Publications

On the predictability of lake surface temperature using air temperature in a changing climate: A case study for Lake Tahoe (U.S.A.) 2018

Piccolroaz, Sebastiano; Healey, N. C.; Lenters, J. D.; Schladow, S. G.; Hook, S. J.; Sahoo, G. B.; Toffolon, Marco, "On the predictability of lake surface temperature using air temperature in a changing climate: A case study for Lake Tahoe (U.S.A.)" in LIMNOLOGY AND OCEANOGRAPHY, v. 2018, n. 63.1 (2018), p. 243-261. - URL: http://onlinelibrary.wiley.com/doi/10.1002/lno.10626/abstract . - DOI: 10.1002/lno.10626

Can we predict long-term trends of lake surface temperature based on air temperature alone? We explore this question by analyzing the performance of a hybrid model (air2water) as a predictive tool for defining scenarios of lake surface temperature in the framework of climate change studies. Employing Lake Tahoe (U.S.A.) as a case study, we apply the model using different air temperature datasets (in situ measurements, gridded observations, and downscaled General Circulation Models). Through a data-driven calibration of the model parameters based on surface water temperature records, we show that air2water provides good performance (root mean square error ∼ 0.5°C, on a monthly scale) regardless of the input dataset. The model is able to accurately capture the historical long-term trend and interannual fluctuations over decades (from 1969 to present), using only 7 yr of monthly measurements of surface water temperature for calibration. Additionally, when used to predict future surface water temperature of the lake, air2water produces the same projections irrespective of the air temperature dataset used to drive the model. This is certainly desirable, but not immediately expected when using a relatively simple model. Overall, the results suggest the high potential and robustness of air2water as a predictive tool for climate change assessment. Lake surface temperature warming of up to 1.1°C (RCP 4.5) and 2.9°C (RCP 8.5) was simulated at the end of the 21st century during summer months in Lake Tahoe. Such a scenario, if realized, would lead to serious consequences on lake water chemistry, primary productivity, plankton community structure, and nutrient cycling.

2018 journal paper

Large Eddy Simulation (LES) of wind-driven circulation in a peri-alpine lake: Detection of turbulent structures and implications of a complex surrounding orography 2017

Santo, Marco A.; Toffolon, Marco; Zanier, Giulia; Giovannini, Lorenzo; Armenio, Vincenzo, "Large Eddy Simulation (LES) of wind-driven circulation in a peri-alpine lake: Detection of turbulent structures and implications of a complex surrounding orography" in JOURNAL OF GEOPHYSICAL RESEARCH. OCEANS, v. 2017, n. 122 (2017), p. 4704-4722. - URL: http://onlinelibrary.wiley.com/doi/10.1002/2016JC012284/abstract . - DOI: 10.1002/2016JC012284

We investigate wind-driven circulation in a peri-alpine lake (Lake Ledro - Italy) using LES-COAST. Lake Ledro is interesting because its own dimensions are suited for LES and it is surrounded by complex orography, affecting wind distribution. We consider the winter condition when stratification is nearly absent. Two types of time-varying wind stress are used: spatially homogeneous and spatially inhomogeneous respectively. The analysis of the eddy viscosities shows substantial differences with respect to the ocean case characterized by absence of coastal boundaries and homogeneous, steady wind. The quantities exhibit a noticeable inhomogeneous behavior: the horizontal eddy viscosity is larger in the water body far from the boundaries, whereas the vertical one is larger close to the lateral boundaries due to the presence of a boundary layer. The energetic bottom boundary layer, typically occurring in lakes, is not present. This because of the intrinsic unsteadiness of the thermal wind blowing over the lake and due to the absence of large amplitude internal waves, the latter present only in case of stable stratification. In the inhomogeneous wind case, up-welling and down-welling areas are not confined along the shoreline only, but are also generated in the water body due to substantial horizontal velocity divergence, and turbulent mixing, quantified by eddy viscosities, TKE and its dissipation rate, appears enhanced with respect to the homogeneous wind case. Finally, downwelling/upwelling areas along the windward/leeward coastline respectively were observed, whose quantitative estimation may give explanation for the bloom of cyanobacteria at the lake surface observed in winter.

