| Popis: |
River pollution is the contamination of river water by pollutant being discharged directly or indirectly on it. Depending on the degree of pollutant concentration, subsequent negative environmental effects such as oxygen depletion and severe reductions in water quality may occur which affect the whole environment. River pollution can then cause a serious threat for fresh water and as well as the entire living creatures. Dispersion in natural stream is the ability of a stream to dilute soluble pollutants. Different types of pollution, such as accidental spill of toxic chemicals, industrial waste, intermittent discharge from combined sewer overflows and temperature variations produced by thermal outflows, may generate a cloud whose longitudinal spreading strongly affects the pollutant concentration dynamics. Pollutants discharging form a point source is easier to control where as pollutant discharging from non point sources arehardlycontrollable and may represent severe threat to the river ecosystem. The longitudinal dispersion coefficient is used to describe the change in characteristics of a solute cloud from an initial state of high concentration and low spatial variance to a downstream state of lower concentration and higher spatial variance. Therefore, in order to correctly estimate the degree of pollutionwithin a stream and ensure an efficient and informed management of riverine environments,a reliable estimationof the dispersion withinthe stream is a crucial concern. The objective of my research is to develop a mathematical model for determining the dispersion in alluvial river. In order to achieve the goal, a model has been developed which provides an analytical relation for the prediction of the dispersion coefficient in natural streams, given the planimetric configuration of the river and the relevant hydrodynamic and morphodynamic parameters (i.e., width to depth ratio, the sediment grain size, scaled with the flow depth, the Shields stress). One of the most striking features of alluvial rivers is their tendency to develop regular meandering plan forms. Their geometry is in fact characterized by a sequence of symmetrical curves which amplify over time due to erosion processes at the outer bank and deposition at the inner bank. This planimetric pattern affects both the hydrodynamics of the river and the distribution of bed elevations, as well as its hydraulic response, as the average bed slope is progressively reduced along with the flow cross sections. The flow filed that establishes in meandering rivers has clearly a great relevance on the behavior of the pollutant cloud and hence on the dispersion that drives its microscopic evolution. To develop a dispersion coefficient predicting model, the analytical models of flow field establishing in the cross section of a straightriver [TubinoansColombini, 1992] and of a meandering river [Frascati and Lanzoni, 2013] aredeveloped. The two dimensional mass balance equation governing the dynamics of a pollutant is then solved using asymptoticexpression and Morse and Feshbach[1953] formalism. Finally, using the two dimensional spatial distributions of the concentration, the flow depth and the velocity, the dispersion coefficient are obtained. For straight rivers the cross-sectional velocityand the theoretically predicted dispersion coefficients with the field datacollected by Godfrey and Frederick (1970)in two rivers (Clinch River, Copper Creek). The comparison is reasonably good. The performance of the model is also tested with reference to the predictions provided by the model proposed by Deng (2001). The resultant model is found to give prediction closer to 80% of the experimental data,a much better performance agreement with respect to the model of Deng (2001). The results of the model developed to estimate the dispersion coefficients in meandering river, have been compared with the experimental data available in experimental and referring to six different rivers. Also in this case the agreement between the dispersion coefficient predicted theoretically and those calculated on the basis of tracer tests is quite good and better than that ensured by the other theoretical and empirical predictors available in literature |
