Bedload Monitoring Research Articles

Bedload monitoring under conditions of ultra-high suspended sediment concentrations

The bedload response of the Moulin Ravine, a small alluvial system draining a very active Mediterranean badlands landscape entrenched into Jurassic black marls of the Southern French Prealps, has been investigated using an automatic Reid bedload slot sampler. This site is known for its exceptional sediment transport conditions thanks to a long-term monitoring program that started in the late 1980s, revealing a mean annual bedload yield of 2810tkm−2yr−1, and suspended sediment concentrations (SSCs) during flow events commonly reaching 100gL−1. With the deployment of the slot sampler, it has been possible to record instantaneous bedload fluxes during 10 s time increments and to investigate bedload response under flow conditions with ultra-high SSCs. Bedload records cover 4 flashy summer flow events induced by heavy convective storms including a 20-yr return period event. Due to the very high SSC conditions these events challenge bedload monitoring. Even if slot sampling has been recognized as insensitive to fine sediments (silts and clays), it has never been tested in such exceptional muddy flow conditions. The bedload slot sampler performed well in such conditions. A flow-invariant proportion of fines (∼15–20%) was captured in the slot sampler during flows. This proportion is equivalent to its content in the active bedload layer during summer flows, suggesting that fines enter the slot embedded with coarse particles. Instantaneous bedload fluxes recorded in the Moulin are amongst the highest hitherto reported values worldwide, providing evidence of the exceptional sediment transport conditions of marly alpine badlands. The dimensionless entrainment threshold is one order of magnitude higher than commonly reported for gravel-bed rivers, likely reflecting the cohesion effect of fines intruded in the channel surface and subsurface.

Bedload pulses in a hydropower affected alpine gravel bed river

This study investigated the sediment resupply and transport dynamics at the Upper Drau River upstream of Lienz (Eastern Tyrol, Austria). Due to a hydropower plant, a 24km long river reach of this alpine gravel bed river is under residual flow conditions, although sediment is still resupplied into the reach through many active torrents and tributaries. As a result, sediment deposition in the residual flow reach intensified, hence increasing maintenance efforts to stabilize this river section and ensure flood protection. In combination with a new sediment management program, a continuous bedload monitoring system was installed 2km downstream of the residual reach in 2001 to support the development of adapted sediment management strategies. The surrogate bedload monitoring system consists of 16 impact plate geophones, installed over a 17m wide cross section. The unprecedented 15-year dataset of high-resolution bedload intensity revealed a complex process of gravel storage and intermittent resupply from the residual reach, allowing the authors a detailed analysis of frequently occurring bedload pulses.These transport features are triggered by increased discharges during floods in the residual reach and created pronounced anticlockwise bedload hysteresis or, with a temporal shift to the event peak, caused distinct shifts in the bedload activity downstream. Bedload pulses produce very high bedload fluxes while in transit, tend to increase bedload flux in the post-event phase, and can alter and reduce the upstream sediment storage leading to a lowering of bedload availability for future pulses. The observed time lags between main discharge events and the arrival of the macro-pulses are correlated with mean water discharge during pulse propagation, thus enabling a prediction of the pulse arrival at the monitoring station solely based on the hydrograph. In combination with the hydrological setup of the reach, the observed bedload pulse time lags allowed an estimation of pulse velocities in the range 0.002−0.05ms−1.

Passive Acoustic Monitoring of Bed Load for Fluvial Applications

Passive Acoustic Monitoring of Bed Load for Fluvial Applications

Development and installation of bedload monitoring systems with submerged load cells

Bedload governs riverbed channel variations and morphology,it isnecessarytodeterminebedload discharge through an arbitrary cross sectionin a mountain river. A new system with submerged load cells has been developed to directly measure bedload discharge. The system consists of:(1) anironbox which is 1 m long, 0.5 m wide and 0.1 m in depth, (2) two submerged load cells 0.7 m apart, (3) a pressure sensor and,(4) an electromagnetic velocity meter. This system has been designed to exclude the effect of the hydraulic pressure of water on direct measurements ofbedload particle weight. Initialtestsin a laboratorywere conducted to examine the accuracy of measurements with the system underaerial conditions. The system has beeninstalled in the supercritical flume in Ashi-arai-dani River of the Hodaka Sedimentation Observatory of the Disaster Prevention Research Institute (DPRI) of Kyoto Universityto obtain bedload dischargeunder natural conditions. Flume tests were conductedin this channelby artificial supply of uniform sediment particles ofseveral grain sizes.The averagevelocity of the sediment particles near the bed wasestimated usingcross-correlation functions forweightwaves obtained bythe two load cells.Bedload dischargecalculations were based ontime integration oftheproduct of sediment velocity andsedimentweight obtained bythe twoload cells. Thisstudyclarifies thereasons whybedload measurements are difficult, and providessomesolutions using the monitoring systems with submerged load cells through the field measurements. Additionally, the applicabilityofbedload measurement with the submerged load cellsis explained based onexperimental artificial sediment supplydata.