2017 journal paper

Ecogeomorphic relations between marsh surface elevation and vegetation properties in a temperate multi-species salt marsh 2017

Belliard, Jean-philippe, Laurent, Louis-marie; Temmerman, Stijn; Toffolon, Marco, "Ecogeomorphic relations between marsh surface elevation and vegetation properties in a temperate multi-species salt marsh" in EARTH SURFACE PROCESSES AND LANDFORMS, v. 42, n. 6 (2017), p. 855-865. - URL: http://onlinelibrary.wiley.com/doi/10.1002/esp.4041/abstract . - DOI: 10.1002/esp.4041

Salt marshes represent a fascinating example of an ecosystem whose dynamic response to climate change is chiefly governed by a two-way bio-physical coupling between marsh ecology and geomorphology. Relationships between vegetation biomass and marsh surface elevation, and between vegetation biomass and its physical properties that influence sedimentation rates, have been progressively formulated in the literature in order to provide mechanistic understandings and mathematical model descriptions of these ecogeomorphic feedbacks. In this study, a field survey was conducted in a temperate salt marsh grown by multiple halophyte species in order to quantify and validate these empirical relationships, yet in a location characterized by different climatic and ecological conditions from the locations where these relationships were initially derived. Regression analysis revealed that vegetation biomass can be expressed as a linearly increasing function of marsh elevation, providing therefore a direct empirical validation for such a relationship previously reported in the literature and implemented in some ecogeomorphic models. However, previously documented allometric relationships between total standing biomass and vegetation morphometrics – namely stem diameter, stem density and projected plant area per unit volume – were not confirmed by our results, which only showed an allometric scaling for stem height. These results suggest that previously documented formulations of mineral sediment trapping processes modulated by plants, which are partly derived on the basis of these allometric relationships, are not generally validated for multi-species salt marshes. Therefore, existing models that apply these process-based equations to study marsh evolution in a multi-species context may not capture in detail the vegetation-induced geomorphic work.

2017 journal paper

A plunge into the depths of Italy’s Lake Garda 2017

Toffolon, M.; Piccolroaz, S.; Dijkstra, H., "A plunge into the depths of Italy’s Lake Garda" in EOS, v. 98, (2017). - URL: https://eos.org/meeting-reports/a-plunge-into-the-depths-of-italys-lake-garda . - DOI: 10.1029/2017EO074499

2017 journal paper

Comparing simple models to predict river water temperature 2016

Piccolroaz, Sebastiano; Calamita, Elisa; Majone, Bruno; Gallice, Aurélien; Siviglia, Annunziato; Toffolon, Marco, "Comparing simple models to predict river water temperature" in 33rd SIL Congress - Book of Abstracts, Torino: International Society of Limnology, 2016. Proceedings of: SIL2016, Torino, 31st July - 05th August 2016

2016 extended abstract

Editorial 2016

Toffolon, Marco, "Editorial" in WATER MANAGEMENT, v. 169, n. 5 (2016), p. 199-200. - URL: http://www.icevirtuallibrary.com/content/serial/wama . - DOI: 10.1680/jwama.2016.169.5.199

2016 journal paper

Prediction of river water temperature: a comparison between a new family of hybrid models and statistical approaches 2016

Piccolroaz, Sebastiano; Elisa, Calamita; Majone, Bruno; Aurélien, Gallice; Annuziato, Siviglia; Toffolon, Marco, "Prediction of river water temperature: a comparison between a new family of hybrid models and statistical approaches" in HYDROLOGICAL PROCESSES, v. 30, n. 21 (2016), p. 3901-3917. - DOI: 10.1002/hyp.10913

River water temperature is a key physical variable controlling several chemical, biological and ecological processes. Its reliableprediction is a main issue in many environmental applications, which however is hampered by data scarcity, when using datademandingdeterministic models, and modelling limitations, when using simpler statistical models. In this work we test a suite ofmodels belonging to air2stream family, which are characterized by a hybrid formulation that combines a physical derivation ofthe key equation with a stochastic calibration of parameters. The air2stream models rely solely on air temperature andstreamflow, and are of similar complexity as standard statistical models.The performances of the different versions of air2stream in predicting river water temperature are compared with those of themost common statistical models typically used in the literature. To this aim, a dataset of 38 Swiss rivers is used, which includesrivers classified into four different categories according to their hydrological characteristics: low-land natural rivers, lake outlets,snow-fed rivers and regulated rivers. The results of the analysis provide practical indications regarding the type of model that ismost suitable to simulate river water temperature across different time scales (from daily to seasonal) and for differenthydrological regimes. A model intercomparison exercise suggests that the family of air2stream hybrid models generallyoutperforms statistical models, while cross-validation conducted over a 30-year period indicates that they can be suitably adoptedfor long-term analyses.

