|SLIM : A model for the land-sea continuum and beyond
Deleersnijder, E. ; Blaise, S. ; Delandmeter, P. ; Fichefet, T. ; Hanert, E. ; Lambrechts, J. ; Bars, Y. Le; Legat, V. ; Naithani, J. ; Pham Van, C. ; Remacle, J.F. ; Soares-Frazao, S. ; Thomas, C. ; Vallaeys, V. ; Vincent, D. ; Hoitink, T. ; Sassi, M. ; Dehant, V. ; Karatekin, O. ; Wolanski, E. - \ 2016
In: Sustainable Hydraulics in the Era of Global Change - Proceedings of the 4th European Congress of the International Association of Hydroenvironment engineering and Research, IAHR 2016. - CRC Press/Balkema - ISBN 9781138029774 - p. 741 - 744.
The Second-generation Louvain-la-Neuve Ice-ocean Model (SLIM, www.climate.be/slim_flyer) deals with the equations governing sea-ice, geophysical, environmental and groundwater phenomena by means of the (discontinuous Galerkin) finite element method on 1D, 2D or 3D unstructured meshes. To take advantage of state-of-the-art developments, SLIM is also being interfaced with existing tools (often based on radically different numerical methods), such as the well-known and widely used General Ocean Turbulence Model (www.gotm.net, GOTM). The post-processing of the results is achieved with the help of usual statistical and computer graphics methods. Other techniques are also resorted to, such as tracer and timescale methods derived from CART (Constituent-oriented Age and Residence time Theory, www.climate.be/cart) or network science tools (sites.uclouvain.be/networks) (Thomas et al. 2014).The hydrodynamics simulated by the aforementioned finite element model can be introduced into a number of SLIM-based environmental modules, which are capable of representing sediment transport (Delandmeter et al. 2015), as well as the fate of some classes of contaminants, namely microbiological pollutants (de Brauwere et al. 2014), endocrine disrupting compounds, heavy metals (Elskens et al. 2014) or radionuclides. A simple ecological model is being developed, whose aim is to simulate the evolution of various species of phyto- and zoo-plankton (Naithani et al. 2016).
Simulations of the flow in the Mahakam river–lake–delta system, Indonesia
Pham van, Chien ; Brye, Benjamin de; Deleersnijder, Eric ; Hoitink, A.J.F. ; Sassi, Maximiliano ; Spinewine, Benoit ; Hidayat, Hidayat ; Soares-Frazão, Sandra - \ 2016
Environmental Fluid Mechanics (2016). - ISSN 1567-7419 - p. 603 - 633.
Coupled 1D/2D model - Mahakam River - River–lake–delta system - SLIM
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.
Morphological change in the Pearl River Delta, China
Zhang, W. ; Xu, Y. ; Hoitink, A.J.F. ; Sassi, M.G. ; Zheng, J. ; Chen, X. ; Zhang, C. - \ 2015
Marine Geology 363 (2015). - ISSN 0025-3227 - p. 202 - 219.
Morphological changes in the Pearl River Delta (PRD) have been investigated using bathymetric charts, underwater Digital Elevation Models, remote sensing data and Geographic Information Systems. Water depths were extracted from digitized charts to explore the accretion–erosion characteristics of three estuarine environments, and to provide quantitative estimates of changes in sediment volumes. Multi-temporal satellite images have been used, in combination with topographical data, to analyze the coastline changes. PRD has gained an abundant amount of sediment of almost 9.45 × 105 km3 above the 10-m isobath in the period roughly between 1970 and 2010; the average sedimentation rate was 3.15 × 104 km3/yr. Between 1976 and 2006, the coastline extended seaward by 579.2 m on average, with a mean net extension rate of 19.3 m/yr. The results suggest that the PRD experienced a major phase of accretion, with net erosion only in some local zones. Coastline extension, associated with major morphological changes, has accelerated in recent decades. Changes in boundary conditions, such as sea-level rise, seem to have relatively minor impacts on the dramatic changes in the morphology of the estuaries. The seaward extension of the coastlines shows an increasing trend whereas the sediment supply from the delta displays a decreasing trend. A detailed comparative analysis demonstrates that land reclamation in the PRD is the most significant factor that progressively alters the delta morphology, overwhelming the effects of subsidence and sediment supply.
