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Resulting from a collaboration with Perm State University, this paper investigates river mixing. A rapid downstream weakening of the processes that drive the intensity of transverse mixing at the confluence of large rivers has been identified in the literature and attributed to the progressive reduction in channel scale secondary circulation and shear-driven mixing with distance downstream from the junction. These processes are investigated in this paper using a three-dimensional computation of the Reynolds averaged Navier Stokes equations combined with a Reynolds stress turbulence model for the confluence of the Kama and Vishera rivers in the Russian Urals. Simulations were carried out for three different configurations: an idealized planform with a rectangular cross-section (R), the natural planform with a rectangular cross-section (P), and the natural planform with the measured bathymetry (N), each one for three different discharge ratios. Results show that in the idealized configuration (R), the initial vortices that form due to channel-scale pressure gradients decline rapidly with distance downstream. Mixing is slow and incomplete at more than 10 multiples of channel width downstream from the junction corner. However, when the natural planform and bathymetry are introduced (N), rates of mixing increase dramatically at the junction corner and are maintained with distance downstream. Comparison with the P case suggests that it is the bathymetry that drives the most rapid mixing and notably when the discharge ratio is such that a single channel-scale vortex develops aided by curvature in the post junction channel. This effect is strongest when the discharge of the tributary that has the same direction of curvature as the post junction channel is greatest. A comprehensive set of field data are required to test this conclusion. If it holds, theoretical models of mixing processes in rivers will need to take into account the effects of bathymetry upon the interaction between river discharge ratio, secondary circulation development, and mixing rates. An Open Access copy of the paper is available here.

Mancini, D. and Lane, S.N., 2020. Changes in sediment connectivity following glacial debuttressing in an Alpine valley system. Geomorphology, 352, UNSP 106987

Increasing air temperature and declining winter snowfalls are resulting in rapid glacier recession and the expansion of proglacial margins in Alpine regions. Such margins include substantial debris accumulations (e.g. frontal/lateral moraine ridges; till-covered and steep valley sidewalls) which may be unstable due to glacial debuttressing. Rainfall, snowmelt and ice melt out may then cause mass movements. Here, we quantify the decadal-scale erosion and deposition patterns and changes in connectivity for two valley sidewall geomorphological systems following retreat of the Glacier d’Otemma, Switzerland. We apply archival digital photogrammetric methods to the period 1964 to 2009 to determine high resolution digital elevation models. These were differenced to calculate patterns of erosion and deposition and to quantify the evolution of sediment connectivity. We found that gully headward erosion (rates between ca. −10.6 mm/year and −1002.1 mm/year) was the main geomorphological process during glacier thinning but increasing depositional rates downslope of the gullies (ca. +21.3 to +298.5 mm/year) were recorded in the following years associated with significant alluvial fan growth at the slope base. While gullying enhanced connectivity by removing glacially conditioned sediment transfer buffers, so connecting side-slopes to upstream sediment sinks (the upslope contributing area between 1964 and 2009 increased by +73.8% and +195.1% in each subsystem), alluvial fans reduced the rates of sediment transfer to the rapidly enlarging glacial forefields. The detail of these responses is conditioned by three generic processes: (1) the wider geomorphic setting – here, the presence of a moraine bastion as a primary part of the sediment cascade strongly influenced gully morphology evolution and the likely length of the paraglacial period length; (2) the thickness of sediment left by the retreating glacier which controlled the influence of bedrock topographic buffers on connectivity; and (3) the extent to which diffusive drainage systems develop in response to the deposition at the hillslope base, which tend to disconnect sediment flux. Post-glacially, gully development has a self-limiting effect on sediment connectivity in that while gullying increases sediment connectivity, the eroded sediment leads to deposition on the alluvial plain that reduces sediment connectivity. Please email davide.mancini@unil.ch if you would like a copy.

