which present a high level of vulnerability for people to a potential coastal flood event. This work is presented on Noirmoutier Island which was few impacted by Storm Xynthia. However, it presents some common points with the most hit territories during the storm: 70% of the territory is under Xynthia level; 65% of the houses located in this low-lying area are single-storey constructions This map shows us different vulnerability between the four towns of the island.