2016 journal paper

Ecogeomorphological feedbacks of water fluxes, sediment transport and vegetation dynamics in rivers and estuaries 2016

D'Alpaos, Andrea; Toffolon, Marco; Camporeale, Carlo, "Ecogeomorphological feedbacks of water fluxes, sediment transport and vegetation dynamics in rivers and estuaries" in ADVANCES IN WATER RESOURCES, v. 93, (2016), p. 151-155. - URL: http://www.elsevier.com/inca/publications/store/4/2/2/9/1/3/index.htt . - DOI: 10.1016/j.advwatres.2016.05.019

2016 journal paper

An analytical approach to determining resonance in semi-closed convergent tidal channels 2016

Cai, Huayang; Toffolon, Marco; Savenije, Hubert H. G., "An analytical approach to determining resonance in semi-closed convergent tidal channels" in COASTAL ENGINEERING JOURNAL, v. 58, n. 3 (2016), p. 1650009. - URL: http://www.worldscinet.com/cej/mkt/archive.shtml . - DOI: 10.1142/S0578563416500091

An analytical model is used to investigate the resonant behavior in a semi-closed channel. The main integral quantities of the tidal wave are obtained by means of a linearized one-dimensional model as a function of three dimensionless parameters, representing cross-section convergence, friction and distance to the closed boundary. Arbitrary along-channel variations of width and depth are accounted for by using a multi-reach approach, whereby the main tidal dynamics are reconstructed by solving a set of linear equations satisfying the continuity conditions of water level and discharge at the junctions of the sub-reaches. We highlight the importance of depth variation in the momentum equation, which is not considered in the classical tidal theory. The model allows for a direct characterization of the resonant response and for the understanding of the relative importance of the controlling parameters, highlighting the role of convergence and friction. Subsequently, the analytical model is applied to the Bristol Channel and the Guadalquivir estuary. The proposed analytical relations provide direct insights into the tidal resonance in terms of tidal forcing, geometry and friction, which will be useful for the study of semi-closed tidal channels that experience relatively large tidal ranges at the closed end.

2016 journal paper

Biomorphodynamic modelling of inner bank advance in migrating meander bends 2016

Zen, Simone; Zolezzi, Guido; Toffolon, Marco; Gurnell, Angela M., "Biomorphodynamic modelling of inner bank advance in migrating meander bends" in ADVANCES IN WATER RESOURCES, v. 93, (2016), p. 166-181. - URL: http://www.elsevier.com/inca/publications/store/4/2/2/9/1/3/index.htt . - DOI: 10.1016/j.advwatres.2015.11.017

We propose a bio-morphodynamic model at bend cross-sectional scale for the lateral migration of river meander bends, where the two banks can migrate separately as a result of the mutual interaction between river flow, sediments and riparian vegetation, particularly at the interface between the permanently wet channel and the advancing floodplain. The model combines a non-linear analytical model for the morphodynamic evolution of the channel bed, a quasi-1D model to account for flow unsteadiness, and an ecological model describing riparian vegetation dynamics. Simplified closures are included to estimate the feedbacks among vegetation, hydrodynamics and sediment transport, which affect the morphology of the river-floodplain system. Model tests reveal the fundamental role of riparian plants in generating bio-morphological patterns at the advancing floodplain margin. Importantly, they provide insight into the biophysical controls of the ‘bar push’ mechanism and into its role in the lateral migration of meander bends and in the temporal variations of the active channel width.

