Andrea Bottacin-Busolin

Solute transport in rivers with multiple storage zones: The STIR model

Solute transport in rivers is controlled by surface hydrodynamics and by mass exchanges between the surface stream and distinct retention zones. This paper presents a residence time model for strea ...

Hyporheic flows in stratified beds

Surface-subsurface exchange fluxes are receiving increasing interest because of their importance in the fate of contaminants, nutrients, and other ecologically relevant substances in a variety of aquatic systems. Solutions have previously been developed for pore water flows induced by geometrical irregularities such as bed forms for the cases of homogeneous sediment beds and idealized heterogeneous beds, but these solutions have not accounted for the fact that streambed sediments are subject to sorting processes that often produce well-defined subsurface structures. Sediments at the streambed surface are often coarser than the underlying material because of size-selective sediment transport, producing relatively thin armor layers. Episodic erosional and depositional processes also create thick layers of different composition within the porous medium, forming stratified beds. A series of experiments were conducted to observe conservative solute transport in armored and stratified beds. An analytical solution was developed for advective exchange with stratified beds and provides appropriate scaling of the physical variables that control exchange flows. The results show that armor layers are too thin to significantly alter the advective pumping process but provide significant solute storage at short time scales. Stratified beds with layers of significant thickness favor development of horizontal flow paths within the bed and change the rate of solute transfer across the stream-subsurface interface compared to homogeneous beds.

Combined role of advective pumping and mechanical dispersion on time scales of bed form–induced hyporheic exchange

This study analyzes the effect of advective pumping and pore scale dispersion on bed form-induced hyporheic exchange. Advection and dispersion play a competitive role in the exchange dynamics betwe ...

Spectral scaling of heat fluxes in streambed sediments

Advancing our predictive capabilities of heat fluxes in streams and rivers is important because of the effects on ecology and the general use of heat fluxes as analogues for solute transport. Along ...

Probabilistic description of grain resistance from simultaneous flow field and grain motion measurements

Experiments were carried out using a mobile gravel bed placed in a tilting flume with a modified particle image velocimetry (PIV) system. Individual grain movements were surveyed using data from time series of images. Near-bed velocity flow field measurements were made simultaneously above the same area of the sediment surface by applying cross-correlation techniques to the collected plan view images. Statistics of grain motions were collected through a semiautomatic procedure. Significant changes in the flow field were observed in the proximity of the entrained or deposited particles. A strong correlation is shown between the changes in the local streamwise and lateral velocity and the movement of the grains. The theory of Grass is revisited and developed based on the experimental results. The probability distribution of individual grain resistance has been derived from the statistics of the near-bed velocity field and of the entrainment risk.

Stochastic determination of entrainment risk in uniformly sized sediment beds at low transport stages: 2. Experiments

Fluvial sediment transport is caused by a complex interaction of interdependent grain and fluid processes many of which are stochastic in nature and cannot be adequately represented by deterministic equations. Random variable analysis has been used previously but limited data are available to describe the variability of grain resistance combined with particle arrangements, and thus validate such analysis. In this study low to medium bed load transport tests were carried out in a flume where sediment movement was monitored using a three-camera 3D PIV system. Simultaneous grain motion and flow velocity measurements were made on a plane located slightly above and parallel to the sediment bed. Detailed statistical velocity information was acquired to model the velocity distribution at the bed level. This was combined with the joint probabilistic distribution of particle exposures and grain resistance to motion, which were obtained from discrete particle modeling (DPM) simulations. DPM simulations were used to provide a stochastic mathematical description of the risk that a stationary particle is entrained by the flow. Predictions from the stochastic model equations replicated the observed pulsation in sediment transport. This demonstrates that it is possible to simulate sediment entrainment and transport at a high resolution by adequately modeling all the sub-processes. A number of flow patterns were identified that caused large fluctuations of the entrainment rate. These all exhibit high velocity flow structures, but they selectively cause the dislodgement of individual particles located at different positions. This selective behavior follows from the variability of the interaction between the near-bed flow and the particles having different exposure.

