M. G. Sassi

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.

Variability of residual fluxes of suspended sediment in a multiple tidal-inlet system: the Dutch Wadden Sea

In multiple tidal-inlet systems such as the Dutch Wadden Sea, the exchange of sediments between the coastal lagoon and the adjacent sea is controlled by the combined effect of the tides, wind-driven flows, and density-driven flows. We investigate the variability of residual (tidally averaged) fluxes of suspended sediment with the three-dimensional numerical model GETM in relation to forcing mechanisms and model parameters. Sediment transport is modeled with three sediment classes. A modified Partheniades-Krone formulation describes the erosion and deposition fluxes from a single-layer sediment pool. The model is initialized with a uniform sediment pool for each class and the spin up period amounts to six months. Simulations span 1 year. Comparisons with observations show that model results are fairly realistic. Residual fluxes of water and suspended sediment are episodic in nature and vary strongly throughout the year, mainly due to wind variability. The net balance between import and export of material is very sensitive to model parameters. Residual fluxes are sensitive to the geographical orientation and location of the inlets, and the effect of driving mechanisms on the residual fluxes and concentrations can be organized hierarchically, with wind forcing having the largest effect on concentration levels and variability.

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.

Residual water transport in the Marsdiep tidal inlet inferred from observations and a numerical model

At tidal inlets, large amounts of water are exchanged with the adjacent sea during the tidal cycle.The residual flows, the net effect of ebb and flood, are generally small compared with the gross flux;they vary in magnitude and sign from one tidal period to the other; and their long-term mean variesfrom year to year. Here, we focus on the temporal variability of the residual flows in the Marsdieptidal inlet, which is the western-most inlet of the Wadden Sea, a tidal lagoon along the coasts of theNetherlands, Germany, and Denmark. We compare the transport from a high-resolution numericalmodel with the transport from velocity profile data collected beneath a ferry that crosses the inletdaily. The comparison works in two ways: for the areas and times covered by the measurements, thedata serve to validate the model, and conversely, the model is employed to assess the consequencesof spatial and temporal gaps in the data. Modeled and observed transports over the region of theflow that is covered by the acoustic Doppler current profiler are in good agreement for gross andresidual quantities. Results indicate that uncertainties due to spatial gaps can be overcome with asimple extrapolation approach applied to the velocity profiles, whereas uncertainties due to temporalgaps are more problematic and leave large discrepancies in the residuals.

Estuarine mesozooplankton dynamics on a short-term time scale: role of semidiurnal tidal cycle

Wageningen University Hydrology and Quantitative Water Management, Environmental Sciences Group (Atlas, Droevendaalsesteeg 4, 6708 PB, Wageningen, The Netherlands) Descriptors: Mesozooplankton, Acartia tonsa , Tidal variability, Estuaries, Argentina. Descritores: Mesozooplancton, Acartia tonsa , Ciclo de marea, Estuario, Argentina. In estuaries and coastal areas, planktonic organisms are subject to tidal, diurnal and seasonal environmental changes (MARQUES et al., 2009). Knowledge of the variability of zooplankton communities on different spatial and temporal scales is a prerequisite for the understanding of ecosystem dynamics. Tidal currents and estuarine circulation are well known phenomena that contribute to both vertical and horizontal redistribution of planktonic organisms (MORGADO et al., 2003. Moreover, biological mechanisms may also account for a significant part of the temporal variation in zooplankton community structure (MARQUES et al., 2009). Planktonic communities have presumably developed adaptations in order to compensate for or resist dispersive losses (LAM-HOAI et al., 2006), thus resulting in recurrent patterns of zooplankton distribution. The Bahia Blanca Estuary constitutes a mesotidal system with a semidiurnal tidal cycle, the tides being one of the most important sources of energy (PERILLO; PICCOLO, 1991). The temporal and spatial dynamics of the mesozooplankton in this estuary have been extensively investigated (e.g. HOFFMEYER 2004; HOFFMEYER et al., 2008) but none of these studies has considered the short-term variability associated with tidal cycles. The objectives of the present study were to describe the influence of the tidal cycle on mesozooplankton variability and to compare the mesozooplankton distribution of two zones in the main channel of the estuary: its northern margin and the deeper central zone. The Bahia Blanca Estuary is a coastal plain system (38°45´S; 62°22´W) on the Atlantic coast of Argentina (Fig. 1). It is formed by several NW to SE tidal channels separated by extensive intertidal flats, low marshes and islands (PICCOLO; PERILLO, 1990). The tides and winds have been considered the main factors controlling the water turbulence processes (PICCOLO; PERILLO, 1990), especially in the inner zone where the tidal height is maximal (tidal range of up to 3.6 m). Tidal currents are reversible with maximum surface velocities of about 1.3 m s

