B. Vermeulen

Continuous measurements of discharge from a horizontal acoustic Doppler current profiler in a tidal river

[1] Acoustic Doppler current profilers (ADCPs) can be mounted horizontally at a river bank, yielding single-depth horizontal array observations of velocity across the river. This paper presents a semideterministic, semistochastic method to obtain continuous measurements of discharge from horizontal ADCP (HADCP) data in a tidal river. In the deterministic part, single-depth velocity data are converted to specific discharge by applying the law of the wall, which requires knowledge of local values of the bed roughness length (z0). A new filtration technique was developed to infer cross-river profiles of z0 from moving boat ADCP measurements. Width-averaged values of z0 were shown to be predominantly constant in time but differed between ebb and flood. In the stochastic part of the method, specific discharge was converted to total discharge on the basis of a model that accounts for the time lag between flow variation in the central part of the river and flow variation near the banks. Model coefficients were derived using moving boat ADCP data. The consistency of mutually independent discharge estimates from HADCP measurements was investigated to validate the method, analyzing river discharge and tidal discharge separately. Inaccuracy of the method is attributed primarily to mechanisms controlling transverse exchange of momentum, which produce temporal variation in the discharge distribution over the cross section. Specifically, development of river dunes may influence the portion of the discharge concentrated within the range of the HADCP.

Discharge estimation from H-ADCP measurements in a tidal river subject to sidewall effects and a mobile bed

Horizontal acoustic Doppler current profilers (H-ADCPs) can be employed to estimate river discharge based on water level measurements and flow velocity array data across a river transect. A new method is presented that accounts for the dip in velocity near the water surface, which is caused by sidewall effects that decrease with the width to depth ratio of a channel. A boundary layer model is introduced to convert single-depth velocity data from the H-ADCP to specific discharge. The parameters of the model include the local roughness length and a dip correction factor, which accounts for the sidewall effects. A regression model is employed to translate specific discharge to total discharge. The method was tested in the River Mahakam, representing a large river of complex bathymetry, where part of the flow is intrinsically three-dimensional and discharge rates exceed 8000 m3 s-1. Results from five moving boat ADCP campaigns covering separate semidiurnal tidal cycles are presented, three of which are used for calibration purposes, whereas the remaining two served for validation of the method. The dip correction factor showed a significant correlation with distance to the wall and bears a strong relation to secondary currents. The sidewall effects appeared to remain relatively constant throughout the tidal cycles under study. Bed roughness length is estimated at periods of maximum velocity, showing more variation at subtidal than at intratidal time scales. Intratidal variations were particularly obvious during bidirectional flow conditions, which occurred only during conditions of low river discharge. The new method was shown to outperform the widely used index velocity method by systematically reducing the relative error in the discharge estimates

Coupled ADCPs can yield complete Reynolds stress tensor profiles in geophysical surface flows

[1 ] We introduce a new technique to measure profiles of each term in the Reynolds stress tensor using coupled acoustic Doppler current profilers (ADCPs). The technique is based on the variance method which is extended to the case with eight acoustic beams. Methods to analyze turbulence from a single ADCP rely on questionable assumptions on turbulence anisotropy ratios and on the requirement of perfect vertical alignment, which are no longer necessary with the coupled system. We apply the technique successfully to data collected in the Mahakam River, East Kalimantan, Indonesia. Citation: Vermeulen, B., A. J. F. Hoitink, and M. G. Sassi (2011), Coupled ADCPs can yield complete Reynolds stress tensor profiles in geophysical surface flows, Geophys. Res. Lett., 38, L06406, doi:10.1029/2011GL046684.

Improved flow velocity estimates from moving-boat ADCP measurements

Acoustic Doppler current profilers (ADCPs) are the current standard for flow measurements in large-scale open water systems. Existing techniques to process vessel-mounted ADCP data assume homogeneous or linearly changing flow between the acoustic beams. This assumption is likely to fail but is nevertheless widely applied. We introduce a new methodology that abandons the standard assumption of uniform flow in the area between the beams and evaluate the drawbacks of the standard approach. The proposed method strongly reduces the extent over which homogeneity is assumed. The method is applied to two field sites: a mildly curved bend near a junction featuring a typical bend flow and a sharply curved bend that features a more complex sheared flow. In both cases, differences are found between the proposed method and the conventional method. The proposed technique yields different results for secondary flow patterns compared with the conventional method. The velocity components estimated with the conventional method can differ over 0.2 m/s in regions of strong shear. We investigate the number of repeat transects necessary to isolate the mean flow velocity vector from the raw ADCP signal, discarding the influences of noise, positioning and projection errors, and turbulence. Results show that several repeat transects are necessary. The minimum number of repeat measurements needed for robust mean velocity estimates is reduced when applying the proposed method.

Tidal controls on river delta morphology

River deltas are shaped by interactions between fluvial and tidal processes. Tides act to stabilize delta morphology, but sediment depletion due to human activities disrupts the balance and leads to erosion and scour.

