AlpWISE : ALPine Water, Ice, Sediment and Ecology

Bakker, M., Antoniazza, G., Odermatt, M. and Lane, S.N., 2019. Morphological response of an Alpine braided reach to sediment-laden flow events. Journal of Geophysical Research – Earth Surface, 124, 1310-28.

Braided gravel?bed rivers show characteristic temporal and spatial variability in morphological change and bedload transport under steady flow and sediment supply rates. Their morphodynamic behavior and long?term evolution in response to nonstationary external forcing is less well known. We studied daily morphological changes in a well?constrained reach of an Alpine braided river that is subject to regulated sediment?laden flows, associated with hydroelectric power exploitation, as well as occasional floods. We found that net reach erosion and deposition were forced by upstream sediment supply, albeit in a nonlinear fashion. The spatial distribution of morphological change and inferred spatially?distributed sediment transport rates varied strongly along the braided reach and between successive sequences of flushing. Local morphological change was driven by two factors: (1) local relief, leading to the preferential filling of topographic lows and erosion of highs, particularly during longer duration floods, which allow braided dynamics to be maintained; and (2) system memory, leading to a negative autocorrelation in bed level changes where erosion was followed by deposition of similar magnitude and vice versa. This effect was associated with the temporary storage of high sediment loads from flushing due to the abrupt on?off nature of these flows and reveals the relatively efficient transport of sediment in a river that is heavily impacted upon by flow abstraction. In general, the internal morphodynamics of the braided river condition their own response to external forcing events and thus sediment transfer.

A copy of the paper can be accessed here.

AGU has published a “Research Spotlight” highlighting a paper by Stuart Lane and Pete Nienow “Decadal?Scale Climate Forcing of Alpine Glacial Hydrological Systems” in Eos. It can be read here.

We have recently made lots of our data newly available in open access format. All the data can be accessed here : These data include

1. 15 minute resolution normalized hydrographs for 6 glaciated basins in south-west Switzerland (search on: Q_1969_2014)
2. Long-term digital elevation models of the Borgne d’Arolla reconstructed from aerial imagery and starting in the 1960s. These are supported by data on flushing of sediment from two intakes (search on: Bakker M)
3. Short-term (daily time-scale) digital elevation models of a braided river (c. 300 m wide, 1600 m long) over a three week period (search on: Bakker M)
4. Data from a monitoring project (SNSF PNR70 Hyroenv) of macroinvertebrate life in the Borgne d’Arolla (search on: macroinvertebrate)

If you want to look for other data available (e.g. glacier recession, DEMs of hillslopes), choose “advanced search”; then unclick all boxes except “data”

Quantification of climate forcing of glacial hydrological systems at the decadal scale is rare because most measurement stations are too far downstream for glacier impacts to be clearly detected. Here we apply a measure of daily hydrograph entropy to a unique set of reliable, high?altitude gauging stations, dating from the late 1960s. We find a progressive shift to a greater number of days with diurnal discharge variation as well as more pronounced diurnal discharge amplitude. These changes were associated with the onset of rapid warming in the 1980s as well as declining end of winter snow depths as inferred from climate data. In glaciated catchments, lower winter snow depths reduce the magnitude and duration of snowpack buffering and encourage the earlier onset of glacier ice exposure, with associated lower surface albedo and more rapid melt. Together, these processes explain the increase in the observed intensity of diurnal discharge fluctuations.

The paper has been published in Water Resources Research and can be accessed here.

Images from specially?commissioned aeroplane sorties (manned aerial vehicle, MAV), repeat unmanned aerial vehicle (UAV) surveys, and Planet CubeSat satellites are used to quantify dune and bar dynamics in the sandy braided South Saskatchewan River, Canada. Structure?from?Motion (SfM) techniques and application of a depth?brightness model are used to produce a series of Digital Surface Models (DSMs) at low and near?bankfull flows. A number of technical and image processing challenges are described that arise from the application of SfM in dry and submerged environments. A model for best practice is presented and analysis suggests a depth?brightness model approach can represent the different scales of bedforms present in sandy braided rivers with low?turbidity and shallow (< 2?m deep) water. The aerial imagery is used to quantify the spatial distribution of unit bar and dune migration rate in an 18?km reach and three ~1?km long reaches respectively. Dune and unit bar migration rates are highly variable in response to local variations in planform morphology. Sediment transport rates for dunes and unit bars, obtained by integrating migration rates (from UAV) with the volume of sediment moved (from DSMs using MAV imagery) show near?equivalence in sediment flux. Hence, reach?based sediment transport rate estimates can be derived from unit bar data alone. Moreover, it is shown that reasonable estimates of sediment transport rate can be made using just unit bar migration rates as measured from 2D imagery, including from satellite images, so long as informed assumptions are made regarding average bar shape and height. With recent availability of frequent, repeat satellite imagery, and the ease of undertaking repeat MAV and UAV surveys, for the first time, it may be possible to provide global estimates of bedload sediment flux for large or inaccessible low?turbidity rivers that currently have sparse information on bedload sediment transport rates.

