Sediment Production in French Alpine Rivers Pitlick, John; Recking, Alain; Liebault, Fred ...
Water resources research,
December 2021, 2021-12-00, 20211201, Letnik:
57, Številka:
12
Journal Article
Recenzirano
Global compilations of river sediment loads show that mountainous areas produce a high proportion of the sediment transported to the oceans. However, because of the effort involved in measuring ...sediment fluxes in mountain river systems, the loads of these rivers are generally unknown. Here, we present estimates of contemporary sediment loads of 16 gravel bed rivers draining the Ecrins‐Pelvoux Massif in southeast France. Sediment production in this part of the Alps is relatively high and many river segments are either wandering or braided. We model sediment fluxes and annual sediment yields by coupling hydraulic‐based relations for sediment transport with hydrologic‐based relations for flow frequency. Bed load fluxes are modeled for a range of discharges using a function that relates transport rates to excess shear stress. Fluxes are then weighted by the frequency of individual discharges and summed to get the annual bed load for each site. The suspended load is estimated empirically as a fraction of the bed load. Results suggest that bed load fluxes at channel‐forming flows scale almost linearly with downstream increases in discharge. In addition, it appears that annual sediment loads (bed load + suspended load) scale linearly with drainage area. A complementary relation for specific sediment yield suggests that the load per unit drainage area is constant across the range of basins studied. The modeled sediment yields are comparable to previous field‐based estimates of modern sediment yields, and generally lower than estimates developed from analyses of cosmogenic radionuclides.
Key Points
Relations for flow frequency and sediment transport are used to model sediment fluxes of 16 rivers in southeastern France
Annual sediment fluxes (t/year) scale linearly with drainage area, specific sediment yields (t/km2/year) exhibit no trend with drainage area
Contemporary denudation rates are roughly half the long‐term denudation rates determined from analyses of cosmogenic nuclides
Estimates of fluvial sediment discharge from in situ instruments are an important component of large‐scale sediment budgets that track long‐term geomorphic change. Suspended sediment load can be ...reliably estimated using acoustic or physical sampling techniques; however, bedload is difficult to measure directly and can consequently be one of the largest sources of uncertainty in estimates of total load. We propose a physically informed predictive empirical model for bedload sand flux as a function of variables that are measured using existing acoustic or physical sampling techniques. This model depends on the assumption that concentration and grain size in suspension are in equilibrium with reach‐averaged boundary conditions. Bayesian inference is used to fit model parameters to data from eight sand‐bed rivers and to simulate bedload flux over the available gage record at one site on the Colorado River in Grand Canyon National Park. We find that the cumulative bedload flux during the 9 year period from 2008 to 2016 was 5% of the cumulative suspended sand load; however, instantaneous bedload flux ranged from as little as 1% of instantaneous suspended sand load to as much as 75% of instantaneous suspended sand load due to fluctuations in flow strength and sediment supply. Changes in bedload flux at a constant discharge are indicative of short‐term sediment supply enrichment and depletion. Long‐term average bedload flux cannot be expected to remain constant in the future as the river adjusts to changes in sediment runoff and the dam‐regulated discharge regime.
Key Points
Bedload flux is predicted from variables that are measured at acoustic suspended sediment monitoring stations
Bayesian modeling extends the utility of this approach to a wide range of conditions and rivers
Predicted bedload flux provides an indicator of short‐term sediment supply enrichment and depletion
Previous measurements of bed load transport in gravel bed streams revealed a large temporal and spatial variability of bed load transport rates. Using an impact plate geophone system, continuous bed ...load transport measurements were made during 6 years in two mountain streams in Austria. The two streams have a snow‐melt and glacier‐melt dominated hydrologic regime resulting in frequent transport activity during the summer half year. Periods of days to weeks were identified which are associated with approximately constant Shields values that indicate quasi‐stable bed conditions. Between these stable periods, the position of the bed load transport function varied while its steepness remained approximately constant. For integration time scales of several hours to 1 day, the fluctuations in bed load transport decreased and the correlation between bed load transport and water discharge increased. For integration times of about 70–100 days, bed load transport is determined by discharge or shear stress to within a factor of about 2, relative to the 6 year mean level. Bed load texture increased with increasing mean flow strength and mean transport intensity. Weak and predominantly clockwise daily hysteresis of bed load transport was found for the first half of the summer period.
Plain Language Summary
The study presents observations from continuous bedload transport measurements made in two mountain streams during six summers. These measurements were made with acoustic sensors fixed underneath steel plates that were installed in a sill across the stream width. The paper primarily discusses the variability of bedload transport for a given discharge, and the daily and seasonal variabilities observed for similar discharge variations in the two streams. These results are among the first reported for such a long period of continuous observations on bedload transport, since they rely on a surrogate measuring technique that has only recently been installed in a number of gravel‐bed streams worldwide.