Prediction of Discharge in a Tidal RIver Using Artificial Neural Networks
Hidayat, H. ; Hoitink, A.J.F. ; Sassi, M.G. ; Torfs, P.J.J.F. - \ 2014
Journal of Hydrologic Engineering 19 (2014)8. - ISSN 1084-0699 - 8 p.
Discharge predictions at tidally affected river reaches are currently still a great challenge in hydrological practices. In tidal rivers, water levels are not uniquely a function of streamflow. Here, the possibility to predict discharge from water level information from gauge stations at sea and in the river is explored. A hindcast model is established for a tide-dominated lowland site along the Mahakam River (East Kalimantan, Indonesia), using an artificial neural network (ANN) model. The input data for the ANN are gradually increased, by adding new input data in each step. The results show that the inclusion of data from tide predictions at sea leads to an improved model performance. The optimized ANN-based hindcast model produces a good discharge estimation, as shown by a consistent performance during both the training and validation periods. Using this model, discharge can be predicted from astronomical tidal predictions at sea plus water level measurements from a single station at an upstream location. Alternatively, the ANN model can be used as a tool for data gap filling in a disrupted discharge time-series based on a horizontal acoustic Doppler current profiler (H-ADCP). A forecast model is developed for the same river site that is located near the city of Samarinda. To this end, water level data, predicted tide levels, and at-site historical data are considered as input for the model. The discharge time-series derived from H-ADCP data are used for calibration and validation of the multistep ahead discharge forecast model. A good performance is obtained for predictions with a forecast lead time up to two days. Read More: http://ascelibrary.org/doi/10.1061/%28ASCE%29HE.1943-5584.0000970
Improved flow velocity estmates from oving-boat ADCO measurements
Vermeulen, B. ; Sassi, M.G. ; Hoitink, A.J.F. - \ 2014
Water Resources Research 50 (2014)5. - ISSN 0043-1397 - p. 4186 - 4196.
doppler current profiler - suspended sediment - turbulence measurements - acoustic measurement - river - discharge - transport - division - channel - vessel
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
Sensitivity of power functions to aggregation: bias and uncertainty in radar rainfall retrieval
Sassi, M.G. ; Leijnse, H. ; Uijlenhoet, R. - \ 2014
Water Resources Research 50 (2014)10. - ISSN 0043-1397 - p. 8050 - 8065.
zero-rainfall - semi-variance - resolution - hydrology - time - multifractals - distributions - attenuation - variability - netherlands
Rainfall retrieval using weather radar relies on power functions between radar reflectivity Z and rain rate R. The nonlinear nature of these relations complicates the comparison of rainfall estimates employing reflectivities measured at different scales. Transforming Z into R using relations that have been derived for other scales results in a bias and added uncertainty. We investigate the sensitivity of Z-R relations to spatial and temporal aggregation using high-resolution reflectivity fields for five rainfall events. Existing Z-R relations were employed to investigate the behavior of aggregated Z-R relations with scale, the aggregation bias, and the variability of the estimated rain rate. The prefactor and the exponent of aggregated Z-R relations systematically diverge with scale, showing a break that is event-dependent in the temporal domain and nearly constant in space. The systematic error associated with the aggregation bias at a given scale can become of the same order as the corresponding random error associated with intermittent sampling. The bias can be constrained by including information about the variability of Z within a certain scale of aggregation, and is largely captured by simple functions of the coefficient of variation of Z. Several descriptors of spatial and temporal variability of the reflectivity field are presented, to establish the links between variability descriptors and resulting aggregation bias. Prefactors in Z-R relations can be related to multifractal properties of the rainfall field. We find evidence of scaling breaks in the structural analysis of spatial rainfall with aggregation.
|Improved processing of vessel mounted ADCP data in highly sheared turbulent flows
Vermeulen, B. ; Sassi, M.G. ; Hoitink, A.J.F. - \ 2013
|Sensitivity of Z–R relations to spatial and temporal aggregation based on high-resolution X-band radar data
Sassi, M.G. ; Leijnse, H. ; Uijlenhoet, R. - \ 2013
In: Book of Abstracts of the 11th International Precipitation Conference, Ede, the Netherlands, 30 |June - 03 July, 2013. - Wageningen : - p. 145 - 145.
|The hydrological effects of using high- vs. coarse-resolution rainfall products in a small Dutch lowland catchment
Terink, W. ; Sassi, M.G. ; Eertwegh, G.A.P.H. van den; Leijnse, H. ; Uijlenhoet, R. - \ 2013
In: Book of Abstracts of the 11th International Precipitation Conference, Ede, the Netherlands, 30 June - 03 Luly, 2013. - Wageningen : - p. 181 - 181.
