Search

Prediction of bedload rates in gravel-bed rivers at low-to-moderate flow conditions, where bedload movement is intermittent, remains a challenging problem. While the virtual velocity concept provides a useful approach to bedload rate estimation in the intermittent movement regime, virtual velocity estimation remains hindered by a lack of tools for predicting mean sediment resting time. As a first step toward sediment resting time estimation in gravel beds, the present study develops a semi-theoretical resting time model applicable to nonuniform gravel-sized spherical particles. The model is based on the consideration that interactions of near-bed flow with bed material lead to mobilization of individual resting particles during hydrodynamic momentum transfer events (i.e., impulses). Thus, resting time is affected by impulse magnitude and timing. The primary premise underpinning model development is that an instantaneous velocity time-series generation approach based on the velocity spectrum can be used to mimic hydrodynamic impulses and simulate resting times. Based on past findings, two model parameters are considered important to advancing resting time predictions in gravel beds. First, the relative particle size allows size-fractional resting time predictions for a nonuniform sediment mixture. Second, the hindrance coefficient accounts for hiding effects and enables resting time predictions for different bed structure types. To provide calibration and verification data, laboratory experiments documenting impulse statistics and mean resting times for a range of flow and relative particle size conditions were also performed. The verified model exhibits mean resting times with similar magnitude and trends with increasing stress compared with experimental verification data. [ABSTRACT FROM AUTHOR]

Bedload is rarely measured in Indian rivers. It is recommended that 5% of suspended load can be taken as bedload in the absence of measured bedload. The present study validates this by direct physical measurement of bedload using the Helley–Smith sampler in an ephemeral mountain stream of Gujarat, India. It was observed that, on an average, the bedload formed 3.97% of the suspended load. The measured bedload flux was 1.02 tonnes/ day. To overcome the need and dependability on actual physical bedload measurement, a bedload rating curve against specific discharge was developed to predict the bedload rate in the study reach. Few prominent existing bedload equations selected from the literature were tested against the measured bedload, which over-predicted the bedload transport rate with a discrepancy ratio greater than 2 and RMSE 2.4–48. A calibration coefficient ξ = 0.00167 was introduced in the widely used Recking (2013) equation for the study reach resulting in an improvement of the coefficient of variation as 1.92 and RMSE as 1.35. [ABSTRACT FROM AUTHOR]

Trapezoidal-shaped hydrographs are typical of anthropized rivers, as this form is generally associated with the release of water from hydropower dams. To investigate how such unnatural waves can affect bedload rate, preliminary flume experiments were performed in Krakow, Poland, looking at bedload transport rate, bed shear stress and bed morphology. In addition, close-range bed surface photogrammetry was used to investigate bed changes due to the passage of the flood wave. Three scenarios, having the same water volume but different wave magnitudes, were tested. The lowest wave showed almost no sediment transport and no visible changes in bed morphology, while higher waves changed the bed morphology, creating erosion and accumulation zones. The highest wave was characterized by an 8-shaped hysteresis of the bedload rate with a peak during the wave maximum. The lag time between the maximum bedload rate and the wave plateau was longer than expected due to zero-slope conditions. [ABSTRACT FROM AUTHOR]

The dynamics of sandy rivers is a complex phenomenon linking the interactions between dunes, flow and sediment transport. Despite an existing large amount of research, few comparative studies exist for rivers with major hydro-sedimentological differences. This work uses detailed dune measurements from the Paraná and Tercero rivers, Argentina. Significant differences were found in the representative roughness height scale, dune steepness, lee angles, velocity profiles and flow recirculation. It is found that whereas the hydraulic resistance for the Tercero River scales with the size of large dunes, the Paraná River needs to develop an intermediate roughness scale (the small superimposed dunes) to accommodate the required balance between gravity and friction. Suspended bed sediment increase is inversely related to the dune steepness and lee side angles as advanced by previous findings. The suspension number seems to be a paramount metric that helps explain differences regarding dune geometry within a wide range of flow conditions. [ABSTRACT FROM AUTHOR]

• Lateral sediment flux from hillslopes is directly connected to the curve number. • The curve number summarized the resistance from the soil-group complex. • The lateral sedimentologic connectivity was well represented by the curve number. • The soil-cover complex in a watershed is closely related to watershed erodibility. The flux of suspended sediment and bedload in a watershed is closely linked to the resistance of soil, known as erodibility. However, the relation between the resistance generated by soil to sediment transport from the hillslope to the river flow is still a challenge. Typically, the bedload flux is related to the shear stress of the main channel. However, the link between lateral sediment flux from hillslopes to suspended sediment flux of the main channel and the relation between bedload and suspended sediment flux to soil erodibility need to be developed. For three Brazilian watersheds, suspended sediment and bedload transport, soils, and hydrological data were obtained to determine watershed soil erodibility. The landscape connectivity expressed by hydrological-sediment connectivity was applied to determine the lateral sediment transport to the primary watershed channel. The flow resistance and lateral suspended sediment discharge were represented by the curve number, which summarized the resistance from the soil-group complex leading to soil erodibility. The curve number addressed the lateral flow and suspended sediment transport to represent the lateral connectivity on the landscape. Sediment transport (sand and suspended sediment) from hillslopes was added to the bedload transport along the longitudinal axis of the watershed, and the watershed soil erodibility (K W) was, thus, obtained for each of the three evaluated watersheds. [ABSTRACT FROM AUTHOR]