Simulations of the flow in the Mahakam river–lake–delta system, Indonesia

Abstract Large rivers often present a river–lake–delta system, with a wide range of temporal and spatial scales of the flow due to the combined effects of human activities and various natural factors, e.g., river discharge, tides, climatic variability, droughts, floods. Numerical models that allow for simulating the flow in these river–lake–delta systems are essential to study them and predict their evolution under the impact of various forcings. This is because they provide information that cannot be easily measured with sufficient temporal and spatial detail. In this study, we combine one-dimensional sectional-averaged (1D) and two-dimensional depth-averaged (2D) models, in the framework of the finite element model SLIM, to simulate the flow in the Mahakam river–lake–delta system (Indonesia). The 1D model representing the Mahakam River and four tributaries is coupled to the 2D unstructured mesh model implemented on the Mahakam Delta, the adjacent Makassar Strait, and three lakes in the central part of the river catchment. Using observations of water elevation at five stations, the bottom friction for river and tributaries, lakes, delta, and adjacent coastal zone is calibrated. Next, the model is validated using another period of observations of water elevation, flow velocity, and water discharge at various stations. Several criteria are implemented to assess the quality of the simulations, and a good agreement between simulations and observations is achieved in both calibration and validation stages. Different aspects of the flow, i.e., the division of water at two bifurcations in the delta, the effects of the lakes on the flow in the lower part of the system, the area of tidal propagation, are also quantified and discussed.

On the use of horizontal acoustic Doppler profilers for continuous bed shear stress monitoring

Abstract Continuous monitoring of bed shear stress in large river systems may serve to better estimate alluvial sediment transport to the coastal ocean. Here we explore the possibility of using a horizontally deployed acoustic Doppler current profiler (ADCP) to monitor bed shear stress, applying a prescribed boundary layer model, previously used for discharge estimation. The model parameters include the local roughness length and a dip correction factor to account for sidewall effects. Both these parameters depend on river stage and on the position in the cross-section, and were estimated from shipborne ADCP data. We applied the calibrated boundary layer model to obtain bed shear stress estimates over the measuring range of the HADCP. To validate the results, co-located coupled ADCPs were used to infer bed shear stress, both from Reynolds stress profiles and from mean velocity profiles. From HADCP data collected over a period of 1.5 years, a time series of width profiles of bed shear stress was obtained for a tidal reach of the Mahakam River, East Kalimantan, Indonesia. A smaller dataset covering 25 hours was used for comparison with results from the coupled ADCPs. The bed shear stress estimates derived from Reynolds stress profiles appeared to be strongly affected by local effects causing upflow and downflow, which are not included in the boundary layer model used to derive bed shear stress with the horizontal ADCP. Bed shear stresses from the coupled ADCP are representative of a much more localized flow, while those derived with the horizontal ADCP resemble the net effect of the flow over larger scales. Bed shear stresses obtained from mean velocity profiles from the coupled ADCPs show a good agreement between the two methods, and highlight the robustness of the method to uncertainty in the estimates of the roughness length.