Key Points
Periods of days to weeks with approximately constant Shields values reflect quasi‐stable bed conditions
For integration times of about 70–100 days bed load transport is determined by water discharge to within a factor of about 2 relative to the 6 year mean level
Only weak daily and seasonal hysteresis effects can be detected
We use four stream segments along a wood‐rich, pool–riffle mountain stream in the Southern Rockies of Colorado, USA to examine how spatial variations in wood load and variations in discharge during ...and after the snowmelt peak flow influence the magnitude of surface and subsurface transient storage. Segments range in complexity from a single channel with no large wood to an anabranching channel with closely spaced, channel‐spanning logjams. Discharges at which transient storage was assessed range from base flow to snowmelt peak flow. To explore these relations, we used 10 geomorphic variables representing channel morphology and bed substrate, four wood‐related variables representing wood load and associated backwater storage, and two measures of skewness from instream and bulk electrical conductivity breakthrough curves during tracer tests. Instream curves reflect surface and subsurface transient storage, whereas bulk curves primarily represent subsurface transient storage. Higher values of skewness indicate greater retention, and we used the values here as a metric of increased transient storage. Although limited sample size restricts the power of our results, our findings suggest that stream segments with greater instream large wood loads have more and larger pools, greater storage of fine sediment and particulate organic matter, and higher values of skew from instream conductivity. The results also suggest that the presence of instream wood, rather than changes in channel morphology associated with wood, is the most important driver of transient storage. This implies that river management designed to foster transient storage should focus on retaining instream large wood. We did not find significant correlations between geomorphic or wood‐related variables and the skew estimated from bulk conductivity, which may reflect the relatively thin alluvium present in the field area and the prevalence of surface transient storage in this system.
Logjam with electrical resistivity sensors deployed across the channel upstream (at left).
This study investigates the influence of bed‐load transport on flow resistance and bed stability in steep step‐pool channels. A total of 86 flume experiments was performed. Stable step‐pool sequences ...were formed with increasing discharge under clear‐water conditions. The addition of fine bed‐load material to the flow led to a decrease in bed roughness and an increased mobility of step‐forming clasts. Velocity measurements showed that bed‐load transport significantly decreased flow resistance and increased near‐bed velocity for the conditions investigated. A set of existing resistance equations was tested against our data, of which the hydraulic geometry relation performed best. However, none of the existing equations was able to reproduce flow resistance during active bed‐load transport. Two new resistance equations based on a hydraulic geometry approach are proposed for conditions with and without bed‐load transport. The increase in near‐bed velocity associated with bed‐load transport was identified as the main cause for the observed increase in step mobility associated with bed‐load transport. For flows with bed‐load transport, step‐forming clasts were mobilized at discharges some 10% to 30% lower than under clear‐water conditions.
Key Points
Bed‐load transport of fine material decreases flow resistance in step‐pool channels
Bed‐load transport increases near‐bed velocity and enhances the mobility of step‐forming clasts
Hydraulic geometry relations are adequate to predict bulk flow velocity in step‐pool channels with and without active bed‐load transport
Input of organic matter into stream channels is the primary energy source for headwater ecosystems and ultimately carbon to the oceans and hence is an important component of the global carbon cycle. ...Here, we quantify organic‐rich fine sediment mobilization, transport, and storage in a Strahler fourth‐order stream during individual intermediate‐sized storm events. By combining measurements of fallout radionuclides (FRNs) 7Be and 210Pb and stable water isotopes with a conceptual model of suspended load trapping by channel margins, we find that the channel bed was consistently a source of suspended load to the channel margins. Relative to storage on the channel margins, suspended load export increased through the spring and summer, perhaps related to the in‐channel decomposition of organic debris as indicated by its FRN exposure age and changing bulk δ13C composition. Trapping of suspended load by riparian margins limits sediment transport distances, which, given sufficient discharge to fully suspend the load, is nearly independent of stream discharge for sub‐bankfull discharges. Limited data indicate that the fractional size of the channel margins where trapping occurs decreases with increasing watershed area. Increasing transport length and decreasing fractional margin area with increasing watershed area results in a systematic downstream decoupling of the channel from local terrestrial organic matter exchange. These findings provide a framework for understanding suspended load dynamics in formerly glaciated regions where sediment production and fluxes are generally low and thus the annual input of organic debris is a major component of suspended load budget.
Plain Language Summary
The decomposition of organic‐rich debris (leaves, twigs, etc.) within stream channels serves as an important organic carbon source to stream margins or banks. During moderate storm events, we observed that the channel bed was consistently a source of organic‐rich suspended load that is then trapped by the channel margins. Through spring and summer less of the suspended load is trapped by the margins, increasing the fraction of the suspended load exported. This decreased trapping and increased export may be related to changes in the character of the suspended load due to the in‐channel decomposition of organic debris. Trapping of suspended load by channel margins limits the transport distance of suspended load, systematically decoupling the channel from the channel margins with increasing watershed size. These findings provide a framework for understanding suspended load transport in formerly glaciated regions where the annual input of organic debris is a significant component of suspended load budget.
