Flocculation controls in a hypertidal estuary

Estuaries are ephemeral geological formations in constant change, which is being accelerated by human intervention. Fine sediments are an important characteristic of estuarine systems upon which anthropogenic and natural activities depend. An important feature of fine sediments is their cohesiveness, following which individual particles aggregate into the so-called flocs or break-up due to turbulent motions. Throughout the flocculation process (aggregation and break-up), flocs change their properties of size, density and settling velocity with consequences on suspended sediment transport, primary productivity and contamination. The prediction of possible changes in flocculation represents an important tool to help in decision making. Although some knowledge has been gained from laboratory and field investigations, the behaviour of the flocs in real conditions is still not well understood. Even though formulations to describe changes in floc dynamics have been proposed, there is a wide disparity between studies. The aim of this research is to contribute to the knowledge of the flocculation process and propose novel methods for the implementation in numerical models, via a case study in the Dee Estuary. The Dee is a hypertidal Estuary located in Liverpool Bay, with surface tidal currents over 1 m/s. The effects of turbulence generated by the strong tidal currents and waves on floc properties are of particular interest. The possibility of a simple formulation for the flocculation process is also investigated in order to be implemented in a state of the art coupling of hydrodynamic (POLCOMS), turbulence (GOTM) and waves (WAM) numerical models. To achieve these objectives, hourly data of grain size, volume concentration and current velocity from a mooring deployed in the Welsh Channel and water samples from a research vessel taken from 12 February to 9 March on 2008 have been used. The high sampling rate for the currentv elocities enables the calculation of turbulent stress, turbulent kinetic energy, shear rate, Kolmogorov microscale and dissipation. Mass concentrations were obtained froma series of water samples collected from a research vessel during the 12-14th February 2008, which were used to convert volume concentration into mass concentration. Three hydrodynamic regimes have been distinguished from the observations: “current only” (negligible effect of waves), “combined currents-waves” (important effects from both forcings) and “wave dominant”. Quarter-diurnal variability of floc size was present during the first two regimes. Observations showed aggregation of flocs during periods of low turbulence with higher magnitude during low water slack than during high water slack. Break-up occurred concurrently with high turbulence periods during flood and ebb phases with higher magnitudes after ebb. Differences are likely due to turbulent stress asymmetries related with mixing and periodic stratification even though freshwater input was low. The “current only” regime was used to investigate the changes in floc settling velocities in relation with turbulent stress. A simple semi-empirical formulation was proposed and implemented in the numerical models. This expression depends on a single variable, which can be obtained from the turbulence model, and is both physically and mathematically correct. Model results qualitatively reproduced the neap-spring variability and the quarter-diurnal variability of floc settling velocities and suspended sediment concentration. During the “combined currents-waves” regime, waves were tidally modulated and led to enhanced aggregation and break-up, with higher floc size range than during the “current only” regime. Wave tidal modulation and quarter-diurnal variability of floc size were lost when waves were dominant. Flocs sizes exhibited a low range related to wave height. Inverse relationships between turbulent properties and median grain size were found for the three regimes, with higher scatter of data for the Kolmogorov microscale and shear rate due to different floc behaviour during flood and ebb phases. Turbulent kinetic energy showed a better relationship with floc size, which suggests its use as a floc size predictor instead of turbulent stresses

Effective energy controls on flocculation under various wave-current regimes

Transport of sediments is a critical process in the coastal zone because of its relation with coastal erosion, productivity and pollution. Of particular interest are the dynamics of suspended cohesive sediments, known as flocs, which can aggregate and break-up during the flocculation process. This changes their size, density, settling velocity and overall transport. Even though turbulence is widely accepted to be an important control on floc aggregation and break-up, specific and detailed floc behaviour is still not fully understood. The present study seeks to help in the understanding of the intra-tidal turbulence-induced flocculation under different current-wave regimes. Observations of floc size and currents at high sample rates are used to investigate the changes throughout a fortnightly cycle. The occurrence of waves at different stages during the sampling period enabled determination of three regimes of currents dominant, combined waves and currents, and wave dominant. The first two regimes showed quarter-diurnal floc size variability with aggregation during low turbulence (slack waters) and higher floc aggregation magnitude on low water slack. Break-up occurred with high turbulence (flood and ebb) with higher magnitude after ebb. During the “currents-waves” regime, waves were tidally modulated and led to enhanced aggregation and break-up, with larger floc size range than during the “current dominant” regime. Wave tidal modulation and quarter-diurnal variability of floc size were lost when waves were dominant. Flocs sizes exhibited a low range related to wave height. Inverse relationships between turbulent properties and median floc size were found for the three regimes, with higher scatter of data for the Kolmogorov microscale and shear rate due to different floc behaviour during flood and ebb phases. Effective kinetic energy obtained from the combined effect of both currents and waves seems to have a better relationship with floc size, which suggests its use as a floc size predictor instead of shear stress

