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<p>Along with the development of “classical” optically stimulated luminescence (OSL) dating, portable luminescence readers have been increasingly employed in a wide breadth of geomorphological settings over the last decade. Portable measurements of bulk samples along depth profiles yield infra-red and optically stimulated luminescence net intensities (IRSL/OSL). The signal build-up depends on (i) the burial duration of the sediment, (ii) the dose rate, (iii) the inherited dose at time of burial, (iv) the mineral composition and (v) the luminescence sensitivity of the minerals. In fluvial landscapes, however, the handful of existing case studies mostly focused on the sole identification of alluvium’s unconformities via luminescence profiling. We argue here that this approach shall go beyond the mere one- (or sometimes two-) dimensional sedimentary screening and steer towards the production of three-dimensional chronostratigraphical information.</p><p>This study presents the outcomes of a luminescence-profiling approach performed in a wide array of fluvial depositional systems located in the Upper Rhine Graben and the adjacent Vosges Mountains (NE France). They include:</p><ul><li>modern and historical overbank fines from the main stem (Rhine) and its tributaries (Bruche and Sauer Rivers);</li> <li>palaeomeanders and swale-and-ridge topography (Bruche);</li> <li>Holocene terrace deposits (Bruche);</li> <li>fluvio-glacial deposits (probably) from the Last Glacial Maximum (Cleurie Valley).</li> </ul><p>We thus explore the ability of the reader to measure signals of varying intensities in different morpho-sedimentary units of clearly distinct ages. The results globally show:</p><ul><li>a consistent pattern of downward increasing OSL/IRSL signal intensities, although some sharp contrasts (i.e., shifts of one order of magnitude in signal intensities) in some sequences could be identified (e.g., Rhine);</li> <li>a larger scatter of OSL/IRSL signal intensities in the sandy fraction than in the silty fraction, probably reflecting the influence of grain size on the signal accumulation (requiring further investigation);</li> <li>the potential of the portable reader as a rapid and efficient tool for tracing historical overbank fine deposition in floodplains.</li> </ul>

<p>High frequency measurements of the concentration and grain size of suspended sand in rivers remain a scientific challenge due to the strong spatio-temporal variability. Applying a hydroacoustic multi-frequency method can improve temporal resolution compared to the classical approach by solid gauging (water sampling) and provides an interesting surrogate for suspended sediment concentration and grain size in rivers characterized by a bimodal suspension. The aim of this study is to establish time series of concentration and grain size of suspended sand in the Isère River (France) using a hydroacoustic method. Measurements with 400 and 1000 kHz Horizontal Acoustic Doppler Current Profilers (HADCP) are used to determine the acoustic attenuation and backscatter. Using frequent isokinetic water samples obtained with a US P-06 sampler close to the ensonified volume, a relation between the acoustic signal and the sediment concentration and grain size can be determined. In a next step, regular solid gaugings help to establish a relation between the concentration and grain size in the ensonified volume and on average in the river cross-section. Finally, time series of concentration and grain size of suspended sand may be established based on this relation. Results show a good correlation between the concentration of fine-grained sediments and acoustic attenuation as well as between the sand concentration and backscatter. While the acoustic signature of fine sediments is mostly driven by concentration changes, the acoustic signature of the sand fraction is impacted by changes not only in concentration but also in grain size distribution (the median diameter  varying between 150 and 400 µm). The homogeneity of concentration and grain size along the acoustic beam seems to be a main factor for successfully establishing concentration time series based on a cell-by-cell analysis.</p>