The paper is published in Earth Surface Processes and Landforms and can be accessed here.

Shallow rivers provide important habitat for various aquatic and terrestrial species. The bathymetry of such environments is, however, difficult to measure as devices and approaches have been traditionally developed mainly for deeper waters. This study addresses the mapping of shallow waterbathymetry with high spatial resolution and accuracy by comparing three remote sensing (RS) approaches: one based on echo sounding (active RS) and two on photogrammetry (passive RS): bathymetric Structure from Motion (SfM) and optical modelling. The tests were conducted on a 500?m long and ~30?m wide reach of sand-bedded meandering river: (1) during a rising spring flood (Q?=?10–15?m3/s) with medium turbidity and high water color and; (2) during autumn low discharge (Q?=?4?m3/s) with low turbidity and color. Each method was used to create bathymetric models. The models were compared with high precision field measurements with a mean point spacing of 0.86?m. Echo sounding provided the most accurate (ME~?0.02?m) and precise (SDE?=?±?0.08?m) bathymetric models despite the high degree of interpolationneeded. However, the echo sounding-based models were spatially restricted to areas deeper than 0.2?m and no small scale bathymetric variability was captured. The quality of the bathymetric SfM was highly sensitive to flow turbidity and color and therefore depth. However, bathymetric SfM suffers less from substrate variability, turbulent flow or large stones and cobbles on the river bed than optical modelling. Color and depth did affect optical model performance, but clearly less than the bathymetric SfM. The optical model accuracy improved in autumn with lower water color and turbidity (ME?=??0.05) compared to spring (ME?=??0.12). Correlations between the measured and modelled depth values (r?=?0.96) and the models precision (SDE?=?0.09–0.11) were close to those achieved with echo sounding. Shadows caused by riparian vegetation restricted the spatial extent of the optical models.

The paper is published in the journal Geomorphology and can be accessed here.

Publié dans le Bulletin de la Murithienne, 135, 39-53, 2018. Cliquer ici pour l’obtenir

Alors que tous s’accordent dans l’idée de conduire une gestion environnementale optimale, c’est-à-dire de déterminer des mesures efficaces qui permettent à la balance énergie-écologie de couvrir l’ensemble des objectifs, l’enjeu réside parfois dans le manque de connaissances concernant le fonctionnement des systèmes naturels, et des écosystèmes notamment. Les environnements alpins, moins bien maitrisés, devraient faire l’objet d’une attention toute particulière, d’autant plus en cette année des Risques 2018. Cet article expose les principaux résultats de recherche du groupe AlpWISE ( wp.unil.ch / alpenv ) des scientifiques de l’Université de Lausanne, sous la direction du Professeur Dr. Stuart Lane. Il s’agit de déterminer les réponses des éléments du paysage au changement climatique rapide par une approche intégrative regroupant en particulier géographie, géomorphologie et écologie. La figure 1 présente un résumé des interactions du paysage discutées, avec le Val d’Arolla pour terrain d’étude. Die alpine Umwelt angesichts des Klimawandels : Gletscher, Gewässer und Landschaften, wo sind die Herausforderungen ? Obwohl sich alle einig sind, dass wir den Umgang mit der Umwelt optimieren müssen, das heisst, dass wir effiziente Massnahmen zur Herstellung des Gleichgewichtes zwischen Energie und Ökologie ergreifen wollen, besteht das Problem manchmal darin, dass wir zu wenig über die Funktionsweise der natürlichen Systeme und der Ökosysteme im Speziellen wissen. Die alpine Umgebung, welche weniger bekannt ist, besondere Beachtung erhalten, umso mehr in diesem Jahr des Risikos 2018. Dieser Artikel beschreibt die wichtigsten Forschungsresultate der Gruppe AlpWISE der Wissenschaftler der Universität Lausanne, unter der Leitung des Professors Dr. Stuart Lane. Ziel ist es, die Reaktionen verschiedener Landschaftselemente auf den schnellen Klimawandel mit Hilfe einer integrativen Methode zu bestimmen, welche Geografie, Geomorphologie und Ökologie vereint. Abbildung 1 zeigt eine Zusammenfassung der Prozesse in der Landschaft diskutiert, mit Hilfe des Val d’Arolla als Untersuchungsgebiet.