Modelling tidal stream turbines in a three-dimensional wave-current fully coupled oceanographic model

© 2017 The Author(s) A tidal turbine simulation system is developed based on a three-dimensional oceanographic numerical model. Both the current and turbulent controlling equations are modified to account for impact of tidal turbines on water velocity and turbulence generation and dissipation. High resolution mesh size at the turbine location is assigned in order to capture the details of hydrodynamics due to the turbine operation. The system is tested against comprehensive measurements in a water flume experiment and results of Computational Fluid Dynamics (CFD) simulations. The validation results suggest that the new modelling system is proven to be able to accurately simulate hydrodynamics with the presence of turbines. The developed turbine simulation system is then applied to a series of test cases in which a standalone turbine is deployed. Here, complete velocity profiles and mixing are realized that could not have been produced in a standard two-dimensional treatment. Of particular interest in these cases is an observed accelerated flow near the bed in the wake of the turbine, leading to enhanced bottom shear stress (∼2 N/m 2 corresponding to the critical stress of a range of fine gravel and finer sediment particles)

Three-dimensional modelling of suspended sediment transport in the far wake of tidal stream turbines

A three-dimensional tidal turbine simulation based on an oceanographic numerical model has been tested for suspended sediment calculation, particularly in the wake of a standalone tidal turbine. The results suggest a need for further improvement of the model in order to obtain correct predictions of suspension strength of the wake and suspended sediment concentration under the influence of a turbine (compared to measured data). Due to the wide use of FVCOM in coastal applications where turbines are commonly installed, it proves necessary to address this issue. Two approaches with respect to modifying bed shear stress and turbulent mixing calculations in the presence of a turbine are proposed and tested in this research. Using data collected in the laboratory as reference, the turbulent mixing enhancement approach is shown to be effective. A series of tests are carried out to identify the impact of the turbine on suspended sediment transport in its vicinity. The results suggest that the impact is highly dependent upon the sediment grain size

Three-dimensional modelling of suspended sediment transport in the far wake of tidal stream turbines

A three-dimensional tidal turbine simulation based on an oceanographic numerical model has been tested for suspended sediment calculation, particularly in the wake of a standalone tidal turbine. The results suggest a need for further improvement of the model in order to obtain correct predictions of suspension strength of the wake and suspended sediment concentration under the influence of a turbine (compared to measured data). Due to the wide use of FVCOM in coastal applications where turbines are commonly installed, it proves necessary to address this issue. Two approaches with respect to modifying bed shear stress and turbulent mixing calculations in the presence of a turbine are proposed and tested in this research. Using data collected in the laboratory as reference, the turbulent mixing enhancement approach is shown to be effective. A series of tests are carried out to identify the impact of the turbine on suspended sediment transport in its vicinity. The results suggest that the impact is highly dependent upon the sediment grain size

Variation in stem mortality rates determines patterns of above-ground biomass in Amazonian forests: implications for dynamic global vegetation models

Understanding the processes that determine above-ground biomass (AGB) in Amazonian forests is important for predicting the sensitivity of these ecosystems to environmental change and for designing and evaluating dynamic global vegetation models (DGVMs). AGB is determined by inputs from woody productivity [woody net primary productivity (NPP)] and the rate at which carbon is lost through tree mortality. Here, we test whether two direct metrics of tree mortality (the absolute rate of woody biomass loss and the rate of stem mortality) and/or woody NPP, control variation in AGB among 167 plots in intact forest across Amazonia. We then compare these relationships and the observed variation in AGB and woody NPP with the predictions of four DGVMs. The observations show that stem mortality rates, rather than absolute rates of woody biomass loss, are the most important predictor of AGB, which is consistent with the importance of stand size structure for determining spatial variation in AGB. The relationship between stem mortality rates and AGB varies among different regions of Amazonia, indicating that variation in wood density and height/diameter relationships also influences AGB. In contrast to previous findings, we find that woody NPP is not correlated with stem mortality rates and is weakly positively correlated with AGB. Across the four models, basin-wide average AGB is similar to the mean of the observations. However, the models consistently overestimate woody NPP and poorly represent the spatial patterns of both AGB and woody NPP estimated using plot data. In marked contrast to the observations, DGVMs typically show strong positive relationships between woody NPP and AGB. Resolving these differences will require incorporating forest size structure, mechanistic models of stem mortality and variation in functional composition in DGVMs

