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- Hjulström, Filip, 1902-1982
(författare)
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Studies of the morphological activity of rivers as illustrated by the River Fyris
- 1935
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- (Summary)The investigation bears upon a determination of the rate of the mechanical and chemical denudation within the Fyris river-basin north of Uppsala in Central Sweden. The introduction deals with the morphological activity of rivers in general and the different methods (the sampling and the plot methods) for the determination of the rate of degradation.In order to state the reason for the method of investigation as well as for the interpretation of the results there is given an account of the dynamics of streams (Ch. I) and some reflections upon the influence of the stream on solid material at the bottom and in suspension (Ch. II). – A short account of the falling of the water calls attention to the tendency of the water to spread out in drops, whereby the erosion is reduced. – The critical velocity for the transition from streamline to turbulent flow (or inversely) has an especially great interest as to the erosion and is the object for some experiments. It is shown that the increase in viscosity by adding silt to the water has almost no signification, compared to that of the temperature. The critical value of Reynolds´ number for flow in thin watersheets above sand, like the run-off of rain-water, is determined. The result is, that the Rcrit. is very low and that, therefore, the rain-wash is caused by water in turbulent motion (p. 233–243). As to the characteristics of turbulence treated at some length here, the pulsations, and their importance in erosion are especially noticed; the variations in the contents of silt in the case of erosion (p. 251–255) as well as without erosion (p. 388) have been examined, partly by means of the Falck meter.Chapter II gives an account of the theories of the influence of the hydrodynamic upthrust and the Austausch-process. The results in meteorology about turbulence are applied to streaming of water in a river. The equilibrium conditions, in regard to the distribution of silt have been examined. An experiment has been carried out in order to determine the Austausch-coefficient at different points of a cross-profile, and shows a very complicated distribution.Chapter III discusses some problems of erosion, transportation and deposition. From old and new investigations a new curve is drawn, showing the relation between grain-size and erosion velocity. In a section about erosion of solid rocks an attempt is made to show that besides evorsion and direct wearing by silt-laden water another, hitherto unobserved mode of erosion may exist, namely by cavitation-erosion. The agreement in appearance between the »Sichelwanne», described by Ljungner and the forms, caused by cavitation-erosion is pointed out. This mode of erosion may, in all probability, mainly have some affect below great ice-sheets with great hydrostatic pressure and in water-falls, where the velocity of the water is great. – The problem of transportation of different materials over a bed, consisting of a finer size of particles has been examined. The conclusion is, that only for a bed, consisting of special groups of material transportation of some material without erosion may exist. The stratigraphical significance of this conclusion is pointed out (p. 323–327). As to the transportation of bedload the problem of stability is applied to the motion of the bottom-layer. It is seen that the possibility of the occurrence of a bottom-layer with laminary motion is greatest in the case of erosion. The attempts to find a method for determining the amount of transportation of the bed-load brought about an examination of the mode of transportation along the bottom. The origin of the dune mode of traction is by laboratory-studies shown to be connected with the occurrence of pulsations. As to the wave-length it will be seen that there is a certain accordance with the results of research in the stability-problem, though these are not valid for streaming near a boundary. The presence of stream ripples in an unchanged form signifies equilibrium without erosion or deposition. The transportation of the sand as a uniform layer generally signifies erosion. – A short section deals with the capacity of a stream; it is suggested that a river with access to material of all grain-dimensions and the velocity necessary to erode them has no maximum-load of material transported. Hovever, a stream which transports rolling matter without access to anything else can only transport a certain amount of this matter.Chapter IV deals with the degradation of the Fyris river-basin. A summary of previous investigations in connection with the geographical, geological, climatological and hydrological conditions is given. An especially aggrevating circumstance for the present investigation was offered by the damming up of the Fyris-water by the Mälar lake; the rating-curve for the Uppsala Nedre gauge station is not reliable for low water-levels. The error caused by this circumstance, in the total sum of the material transported from the area concerned, however, only amounts to a few per cent. The method of investigation has demanded especially great care, as the material has a very fine grain-size, and partly consists of colloids. Different methods of procuring the samples are critically discussed. A practical method of analysis is worked out, based upon adsorption by asbestos.For five years (July 1929–June 1934) analyses of water-samples have been carried out. It is seen that the River Fyris, on an average, each year carries away from the drainage-area north of Uppsala sedimentary matter to an amount of more than 5500 tons per year and dissolved matter to about 62000 tons, or together 57370 tons. Per sq.-km. this forms 56.1 tons per year. The variations in the contents of silt are put in relation to several, mainly climatological, factors. The erosion is mainly interfluvial, and caused by rain-wash. The spring flood at the melting of the cover of snow transports the greatest load of sedimentary matter but also the autumn rains cause transportation of large masses. The two other periods, the cold and the warm, have no great transportation. The mechanical and the chemical composition of the material have been examined.
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