Hydraulic response in flooded stream networks

Average water travel times through a stream network were determined as a function of stage (discharge) and stream network properties. Contrary to most previous studies on the topic, the present work allowed for streamflow velocities to vary spatially (for most of the analyses) as well as temporally. The results show that different stream network mechanisms and properties interact in a complex and stage-dependent manner, implying that the relative importance of the different hydraulic properties varies in space and over time. Theoretical reasoning, based on the central temporal moments derived from the kinematic-diffusive wave equation in a semi-2-D formulation including the effects of flooded cross sections, shows that the hydraulic properties in contrast to the geomorphological properties will become increasingly important as the discharge increases, stressing the importance of accurately describing the hydraulic mechanisms within stream networks. Using the physically based, stage-dependent response function as a parameterization basis for the streamflow routing routine (a linear reservoir) of a hydrological model, discharge predictions were shown to improve in two Swedish catchments, compared with a conventional, statistically based parameterization scheme. Predictions improved for a wide range of modeled scenarios, for the entire discharge series as well as for peak flow conditions. The foremost novelty of the study lies in that the physically based response function for a streamflow routing routine has successfully been determined independent of calibration, i.e., entirely through process-based hydraulic stream network modeling.

Effects of intake-entrance profiles on free-surface vortices

ABSTRACT Intake free-surface vortices can cause efficiency losses, flow fluctuations and even structural damages. Experiments were performed to examine the effect of entrance shapes on the critical submergence. Seven entrance shapes were devised and tested, including a square-edged, a bell-mouthed, three symmetrical conical and two conical profiles with eccentricity. The focus of the study was on a range of Froude numbers from 0.25 to 0.65. The square-edged shape appeared to show the highest local head-loss compared to other shapes. Steady counter-clockwise vortices characterize all the intake profiles except in a narrow water tank. The experiments show both discrepancy and similarity between the intake profiles. The critical submergence of the bell-mouthed intake is lower when compared to the square-edged shape. For the other profiles, it is proportional to the Froude number. A closer sidewall may lead to larger critical submergence in the case of weak circulations. The results demonstrate that the intake-entrance profile has an important effect on the critical submergence.

Effects of Vegetation Density and Wetland Aspect Ratio Variation on Hydraulic Efficiency of Wetlands

Hydraulic efficiency of wetlands, evaluated through retention time and mixing levels, was investigated as a function of wetland shape and vegetation density using a two-dimensional numerical model. The numerical model was applied to four different aspect ratios of a rectangular wetland (i.e. 1:1 to 1:4) with 1-ha area and vegetation density varying from 20 to 1500 stems/m2. The results, modeled velocity field and the simulated transport of a continuously injected tracer, were used to develop Residence Time Distribution graphs (RTDs). Analysis of RTDs showed that the efficiency measure related to retention time, e, and the measure of mixing, λ p , improved for denser vegetation before reaching to a constant value. It was also observed that narrow rectangular-shaped wetlands (higher aspect ratio) have better efficiency than square wetlands. The results from the study provide a quantitative understanding of hydraulic efficiency in connection with wetland vegetation and shape which may help engineers to design more efficient and cost-effective water systems.

Incorporating Hydrologic Routing into Reservoir Operation Models: Implications for Hydropower Production Planning

Increased reliance on variable and intermittent energy sources is likely to lead to a change in the production strategies of hydropower, thereby increasing the importance of accurate forecasting of production. For optimization models applied to water reservoirs, the computational cost increases with the number of reservoirs and future time-steps considered, often requiring simplification of the physical description of the flow dynamics. Here it is demonstrated that deficiency of the model of the flow dynamics on stream-reaches gives rise to errors in short-term planning, which leads to sub-optimal production. Here a simplified hydraulic model based on the kinematic-diffusion wave model was incorporated in the optimization of reservoir production planning. The time-lag distributions of the streams were evaluated for River Dalälven and implemented in a computationally efficient form of the kinematic-diffusion wave equation incorporated in a production optimization algorithm for a series of reservoirs. Compared to using a single time-lag for the water transfer on flow reaches between hydropower stations, the wave diffusion was found to affect the management as a deviation between the actual production and the planned production. The deviation was found to increase with increasing short-term regulation and decreasing Peclet number below about 10. For a sufficiently high Peclet number and long wavelength characterizing individual stream reaches, the distribution of time-lags become sufficiently narrow to motivate being replaced by a simpler description such as the constant time-lag.