Simplicity and complexity of bed load response during flash floods in a gravel bed ephemeral river: A 10 year field study

Cohen, Hai; Laronne, Jonathan B.; Reid, Ian

doi:10.1029/2010wr009160

Cited by 33 publications

(40 citation statements)

References 82 publications

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“…In the 2008–2009 dataset, maximum channel‐average bedload flux was 0.35 kg m ‐1 s ‐1 (a non‐dimensional bedload flux i b * = 0.02) and this was produced at a channel‐average shear stress of 25.3 N m ‐2 (τ * = 0.100). In the 1991–2001 database at the same level of shear stress (Cohen et al ., , Figure ), bedload flux is 3.6 kg m ‐1 s ‐1 (i b * = 0.17) at the left‐hand edge of the data envelope and 2.2 kg m ‐1 s ‐1 (i b * = 0.1) according to the linear least‐squares relation, 10.3 and 6.3 times the maximum 2008–2009 value, respectively. Moreover, during 2008–2009, the highest channel average shear stress (28 N m ‐2 , τ * = 0.110) generated bedload fluxes no larger than 0.3 kg m ‐1 s ‐1 (i b * = 0.01), whereas during 1991–2001, the same channel average shear stress produced bedload fluxes in the range 1.1 to 4.2 kg m ‐1 s ‐1 (i b * = 0.05 to 0.20), 3.5 to 14 times larger.…”

Section: Resultsmentioning

confidence: 99%

Effect of changes in fine‐grained matrix on bedload sediment transport in a gravel‐bed river

Barzilai

Laronne

Reid

2012

Earth Surf Processes Landf

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While clay and silt matrices of gravel‐bed rivers have received attention from ecologists concerned variously with the deteriorating environments of benthic and hyporheic organisms, their impact on sediment entrainment and transport has been explored less. A recent increase of such a matrix in the bed of Nahal Eshtemoa, an ephemeral river of the northern Negev, has more than doubled the boundary shear stress needed to initiate bedload, from 7 N m‐2 (τ* = 0.027) during the flash floods of 1991–2001 to 15 N m‐2 (τ* = 0.059) during those of 2008–2009. The relation between bedload flux and boundary shear stress continues to be well‐defined, but it is displaced. The matrix now contains a significant amount of silt and clay size material. The reasons for the increased entrainment threshold of bedload are explored. Large‐scale laser scanning of the dry bed reveals a reduction in grain‐scale morphological roughness, while artificial in situ tests of matrix integrity indicate considerable cohesion. The implications for adopting bed material sampling strategies that account for matrix development are assessed. Copyright © 2012 John Wiley & Sons, Ltd.

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Section: Resultsmentioning

confidence: 99%

Effect of changes in fine‐grained matrix on bedload sediment transport in a gravel‐bed river

Barzilai

Laronne

Reid

2012

Earth Surf Processes Landf

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“…This factor should represent the flow duration curve from the previous high‐magnitude event able to restructure the bed (e.g., mobilizing even the D 84 as fully mobile conditions in the sense of Wilcock and McArdell [1997]). Also, because timings and temporal sequences of flood events can have a strong influence on bed load transport [ Cohen et al , 2010; Recking et al , 2012] this correction factor for bed load transport formulas should also include a term characterizing the temporal sequence of disturbance of different magnitude. The present study reveals that differences between rising and falling limb of hydrographs, mainly due to changes in the degree of grain organization in the mobile armor layer, could be quantified by analyzing simple statistical moments and second‐order structure functions.…”

Section: Discussionmentioning

confidence: 99%

The effect of hydrographs on bed load transport and bed sediment spatial arrangement