2016 journal paper

Interactions between river stage and wetland vegetation detected with a Seasonality Index derived from LANDSAT images in the Apalachicola delta, Florida 2016

la Cecilia, Daniele; Toffolon, Marco; Woodcock, Curtis E.; Fagherazzi, Sergio, "Interactions between river stage and wetland vegetation detected with a Seasonality Index derived from LANDSAT images in the Apalachicola delta, Florida" in ADVANCES IN WATER RESOURCES, v. 89, (2016), p. 10-23. - URL: http://www.elsevier.com/inca/publications/store/4/2/2/9/1/3/index.htt . - DOI: 10.1016/j.advwatres.2015.12.019

The distribution of swamp floodplain vegetation and its evolution in the lower non-tidal reaches of the Apalachicola River, Florida USA, is mapped using Landsat Thematic Mapper and Enhanced Thematic Mapper Plus (TM/ETM+) images captured over a period of 29 years. A newly developed seasonality index (SI), the ratio of the NDVI in winter months to the summer months, shows that the hardwood swamp, dominated by bald cypress and water tupelo, is slowly replaced by bottomland hardwood forest. This forest shift is driven by lower water levels in the Apalachicola River in the last 30 years, and predominantly occurs in the transitional area between low floodplains and high river banks. A negative correlation between maximum summer NDVI and water levels in winter suggests the growth of more vigorous vegetation in the vicinity of sloughs during years with low river flow. A negative correlation with SI further indicates that these vegetation patches are possibly replaced by species typical of drier floodplain conditions.

2016 journal paper

Sediment and vegetation spatial dynamics facing sea-level rise in microtidal salt marshes: Insights from an ecogeomorphic model 2016

Belliard, Jean-philippe, Laurent, Louis-marie; Di Marco, Nicola; Carniello, Luca; Toffolon, Marco, "Sediment and vegetation spatial dynamics facing sea-level rise in microtidal salt marshes: Insights from an ecogeomorphic model" in ADVANCES IN WATER RESOURCES, v. 93, (2016), p. 249-262. - URL: http://www.elsevier.com/inca/publications/store/4/2/2/9/1/3/index.htt . - DOI: 10.1016/j.advwatres.2015.11.020

Modeling effort s have considerably improved our understanding on the chief processes that govern the evolution of salt marshes under climate change. Yet the spatial dynamic response of salt marshes to sea-level rise that results from the interactions between the tidal landforms of interest and the presence of bio-geomorphic features has not been addressed explicitly. Accordingly, we use a modeling framework that integrates the co- evolution of the marsh platform and the embedded tidal networks to study sea-level rise effects on spatial sediment and vegetation dynamics in microtidal salt marshes considering different ecological scenarios. The analysis unveils mechanisms that drive spatial variations in sedimentation rates in ways that increase marsh resilience to rising sea-levels. In particular, marsh survival is related to the effectiveness of transport of sediments toward the interior marshland. This study hints at additional dynamics related to the modulation of channel cross-sections affecting sediment advection in the channels and subsequent delivery in the inner marsh, which should be definitely considered in the study of marsh adaptability to sea-level rise and posterior management.

2016 journal paper

Metal fate and effects in estuaries: A review and conceptual model for better understanding of toxicity 2016

de Souza Machado, Anderson Abel; Spencer, Kate; Kloas, Werner; Toffolon, Marco; Zarfl, Christiane, "Metal fate and effects in estuaries: A review and conceptual model for better understanding of toxicity" in SCIENCE OF THE TOTAL ENVIRONMENT, v. 541, (2016), p. 268-281. - URL: www.elsevier.com/locate/scitotenv . - DOI: 10.1016/j.scitotenv.2015.09.045

Metal pollution is a global problemin estuaries due to the legacy of historic contamination and currently increasing metal emissions. However, the establishment of water and sediment standards or management actions in brackish systems has been difficult because of the inherent transdisciplinary nature of estuarine processes. According to the European Commission, integrative comprehension of fate and effects of contaminants in different compartments of these transitional environments (estuarine sediment, water, biota) is still required to better establish, assess and monitor the good ecological status targeted by the Water Framework Directive. Thus, the present study proposes a holistic overview and conceptual model for the environmental fate of metals and their toxicity effects on aquatic organisms in estuaries. This includes the analysis and integration of biogeochemical processes and parameters, metal chemistry and organism physiology. Sources of particulate and dissolved metal, hydrodynamics, water chemistry, and mechanisms of toxicity are discussed jointly in a multidisciplinary manner. It is also hypothesized how these different drivers of metal behaviour might interact and affect metal concentrations in diverse media, and the knowledge gaps and remaining research challenges are pointed. Ultimately, estuarine physicochemical gradients, biogeochemical processes, and organismphysiology are jointly coordinating the fate and potential effects of metals in estuaries, and both realistic model approaches and attempts

