Magnetic tracing of fine-sediment over pool-riffle morphology

Field studies documenting fine-sediment (< 2 mm) transport in gravel-bed rivers are rare. For the first time in a fluvial environment, a technique that enhances the magnetic susceptibility of sand is used to trace its longitudinal dispersion and storage. This paper describes the methodology behind the artificial magnetic enhancement of iron-stained sand, and presents the results from sand tracing exercises conducted on two gravel-bed channels with pool-riffle morphology; one unregulated and sinuous in nature (site A), the other regulated and straight (site B), both situated on the River Rede Northumberland, UK. Two tonnes of magnetically enhanced tracer sand was introduced to site A and four tonnes to site B, to provide information on fine-sediment storage dynamics, interaction of fines with the stream bed, and rates of movement, expressed as virtual velocity (Vi). Sand transport pathways appeared to differ between the reaches; for site A, sand storage was found on bars and riffle margins with no storage or signs of transport through pools, and in contrast pool storage of tracer was a key feature shown at site B. Topographic forcing may cause differences in sediment sorting at site A; topographic highs tend to have low sand transport rates with sand grains becoming congested in these areas, whereas topographic lows show higher transport rates resulting in greater dispersion. Supply limitation of sand on the falling limb of the hydrograph may also become an issue in the topographic lows at this site. Hydrograph differences between the regulated and unregulated reaches could also play a role; however this could not be quantified in this study. There was no evidence of sand infiltration into the bed at site A; however marginal evidence for infiltration into the near-surface (0–15 cm) substrate voids was found at site B. The general lack of evidence for significant infiltration may reflect limited availability of void space in substrate framework gravels. Tracer sand was transported over the bed surface, with little vertical interaction with the substrate, despite periods of gravel mobilisation at site A. Vi over the study duration for site A was 2.28 m day− 1, and 0.28 m day− 1 for site B. These values are greater than those calculated using existing predictive equations developed from gravel tracer data, possibly reflecting differences in the mode of transport between bedload and saltation load

Anthropogenic impacts on the water chemistry of a transboundary river system in Southeast Asia

The Red River originating from Yunnan province, China is the second largest river in Vietnam in terms of length and discharge. Combination of water chemistry monitoring data of 4 years (2018–2022) from different sub-basins of the Red River (the Da, Lo, Thao, Tra Ly, and Day) with historical datasets indicates a decline in pH from 8.1 in 2000 to 7.7 in 2021, greater CO2 concentrations and a shift from waters naturally dominated by carbonate weathering to waters dominated by evaporite weathering. Such changes were most apparent in the delta area where heavy human activities have increased influxes of most dissolved chemicals, except SiO2. Evaporite weathering is particularly enhanced by mining and deforestation occurring in upstream regions of both China and Vietnam. Pyrite oxidation, alongside silicate weathering, is enhanced along the Red River Fault Zone but reduced in tributaries with a higher proportion of hydropower reservoirs. Longer water residence times in these large reservoirs (total volume > 2.7x1010 m3) located in the Da and Lo sub-basins have also increased primary productivity, leading to higher evasion/uptake of CO2 and SiO2, lower total dissolved solids (TDS), and higher pH. The total physical and chemical denudation rates of upstream mountain tributaries ranged between 0.107 ± 0.108 and 0.139 ± 0.137 mm yr−1, mainly due to reservoir implementation and instream aquatic biogeochemistry changes. Our findings demonstrate that anthropogenic activities are profound factors impacting the water chemistry of the Red River system