Matthews’ 1992 geoecological model of vegetation succession within glacial forefields describes how following deglaciation the landscape evolves over time as the result of both biotic and abiotic factors, with the importance of each depending on the level of environmental stress within the system. We focus in this paper on how new understandings of abiotic factors and the potential for biogeomorphic feedbacks between abiotic and biotic factors makes further development of this model important. Disturbance and water dynamics are two abiotic factors that have been shown to create stress gradients that can drive early ecosystem succession. The subsequent establishment of microbial communities and vegetation can then result in biogeomorphic feedbacks via ecosystem engineering that influence the role of disturbance and water dynamics within the system. Microbes can act as ecosystem engineers by supplying nutrients (via remineralization of organic matter and nitrogen-fixation), enhancing soil development, either decreasing (encouraging weathering) or increasing (binding sediment grains) geomorphic stability, and helping retain soil moisture. Vegetation can act as an ecosystem engineer by fixing nitrogen, enhancing soil development, modifying microbial community structure, creating seed banks, and increasing geomorphic stability. The feedbacks between vegetation and water dynamics in glacial forefields are still poorly studied. We propose a synthesized model of ecosystem succession within glacial forefields that combines Matthews’ initial geoecological model and Corenblit et al.’s (2007) model to illustrate how gradients in environmental stress combined with successional time drive the balance between abiotic and biotic factors and ultimately determine the successional stage and potential for biogeomorphic feedbacks.

The paper has been accepted for publication in Progress in Physical Geography.

Sediment export from glaciated basins involves complex interactions between ice flow, basal erosion and sediment transfer in subglacial and proglacial streams. In particular, we know very little about the processes associated with sediment transfer by subglacial streams. The Haut Glacier d’Arolla (VS, Switzerland) was investigated during the summer melt season of 2015. LiDAR survey revealed positive surface changes in the ablation zone, indicating glacier uplift, at the end of the morning during the period of peak ablation. Instream measures of sediment transport showed that suspended load and bedload responded differently to diurnal flow variability. Suspended load depended on the availability of fine material whereas bedload depended mainly on the competence of the flow. Interpretation of these results allowed development of a conceptual model of subglacial sediment transport dynamics. It is based upon the mechanisms of clogging (deposition) and flushing (transport/erosion) in sub-glacial channels as forced by diurnal flow variability. Through the melt season, the glacier hydrological response evolves from being buffered by glacier snow cover with a poorly developed subglacial drainage system to being dominated by more rapid ice melt with a more hydraulically efficient subglacial channel system. The resultant changes in the shape of diurnal discharge hydrographs, and notably higher peak flows and lower base flows, causes sediment transport to become discontinuous, with overnight clogging and late morning flushing of subglacial channels. Overnight clogging may be sufficient to reduce subglacial channel size, creating temporarily pressurised flow and lateral transfer of water away from the subglacial channels, leading to the late morning glacier surface uplift. However, without further data, we cannot exclude other hypotheses for the uplift.

The paper is in press with the journal Earth Surface Processes and Landforms and can be accessed here.

Improved water management strategies necessitate a solid understanding of environmental impacts associated with various flow release policies. Habitat suitability models use hydrodynamic simulations to generate weighted usable area curves, which are useful in characterizing the ecological suitability of flow release rules. However, these models are not conveniently run to resolve the hydrodynamics at the smaller scales associated with macroroughness elements (e.g., individual stones), which produce wakes that contribute significantly to habitat suitability by serving as shelter zones where fishes can rest and feed. In this study, we propose a robust environmental indicator that considers the habitat generated by the wakes downstream of stones and can thus be used to assess the environmental efficiency of flow release rules for impounded streams. We develop an analytical solution to approximate the wake areas behind macroroughness elements, and the statistical distribution of wake areas is then found using the derived distribution approach. To illustrate the concept, we apply our theory to four exemplary river streams with dispersed stones having different statistical diameter size distributions, some of which allow for an analytical expression of the weighted usable area. We additionally investigate the impact of spatiotemporal changes in stone size distributions on the usable area and the consequent threshold flows. Finally, we include the proposed environmental indicator to solve a multiobjective reservoir optimization problem. This exemplifies its practical use and allows stakeholders to find the most favorable operational rules depending on the macroroughness characteristics of the impounded stream.

The paper has been accepted for publication in Water Resources Research and can be accessed here.