| 1. |
- Bottacin-Busolin, Andrea, 1980-, et al.
(författare)
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Combined role of advective pumping and mechanical dispersion on time scales of bed form-induced hyporheic exchange
- 2010
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Ingår i: Water resources research. - : American Geophysical Union (AGU). - 0043-1397 .- 1944-7973. ; 46, s. W08518-
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Tidskriftsartikel (refereegranskat)abstract
- 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 between the porous medium and the overlying stream: Advective fluxes first lead solutes deep into the bed and then back to the stream water, whereas dispersive fluxes favor the transfer of solutes deep into the bed leading to a permanent mass retention. The combined effect of advective exchange and dispersive fluxes produces complexity in the shape of the tails of the residence time distributions (RTDs), which follow at various stages of the process either a power law or an exponential decay. The seepage velocity induced by the stream gradient and, in case of a moving bed, the celerity of the translating bed forms limit the thickness of the advective hyporheic zone, inducing the RTDs to decrease rapidly at late time. This rapid decay can be preceded by a temporal region where the probability density functions (pdf's) tend to be inversely proportional to the square of time, and is followed by a region dominated by dispersion where the pdf's tend to be inversely proportional to the 3/2 power of time. The process shows distinct temporal ranges identified here by appropriate dimensionless parameters. Because of this complex exchange dynamics, models considering pure advection in the porous medium can significantly underestimate solute transfer at long time scales, whereas purely diffusive models of hyporheic exchange appear inadequate to represent the physical processes at an intermediate stage.
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| 2. |
- Bottacin-Busolin, Andrea, 1980-, et al.
(författare)
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Effects of Streambed Morphology and Biofilm Growth on the Transient Storage of Solutes
- 2009
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Ingår i: Environmental Science and Technology. - : American Chemical Society (ACS). - 0013-936X .- 1520-5851. ; 43:19, s. 7337-7342
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Tidskriftsartikel (refereegranskat)abstract
- Microbial biofilms are the prime site of nutrient and contaminant removal in streams. It is therefore essential to understand how biofilms affect hydrodynamic exchange, solute transport and retention in systems where geomorphology and induced hydrodynamics shape their growth and structure. We experimented with large-scale streamside flumes with streambed landscapes constructed from graded bedforms of constant height and wavelength. Each flume had a different bedform height and was covered with a layer of gravel as substratum for benthic microbial biofilms. Biofilms developed different biomass and physical structures in response to the hydrodynamic conditions induced by the streambed morphology. Step injections of conservative tracers were performed at different biofilm growth stages. The experimental breakthrough curves were analyzed with the STIR model, using a residence time approach to characterize the retention effects associated with biofilms. The retained mass of the solute increased with biofilm biomass and the biofilm-associated retention was furthermore related to bedform height We tentatively relate this behavior to biofilm structural differentiation induced by bed morphology, which highlights the strong linkage between geomorphology, hydrodynamics, and biofilms in natural streams and provide important clues for stream restoration.
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| 3. |
- Bottacin-Busolin, Andrea, 1980-, et al.
(författare)
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Evidence of distinct contaminant transport patterns in rivers using tracer tests and a multiple domain retention model
- 2011
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Ingår i: Advances in Water Resources. - : Elsevier. - 0309-1708 .- 1872-9657. ; 34:6, s. 737-746
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Tidskriftsartikel (refereegranskat)abstract
- Solute transport in rivers is controlled by surface hydrodynamics and by mass exchanges with distinct retention zones. Surface and hyporheic retention processes can be accounted for separately in solute transport models with multiple storage compartments. In the simplest two component model, short term storage can be associated to in-channel transient retention, e.g. produced by riparian vegetation or surface dead zones, and the long-term storage can be associated to hyporheic exchange. The STIR (Solute Transport In Rivers) multiple domain transport model is applied here to tracer test data from three very different Mediterranean streams with distinctive characteristics in terms of flow discharge, vegetation and substrate material. The model is used with an exponential residence time distribution (RTD) to represent surface storage processes and two distinct modeling closures are tested to simulate hyporheic retention: a second exponential RTD and a power-law distribution approximating a known solution for bedform-induced hyporheic exchange. Each stream shows distinct retention patterns characterized by different timescales of the storage time distribution. Both modeling closures lead to very good approximations of the observed breakthrough curves in the two rivers with permeable bed exposed to the flow, where hyporheic flows are expected to occur. In the one case where the occurrence of hyporheic flows is inhibited by bottom vegetation, only the two exponential RTD model is acceptable and the time scales of the two components are of the same magnitude. The significant finding of this work is the recognition of a strong signature of the river properties on tracer data and the evidence of the ability of multiple-component models to describe individual stream responses. This evidence may open a new perspective in river contamination studies, where rivers could possibly be classified based on their ability to trap and release pollutants.
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| 4. |
- Bottacin-Busolin, Andrea, 1980-, et al.
