Vízügyi Közlemények, 1998 (80. évfolyam)

3. füzet - Győrke Olivér: Folyószabályozási kismintavizsgálatok

Folyószabályozási kismintavizsgálatok 435 Hydraulic scale models of river training facilities by Olivér GYÖRKÉ C.E. The very last study of the late author, sent to our editorial office, is dealing with the options of hydraulic scale models which have remained after the conquest of the numeric computer models. It seems that the movable-bed alluvial river channel models represent the only field where scale models are competitive with numerical computer models. The author advocates that hydraulic scale are actually "computers" built for a single purpose, for the simulation and analysis of the flow and transport conditions of a given site for given water engineering tasks, under complex geometrical and morphological boundary conditions. A further advantage of the scale model is that it visualizes the phenomena thus enabling the tracking of processes and the final engineering solution can be designed on the basis of the expectable results of various management strategies. Some of the river engineering tasks can be studied on fixed-channel scale models. Design of scale models becomes even more complicated when sediment discharge and the related channel changes are to be studied, in addition to the flow conditions. In this case a movable-bed scale model is needed. In studying natural channels, also in their hydraulic scale models, the loss of head should be split into its components. Figure 1. shows the head loss hv [m] and its components hs and hA of steady state spatially varying flow for a river section of L km length. The conversion of actual channel roughness to the model value is illustrate by Figure 2. The author suggests the e=dç>o [m] value to be taken into account as the value of absolute roughness of natural channels. Steps of constructing fixed-bed hydraulic scale models are shown in Table I. In movable-bed scale models conditions should be such as to assure sediment transport processes similar to the prototype, in order to be able to simulate scouring and settling processes, that is the changes of the channel. A basic precondition of assuring the similarity of sediment movement in the model and in the prototype is to establish similar flow pattern and the similarity of the distribution of near-bottom flow patterns. In the case of movable-bed scale models the Hungarian practice is that the same material is used for constructing the channel as for input bed-load. The author draws the attention to the relationship between the particle diameter and the factor of channel-bed stability and to additional improved version of this relationship which includes relative density and temperature as additional parameters, that is based on basic relationships developed on the basis of the "friction theory". According to "friction theory" the "sediment moving force" is considered by the product of slope and depth (I*h) which is proportional to the force that causes the displacement of the sediment particle. Another theory on which studies of sediment movement can be based is the "impact force" theory according to which the quantity proportional to the force causing particle displacement shall be determined in function of the flow velocity (particularly with the near-bottom flow velocity). The author advocates that in hydraulic scale models the dimensions should be defined, in a number of cases (such as for studying channel erosion downstream of river dams or in the vicinity of piers, bottom dikes, etc.) on the basis of the formulas developed by the author on the basis of the "impact force theory". Some of the existing relationships available among parameters like the critical (bottom) flow velocity for initiating bed-load sediment movement and the characteristics of the stream, sediment and other quantities (such as the channel material and roughness conditions) and also the increased flow velocity due to turbulent fluctuation usually enable the use of model bed materials lighter than quartz sand. Figure 3 gives, on the basis of various authors, the relationship between critical flow velocity and the diameter of the sediment particle for various ranges of the channel roughness.