2016 journal paper

An ecogeomorphic model of tidal channel initiation and elaboration in progressive marsh accretional contexts 2015

Belliard, Jean-philippe, Laurent, Louis-marie; Toffolon, Marco; Carniello, L.; D'Alpaos, A., "An ecogeomorphic model of tidal channel initiation and elaboration in progressive marsh accretional contexts" in JOURNAL OF GEOPHYSICAL RESEARCH. EARTH SURFACE, v. 120, n. 6 (2015), p. 1040-1064. - URL: https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1002/2015JF003445 . - DOI: 10.1002/2015JF003445

The formation and evolution of tidal networks have been described through various theories which mostly assume that tidal network development results from erosional processes, therefore emphasizing the chief role of external forcing triggering channel net erosion such as tidal currents. In contrast, in the present contribution we explore the influence of sediment supply in governing tidal channel initiation and further elaboration using an ecogeomorphic modeling framework. This deliberate choice of environmental conditions allows for the investigation of tidal network growth and development in different sedimentary contexts and provides evidences for the occurrence of both erosional and depositional channel‐forming processes. Results show that these two mechanisms in reality coexist but act at different time scales: channel initiation stems from erosional processes, while channel elaboration mostly results from depositional processes. Furthermore, analyses suggest that tidal network ontogeny is accelerated as the marsh accretional activity increases, revealing the high magnitude and prevalence of the depositional processes in governing the morphodynamic evolution of the tidal network. On a second stage, we analyze the role of different initial topographic configurations in driving the development of tidal networks. Results point out an increase in network complexity over highly perturbed initial topographic surfaces, highlighting the legacy of initial conditions on channel morphological properties. Lastly, the consideration that landscape evolution depends significantly on the parameterization of the vegetation biomass distribution suggests that the claim to use uncalibrated models for vegetation dynamics is still questionable when studying real cases.

2015 journal paper

Minimal model for double diffusion and its application to Kivu, Nyos, and Powell Lake 2015

Toffolon, Marco; Wüest, Alfred; Sommer, Tobias, "Minimal model for double diffusion and its application to Kivu, Nyos, and Powell Lake" in JOURNAL OF GEOPHYSICAL RESEARCH. OCEANS, v. 120, (2015), p. 6202-6224. - URL: http://onlinelibrary.wiley.com/doi/10.1002/2015JC010970/abstract . - DOI: 10.1002/2015JC010970

Double diffusion originates from the markedly different molecular diffusion rates of heat and salt in water, producing staircase structures under favorable conditions. The phenomenon essentially consists of two processes: molecular diffusion across sharp interfaces and convective transport in the gravitationally unstable layers. In this paper, we propose a model that is based on the one‐dimensional description of these two processes only, and—by self‐organization—is able to reproduce both the large‐scale dynamics and the structure of individual layers, while accounting for different boundary conditions. Two parameters characterize the model, describing the time scale for the formation of unstable water parcels and the optimal spatial resolution. Theoretical relationships allow for the identification of the influence of these parameters on the layer structure and on the mass and heat fluxes. The performances of the model are tested for three different lakes (Powell, Kivu, and Nyos), showing a remarkable agreement with actual microstructure measurements.

2015 journal paper

The role of stratification on lakes' thermal response: The case of Lake Superior 2015

Piccolroaz, Sebastiano; Toffolon, Marco; Majone, Bruno, "The role of stratification on lakes' thermal response: The case of Lake Superior" in WATER RESOURCES RESEARCH, v. 2015, n. 51 (2015), p. 7878-7894. - DOI: 10.1002/2014WR016555

During the last several decades, the Great Lakes region has been experiencing a significant risein temperatures, with the extraordinary summer warming that affected Lake Superior in 1998 as an exampleof the marked response of the lake to increasingly warmer atmospheric conditions. In this work, we combinethe analysis of this exceptional event with some synthetic scenarios, to achieve a deeper understandingof the main processes driving the thermal dynamics of surface water temperature in Lake Superior. Theanalysis is performed by means of the lumped model air2water, which simulates lake surface temperatureas a function of air temperature alone. The model provides information about the seasonal stratificationdynamics, suggesting that unusual warming events can result from two factors: anomalously high summerair temperatures, and increased strength of stratification resulting from a warm spring. The relative contributionof the two factors is quantified using the model by means of synthetic scenarios, which provide asimple but effective description of the positive feedback between the thermal behavior and the stratificationdynamics of the lake.