Quantified turbulent diffusion of suspended sediment using acoustic Doppler current profilers
Sassi, M.G. ; Hoitink, A.J.F. ; Vermeulen, B. - \ 2013
Geophysical Research Letters 40 (2013)21. - ISSN 0094-8276 - p. 5692 - 5697.
reynolds stress - boundary-layer - open channels - flow - suspension - transport - fluxes - adcp - sand
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 Prandtl-Schmidt number are coherent with corresponding profiles of the sediment diffusivity, rather than with profiles of the eddy viscosity.
On the use of horizontal acoustic doppler profilers for continuous bed shear stress monitoring
Vermeulen, B. ; Hoitink, A.J.F. ; Sassi, M.G. - \ 2013
International Journal of Sediment Research 28 (2013)2. - ISSN 1001-6279 - p. 260 - 268.
broad-band adcp - tidal channel - turbulence - velocity - flow - layer
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.
River flow controls on tides an tide-mean water level profiles in a tidel freshwater river
Sassi, M.G. ; Hoitink, A.J.F. - \ 2013
Journal of Geophysical Research: Oceans 118 (2013)9. - ISSN 2169-9275 - p. 4139 - 4151.
wavelet transform - salmonid habitat - mahakam delta - discharge - propagation - estuaries - channels - friction - impacts - cycles
 Tidal rivers feature oscillatory and steady gradients in the water surface, controlled by interactions between river flow and tides. The river discharge attenuates the tidal motion, and tidal motion increases tidal-mean friction in the river, which may act as a barrier to the river discharge. Time series of tidal water level amplitudes at five gauge stations along the River Mahakam in Indonesia, and tidal flow velocity amplitudes at a discharge monitoring station were obtained applying wavelet analysis. Temporal variations in tidal damping coefficients for quaterdiurnal, semidiurnal, and diurnal tidal species were quantified from the observed amplitude profiles. The analysis shows that tidal damping during the rising limb of a discharge wave differs from damping during the falling limb. Wavelet cross-correlations between surface levels yielded empirical estimates of wave numbers. An empirical relation between tidal damping and river flow is derived to describe subtidal bottom friction along an idealized tidal river resembling the Mahakam. The subtidal friction is decomposed into contributions from the river flow only, from river-tide interaction, and from tidal asymmetry. Even for high river flow and low tidal velocity, river flow enhances friction attributed to river-tide interaction, causing subtidal water level setup. A simple multilinear regression model using subtidal bottom friction is employed to predict subtidal water levels at locations upstream, with a relatively good agreement between predictions and observations. The explicit expression shows the nonlinear dependence of subtidal friction on river flow velocity, explaining the complex behavior of tidal-mean surface level profiles.
Sediment discharge division at two tidally influenced river bifurcations
Sassi, M.G. ; Hoitink, A.J.F. ; Vermeulen, B. ; Hidayat, H. - \ 2013
Water Resources Research 49 (2013)4. - ISSN 0043-1397 - p. 2119 - 2134.
depth-integrated model - open-channel flow - suspended sediment - settling velocity - secondary flow - load transport - boundary-layer - particles - suspension - roughness
 We characterize and quantify the sediment discharge division at two tidally influenced river bifurcations in response to mean flow and secondary circulation by employing a boat-mounted acoustic Doppler current profiler (ADCP), to survey transects at bifurcating branches during a semidiurnal tidal cycle. The ADCP collecting flow velocity and acoustical backscatter data was used to quantify suspended sediment discharge, adopting a recently introduced calibration procedure. Measured profiles of flow velocity and sediment concentration allowed us to compute spatiotemporal distributions of the shear velocity, the roughness length and the Rouse number. Spatiotemporal distributions of the settling velocity were obtained by combining the Rouse number and shear velocity estimates with in situ measurements from a laser particle size analyzer. Bed-load transport rates were inferred from shear stress estimates. The concentration field shows a direct response to bed shear stress, stressing the alluvial context of the system. The flow in the bifurcation regions is characterized by counter rotating secondary-flow cells, which stretch over the full width and depth of the cross sections in the downstream branches, and persist throughout the entire tidal cycle. The pattern of secondary flow suggests the flow approaching the bifurcation is concentrated in two independent threads. A two-cell structure inhibits the exchange of sediment that would occur in case a single cell would stretch over the full channel width. The division of suspended sediment primarily depends on the upstream transverse profile of the suspended sediment concentration, which is in turn dependent on geometrical factors such as upstream curvature.
