Shallow Gravel-bed Rivers Research Articles

The Grain-size Patchiness of Braided Gravel-Bed Streams – example of the Urumqi River (northeast Tian Shan, China)

Abstract. In gravel-bed rivers, sediments are often sorted into patches of different grain-sizes, but in braided streams, the link between this sorting and the channel morpho-sedimentary elements is still unclear. In this study, the size of the bed sediment in the shallow braided gravel-bed Urumqi River is characterized by surface-count and volumetric sampling methods. Three morpho-sedimentary elements are identified in the active threads of the river: chutes at flow constrictions, which pass downstream to anabranches and bars at flow expansions. The surface and surface-layer grain-size distributions of these three elements show that they correspond to only two kinds of grain-size patches: (1) coarse-grained chutes, coarser than the bulk river bed, and (2) finer-grained anabranches and bars, consistent with the bulk river bed. In cross-section, the chute patches are composed of one coarse-grained top layer, which can be interpreted as a local armour layer overlying finer deposits. In contrast, the grain size of the bar-anabranch patches is finer and much more homogeneous in depth than the chute patches. Those patches, which are features of lateral and vertical sorting associated to the transport dynamics that build braided patterns, may be typical of active threads in shallow gravel-bed rivers and should be considered in future works on sorting processes and their geomorphologic and stratigraphic results.

Continuous monitoring of a dam flush in a shallow river using two crossing ultrasonic transmission lines

Continuous measurements of streamflow and water temperature were carried out in a shallow gravel-bed river during dam flushing operations using a new shallow acoustic system with two crossing transmission lines. The fluvial acoustic system (FAS) was equipped with four 25 kHz broadband omnidirectional transducers. The reciprocal sound transmissions were performed between the two pairs of acoustic stations, located on both sides of the river. The four-station FAS enabled the measurement of the stream direction in addition to the depth- and range-averaged sound speed, and therefore water velocity. Discharge values computed from FAS reports were compared to those estimated by the moving-boat acoustic Doppler current profiler (ADCP) and rating curve (RC) methods. FAS estimates were in good agreement with ADCP and RC estimates over a range of 50 to 180 m3 s−1. The temperature gradients induced strong sound scattering for the 25 kHz FAS. The fine suspended sediment particles also induced a gradual decrease in the signal-to-noise ratio of the FAS. Nevertheless, the FAS has been proved to be a reliable and accurate technique for the continuous measurement of cross-sectional average velocity/water temperature.

Mapping gravel bed river bathymetry from space

Understanding river form and behavior requires an efficient means of measuring channel morphology. This study evaluated the potential to map the bathymetry of two clear‐flowing, shallow (<3 m deep) gravel bed rivers <60 m wide from 2 m‐pixel WorldView2 satellite images. Direct measurements of water column optical properties were used to quantify constraints on depth retrieval. The smallest detectable change in depth was 0.01–0.04 m and the maximum detectable depth was 5 m in green bands but <2 m in the near‐infrared; lower sensor radiometric resolution yields less precise estimates over a smaller range. An algorithm for calibrating a band ratioXto field measurements of depthdproved effective when applied to spectra extracted from images (R2= 0.822 and 0.594 for the larger and smaller stream, respectively) or measured in the field (R2= 0.769 and 0.452). This procedure also identified optimal wavelength combinations, but different bands were selected for each site. Accuracy assessment of bathymetric maps produced using various calibration approaches and image types indicated that: 1) a lineardvs.Xrelation provided depth estimates nearly as accurate as a quadratic formulation; 2) panchromatic and pan‐sharpened multispectral images with smaller 0.5 m pixels did not yield more reliable depth estimates than the original images; and 3) depth retrieval was less reliable in pools due to saturation of the radiance signal. This investigation thus demonstrated the feasibility, as well as the limitations, of measuring the bathymetry of clear, shallow gravel bed rivers from space.