(författare)
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Misurazioni del campo di moto in prossimità di grani in movimento
- 2006
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Konferensbidrag (refereegranskat)abstract
- È stata condotta una serie di esperimenti utilizzando una canaletta di laboratorio a fondo inclinabile ed un avanzato sistema PIV (Particle Image Velocimetry). L’apparato sperimentale ha permesso un’indagine a scala di grano delle condizioni di movimento incipiente. Sul fondo della canaletta si è disposto un letto di materiale granulometrico uniforme, che nelle prove è stato sottoposto all’azione continua di una portata d’acqua costante. Il regime idrodinamico approssima il moto uniforme della corrente e la condizione di incipiente movimento dei grani. Ciascun test è stato condotto per una durata di 10 – 18 ore, durante la quale sono state registrate misurazioni per periodi di circa cinque minuti ad intervalli regolari. Il campo di moto istantaneo è stato misurato all’interno di un’area di circa 400 cm2, posta su un piano inizialmente parallelo al letto, ad una distanza da esso di circa 5 mm. Un apposito software ha consentito la costruzione delle matrici del campo di velocità istantaneo, tramite la tecnica di cross-correlazione delle immagini applicata a particelle traccianti. Mediante una fotocamera digitale si sono ottenute, inoltre, delle sequenze di foto della superficie dei sedimenti, ognuna delle quali è contemporanea ad una misura del campo di moto. Tramite una procedura semi-automatica di elaborazione dei dati è stata prodotta una statistica riguardante il moto dei sedimenti. Analizzando le sequenze è stato possibile riconoscere lo spostamento dei grani superficiali; in seguito, osservando l’andamento temporale delle velocità in prossimità delle particelle in movimento, sono stati individuati dei cambiamenti del campo di moto locale che si verificano contemporaneamente alla loro rimozione o deposizione. Tali variazioni di velocità, che possono interessare la componente longitudinale e laterale o le rispettive fluttuazioni dalla media, appaiono una diretta conseguenza del moto delle particelle. È inoltre rilevante la correlazione tra “salti” positivi di velocità in corrispondenza di un grano che viene mosso e “salti” negativi nel caso di un grano che si deposita.
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| 5. |
- Bottacin-Busolin, Andrea, 1980-, et al.
(författare)
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Modellazione a tempi di residenza del trasporto di soluti nei corsi d'acqua: Applicazione al fiume Yarqon, Israele
- 2008
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Il trasporto di soluti nei corsi d’acqua è controllato dall’idrodinamica superficiale e da scambi di massa con distinte zone di ritenzione. La propagazione a valle delle sostanze trasportate è generalmente ritardata dall’immagazzinamento temporaneo in zone morte superficiali, tipicamente zone vegetate o sacche di ritenzione laterali, e nei sedimenti sottostanti, nella cosiddetta zona iporeica. In questo articolo viene presentato un modello concettuale per il trasporto di soluti nei corsi d’acqua (STIR, Solute Transport In Rivers) nel quale i singoli processi di scambio sono rappresentati separatamente da una probabilità di “uptake” e da un’appropriata distribuzione dei tempi di residenza. Ciò rende il modello flessibile e modulare, e consente di incorporare l’effetto di una varietà di processi d’immagazzinamento e reazioni chimiche in modo dettagliato. La capacità del modello di rappresentare situazioni reali è qui dimostrata dall’applicazione al fiume Yarqon, in Israele.
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| 6. |
- Bottacin-Busolin, Andrea, 1980-, et al.
(författare)
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Probabilistic description of grain resistance from simultaneous flow field and grain motion measurements
- 2008
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Ingår i: Water resources research. - : American Geophysical Union (AGU). - 0043-1397 .- 1944-7973. ; 44:9, s. W09419-
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 7. |
- Marion, Andrea, et al.
(författare)
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Hyporheic flows in stratified beds
- 2008
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Ingår i: Water resources research. - : American Geophysical Union (AGU). - 0043-1397 .- 1944-7973. ; 44:9, s. W09433-
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 8. |
- Marion, Andrea, et al.
(författare)
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Near-bed flow field measurements at threshold of sediment motion
- 2007
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Ingår i: PROCEEDINGS OF THE CONGRESS - INTERNATIONAL ASSOCIATION FOR HYDRAULIC RESEARCH. ; , s. 401-
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Konferensbidrag (refereegranskat)abstract
- Several experimental tests were carried out using a gravel bed tilting flume and an advanced Particle Image Velocimetry (PIV) system. Sediment movement at the threshold was surveyed at the grain scale under uniform flow conditions. A natural river gravel with well sorted sediment was used. Flow field measurements were made over an area of approximately 400 cm2, on a plane located at 5 mm above the original sediment bed. Image processing and cross-correlation techniques were used to obtain flow velocities. Sequences of images of the bed surface were taken simultaneously with the measurements of the flow field. Statistics of the movements of the grains were collected through a semi-automatic procedure. Significant changes in the local flow field were observed in the proximity of the entrained or deposited particles. Such variations of velocity, which can be either in terms of the longitudinal and lateral components or in terms of the relevant fluctuations from the time average, appear as a direct effect of the removal or deposition of individual grains. A strong correlation is shown between the changes in the streamwise velocity and the movement of the particles: local velocity increases were observed to correspond with the entrainment of grains, while local velocity reductions occurred as a grain was deposited.
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| 9. |
- Marion, Andrea, et al.
(författare)
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Solute transport in rivers with multiple storage zones : The STIR model
- 2008
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Ingår i: Water resources research. - Washington, DC : American Geophysical Union (AGU). - 0043-1397 .- 1944-7973. ; 44:10, s. W10406-
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Tidskriftsartikel (refereegranskat)abstract
- 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 stream transport of solutes, Solute Transport in Rivers (STIR), that accounts for the effect of the stream-subsurface interactions on river mixing. A stochastic approach is used to derive a relation between the in-stream solute concentration and the residence time distributions (RTDs) in different retention domains. Particular forms of the RTD are suggested for the temporary storage within surface dead zones and for bed form-induced hyporheic exchange. This approach is advantageous for at least two reasons. The first advantage is that exchange parameters can generally be expressed as functions of physical quantities that can be reasonably estimated or directly measured. This gives the model predictive capabilities, and the results can be generalized to conditions different from those directly observed in field experiments. The second reason is that individual exchange processes are represented separately by appropriate residence time distributions, making the model flexible and modular, capable of incorporating the effects of a variety of exchange processes and chemical reactions in a detailed way. The capability of the model is illustrated with an example and with an application to a field case. Analogies and differences with other established models are also discussed.
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| 10. |
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