Discharge regimes, tides and morphometry in the Mahakam delta channel network
Sassi, M.G. - \ 2013
Wageningen University. Promotor(en): Remko Uijlenhoet, co-promotor(en): Ton Hoitink. - [S.l.] : s.n. - ISBN 9789461734679 - 186
delta - afvoer - getijden - kanalen - kanalen, klein - morfometrie - sediment - rivieren - indonesië - deltas - discharge - tides - canals - channels - morphometrics - sediment - rivers - indonesia
The Mahakam delta in Indonesia constitutes a text book example of a mixed tide and fluvial dominated delta. Understanding the factors that control the division of water and sediment discharge over channels in the delta is relevant in the contexts of geology, ecology and river engineering. In the Mahakam river and its delta, the tide interacts with the river outflow. River-tide interaction exerts an influence on the discharge regimes and on the division of water and sediment at the bifurcations in the delta. Bifurcations control the dispersal of sediments that eventually govern the shape and evolution of the delta. In this thesis, spatial and temporal aspects of delta evolution are shown to be reflected in scaling relations between the geometric properties of delta channels and the discharge conveyed by the channels, which is known as downstream hydraulic geometry (HG). Downstream HG relations as established in this research, feature a transition from the landward part to the seaward part of the delta characterized by a clear break in scaling behavior. The variation of river discharge throughout the network is largely impacted by river-tide interaction, which is captured by downstream HG relations.
|Sediment discharge division at two tidally influenced river bifurcations
Hoitink, A.J.F. ; Sassi, M.G. ; Vermeulen, B. ; Hidayat, H. - \ 2012
|Methods to estimate river discharge from horizontally deployed acoustic Doppler current profilers
Vermeulen, B. ; Hoitink, A.J.F. ; Sassi, M.G. ; Buschman, F.A. ; Hidayat, H. - \ 2012
|Measuring profiles of turbulence with two coupled acoustic Doppler current profilers
Vermeulen, B. ; Sassi, M.G. ; Hoitink, A.J.F. - \ 2012
|H-ADCP discharge monitoring of a large tropical river
Hidayat, H. ; Sassi, M.G. ; Vermeulen, B. - \ 2012
River flow can be continuously monitored through velocity measurements with an acoustic Doppler current profiler, deployed horizontally at a river bank (H-ADCP). This approach was adopted to obtain continuous discharge estimates at two cross-sections in the River Mahakam, i.e. at an upstream station located about 300 km from the river mouth and at a downstream station located about 15 km from the river mouth in the Mahakam delta. We applied both the standard index velocity method and a recently developed methodology to obtain a continuous time-series of discharge from the H-ADCP data. Measurements with a boat-mounted ADCP were used for calibration and validation of the model to translate H-ADCP velocity to discharge. The new method 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. A regression model is employed to translate specific discharge to total discharge. The upstream discharge station represents an area influenced by variable backwater effects from lakes, tributaries and floodplain ponds, and by tides. Discharge rates at this station exceeded 3250 m3 s ˆ’1. Backwater effects from lakes were shown to be significant, whereas interaction of the river flow with tides systematically impact discharge variation. Despite the complexity of feedbacks between the river flow and the tidal motion, tides are shown to have a predictable modulating effect on discharge, which is most apparent in the fortnightly frequency band. At the downstream station, discharge rates exceeded 8000 m3 s ˆ’1. Intratidal variations were most 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 in these poorly gauged sites.
|Measuring profiles of tubulence with two coupled acoustic Doppler current profilers
Vermeulen, B. ; Hoitink, A.J.F. ; Sassi, M.G. - \ 2012
|Downstream hydraulic geometry as a tool to fingerprint tidal influence in river deltas
Sassi, M.G. ; Hoitink, A.J.F. - \ 2012