Sharp bends associated with deep scours in a tropical river: The river Mahakam (East Kalimantan, Indonesia)

Autogenic scouring in sharp river bends has received ample attention in laboratory and modeling studies. These studies have significantly advanced our understanding of how flow processes are influenced by strong curvature and how they affect the bathymetry. Here we present a 300 km reach of the Mahakam River in Indonesia, which features several sharp bends (W/R > 0.5), providing a unique field data set to validate existing knowledge on sharp bends. Scour depths were found to strongly exceed what can be expected based on existing understanding of sharp bends and are highly correlated with curvature. A comprehensive stream reconnaissance was carried out to compare the occurrence of sharp bends and deep scours with lateral bank migration. Histograms of the occurrence of erosive, stable, advancing, and bar-type banks as a function of curvature quantify the switch from a mildly curved bend regime to a sharp bend regime. In mild bends, outer banks erode and inner banks advance. In sharp bends the erosion pattern inverts. Outer banks stabilize or advance, while inner banks erode. In sharply curved river bends, bars occur near the outer banks that become less erosive for higher curvatures. Inner banks become more erosive for higher curvatures but nevertheless accommodate the larger portion of exposed bars. No relation was found between the land cover adjacent to the river and the occurrence of sharp bends. Soil processes may play a crucial role in the formation of sharp bends, which is inferred from iron and manganese concretions observed in the riverbanks, indicating ferric horizons and early stages of the formation of plinthic horizons. Historical topographic maps show the planform activity of the river is low, which may relate to the scour holes slowing down planimetric development.

Flow structure caused by a local cross‐sectional area increase and curvature in a sharp river bend

Horizontal flow recirculation is often observed in sharp river bends, causing a complex three-dimensional flow structure with large implications for the morphological and planimetric development of meanders. Several field observations in small-scale systems show that sharp bends are often found in association with a strong increase in cross-sectional area, the deposition of outer bank benches, and reattachment bars near the inner bank. Recent studies show that these bends can also occur in large-scale systems. In this study, we present field measurements of a sharp bend in the Mahakam River, East Kalimantan, Indonesia. The cross-sectional area increases by a factor of 3 compared with the reach-averaged cross-sectional area. Along a river reach of about 150 km, cross-sectional area correlates strongly with curvature. The field measurements are analyzed together with the results from numerical simulation with a three-dimensional finite element model, which yields a comprehensive view of the intricate flow structure. In turn, the model is used to validate a new equation that captures the water surface topography dependence on cross-sectional area variation and curvature. The results show the importance of the increase in cross-sectional area in the development of horizontal recirculation. Vertical acceleration of the flow into the scour causes the pressure to deviate from a hydrostatic pressure distribution. Strong downflow (up to 12 cm s -1 ) advects longitudinal momentum toward the bed, causing the flow to concentrate in the lower part of the cross section. This increases the velocity magnitude throughout the cross section, which is expected to maintain the large scour depth found in several bends along the Mahakam River.

Quantified turbulent diffusion of suspended sediment using acoustic Doppler current profilers

[1] Collocated profiles of the Reynolds stress tensor and eddy covariance fluxes are obtained to derive vertical profiles of turbulent momentum and sediment diffusivity in a tidal river, using coupled acoustic Doppler current profilers (ADCPs). Shear and normal stresses are obtained by combining the variances in radial velocities measured by the ADCP beams. The covariances between radial velocities and calibrated acoustic backscatter allow the determination of the three Cartesian components of the turbulent flux of suspended sediment. The main advantage of this new approach is that flow velocity and sediment concentration measurements are exactly collocated, and allowing for profiling over longer ranges, in comparison to existing techniques. Results show that vertical profiles of the inverse turbulent PrandtlSchmidt number are coherent with corresponding profiles of the sediment diffusivity, rather than with profiles of the eddy viscosity. Citation: Sassi, M. G., A. J. F. Hoitink, and B. Vermeulen (2013), Quantified turbulent diffusion of suspended sediment using acoustic Doppler current profilers, Geophys. Res. Lett., 40, 5692–5697, doi:10.1002/2013GL058299.

Distributary channels in the fluvial to tidal transition zone

Coastal lowland plains under mixed fluvial-tidal influence may form complex, composite channel networks, where distributaries blend the characteristics of mouth bar channels, avulsion channels, and tidal creeks. The Kapuas coastal plain exemplifies such a coastal plain, where several narrow distributaries branch off the Kapuas River at highly asymmetric bifurcations. A comprehensive geomorphological analysis shows that trends in the channel geometry of all Kapuas distributaries are similar. They consist of a short, converging reach near the sea and a nonconverging reach upstream. The two parts are separated by a clear break in scaling of geometrical properties. Such a break in scaling was previously established in the Mahakam Delta, which suggests that this may be a general characteristic in the fluvial to tidal transition zone. In contrast to the geometrical trend similarities, a clear difference in bed material between the main and side distributaries is found. In the main distributary, a continuous trend of downstream fining is established, similar to what is often found in lowland rivers. In the side distributaries, bed material coarsens in the downstream direction. This indicates an undersupply of sediment to the side distributaries, which may contribute to their long-term stability as established from historical maps. Tides may be the main agent preventing fine sediment to settle, promoting residual transport of fine material to the coastal ocean.

Multiscale structure of meanders

River meander planforms can be described based on wavelet analysis, but an objective method to identify the main characteristics of a meander planform over all spatial scales is yet to be found. Here we show how a set of simple metrics representing meander shape can be retrieved from a continuous wavelet transform of a planform geometry. We construct a synoptic multiple looping tree to establish the meander structure, revealing the embedding of dominant meander scales in larger-scale loops. The method can be applied beyond the case of rivers to unravel the meandering structure of lava flows, turbidity currents, tidal channels, rivulets, supraglacial streams, and extraterrestrial flows.