Key Points
We observe seasonal variations in the ratio of suspended load exported from the watershed versus stored in channel margins
Greater fractional area of deposition along margins of headwaters facilitates trapping of suspended load that limits suspended load export
Increasing transport length with increasing watershed area systematically decouples the channel from terrestrial organic exchange
Measurement and estimation of bed load transport in gravel bed rivers are highly affected by its temporal fluctuations. Such variability is primarily driven by the flow regime but is also associated ...with a variety of inherent channel processes, such as flow turbulence, grain entrainment, and bed forms migration. These internal and external controls often act at comparable time scales, and are therefore difficult to disentangle, thus hindering the study of bed load variability under unsteady flow regime. In this paper, we report on laboratory experiments performed in a large, mobile bed flume where typical hydromorphological conditions of gravel bed rivers were reproduced. Data from a large number of replicated runs, including triangular and square‐wave hydrographs, were used to build a statistically sound description of sediment transport processes. We found that the inherent variability of bed load flux strongly depends on the sampling interval, and it is significantly higher in complex, wandering or braided channels. This variability can be filtered out by computing the mean response over the experimental replicates, which allows us to highlight two distinctive phenomena: (i) an overshooting (undershooting) response of the mean bed load flux to a sudden increase (decrease) of discharge, and (ii) a clockwise hysteresis in the sediment rating curve. We then provide an interpretation of these findings through a conceptual mathematical model, showing how both phenomena are associated with a lagging morphological adaptation to unsteady flow. Overall, this work provides basic information for evaluating, monitoring, and managing gravel transport in morphologically active rivers.
Key Points
Bed load transport exhibits an inherent variability that depends on discharge and bed morphology
Adaptation of bed topography to unsteady flow produces hysteresis in the bed load rating curve
The clockwise hysteresis depends on flood duration relative to the time scale of bed evolution
A field investigation has been undertaken to characterize the event‐based bed load transport dynamics of a highly urbanized gravel bed stream. A combination of direct bed load and tracer particle ...measurements were taken over a 3 year period during which time approximately 30 sediment mobilizing events occurred. Sediment transport measurements were used to calibrate a fractional bed load transport model and combined with hydrometric data which represent four different land use conditions (ranging from rural to highly urbanized) to analyze the differences in discharge magnitude and frequency and its impact on sediment transport. Fractional transport analysis of the bed load measurements indicates that frequent intermediate discharge events can mobilize sand and fine gravel to an approximate equally mobile condition, however, the transport rates at these discharges exhibit greater variability than at discharges above the bankfull discharge. Path lengths of the coarse fraction, measured using tracer clasts, are insensitive to peak discharge, and instead transport at distances less than those reported in other gravel bed channels, which is attributed to the shorter duration discharge events common to urban streams. The magnitude‐frequency analysis reveals that the frequency, time, and volume of competent sediment mobilizing events are increasing with urbanization. Variability in effective discharges suggests that a range of discharges, spanning between frequent, low magnitude events to less frequent, high magnitude events are geomorphically significant. However, trends in the different land use scenarios suggest that urbanization is shifting the geomorphic significance toward more frequent, lower magnitude events.
Plain Language Summary
Urbanization is known to have impacts on river health, typically resulting in a degraded ecosystem and erosion problems. This article investigates the impact of urbanization on sediment transport, which is known to be a primary factor controlling how rivers evolve over time. Both field measurements and analytical modeling were employed encompassing different approaches to achieve the study objectives. The increases in floods common to urbanization change the way sediment moves in a gravel bed river in Toronto, Ontario, Canada. It is also shown that more frequent, lower magnitude floods (which are known to increase with urbanization) are becoming more important for controlling how gravel bed rivers are evolving over time.
Key Points
Sediment mobility of a highly urbanized stream was studied over a period of 3 years
The impact of urbanization was assessed using a field calibrated geomorphic work‐based analysis
Urbanization is increasing the geomorphic significance of more frequent, low magnitude events
AbstractDespite limitations in reproducing complex bedload sediment transport processes in rivers, formulas have been preferred over collection and analysis of field data due to the high cost and ...time-consuming nature of bedload discharge measurements. However, the performance of such formulas depends on the hydraulic and sedimentological conditions they attempt to describe. The availability of field measurements provides a unique opportunity to test bedload transport formulas to better guide formula selection. Hydraulic parameters and bedload discharge data from the Lower Minnesota River and two of its tributaries were used to evaluate nine bedload transport formulas using three different indices. The bedload data for the different sites were collected by the United States Geological Survey (USGS) from 2011 through 2014, with bed material varying from very coarse to medium sand. The formulas calculated higher bedload rates than were measured due to a combination of site-specific physical characteristics, including the presence of bed forms (dunes), and sampling uncertainties. Because of the lack of reproducibility of the tested formulas, five power functions, based on the relation between the specific unit power (independent hydraulic variable) and the USGS measured data (dependent variable), were derived as provisional equations to estimate the bedload discharge on the Lower Minnesota River and tributaries.
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