2015 journal paper

A hybrid model for river water temperature as a function of air temperature and discharge 2015

Toffolon, Marco; Piccolroaz, Sebastiano, "A hybrid model for river water temperature as a function of air temperature and discharge" in ENVIRONMENTAL RESEARCH LETTERS, v. 10, n. 11 (2015), p. 114011. - URL: http://iopscience.iop.org/article/10.1088/1748-9326/10/11/114011/pdf . - DOI: 10.1088/1748-9326/10/11/114011

Water temperature controls many biochemical and ecological processes in rivers, and theoretically depends on multiple factors. Here we formulate a model to predict daily averaged river water temperature as a function of air temperature and discharge, with the latter variable being more relevant in some specific cases (e.g., snowmelt-fed rivers, rivers impacted by hydropower production). The model uses a hybrid formulation characterized by a physically based structure associated with a stochastic calibration of the parameters. The interpretation of the parameter values allows for better understanding of river thermal dynamics and the identification of the most relevant factors affecting it. The satisfactory agreement of different versions of the model with measurements in three different rivers (root mean square error smaller than 1oC, at a daily timescale) suggests that the proposed model can represent a useful tool to synthetically describe medium- and long-term behavior, and capture the changes induced by varying external conditions.

2015 journal paper

HOW LAKES RESPOND TO AIR TEMPERATURE CHANGES: A LUMPED MODEL FOR LONG-TERM PREDICTIONS 2015

Toffolon, Marco; Piccolroaz, Sebastiano; Majone, Bruno, "HOW LAKES RESPOND TO AIR TEMPERATURE CHANGES: A LUMPED MODEL FOR LONG-TERM PREDICTIONS" in ASLO 2015 Aquatic Sciences Meeting: PROGRAM BOOK, Waco, Texas: ASSOCIATION FOR THE SCIENCES OF LIMNOLOGY AND OCEANOGRAPHY, 2015. Proceedings of: ASLO 2015, Granada, Spain, 22-27/02/2015. - URL: https://www.sgmeet.com/aslo/granada2015/static/files/aslo2015-pgmbk-web.pdf

2015 extended abstract

Explaining water temperature changes in Lake Superior by means of a simple lumped model 2014

Piccolroaz, Sebastiano; Toffolon, Marco; Majone, Bruno, "Explaining water temperature changes in Lake Superior by means of a simple lumped model" in 17th Workshop on Physical Processes in Natural Waters, Trento: Università degli Studi di Trento, 2014, p. 66-67. - ISBN: 9788884435507. Proceedings of: PPNW2014, Trento, 1-4 July 2014. - [download]

2014 extended abstract

Crossing the Boundaries of Physical Limnology 2014

Toffolon, Marco; Piccolroaz, Sebastiano; Bouffard, D., "Crossing the Boundaries of Physical Limnology" in EOS, v. 95, n. 44 (2014), p. 403-403. - DOI: 10.1002/2014EO440009

2014 journal paper

Proceedings of the 17th International Workshop on Physical Processes in Natural Waters 2014

Toffolon, Marco; Piccolroaz, Sebastiano (edited by), "Proceedings of the 17th International Workshop on Physical Processes in Natural Waters", by Autori Vari, Trento: Università degli Studi di Trento, 2014, 112 p. - ISBN: 9788884435514. - [download] . - DOI: 10.15168/11572_97431

2014 book

Is Air Temperature Enough to Predict Lake Surface Temperature? 2014

Piccolroaz, Sebastiano; Toffolon, Marco; Majone, Bruno, "Is Air Temperature Enough to Predict Lake Surface Temperature?" in Eos Trans. AGU, Washington: AGU, 2014, p. 1-1. Proceedings of: AGU Fall Meeting 2014, San Francisco (USA), 15 - 19th December 2014

2014 extended abstract

Linking the river to the estuary: influence of river discharge on tidal damping 2014

H. Cai; H.H.G. Savenije; M. Toffolon, "Linking the river to the estuary: influence of river discharge on tidal damping" in HYDROLOGY AND EARTH SYSTEM SCIENCES, v. 18, (2014), p. 287-304. - DOI: 10.5194/hess-18-287-2014