Evolutionary Heritage Influences Amazon Tree Ecology

Lineages tend to retain ecological characteristics of their ancestors through time. However, for some traits, selection during evolutionary history may have also played a role in determining trait values. To address the relative importance of these processes requires large-scale quantification of traits and evolutionary relationships among species. The Amazonian tree flora comprises a high diversity of angiosperm lineages and species with widely differing life-history characteristics, providing an excellent system to investigate the combined influences of evolutionary heritage and selection in determining trait variation. We used trait data related to the major axes of life-history variation among tropical trees (e.g. growth and mortality rates) from 577 inventory plots in closed-canopy forest, mapped onto a phylogenetic hypothesis spanning more than 300 genera including all major angiosperm clades to test for evolutionary constraints on traits. We found significant phylogenetic signal (PS) for all traits, consistent with evolutionarily related genera having more similar characteristics than expected by chance. Although there is also evidence for repeated evolution of pioneer and shade tolerant life-history strategies within independent lineages, the existence of significant PS allows clearer predictions of the links between evolutionary diversity, ecosystem function and the response of tropical forests to global change

Phylogenetic diversity of Amazonian tree communities

Aim: To examine variation in the phylogenetic diversity (PD) of tree communities across geographical and environmental gradients in Amazonia. Location: Two hundred and eighty-three c. 1 ha forest inventory plots from across Amazonia. Methods: We evaluated PD as the total phylogenetic branch length across species in each plot (PDss), the mean pairwise phylogenetic distance between species (MPD), the mean nearest taxon distance (MNTD) and their equivalents standardized for species richness (ses.PDss, ses.MPD, ses.MNTD). We compared PD of tree communities growing (1) on substrates of varying geological age; and (2) in environments with varying ecophysiological barriers to growth and survival. Results: PDss is strongly positively correlated with species richness (SR), whereas MNTD has a negative correlation. Communities on geologically young- and intermediate-aged substrates (western and central Amazonia respectively) have the highest SR, and therefore the highest PDss and the lowest MNTD. We find that the youngest and oldest substrates (the latter on the Brazilian and Guiana Shields) have the highest ses.PDss and ses.MNTD. MPD and ses.MPD are strongly correlated with how evenly taxa are distributed among the three principal angiosperm clades and are both highest in western Amazonia. Meanwhile, seasonally dry tropical forest (SDTF) and forests on white sands have low PD, as evaluated by any metric. Main conclusions: High ses.PDss and ses.MNTD reflect greater lineage diversity in communities. We suggest that high ses.PDss and ses.MNTD in western Amazonia results from its favourable, easy-to-colonize environment, whereas high values in the Brazilian and Guianan Shields may be due to accumulation of lineages over a longer period of time. White-sand forests and SDTF are dominated by close relatives from fewer lineages, perhaps reflecting ecophysiological barriers that are difficult to surmount evolutionarily. Because MPD and ses.MPD do not reflect lineage diversity per se, we suggest that PDss, ses.PDss and ses.MNTD may be the most useful diversity metrics for setting large-scale conservation priorities

Adaptación a los cambios ambientales y territoriales

En este libro se abordan temáticas que destacan la adaptación de los distintos sectores de población a los cambios ambientales y territoriales, la cual muestra las respuestas a la incidencia de los estímulos del entorno, económico, social y ambiental. Así, se destaca la exposición de la población a los efectos destructivos de las amenazas y peligros naturales, lo que ha despertado interés en conocer sus causas, prevenir y mitigar el daño. A través de la revisión de estudios se induce la aprehensión de un tema que adquiere importancia en el contexto de los impactos globales, regionales y locales que se producen como consecuencia de la vulnerabilidad estructural característica de los países en desarrollo.En este libro se proponen estrategias de prevención ante la ocurrencia periódica de inundación en San Mateo Atenco, Estado de México y se analizan los factores sociales que inciden en el deterioro del bosque templado en San Lorenzo Huitzitzilapan. También se exponen soluciones para que se mejoren la condición del bosque y la calidad de vida de la población.Proyecto realizado con financiamiento de la Secretaría de Educación Pública-Subsecretaría de Educación Superior-Dirección General de Educación Superior Universitaria. Número del convenio con la SEP: 2017-15-001-017