Over the last two centuries, rivers worldwide have been affected by human interventions, including dams, river training, and gravel mining. Such actions have often involved river fragmentation and sediment starvation. In this regard, gravel augmentation is an increasingly common restoration practice for mitigating sediment starvation in gravel-bed rivers. However, uncertainties remain on how to better implement and design such operations. This is the case for the Rhône River at Péage-de-Roussillon, France, where 6885 m3 of gravels were augmented in 2017. This work raised some concerns from river managers about the potential threat posed to a downstream reservoir from the arrival of the sediment, and explains why they were interested in the timeframe of sediment propagation, and travel distances and velocities of the augmented sediment. To answer these questions, we propose a modelling framework for simulating the long-term downstream propagation of a pulse of augmented sediment, with this framework being based on a combination of particle tracking data collected in the field and bedload transport capacity estimates. This workflow allowed us to successfully model propagation of gravels along the study reach, and provided useful information on how the sediment wave would behave over the long term. We believe that the methodology proposed in this study has much potential for exploring and investigating the kinematics of sediment wave propagation in gravel-bed rivers. [ABSTRACT FROM AUTHOR]

• We examine eight steep slope forested headwater catchments (15–100 ha). • Bedload organic carbon (OC), soil carbon and dissolved OC was measured. • OC in the bedload was found to be significantly related to catchment area. • Dissolved OC export in stream water declined with catchment area. • There was no difference in OC export from the harvested and non-harvest catchments. Qualitative and quantitative understandings of the origin, transport and fate of organic carbon (OC) in forest systems is needed to advance our understanding of biogeochemical cycling as well as catchment management and forest harvesting. Here we present the findings from eight steep slope forested catchments dominated by headwater streams (size range 15–100 ha) in south eastern Australia where bedload, organic carbon in bedload, hillslope soil carbon together with dissolved organic carbon has been measured over multiple years. OC in the bedload was found to be significantly related to catchment area (p < 0.02). Dissolved organic carbon export in stream water declined with catchment area. Combined bedload organic carbon and dissolved organic carbon export was significantly related to catchment area. There was no significant difference between hillslope soil organic carbon and bedload organic carbon concentration. When corrected for area, OC export by bedload was not significantly different to that of dissolved OC export. Using the environmental tracer 137Cs it was found that there was no significant difference in tracer concentration between bedload and hillslope soil. This suggests a direct link between hillslope and channel and delivery of organic carbon to the channel. Of the eight catchments examined here, six had been harvested for timber in previous decades (with area of forest removed ranging from 25 to 60%) while two catchments had minimal disturbance (Control catchments- no harvest). There was no difference in organic carbon export from the harvested and Control catchments. The results demonstrate that although land disturbance had previously occurred the management practices employed in each catchment were effective in the long term. [ABSTRACT FROM AUTHOR]

• We examine eight steep slope forested headwater catchments (15–100 ha). • Bedload organic carbon (OC), soil carbon and dissolved OC was measured. • OC in the bedload was found to be significantly related to catchment area. • Dissolved OC export in stream water declined with catchment area. • There was no difference in OC export from the harvested and non-harvest catchments. Qualitative and quantitative understandings of the origin, transport and fate of organic carbon (OC) in forest systems is needed to advance our understanding of biogeochemical cycling as well as catchment management and forest harvesting. Here we present the findings from eight steep slope forested catchments dominated by headwater streams (size range 15–100 ha) in south eastern Australia where bedload, organic carbon in bedload, hillslope soil carbon together with dissolved organic carbon has been measured over multiple years. OC in the bedload was found to be significantly related to catchment area (p < 0.02). Dissolved organic carbon export in stream water declined with catchment area. Combined bedload organic carbon and dissolved organic carbon export was significantly related to catchment area. There was no significant difference between hillslope soil organic carbon and bedload organic carbon concentration. When corrected for area, OC export by bedload was not significantly different to that of dissolved OC export. Using the environmental tracer 137Cs it was found that there was no significant difference in tracer concentration between bedload and hillslope soil. This suggests a direct link between hillslope and channel and delivery of organic carbon to the channel. Of the eight catchments examined here, six had been harvested for timber in previous decades (with area of forest removed ranging from 25 to 60%) while two catchments had minimal disturbance (Control catchments- no harvest). There was no difference in organic carbon export from the harvested and Control catchments. The results demonstrate that although land disturbance had previously occurred the management practices employed in each catchment were effective in the long term. [ABSTRACT FROM AUTHOR]