Continuous measurements of flow rate in a shallow gravel‐bed river by a new acoustic system

The continuous measurement of river discharge for long periods of time is crucial in water resource studies. However, the accurate estimation of river discharge is a difficult and labor‐intensive procedure; thus, a robust and efficient method of measurement is required. Continuous measurements of flowrate have been carried out in a wide, shallow gravel bed river (water depth ≈ 0.6 m under low‐flow conditions, width ≈ 115 m) using Fluvial Acoustic Tomography System (FATS) that has 25 kHz broadband transducers with horizontally omnidirectional and vertically hemispherical beam patterns. Reciprocal sound transmissions were performed between the two acoustic stations located diagonally on both sides of the river. The horizontal distance between the transducers was 301.96 m. FATS enabled the measurement of the depth‐ and range‐averaged sound speed and flow velocity along the ray path. In contrast to traditional point/transect measurements of discharge, in a fraction of a second, FATS covers the entire cross section of river in a single measurement. The flow rates measured by FATS were compared to those estimated by moving boat Acoustic Doppler Current Profiler (ADCP) and rating curve (RC) methods. FATS estimates were in good agreement with ADCP estimates over a range of 20 to 65 m3 s−1. The RMS of residual between the two measurements was 2.41 m3 s−1. On the other hand the flowrate by RC method fairly agreed with FATS estimates for greater discharges than around 40 m3 s−1. This inconsistency arises from biased RC estimates in low flows. Thus, the flow rates derived from FATS could be considered reliable.

Local‐scale variability of seepage and hydraulic conductivity in a shallow gravel‐bed river

AbstractSeepage rate and direction measured with a seepage metre modified for use in flowing water were greatly variable along a 300‐m reach of a shallow, gravel‐bed river and depended primarily on the local‐scale bed topography. The median value of seepage measured at 24 locations was 24 cm/day, but seepage measured at specific sites ranged from −340 to +237 cm/day. Seepage also varied substantially over periods of hours to days and occasionally reversed direction in response to evolution of the sediment bed. Vertical hydraulic conductivity was related to seepage direction and was larger during upward seepage than during downward seepage; with differences ranging from 4 to 40% in areas of active sediment transport to more than an order of magnitude in areas where current was too slow to mobilize bed sediment. Seepage was poorly related to hydraulic gradient measured over vertical distances of 0·3 m and appeared to be opposite the hydraulic gradient at 18% of the locations where both parameters were measured. Results demonstrate the scale dependence of these measurements in coarse‐grained hyporheic settings and indicate that hydraulic gradients should be determined over a much shorter vertical increment if used to indicate exchange across the sediment–water interface. Published in 2009 by John Wiley & Sons, Ltd.

The development of an automated correction �procedure for digital photogrammetry for the study of wide, shallow, gravel-bed rivers

This paper develops an automated correction procedure for dealing with point errors associated with through-water photogrammetry, for application in the study of clear-water, shallow gravel-bed rivers. The procedure involves combining digital photogrammetry and image analysis techniques to: (i) correct for the effects of refraction at an air–water interface; and (ii) eliminate and reinterpolate points where the bed has not been ‘seen’. The correction procedure was applied to raw digital elevation models (DEMs) generated using digital photogrammetry from 1:3000 scale aerial photography of a small reach of the North Ashburton River, New Zealand. The accuracy of corrected and uncorrected DEMs is evaluated using an independent data set. A measure of ‘geomorphological usefulness’ as well as DEM external reliability is obtained from calculations of water depth distributions and mean bed level. Results show that digital photogrammetry, used in conjunction with image analysis techniques, can successfully be used for extracting high-resolution DEMs of gravel river beds. In exposed areas, errors are small and random, tending to cancel out over large numbers of points. Where water is shallow, and following correction, point elevation errors are statistically no different from those for exposed zones. In deeper water, despite an improvement following application of the correction procedure, elevation errors scale with water depth. The geomorphological potential of photogrammetric survey of large, gravel river beds is demonstrated by the ease and accuracy of calculations of water depth distribution (important for the assessment of a river's ecological and recreational characteristics) and mean bed level (important for the calculation of reach-scale sediment volumes). Copyright © 2000 John Wiley & Sons, Ltd.

Quantification of channel bed morphology in gravel-bed rivers using airborne multispectral imagery and aerial photography

The potential for mapping in-channel morphology within shallow gravel-bed rivers using airborne multispectral imagery and aerial photography is illustrated using a case study from the River Tummel, Scotland. The technique described relies on a good correlation between observed light reflectance levels from a water body and water depth. Measured water depths are regressed against reflectance levels derived from airborne multispectral imagery and black-and-white aerial photographs, to obtain equations that can be used for mapping channel bathymetry. The technique has a great deal of potential for wide-ranging applications, including detailed morphological surveys, assessing in-channel changes and mapping riverine habitats.© 1997 John Wiley & Sons, Ltd.