<p>Measuring the concentration and grain size of suspended sand in rivers continuously remains a scientific challenge due to its pronounced spatio-temporal variability. Vertical and lateral gradients within a river cross-section require spatially-distributed water sampling at multiple verticals and depths. However, this classical approach is time-consuming and offers limited temporal resolution. Sampling is particularly difficult in presence of a bimodal suspension composed of fine sediment and a sand fraction, notably if the fine/sand ratio varies with time. The aim of this study is to establish time series of sand concentration and grain size by improving temporal resolution using an acoustic multi-frequency method based on acoustic attenuation and backscatter to measure the suspension indirectly. Experiences of Moore et al. (2012) and Topping & Wright (2016) with Horizontal Acoustic Doppler Current Profilers (HADCPs) show that dual-frequency inversion can separate the fine sediment fraction dominating acoustic attenuation from the sand fraction dominating acoustic backscatter. Concentration and grain size of suspended sediment, both the fine and sand fraction, can be quantified by signal inversion after correction for transmission losses.</p><p>Applying existing dual-frequency, semi-empirical methods in a typical Piedmont river (River Isère, France) remains a challenge due to the high concentrations and a broad bimodal distribution. Two monostatic HADCPs of 400 and 1000 kHz were installed at a hydrometric station of the Isère at Grenoble Campus where discharge and turbidity have been recorded for more than 20 years. Using frequent isokinetic water samples obtained with US P-72 and US P-06 samplers close to the ensonified volume, a relation between acoustic signal and the sediment concentration and grain size can be determined. Simultaneously, total sand flux and grain size distribution are calculated performing solid gaugings using Delft bottle samples and ADCP measurements in the entire cross-section. The method using index concentration and grain size in the HADCP measurement area is then used to evaluate the total sand flux and average grain size time-series in the cross-section.</p><p>First results show good correlations between the fine sediment concentration and the sediment attenuation for both frequencies. Specific extreme events (e.g. debris flows, dam flushes or spring floods) show distinct signatures in acoustic attenuation, backscatter and ratio between the two frequencies. During a debris flow (concentration up to 5.3 g/l), attenuation reached 1.6 and 3 dB/m for 400 respectively 1000 kHz, but no peak in backscatter intensity, whereas a spring flood (up to 4 g/l with at least 50 % sand) caused major peaks in attenuation and backscatter. Pronounced hysteresis during the events and time-varying ratio between attenuation due to sediments measured by 400 and 1000 kHz indicate suggest that the grain size distribution may vary. Relating sand concentration from physical samples with beam-averaged backscatter may elucidate changes in grain size more precisely. Existing heterogeneities of concentration and grain size along the acoustic beam contradict the homogeneous distribution supposed by the method and require local analysis based on local concentration and grain size characteristics.</p>

<p>In parallel to standard optically stimulated luminescence (OSL) dating, portable OSL readers have been increasingly employed in a wide range of geomorphological settings over the last decade. In fluvial landscapes, most of the OSL signal intensities measured by the portable reader were successfully used either to explore bleaching characteristics of river deposits or to rapidly gain new insights into alluvial stratigraphy via luminescence profiling (Munyikwa et al., 2020). However, going beyond the mere one- (or sometimes two-) dimensional sedimentary screening, the use of portable readers shall steer toward the production of three-dimensional chronostratigraphical information. Against this background, the high lateral mobility of the lowermost Bruche reach (directly upstream of Strasbourg), documented at the decadal scale by Jautzy et al. (2022), thus represents a suitable setting to explore the potential of field-based portable luminescence profiling to provide new insights into both lateral and vertical fluvial dynamics.</p><p>Here, the sampling approach with the portable reader using both blue and infra-red stimulations (BSL and IRSL) is twofold:</p><ul><li>testing the ability of the reader to measure signals of varying intensities in morpho-sedimentary units of different ages, i.e. an early Holocene terrace, historical palaeomeanders and a modern swale-and-ridge system;</li> <li>investigating the gradual lateral shifting and incision of a single palaeomeander in the floodplain recorded by a succession of palaeochannels and former point bar deposits.</li> </ul><p>Preliminary results (i) show that the older the landform, the higher the BSL/IRSL signal intensity, and highlight (ii) a consistent pattern of downward increasing BSL/IRSL signal intensities in the homogeneous fine-grained upper part of all profiles. However, BSL/IRSL signal intensities measured in the sandy fraction (i.e. lower parts of the alluvial sequences or in the swale-and-ridge system) usually record a larger scatter that requires further investigations. This study underlines the potential of the portable reader as a rapid and efficient tool for tracing historical overbank deposition in floodplains; these results will be complemented soon by standard luminescence dating to constrain sedimentation rates.</p><p><em>References:</em></p><p><em>Jautzy, T., Schmitt, L., Rixhon, G., (2022, in press). Historical geomorphological adjustments of an Upper Rhine sub-tributary</em><em> over the two last </em><em>centuries</em><em> </em><em>(</em><em>Bruche River</em><em>,</em><em> France). Géomorphologie, Relief, Processus et Environnements. </em></p><p><em>Munyikwa, K., Kinnaird, T.C., Sanderson, D.C.W. (2020). </em><em>The potential of portable luminescence readers in geomorphological investigations: a review. Earth Surface Processes and Landforms </em><em>DOI: 10.1002/esp.4975. </em></p>