Yu, D., Yin, J., Wilby, R., Lane, S.N., Aerts, J., Lin, N. Liu, M. Yuan, H., Chen, J., Prudhomme, C., Guan, M., Baruch, A., Johnson, W., Tang, X., Kwan, M.P., Yu, L. and Xu, S., 2020. Disruption of emergency response to vulnerable populations during floods. Nature Sustainability, 3, 728–736

Emergency responders must reach urgent cases within mandatory timeframes, regardless of weather conditions. However, flooding of transport networks can add critical minutes to travel times between dispatch and arrival. Here, we explicitly model the spatial coverage of all Ambulance Service and Fire and Rescue Service stations in England during flooding of varying severity under compliant response times. We show that even low-magnitude floods can lead to a reduction in national-level compliance with mandatory response times and this reduction can be even more dramatic in some urban agglomerations, making the effectiveness of the emergency response particularly sensitive to the expected impacts of future increases in extreme rainfall and flood risk. Underpinning this sensitivity are policies leading to the centralization of the Ambulance Service and the decentralization of the Fire and Rescue Service. The results provide opportunities to identify hotspots of vulnerability (such as care homes, sheltered accommodation, nurseries and schools) for optimizing the distribution of response stations and developing contingency plans for stranded sites.

James, M.R., Antoniazza, G., Robson, S. and Lane, S.N., 2020. Mitigating systematic error in topographic models for geomorphic change detection: moving beyond off-nadir imagery. Earth Surface Processes and Landforms, 45, 2251-71

Unmanned aerial vehicles (UAVs) and structure‐from‐motion photogrammetry enable detailed quantification of geomorphic change. However, rigorous precision‐based change detection can be compromised by survey accuracy problems producing systematic topographic error (e.g. ‘doming’), with error magnitudes greatly exceeding precision estimates. Here, we assess survey sensitivity to systematic error, directly correcting topographic data so that error magnitudes align more closely with precision estimates. By simulating conventional grid‐style photogrammetric aerial surveys, we quantify the underlying relationships between survey accuracy, camera model parameters, camera inclination, tie point matching precision and topographic relief, and demonstrate a relative insensitivity to image overlap. We show that a current doming‐mitigation strategy of using a gently inclined (<15°) camera can reduce accuracy by promoting a previously unconsidered correlation between decentring camera lens distortion parameters and the radial terms known to be responsible for systematic topographic error. This issue is particularly relevant for the wide‐angle cameras often integrated into current‐generation, accessible UAV systems, frequently used in geomorphic research. Such systems usually perform on‐board image pre‐processing, including applying generic lens distortion corrections, that subsequently alter parameter interrelationships in photogrammetric processing (e.g. partially correcting radial distortion, which increases the relative importance of decentring distortion in output images). Surveys from two proglacial forefields (Arolla region, Switzerland) showed that results from lower‐relief topography with a 10°‐inclined camera developed vertical systematic doming errors > 0·3 m, representing accuracy issues an order of magnitude greater than precision‐based error estimates. For higher‐relief topography, and for nadir‐imaging surveys of the lower‐relief topography, systematic error was < 0·09 m. Modelling and subtracting the systematic error directly from the topographic data successfully reduced error magnitudes to values consistent with twice the estimated precision. Thus, topographic correction can provide a more robust approach to uncertainty‐based detection of event‐scale geomorphic change than designing surveys with small off‐nadir camera inclinations and, furthermore, can substantially reduce ground control requirements. An Open Access copy is available here.

Secondary circulation in river confluences results in a spatial and temporal variation of fluid motion and a relatively high level of morphodynamic change. Acoustic Doppler current profiler (aDcp) vessel‐mounted flow measurements are now commonly used to quantify such circulation in shallow water fluvial environments. It is well established that such quantification using vessel‐mounted aDcps requires repeated survey of the same cross‐section. However, less attention has been given to how to process these data. Most aDcp data processing techniques make the assumption of homogeneity between the measured radial components of velocity. As acoustic beams diverge with distance from the aDcp probe, the volume of the flow that must be assumed to be homogeneous between the beams increases. In the presence of secondary circulation cells, and where there are strong rates of shear in the flow, the homogeneity assumption may not apply, especially deeper in the water column and close to the bed. To reduce dependence on this assumption, we apply a newly‐established method to aDcp data obtained for two medium‐sized (~60–80 m wide) gravel‐bed river confluences and compare the results with those from more conventional data processing approaches. The comparison confirms that in the presence of strong shear our method produces different results to more conventional approaches. In the absence of a third set of fully independent data, we cannot demonstrate conclusively which method is best, but our method involves less averaging and so in the presence of strong shear is likely to be more reliable. We conclude that it is wise to apply both our method and more conventional methods to identify where data analysis might be impacted upon by strong shear and where inferences of secondary circulation may need to be made more cautiously.