The anatomy of effective discharge: the dynamics of coarse sediment transport revealed using continuous bedload monitoring in a gravel‐bed river during a very wet year

AbstractIndirect, passive approaches for monitoring coarse bedload transport could allow cheaper, safer, higher‐resolution, longer‐term data that revolutionises bedload understanding and informs river management. Here, insights provided by seismic impact plates in a downstream reach of a flashy gravel‐bed river (River Avon, Devon, UK) are explored in the context of plate performance. Monitoring of a centrally‐situated plate (IP1) during an extremely wet 12‐month period demonstrated that impacts were related to discharge as a measure of transport potential (R2 = 0.38) but that factors other than transport limitations are important. Analysis of discrete flow events revealed consistent rising‐limb and falling‐limb impact spikes biased toward the latter for larger events. Such patterns may result from disruption of the upstream armour layer (rising limb) and supply enhancements related to both upstream mass bank failures and/or flood routing of non‐local sediment sources (falling limb). Installation of additional impact plates indicated that plate IP1 was indeed dominantly related to instantaneous discharge, that a three‐plate lateral array somewhat better explained impact variability (R2 = 0.49), and that the bedload track shifts laterally with discharge. Aggregating event‐total IP1 impacts against volumetric discharge further increases explanation as intra‐event and stochastic bedload factors are subsumed but left 26% unexplained variance related to the unsampled bedload mass, inter‐event supply differences, and attributes of plate performance. Annualising the data created an impact‐based 'effective discharge’ for this extremely wet year that was closer to morphological bar‐full in magnitude than bankfull, but the preceding results imply this outcome is related as much to supply limitations as transport limitations. Overall, passive approaches offer a liberating prospect for bedload monitoring, capable of producing insights only achievable through high resolution, extended time periods. Such results could potentially inform threshold conditions and geomorphological effectiveness of flows for future river management strategies. Copyright © 2015 John Wiley & Sons, Ltd.

Bed load transport monitoring using seismic observations in a low‐gradient rural gravel bed stream

AbstractThe characterization of bed load transport in rivers is critical for the fundamental understanding and management of fluvial systems. Bed load monitoring based on seismological observations has recently emerged as a viable noninvasive measurement technique. However, applications of this new approach have been hitherto restricted to the case of sediment transport in steep mountain rivers. Here we further develop and evaluate the approach for a lower gradient gravel bed stream in a rural catchment using seismic observations, in situ hydroacoustic measurements of bed load motion (impact‐plate‐type device), and 3‐D hydromorphodynamic modeling. The results of this joint analysis of seismic measurements, hydroacoustic records, and sediment transport simulations show that the seismic monitoring technique for bed load transport characterization is applicable for a broader range of river systems than previously investigated.

Real time measurements of sediment transport and bed morphology during channel altering flow and sediment transport events

Real-time measurements of bed changes over a reach are a missing piece needed to link bed morphology with sediment transport processes during unsteady flows when the bed adjusts quickly to changing transport rates or visual observation of the bed is precluded by fine sediment in the water column. A new technique is presented that provides continuous measurement of sediment movement over the length of a flume. A bedload monitoring system (BLMS) was developed that makes use of pressure pillows under a false flume bottom to measure sediment and water weights over discrete flume channel sections throughout a flow event. This paper details the construction of the BLMS and provides examples of its use in a laboratory setting to reconstruct bed slopes during unsteady flows and to create a real-time record of sediment transport rates across the flume channel bed during a sediment transporting flow.Data gathered from the BLMS compared well against techniques commonly in use in flume studies. When the BLMS was analyzed in conjunction with bed surface DEMs and differenced DEMs, a complete transport and bed adjustment picture was constructed. The difference DEMs provided information on the spatial extent of bed morphology changes. The BLMS supplied the data record necessary to reconstruct sediment transport records through the downstream channel, including locations and time periods of temporary sediment storage and supply. The BLMS makes it possible to construct a continuous record of the spatial distribution of sediment movement through the flume, including areas of temporary aggradation and degradation. Exciting implications of future research that incorporates a BLMS include a more informed management of river systems as a result of improved temporal predictions of sediment movement and the associated changes in channel slope and bed morphology.