The effect of river discharge on tidal damping in estuaries is explored within one consistent theoretical framework where analytical solutions are obtained by solving four implicit equations, i.e., the phase lag, the scaling, the damping and the celerity equation. In this approach the damping equation is obtained by subtracting the envelope curves of high water and low water occurrence, taking into account that the flow velocity consists of a tidal and river discharge component. Different approximations of the friction term are considered in deriving the damping equation, resulting in as many analytical solutions. In this framework it is possible to show that river discharge affects tidal damping primarily through the friction term. It appears that the residual slope due to nonlinear friction can have a substantial influence on tidal wave propagation when including the effect of river discharge. An iterative analytical method is proposed to include this effect, which significantly improved model performance in the upper reaches of an estuary. The application to the Modaomen and Yangtze estuaries demonstrates that the proposed analytical model is able to describe the main tidal dynamics with realistic roughness values in the upper part of the estuary where the ratio of river flow to tidal flow amplitude is substantial, while a model with negligible river discharge can be made to fit observations only with unrealistically high roughness values.

2014 journal paper

Modelling suspended sediment wave dynamics of reservoir flushing 2014

T.H. Tarekegn; M. Toffolon; M. Righetti; A. Siviglia, "Modelling suspended sediment wave dynamics of reservoir flushing" in Reservoir Sedimentation - Special Session on Reservoir Sedimentation of the 7th International Conference on Fluvial Hydraulics, London: Taylor & Francis Group, 2014, p. 163-173. - ISBN: 9781138026759. Proceedings of: River Flow 2014, Lausanne, Switzerland, 3-5 September 2014

Flushing of fine sediments from reservoirs is one of the most effective techniques to reduce reservoir sedimentation, but the sudden release of high suspended sediment concentration can have adverse effects on the receiving water body. Field observations of sediment flushing operations have shown that the released volume of water and sediments propagate downstream through different types of waves. In particular the hydrodynamic signal travels faster than the sediment signal resulting in the splitting of the two waves. Since the sediment wave lags behind, deposition is enhanced in the tail of the hydrodynamic wave, where velocity decreases and cannot sustain the sediment in suspension: the separation phase of the two waves controls the deposition process of the released suspended sediments. The separation and the interaction between the two waves, especially in the transport of suspended sediments, is controlled by the sediment wave celerity. Neverthless in many sediment transport models the sediment wave celerity is assumed to be the mean flow velocity. We developed a simplified one-dimenional numerical model to study the interaction between the two waves. In the model we introduced a celerity factor that corrects the depth-averaged sediment transport velocity as a function of the shape of the vertical velocity profile and suspended sediment concentration. We observed that the use of the celerity factor in one-dimensional sediment concentration transport enhances the deposition because of the reduced celerity of the sediment wave, which separates sooner from the hydrodynamic wave.

2014 conference paper

Prediction of surface temperature in lakes with different morphology using air temperature 2014

M. Toffolon; S. Piccolroaz; B. Majone; A.-M. Soja; F. Peeters; M. Schmid; A. Wüest, "Prediction of surface temperature in lakes with different morphology using air temperature" in LIMNOLOGY AND OCEANOGRAPHY, v. 59, n. 6 (2014), p. 2185-2202. - URL: http://aslo.org/lo/toc/vol_59/issue_6/2185.pdf . - DOI: 10.4319/lo.2014.59.6.2185

Temperature of the surface layer of temperate lakes is reconstructed by means of a simplified model on the basis of air temperature alone. The comparison between calculated and observed data shows a remarkable agreement (Nash-Sutcliffe efficiency indexes always larger than 0.87, mean absolute errors of approximately 1°C) for all 14 lakes investigated (Mara, Sparkling, Superior, Michigan, Huron, Erie, Ontario, Biel, Zurich, Constance, Garda, Neusiedl, Balaton, and Baikal, in West-to-East order), which present a wide range of morphological and hydrological characteristics. Differently from a pure heat flux balance approach, where the different fluxes are determined on the basis of independent relationships, the input data directly inform parameters of a simple model which, in turn, provides meaningful information about the properties of the real system. The dependence of the model parameters on the main morphological indicators is presented, which allows for a quantitative description of the strong influence of the mean depth of the lake on the thermal inertia and the hysteresis pattern between air and lake surface temperatures.

2014 journal paper