Site specificity of bed load measurement using an acoustic Doppler current profiler

Abstract

[1] Concurrent measurements of bed load transport velocity (v) from the bottom tracking feature of an acoustic Doppler current profiler (aDcp) and bed load transport rate (g b ) from conventional pressure difference samplers are presented. Data sets were collected from both gravel bed and sand bed reaches of the Fraser River, covering a bed material range of 0.25-25 mm. Strong relations, in which v explained >70% of the variability of measured g b , were observed in a gravel bed and a sand bed reach. Differences in correlation between v and g b among the contrasting environments are attributed to variations in both the bed load particle size and aDcp operating parameters. Similar values of v were associated with lower mass transport rates in a sand bed than in a gravel bed reach. A nondimensional data collapse, accounting for differences in bed load particle size, explained 42% of the observed variance of the combined data set. Longer averaging times were required in the gravel bed reach, likely due to the stochastic nature of bed load entrainment in gravels and the resulting heterogeneous bed load velocity field. Bed load transport was modeled using both shear stress models and a kinematic model that utilizes the estimated bed load velocity. The standard shear stress models provided poor matches to the measured bed load transport rates: the sand bed data were overpredicted, and the gravel bed predictions correlated poorly with the measured predictions. Use of the kinematic model yields an estimate for the product of bed load concentration and bed load layer depth. This work highlights the potential of acoustic techniques for estimating bed load.INDEX TERMS: 1815 Hydrology: Erosion and sedimentation; 1824 Hydrology: Geomorphology (1625); 1869 Hydrology: Stochastic processes; 1894 Hydrology: Instruments and techniques; KEYWORDS: bed load, aDcp, acoustic measurement, particle velocity, sediment, river Citation: Rennie, C. D., and P. V. Villard (2004), Site specificity of bed load measurement using an acoustic Doppler current profiler,

Recent (1999–2003) Canadian research on contemporary processes of river erosion and sedimentation, and river mechanics

“…Bedload transport was calculated by attributing biases in the bottom-tracking feature of the instrument to bedload velocity. Results from the ADP corresponded well with results from conventional techniques in the gravel-bed reaches of the Fraser River (Rennie et al, 2002) as well as in the sand-bed reaches (Rennie and Villard, 2003), indicating that the ADP technique is suitable for large rivers in an assumed state of carpet flow transport. Since Einstein's classical work, tracing has become a standard technique in studying sediment transport in gravel-bed rivers.…”

Measurement of the spatial distribution of fluvial bedload transport velocity in both sand and gravel

“…The bedload transport rate measurements tend to corroborate the observed pattern of bedload velocity, although comparison is awkward because the HS was usually located in transition zones between high and low velocity. A calibration curve (data from Rennie and Villard, 2004) for measured bedload velocity to bedload transport rate from the 70 HS samples collected in the reach suggests that g b = 40v, where v is mean primary apparent bedload velocity (m/s) and g b is bedload transport rate (g/s/m) ( Figure 7). This is the functional relation (Davies and Goldsmith, 1972;Mark and Church, 1977), which required an estimate of the ratio of v and g b measurement errors.…”

“…Two outliers (denoted by o) were not included in the regression: it was presumed that these large bedload samples were due to bottom dragging of the Helley-Smith sampler. Data from Rennie and Villard (2004) We compare the bedload velocities of Figure 6a to the near-bed water velocities in Figure 6b. Encouragingly, the patches of highest bedload velocity occurred in the zones of high near-bed water velocity.…”

“…Five-minute samples were collected using a standard Helley-Smith (HS) sampler, with a 76 mm by 76 mm aperture. A further 64 concurrent ADP and bedload samples were collected in the study reach between June 18 and June 22 (Rennie and Villard, 2004). Many of these samples were collected at two stationary positions.…”

Guidance on Setup, Calibration, and Validation of Hydrodynamic, Wave, and Sediment Models for Shelf Seas and Estuaries

“…Attempts to quantify bed load have also exploited the bottom tracking feature of ADCPs in combination with conventional pressure di erence samplers. Together, these instruments can be used to determine the bed load transport velocity and bed load transport rate, respectively (e.g., [78,79]). Rates of bed load transport have also been inferred from rates of bed form migration measured using rotary sonar devices (e.g., [80,81]).…”