Influence of survey strategy and interpolation model on DEM quality

Accurate characterisation of morphology is critical to many studies in the field of geomorphology, particularly those dealing with changes over time. Digital elevation models (DEMs) are commonly used to represent morphology in three dimensions. The quality of the DEM is largely a function of the accuracy of individual survey points, field survey strategy, and the method of interpolation. Recommendations concerning field survey strategy and appropriate methods of interpolation are currently lacking. Furthermore, the majority of studies to date consider error to be uniform across a surface. This study quantifies survey strategy and interpolation error for a gravel bar on the River Nent, Blagill, Cumbria, UK. Five sampling strategies were compared: (i) cross section; (ii) bar outline only; (iii) bar and chute outline; (iv) bar and chute outline with spot heights; and (v) aerial LiDAR equivalent, derived from degraded terrestrial laser scan (TLS) data. Digital Elevation Models were then produced using five different common interpolation algorithms. Each resultant DEM was differentiated from a terrestrial laser scan of the gravel bar surface in order to define the spatial distribution of vertical and volumetric error. Overall triangulation with linear interpolation (TIN) or point kriging appeared to provide the best interpolators for the bar surface. Lowest error on average was found for the simulated aerial LiDAR survey strategy, regardless of interpolation technique. However, comparably low errors were also found for the bar-chute-spot sampling strategy when TINs or point kriging was used as the interpolator. The magnitude of the errors between survey strategy exceeded those found between interpolation technique for a specific survey strategy. Strong relationships between local surface topographic variation (as defined by the standard deviation of vertical elevations in a 0.2-m diameter moving window), and DEM errors were also found, with much greater errors found at slope breaks such as bank edges. A series of curves are presented that demonstrate these relationships for each interpolation and survey strategy. The simulated aerial LiDAR data set displayed the lowest errors across the flatter surfaces; however, sharp slope breaks are better modelled by the morphologically based survey strategy. The curves presented have general application to spatially distributed data of river beds and may be applied to standard deviation grids to predict spatial error within a surface, depending upon sampling strategy and interpolation algorithm

Evaluation of image-based velocity measurements for computing river discharge in real-time

International audienceQuantifying key hydrological parameters, such as the river discharge, during high flow events frequently facestechnical difficulties and is potentially dangerous to operatives. The application of novel hydrological monitor-ing techniques may reduce these risks, and offers the potential to rapidly advance our understanding of operatingprocesses, which may reduce risk to society. We therefore propose a low-cost non-contact alternative method todetermine river discharge in real-time. Our approach is an image-based technique, where video footage is subjectedto optical flow tracking algorithms, enabling the displacement rates of naturally-occurring features on the watersurface to be computed. This is achieved through the application of the Kanade-Lucas-Tomasi (KLT) particle im-age velocimetry (PIV) approach. We installed our real-time camera system at the Environment Agency’s Austin’sBridge Gauging Station on the hydrologically responsive River Dart (SW UK). Our objective was to comparedischarge measurement results obtained with our KLT-PIV method to those from an established and well-provenflow measurement station. We compared our measurements with 73 ADCP transects at discharges ranging from3-101 m3 s-1 and the official rating curve of the Environment Agency. We obtained promising results, our KLT-PIVsurface velocity measurements were predominantly within 10% compared to the Environment Agency’s dischargecomputation. This indicates that the KLT-PIV technique offers potential for the safe and cost-effective real-timeassessment of river flows, even in potentially damaging high magnitude flood events. Therefore, it offers the oppor-tunity for greatly improved flow monitoring in countries with established networks, and for the rapid developmentof monitoring where networks are less well developed

Application of Image-based Velocity Measurements for Computing River Discharge in Real-time

International audienceQuantifying key hydrological parameters, such as the river discharge, during high flow events frequently faces technical difficulties and is potentially dangerous to operatives. The application of novel hydrological monitoring techniques may reduce these risks, and offers the potential to rapidly advance our understanding of operating processes, which may reduce risk to society. We therefore propose a low-cost non-contact alternative method to determine river discharge in real-time. Our approach is an image-based technique, where video footage is subjected to optical flow tracking algorithms, enabling the displacement rates of naturally-occurring features on the water surface to be computed. This is achieved through the application of the Kanade-Lucas-Tomasi (KLT) particle image velocimetry (PIV) approach. We installed our real-time camera system at the Environment Agency’s Austin’s Bridge Gauging Station on the hydrologically responsive River Dart (SW UK). Our objectives were: (i) to compare discharge measurement results obtained with our KLT-PIV method to those from an established and well-proven flow measurement station, (ii) to find out if we can use our surface velocity measurements to estimate river discharge through an index-velocity approach; and (iii) to determine how many ADCP measurements are necessary to produce an acceptable index rating. We compared our measurements with 73 ADCP transects at discharges ranging from 3 – 101 m3 s-1 and obtained promising results showing a highly significant correlation between ADCP derived river velocities and our KLT-PIV surface velocity measurements. This indicates that the KLT-PIV technique offers potential for the safe and cost-effective real-time assessment of river flows, even in potentially damaging high magnitude flood events. Therefore, it offers the opportunity for greatly improved flow monitoring in countries with established networks, and for the rapid development of monitoring where networks are less well developed

