A. J. F. Hoitink

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

Lateral transfer of streamwise momentum caused by a roughness transition across a shallow channel

Research on lateral exchange of streamwise momentum between parallel flows in open channels has mainly been focused on compound channels composed of a main channel and a floodplain, on mixing layer development downstream of river confluences, and on partially vegetated channels. This study aims to establish the mechanisms responsible for streamwise momentum exchange between concurrent parallel flows subject to different bed roughnesses. The contribution of momentum exchange of each mechanism is determined on the basis of flume experiments. For an initially uniform flow that experiences a bed with two parallel lanes of different roughnesses, three mechanisms for exchange of streamwise momentum can be distinguished: cross?channel secondary circulations, turbulent mixing resulting from vortices acting in the horizontal plane, and mass transfer from the decelerating flow over the rough?bottomed lane to the accelerating flow in the parallel smooth?bottomed lane. The mass transfer and associated momentum transfer across the channel cause a gain in longitudinal momentum. The secondary circulations are driven by turbulence anisotropy and feature a main cell that extends over the full water depth, which is centered at the smooth side of the smooth?to?rough transition. The gain of momentum corresponding to the mass transfer in the developing reach is on the same order of magnitude as the momentum exchange by turbulent mixing and of that by secondary circulations in the most downstream position, where the flow is nearly developed. The contribution of the secondary circulations to the exchange of streamwise momentum between the parallel flows gradually becomes dominant over the contribution of turbulent mixing when depth increases.

Hydrodynamic control of the supply of reworked terrigenous sediment to coral reefs in the Bay of Banten (NW Java, Indonesia)

Abstract The Bay of Banten is an example of a shallow-water reef environment adjacent to an eroding delta system, where corals survive in turbid conditions. This study investigated the hydrodynamic controls over residual fluxes of terrigenous sediment in the bay. Observations of wind, waves, currents and suspended sediment concentrations in 1998 and 1999 revealed contrasting dynamic turbidity conditions influenced by tides, monsoon-driven flows and locally generated waves. The most sediment-rich suspensions originate from the shallow coastal margin of an inactive delta, where waves resuspend sediment and small creeks discharge freshwater and sediment. Because of coupling between monsoonal wind, throughflow in the bay and wave height, when wave-induced resuspension peaks seasonally during the northwest monsoon, an eastward throughflow prevents the nearshore reefs in the bay from being directly exposed to turbid water masses generated in the eroding delta. In inshore waters, tidal asymmetry and the spatial variation of current amplitudes cause residual sediment transport, largely depending on the availability of erodible sediment. In the center of the bay, where there is a patch reef complex, the tidal and subtidal currents are weak and therefore here there is a zone where sediment accumulates.

Numerical simulation and analysis of saltwater intrusion lengths in the Pearl River Delta, China

ABSTRACT Zhang, W.; Feng, H.C.; Zheng, J.H., Hoitink, A.J.F.; Van Der Vegt, M.; Zhu, Y., and Cai, H.J., 2013. Numerical simulation and analysis of saltwater intrusion lengths in the Pearl River delta, China. In recent years, large-scale saltwater intrusion has been threatening the freshwater supply in the metropolitan cities surrounding the Pearl River delta (PRD). Therefore, a better understanding of the saltwater intrusion process in this region is necessary for local water resource management. In this paper, a one-dimensional flow and salinity model of the Pearl River networks was established to improve our understanding of saltwater intrusion problems in deltas. The model has high spatial resolution, discretized into 328 reaches and 5108 cross-sections, and the time step is 300 seconds for the hydrodynamic model and 30 seconds for the salinity model on the basis of the Courant–Friedrichs–Lewy condition. The model is calibrated and validated against the field measurements of the water surface elevation, discharge, and salinity at around 40 gauges in 2005 and 2001, respectively. The estimated results are in reasonable agreement with the observational data, suggesting that the model is sufficiently robust to simulate the movement of flow and salinity in the Pearl River networks. The simulated 0.5 parts per thousand salinity isohaline in the Pearl River networks displays a shape similar to “S” and slanting to the right, indicating that the maximum saltwater intrusion length occurs at the Humen outlet. In 2005, the saltwater intrusion lengths intruded far upstream at an average length of 32.4 km from the eight outlets, which is nearly two times that in 2001. Four representative upstream flows were also simulated to acquire quantitative knowledge of the response of the saltwater intrusion to discharges. Finally, historical data were collected to compare the situations of saltwater intrusion in the river networks in the 1960s and 2005. The result implies that the abrupt change in topography due to intensive dredging campaigns in the river networks is probably the most crucial factor leading to the saltwater intrusion outbreaks in large areas of the PRD in recent years.

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.

Field experiment on alternate bar development in a straight sand‐bed stream

Alternate bars in rivers and streams develop as a result of differences in length scales involved in the adjustment of flow and sediment transport to irregularities of the bed. The amount of field evidence supporting theoretical insights is highly limited. Here, we present results from a large-scale field experiment in a 600 m long straight reach. Over a period of almost 3 years, the channel was allowed to evolve autogenously from initially flat bed conditions, subject to discharge variation. Alternate bars developed within 8 months from the start of the experiment. The initial stages of bar development included bar growth, both in wavelength and amplitude, and bar migration. The latter was too limited to classify the bars as being migrating bars; therefore, we classify the bars as nonmigrating bars. Toward the end of the experiment, the regular alternate bar pattern evolved into an irregular pattern and bar amplitude started to decrease. From the start of the experiment we observed a declining channel slope, from 1.8 m km?1 initially to 0.9 m km?1 halfway the experiment, after which it stabilized. We applied two bar theories to establish their predictive capacity. Both bar theories predicted the development of alternate bars under the constructed channel conditions. In response to the declining channel slope, both theories predicted a decreasing likelihood for the development of alternate bars. Our study shows that under field conditions, the applied bar theories may predict the initial stages of bed development.

Wind forcing controls on river plume spreading on a tropical continental shelf

The Berau Continental Shelf is located close to the Equator in the Indonesian Archipelago, hosting a complex of coral reefs along its oceanic edge. The Berau coral reefs have a very high biodiversity, but the area is under serious risk due to river-derived nutrients and sediments. The region is characterized by weak winds, moderate tides, and almost absent Coriolis forcing. Existing knowledge about river plume behavior in tropical environments is limited. The aim of this paper is to investigate the influence of the subtle physical forcing on the dynamics of the Berau river plume. A three-dimensional model (ECOMSED) was calibrated with observational data. The model was forced by freshwater input from the Berau river distributaries, tides at the open boundaries, and measured hourly wind. The model reproduces the freshwater dynamics on the shelf adequately and highlights that the river plume spreads symmetrically for river forcing only. Tides cause vertical mixing and suppress the cross-shelf spreading of the river plume. However, the spreading of the river plume over the shelf is mainly controlled by the weak monsoonal winds, resulting in a seasonal development. During the Southeast Monsoon, the southerly winds push the plume northeastward and cause a stratified water column in the northern part of the continental shelf. Northerly winds during the Northwest Monsoon disperse the plume to the south, promoting a vertically well-mixed water column. The results can be used to predict the possible impact of land-use changes in the steadily developing Berau region on coral reef health.

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.