A copy is available here.

The Swiss National Science Foundation has funded a new project that will involve two PhD students and a technician to work on the behaviour of the areas that form in front of glaciers (proglacial forcefields) as they retreat. They are dramatically increasing in size due to rapid climate warming and glacier recession. Impacts of climate change upon glacier recession are well-established over the timescale of years to decades. Geomorphic response of proglacial margins, of which forefields are one component, has also received significant attention (e.g. consequences of glacial debuttressing; role of vegetation as an ecosystem engineer in stabilising deglaciated terrain). Forefields themselves, especially in Alpine settings, have received less attention notably (1) how they are forced by their upstream boundary condition, glacier runoff and sediment export; and (2) how they filter this signal to influence downstream sediment yield. Quantifying these processes fully and continually at scales from the sub-daily (due to rapid discharge variation following from snow/ice melt) to the seasonal has not yet been attempted but is necessary if we are to understand how changing glacier sediment export translates into downstream sediment delivery. It is of practical importance (e.g. for hydropower management) and scientific interest (e.g. whether measurements of sediment yield can be used to infer glacial erosion rates; how forefield morphodynamics create the habitats upon which new postglacial ecosystems can develop).

The core aim of this project is to undertake the first, coupled study of the relationship between subglacial sediment export, forefield morphodynamics and downstream sediment flux for a retreating Alpine glacier. The project is structured around two broad sets of hypotheses. The first addresses subglacial sediment export, a critical boundary condition that will drive forefield morphodynamics. It seeks to quantify how and over what timescales the marginal zone of a glacier regulates the export of glacially-eroded sediment to its forefield, for both bedload and suspended load. The second focuses upon how the forefield responds to glacial sediment export in terms of morphodynamics and how these morphodynamics in turn filter glacier-exported sediment to drive downstream sediment yield.

The project uses both field data collection and computational modelling. The former focuses upon a representative temperate Alpine valley glacier forefield, Otemma, in Switzerland. In a technician-led work package (WP1) new opportunities for monitoring bedload continuously using acoustic pipe samplers will be combined with standard stage and turbidity monitoring to produce, after calibration, the first season-scale, continuous records of discharge, suspended load and bedload. WP2 will use gauging station data to quantify glacial sediment export and its timescales of variability. It will be supported by ground penetrating radar survey of the subglacial channel in the snout marginal zone and the first attempt to introduce into crevasses and moulins tagged gravel/cobble particles and to track their emergence at the snout. Existing one-dimensional models will be adapted to allow us to simulate how sediment moves through the snout marginal zone under different forcing conditions. WP3 focuses on the forefield, quantifying sub-daily morphodynamics and grain size patterns using UAV systems. These will allow us to quantify how the forefield filters glacier-exported sediment and to relate it to morphodynamics. The HSTAR numerical model will be developed for multiple grain sizes and applied to simulate how different boundary conditions, including the size of the proglacial area, filter the signal of glacier sediment export. WP4 will bring together results from WPs 1-3 to answer the critical science questions: what are the timescales over which glacier sediment export can be used to infer glacial erosion rates? how do proglacial margins filter glacial sediment export to determine basin sediment yield? and how do proglacial morphodynamics evolve as glaciers retreat, so impacting frequencies of forefield disturbance.

Boix Canadell, M. Escoffier, N., Ulseth, A.J., Lane, S.N. and Battin, T.J., 2019. Alpine glacier shrinkage drives shift in dissolved organic carbon export from quasi-chemostasis to transport-limitation. Geophysical Research Letters, 46, 8872-81

The export of dissolved organic carbon (DOC) from catchments is considered as an important energy flux through streams and a major connection between terrestrial and aquatic systems. However, the impact that predicted hydrological changes due to glacier retreat and reduction in snow cover changes will have on DOC export from high?mountain streams remains unclear. In this study, we measured daily runoff and DOC yield during 1 year in Alpine streams draining catchments with different levels of glacier coverage. DOC yield showed a varied response to runoff across the catchments and varied seasonally as a function of the degree of glaciation and vegetation cover. Using space?for?time substitution, our results indicate that the controls on DOC yield from Alpine catchments change from chemostasis to transport limitation as glaciers shrink.

