Showing papers by "Noel Gourmelen published in 2014"

Recent land subsidence caused by the rapid urban development in the Hanoi region (Vietnam) using ALOS InSAR data

Abstract: . Since the 1990s the land subsidence due to the rapid urbanization has been considered a severely destructive hazard in the center of Hanoi City. Although previous studies and measurements have quantified the subsiding deformation in Hanoi center, no data exist for the newly established districts in the south and the west, where construction development has been most significant and where groundwater pumping has been very intensive over the last decade. With a multi-temporal InSAR approach, we quantify the spatial distribution of the land subsidence in the entire Hanoi urban region using ALOS images over the 2007–2011 period. The map of the mean subsidence velocity reveals that the northern bank of the Red River appears stable, whereas some areas in southern bank are subsiding with a mean vertical rate up to 68.0 mm yr−1, especially within the three new urban districts of Hoang Mai, Ha Dong – Thanh Xuan and Hoai Duc – Tu Liem. We interpret the spatial distribution of the surface deformation as the combination of the nature of the unsaturated layer, the lowering of groundwater in the aquifers due to pumping withdrawal capacity, the increase of built-up surfaces and the type of building foundation. The piezometric level in Qp aquifer lowers particularly after 2008, whereas the groundwater level in Qh aquifer remains steady, even if it loses its seasonal fluctuation in urban areas and drawdowns in neighboring water production plants. The time evolution deduced from the InSAR time series is consistent with previous leveling data and shows that the lowering rate of the surface slightly decreases till 2008. The analysis of groundwater levels in instrumented wells shows a correlation between the behavior of groundwater with the urban development and the acceleration of groundwater withdrawal. Also, the time variations suggest that the deformation became non-stationary, with upward and downward transient displacements related to the charge and discharge of the aquifers.

Rapid dynamic activation of a marine‐based Arctic ice cap

Abstract: We use satellite observations to document rapid acceleration and ice loss from a formerly slow-flowing, marine-based sector of Austfonna, the largest ice cap in the Eurasian Arctic. During the past two decades, the sector ice discharge has increased 45-fold, the velocity regime has switched from predominantly slow (~ 101 m/yr) to fast (~ 103 m/yr) flow, and rates of ice thinning have exceeded 25 m/yr. At the time of widespread dynamic activation, parts of the terminus may have been near floatation. Subsequently, the imbalance has propagated 50 km inland to within 8 km of the ice cap summit. Our observations demonstrate the ability of slow-flowing ice to mobilize and quickly transmit the dynamic imbalance inland; a process that we show has initiated rapid ice loss to the ocean and redistribution of ice mass to locations more susceptible to melt, yet which remains poorly understood.

Greenland ice sheet annual motion insensitive to spatial variations in subglacial hydraulic structure

Abstract: We present ice velocities observed with global positioning systems and TerraSAR-X/TanDEM-X in a land-terminating region of the southwest Greenland ice sheet (GrIS) during the melt year 2012-2013, to examine the spatial pattern of seasonal and annual ice motion. We find that while spatial variability in the configuration of the subglacial drainage system controls ice motion at short timescales, this configuration has negligible impact on the spatial pattern of the proportion of annual motion which occurs during summer. While absolute annual velocities vary substantially, the proportional contribution of summer motion to annual motion does not. These observations suggest that in land-terminating margins of the GrIS, subglacial hydrology does not significantly influence spatial variations in net summer speedup. Furthermore, our findings imply that not every feature of the subglacial drainage system needs to be resolved in ice sheet models.

Periodic Glacial Lake Outburst Floods threatening the oldest Buddhist monastery in north-west Nepal

Abstract: Presented manuscript deals with the repeated GLOFs from supraglacial lake in the north-west Nepal, using remotely sensed data, field data, data from climatological reanalysis and flood modelling. I found this topic actual in the frame of ongoing climate change and glacier retreat in Himalayas, and surely fitting into the scope of NHESS. Paper is mostly understandable, language is clear, nevertheless, I suggest some final polishing. Text is accompained by five tables and eleven illustrative figures. List of references contains 40 records. From my point of view, the weakest point of the manuscript is the flood modeling, which I consider to be the one of the aims of this work. With all due respect to the authors, crucial input data (flood hydrograph and peak discharge) seem speculative and even wrong. Firstly, I fundamentaly disagree with the approximation of outflow hydrograph with the Gaussian normal distribution in this case study. Considering the likely mechanism of the flood (lake drainage through the subglacial tunnel), Gaussian normal distribution-like hydrograph is not related to reality anyhow. Such hydrograph should be characterised by steep rising limb and slightly decreasing falling limb reflecting decreasing hydrostatic pressure (see Fig. 1). In addition, please explain, why authors did not use the only relevant field data describing potential flood hydrograph: “The stream level in the village rose early in the afternoon and stayed high for several hours.“ Modelled hydrographs do not reflect this description. I strongly suggest considering the change of input hydrographs for flood modelling in order to get more reliable results.

Determination of glacier velocities at a large spatial scale from optical satellite archives

Abstract: In this paper we present a processing strategy to derive glaciers' velocity fields from a complete satellite archive. All possible pairs with a specified time span are formed, submitted to the same preprocessing and matched together to derive velocity fields. All the results are then selected on a pixel-by-pixel basis based on the confidence of the offset-tracking and merged together by computing the median in a spatio-temporal neighboorhood. The method is applied to 206 images covering the Karakoram region and a period of 3 years. This allows us to estimate a velocity for over 90% of the glaciers' area and to reduce the residuals from 9.9m/yr to 2.1m/yr.