Design and implementation of a passive acoustic bedload monitoring surrogate system

Various methods of employing passive acoustics to monitor bedload transport by listening to noise generated by colliding gravel have been explored in both the lab and field. Expanding upon this research, a hydrophone-based passive bedload-monitoring system was designed, tested, and deployed by researchers at the University of Mississippi and the National Sedimentation Laboratory in Oxford, MS. A series of laboratory experiments was used to measure the dependence of acoustic propagation on various parameters such as depth of the water and placement of the hydrophones. These tests employed a calibrated acoustic transmitter to broadcast tones of various frequencies across a gravel bed. Additional flume tests used a motorized carrier to drag rocks across the gravel bed, allowing a known sediment flux to be measured acoustically without any flow noise. The hydrophone-based system was deployed on the Trinity River, Weaverville, CA, in the summer of 2012 and the Elwha River, Port Angeles, WA, in the summer of 2013. Both deployments were co-incident with physical bedload measurements taken by Graham Mathews and Associates. The design of the acoustic system, methods used to analyze the data, results from laboratory experiments, and preliminary results from field data collection will be presented.

Bedload dynamics and associated snowmelt influence in mountainous and semiarid alluvial rivers

The influence of snowmelt in semiarid alluvial rivers is determined by the river bed configuration and erosion processes. Moreover, the large amount of available sediment in the floodplains of these rivers limits traditional bedload sampling methodologies and suggests the use of large control volumes and long-term studies to understand erosive dynamics. In this study, bedload monitoring in a large control volume (0.2hm3) from 2004 to 2010 was used to study erosive processes of the Guadalfeo River, a semiarid alluvial river in southeastern Spain with high mountain influences and a hydrological regime conditioned by snow dynamics. The methodological approach to testing the performance of different transport models included characterization of the forcing agents (precipitation and snowmelt events), hydraulic configuration of the river, determination of the river bed material (surface and substrate), and one-off measurements for particular events. The results indicate a sediment volume–effective runoff relationship that is consistent with the existence of steep channels with a flashy runoff regime, although the existence of an armored layer derived from snowmelt events greatly controls the transport dynamics, with equivalent fractions of cobble–gravel and sand in the measured bedload. This apparent predominance of near-equal mobility was confirmed by the calibrated hiding function exponent (x≈0.94–0.97). The thresholds for entrainment and transport efficiency are similar to those observed for mountain rivers that exhibit torrential flow, although notable differences can be observed in the dynamics of the processes. The strong performance of the original Meyer-Peter and Müller model (with c=8) is shown to estimate the total transported volume associated with intense events when the variability of d50 is included in the analysis. However, this model does not sufficiently capture transport because of moderate events (rain or snowmelt) and thus underestimates bedload on medium and long timescales. The calibrated model of Parker–Klingeman adequately represents erosive dynamics, with τr50⁎ values between 0.03 and 0.041 for moderate and intense events, respectively, and a flow threshold of 14.2m3s−1 that separates these two situations. Nevertheless, these models based on hiding functions present greater dispersion if the variability of diameters related to cobbles and pebbles is considered. For such cases, the Wilcock and Crowe model performed better for both intense and moderate events. The latter is important in the study area, where snowmelt pulses occasionally exceed the critical flow and break the surface layer with significant sediment contributions. These processes differentiate the transport dynamics from other studies and are distinctive characteristics that are more closely related to mountainous influences in semiarid environments where pulses of intense snowmelt are common. Despite its limitations, the proposed methodology, based on a large check-dam, has proven to be adequate and representative for assessment of erosion processes.