Catchment scale flood management using SCIMAP-Flood: Spatial targeting of flood hazard reduction measures in the East Rapti catchment, Nepal

International audiencePractical approaches for managing flood hazards are moving from mitigation solely at the point of the impact, and towards an integrated catchment management approach which considers the source areas, flow pathways of flood waters and the impacted communities. However, there is uncertainty associated with providing catchment scale solutions which is primarily a function of spatial and temporal variability in patterns of rainfall, land cover, and hydrological connectivity across a catchment. These factors mean that flood waters are not produced in a homogenous way from one event to the next, resulting in a distribution of travel times to points of impact, such as towns and key infrastructure. There must also be careful consideration of the potential for a reduction in the flood hazard in one sub-catchment to increase the hazard elsewhere due to tributary timing and synchronisation

Catchment scale flood management using SCIMAP-Flood: Spatial targeting of flood hazard reduction measures in the East Rapti catchment, Nepal

International audiencePractical approaches for managing flood hazards are moving from mitigation solely at the point of the impact, and towards an integrated catchment management approach which considers the source areas, flow pathways of flood waters and the impacted communities. However, there is uncertainty associated with providing catchment scale solutions which is primarily a function of spatial and temporal variability in patterns of rainfall, land cover, and hydrological connectivity across a catchment. These factors mean that flood waters are not produced in a homogenous way from one event to the next, resulting in a distribution of travel times to points of impact, such as towns and key infrastructure. There must also be careful consideration of the potential for a reduction in the flood hazard in one sub-catchment to increase the hazard elsewhere due to tributary timing and synchronisation

Controls on jökulhlaup-transported buried ice melt-out at Skeiðarársandur, Iceland: Implications for the evolution of ice-marginal environments

High-magnitude jökulhlaups, glacier margin position and ice-thickness have been identified as key controls on sandur evolution. Existing models however have focused primarily on observations made during short windows of time and often do not account for the subsequent modification of proglacial landsystems by repeated jökulhlaups or post-depositional modification due to melt out over decadal time-scales. Digital Elevation Models (DEMs) were used to reconstruct the development of large depressions on Skeiðarársandur, an outwash plain in southeast Iceland. These depressions measure up to 1 km in width and up to 13 m in depth and are associated with ice bodies up to 1 km in length and up to 150 m in height emplaced during a high-magnitude jökulhlaup in 1903 and subsequently buried by jökulhlaups in 1913 and 1922. The continued melting of the Harðaskriða ice bodies over a century following their emplacement, together with subsequent repeated burial, by high-magnitude jökulhlaups demonstrates that jökulhlaups may continue to serve as important controls on sandur evolution on a decadal to centennial timescale (101–102 years). The Harðaskriða depressions developed only following the retreat of the glacier margin after 1945, which highlights the controls of margin position on the evolution of the sandur. Margin position and thickness of the glacier profile was seen to affect not only the distribution and thickness of sediment emplaced during jökulhaups but also the rate and pattern of melt in the decades following the decoupling of the margin from the sandur. The jökulhlaup landsystem model signatures identified at this site may provide a useful analogue for interpreting landforms and strata emplaced by glacier margin fluctuations, jökulhlaups and melt out generated by retreating continental Pleistocene ice sheets