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.
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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.
2019) Revisiting the morphological method in two?dimensions to quantify bed?material transport in braided rivers. Earth Surf. Process. Landforms, 44: 2251– 2267
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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.
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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.