Study of sampling data obtained by a hydrophone for bedload monitoring

Study of sampling data obtained by a hydrophone for bedload monitoring

Towards the hydrologic and bed load monitoring from high-frequency seismic noise in a braided river: The “torrent de St Pierre”, French Alps

We explore the use of seismic noise produced by rivers to monitor the bed load transport in the case of a low-discharge braided river in the French Alps: the “torrent de St Pierre”. For this purpose, we deployed two dedicated seismic networks during summers 2007 and 2008, for which the characteristics of the recorded continuous signal are similar despite changes in the sensor locations. For dry weather conditions, only melting of nearby glaciers controls the supply of water to the stream. In these conditions, the river hydrology and the seismic energy in the 2–80 Hz frequency band both follow a diurnal fluctuation similar to the thermal amplitude. In contrast during rainfall episodes, the temperature variation fails to explain the hydrodynamic changes. Dense cloud covers reduce glacier melting and the recorded seismic energy denotes bursts of high-frequency seismic noise well correlated with water level data. Comparisons between the recorded seismic signals and the collected hydrological and sediment load data indicate that a frequency band of 3–9 Hz best explains the water level changes and thus the seismic waves coming from the flow turbulence. These analyses also reveal the presence of a seismic noise threshold that might be linked to the water shear stress exerted by the flowing water. Using the seismic energy in this frequency band as a proxy of the fluvial shear stress, the seismic–hydrologic relationship may be sensitive to variations in bed load transport. The spectral content of the seismic energy shows patterns consistent with the mobilization of sediment particles. From the interpretations of the seismic wave attenuation of river sources, we finally propose that stations at a distance from the stream less than 50 m are able to record most sediment particles. Farther stations are still useful during extreme events when largest grain sizes are mobilized. More generally this study demonstrates the feasibility of using the river seismic signal to survey bed load transport in various river types from small braided mountain rivers like the “torrent de St Pierre” to the large entrenched Himalayan rivers.

Benefits of design modifications to the Birkbeck bedload sampler illustrated by flash‐floods in an ephemeral gravel‐bed channel

AbstractThe benefits of three simple modifications to the design of a Birkbeck bedload slot‐sampling system that has been continuously operating in Nahal Eshtemoa, Israel, since the early 1990s are demonstrated. The modifications include the deployment of a removable slot cover which delays the accumulation of sediment, so allowing sampling at late stages of a flood and, in conjunction with other samplers, extending the period of sampling during a flood wave; inclusion of a slot the size of which is adjustable so that that the probability of sampling the largest clast sizes in transit as bedload can be increased post‐installation, once knowledge is gained about the bedload grain‐size distribution; and a sampler side‐wall door that allows stratification and textural changes within the accumulated bedload to be identified, so promoting intelligent sampling of the deposit for grain‐size determination. Results from seven flash‐floods are presented and discussed, with recommendations for bedload monitoring, particularly in rivers where sediment flux is high and dynamic sediment records are inevitably short because of instrumental limitations. Copyright © 2006 John Wiley & Sons, Ltd.

Continuous monitoring of bedload flux in a mountain gravel-bed river

An automatic bedload monitoring station has been established on the perennial Tordera River, a Mediterranean mountain gravel-bed stream located in NE Spain. Bedload fluxes were obtained automatically at 1-min intervals using two Birkbeck-type pit bedload samplers aligned across the width of the channel. Flow depth and water-surface slope were both measured continuously and synchronously with bedload. Data for five flow events show that bedload flux varies considerably. Bedload flux is high when it is compared with other perennial streams. For a 15-min interval, maximum channel-average bedload flux was 0.83 kg m −1 s −1. Bedload records also show that there is no direct relationship between bedload flux and grain shear stress. The low correlation between bedload flux and hydraulics arises due to the different thresholds of initiation of motion for each flow event and the variable character of the bed between events. For two individual floods, there is a reasonable relationship between bedload flux and grain shear stress, in part, as a consequence of individual particle entrainment and patch movement over a stable coarse gravel-bed during moderately high discharges. These results demonstrate that the Birkbeck system functions well in a Mediterranean mountain gravel-bed stream with a longitudinal slope of 2%.

Seismic detectors for continuous bed load monitoring in a gravel stream

Abstract A field experiment is described for continuous recording of bed load transport rates in a coarse-grained alluvial channel, using instruments typically available to seismologists. The experiment was carried out in 1990 in the Italian Alps, in a small basin equipped since 1982 with instruments conventionally used in hydrological surveys and bed load transport measurements. In July 1990, six seismometers were placed along the alluvial channel for up to about 150 m upstream of the flow gauging station. Continuous microseismic recordings of seven flood flows were analysed, five being associated with coarse sediment delivery. The mechanism of bed load transport was inferred from two or three microseismic impulse peaks occurring before and after the discharge peak shown in the hydrograph. Those microseismic peaks are thought to reflect the pulsed nature of bed load transport while the periods elapsing between them are interpreted as indicating the duration of the transport process and appeared independent of peak discharges.