Mao¹

2012

J. Geophys. Res.

146

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[1] Field measurements of sediment transport in gravel bed rivers often reveal hysteretic effects due to differences in sediment availability between the rising and falling limbs of a flood hydrograph. However, only a small number of flume studies have analyzed the dynamics of sediment transport during hydrographs. Three types of stepped hydrographs with contrasting durations and magnitudes are simulated here under sediment recirculation conditions. Bed load transport rate and grain size have been measured continuously. The dynamic behavior of the surface armor layer has been explored by analyzing digital photographs and laser scanner surveys of the bed surface taken during hydrographs. The results indicate that sediment transport during the falling limb was lower than during the rising limb in all of the three types of hydrographs. This reduction is more evident for the low-magnitude hydrographs. The grain size of the bed remained virtually constant throughout the hydrographs but the grain size of transported sediments exhibited a counterclockwise hysteresis. Also, a significant increase in the reference shear stress for sediment entrainment was measured during the falling limb of hydrographs. Additionally, a detailed analysis of partial transport dynamics of the bed surface sediments reveals a reduction in sediment mobility during the falling limb of the hydrographs. The difference in sediment entrainment and transport before and after the peak of the hydrographs appears to be caused by a change in the organization of the surface sediments. An analysis of detailed laser scan bed surveys reveals a phase of bed restructuring (lower vertical roughness, clast rearrangement) during the falling limb of hydrographs. Consequently, changes in the degree of organization and complexity of the bed surface are likely the cause of the reduced mobility of sediments, and thus of the reduced sediment transport rate, during the falling limb of the hydrographs.

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“…Although both of these bedform types contribute material to bedload, the majority is derived from the flats and comprises pebbles, granules and sand. Coarser grains, including cobbles and boulders, are mobilized at higher boundary shear stresses and the size-range of the bed material is proportionally represented in the bedload at transport stages ≥ 4.5 τ/τc (Powell et al, 2001;Cohen et al, 2010). This wide range of mobile particle sizes is of particular significance when considering the performance of geophones in this stream environment.…”

Section: Nahal Eshtemoa Field Site and Calibration Measurementsmentioning

confidence: 99%

Bedload transport measurements with impact plate geophones: comparison of sensor calibration in different gravel‐bed streams

Rickenmann

Turowski

Fritschi

et al. 2013

Earth Surf Processes Landf

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119

201

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Indirect bedload transport measurements have been made with the Swiss plate geophone system in five gravel‐bed mountain streams. These geophone sensors record the motion of bedload particles transported over a steel plate mounted flush with the channel bed. To calibrate the geophone system, direct bedload transport measurements were undertaken simultaneously. At the Erlenbach in Switzerland, a moving‐basket sampler was used. At the Fischbach and Ruetz streams in Austria, a Helley–Smith type bedload sampler provided the calibration measurements. A Bunte‐type bedload trap was used at the Rofenache stream in Austria. At the Nahal Eshtemoa in Israel, Reid‐type slot bedload samplers were used. To characterize the response of the geophone signal to bedload particles impacting on the plate, geophone summary values were calculated from the raw signal and stored at one second intervals. The number of impulses, i.e. the number of peaks above a pre‐defined threshold value of the geophone output signal, correlated well with field measured gravel transport loads and was found to be a robust parameter. The relations of impulses to gravel transport loads were generally near‐linear, but the steepness of the calibration relations differed from site to site. By comparing the calibration measurements from the different field sites and utilizing insights gained during preliminary flume experiments, it has been possible to identify the main factors that are responsible for site specific differences in the calibration coefficient. The analysis of these calibration measurements indicates that the geophone signal also contains some information about the grain size distribution of bedload. Copyright © 2013 John Wiley & Sons, Ltd.

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Simplicity and complexity of bed load response during flash floods in a gravel bed ephemeral river: A 10 year field study

Cited by 33 publications

References 82 publications

Effect of changes in fine‐grained matrix on bedload sediment transport in a gravel‐bed river

Effect of changes in fine‐grained matrix on bedload sediment transport in a gravel‐bed river

The effect of hydrographs on bed load transport and bed sediment spatial arrangement

Bedload transport measurements with impact plate geophones: comparison of sensor calibration in different gravel‐bed streams

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