Antoniazza, G., Bakker, M., and Lane, S. N. ( 2019) Revisiting the morphological method in two?dimensions to quantify bed?material transport in braided rivers. Earth Surf. Process. Landforms, 44: 2251– 2267

Research in the 1990s showed that bed?material transport rates could be estimated at the reach scale in both one?dimension and, over small spatial scales (10s of m), in two?dimensions. The limit on the latter was the spatial scale over which it was possible to obtain distributed data on morphological change. Here, we revisit the morphological method given progress in both topographical data acquisition and hydraulic modelling. The bed?material transport needed to conserve mass is calculated in both one and two dimensions for a 1600?m?×?300?m Alpine braided river “laboratory”. High?resolution topographical data were acquired by laser scanning to quantify Digital Elevation Models (DEMs), and morphological changes caused by the flushing of the water intake were derived from repeated surveys. Based on DEMs of differences, 1D bed?material transport rates were calculated using the morphological method. Then, a 2D hydraulic model was combined with a topographic correction to route sediment through the network of braided channels and to obtain a spatially variable estimate of transport in both downstream and cross?stream directions. Monte Carlo simulation was applied to the routing model parameters, allowing identification of the most probable parameter values needed to minimize negative transport. The results show that within?section spatial compensation of erosion and deposition using the 1D treatment leads to substantial local errors in transport rate estimates, to a degree related to braiding intensity. Even though the 2D application showed that a large proportion of the total transport was actually concentrated into one main channel during the studied low flow event, the proportion of transport in secondary anabranches is substantial when the river starts braiding. Investigations of the effects of DEM resolution, competent flow duration and survey frequency related to ‘travelling bedload’ and sequential erosion?deposition emphasized the critical importance of careful data collection in the application of the morphological method.

A copy is available here.

The colonization of proglacial margins by vegetation following glacier recession is a slow process, not least because glacially produced sediments are commonly well drained. Following from human?induced climate change, warming could increase both growth rates and water availability because of glacier melting, so compensating for situations where climate change reduces precipitation. Compensation is likely a function of location, which will control access to meltwater and groundwater, themselves spatially variable. For the Olguin glacier (Torres del Paine, Chile), we test the hypothesis that as climate has warmed and precipitation has fallen, tree growth rate response is dependent upon the access of trees to glacial meltwater. Cores were taken from trees in three revegetating zones: (Z1) proglacial stream proximal, (Z3) proglacial stream distal, and (Z2) intermediate between Z1 and Z3. For trees within each zone, we measured annual tree?ring widths and ?²H values. Z1 growth rates were strongly correlated with temperature and Z3 with precipitation, and Z2 showed a shift from precipitation correlation (i.e., following Z3) to temperature correlation (i.e., following Z1) through time. ?²H values were lowest at Z1, reflecting water of glacial origin, were highest at Z3, reflecting meteoric water supply, and shifted through time at Z2 from meteoric to glacial. Increased water supply associated with temperature?driven glacier recession may compensate for decreasing water supply from precipitation to influence tree growth. This compensation is likely related to the spatial organization of the subsurface flux of glacial melt and leads to different revegetation processes to those envisaged in the classical chronosequence model of vegetation following glacier recession.

The paper has been published in Ecohydrology and a copy is available here.

The Anthropocene has been proposed as a profound, globally synchronous rupture in the history of the Earth System with its current state fundamentally different to that of the Holocene and driven by the geological force of human activity. Here, we show how stratigraphy is being made in a lake that is heavily impacted upon by climate change and human activities. For one of the largest inner-Alpine catchments in the European Alps, we draw attention to how sedimentation rates are a product of non-stationary, reflexive, human actions. In Lake Geneva, we identify both a human-induced climate change (HCC) signature and the effects of a recent economic shock on sediment extraction upon sediment loading to and sedimentation rates in the lake. The HCC signature thus reflects the nature of climate change impacts in this basin, where sediment accumulation rates evolve with climate, but where economic conditions contribute to shifts in the supply of sediment to the lake. Following social theory, we call this glocalization because of the combined importance and inseparability of human impacts across different spatial scales. The nature of human impacts on sediment delivery to the lake mean that the influence of humans is unlikely to be captured in the long-term depositional record.

The paper can be obtained here. The paper was picked up by a number of local media sites, (1), (2), (3) and (4).