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Hydraulic Drag: a Meander-Initiating Mechanism

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Hydraulic Drag: a Meander-Initiating Mechanism MICHAEL A. GORYCKI Department of Geology and Geography, Herbert H. Lehman College of the City University of New Yor\, Bronx, New Yor\ 10468 Hydraulic Drag: A Meander-Initiating Mechanism ABSTRACT INTRODUCTION Previous work has shown that meandering is The natural tendency of streams to meander an intrinsic property of stream flow that is has interested numerous investigators, and as essentially independent of the effects of sedi- a consequence various theories to account for ment. This can be easily demonstrated on a meandering may be found in the literature (see simplified sediment-free stream "table" or Morisawa, 1968, p. 139-141, and Leopold and plate composed of an inclined hydrophobic others, 1964, p. 298-303, for reviews). Attempts plane surface on which a small stream of water have been made to equate stream meandering is allowed to flow. Development of a meander with such factors as energy loss or gain, grade, series from a straight and then sinuous stream erosion and deposition, channel morphology, is induced by increasing water flow. A study grain size of stream sediment, proportion of bed of water motion by injecting ink into the versus suspended load, velocity, channel ob- stream reveals a genetic relationship between structions, condition and composition of the sinuous water motion (meandering thalweg) channel, helical flow, and centrifugal forces. In of straight reaches and the bends of meandering spite of all that has been written on the subject, streams. Dimensional ratios of wave length of however, some workers conclude that there is meandering thalweg to stream width in the no satisfactory or complete explanation for straight stream or stream width to meander meandering (Leopold and others, 1964; King, length and radius of curvature in the meander- 1966; Morisawa, 1968; Schumm and Khan, ing stream closely match those of larger streams 1972). The fact that meander patterns are in nature. seen in some sediment-free streams on glacial ice (Knighton, 1972) or on solid rock well above Variations of water depth along straight or base level, in the meandering thalwegs of meandering experimental streams match, more straight reaches of some streams and also in or less as mirror images, the familiar riffle and the Gulf Stream, suggests that it is an intrinsic pool undulations of the stream bed in straight property of streams to meander. This leads to reaches or the crossings and deeps of meander- the consideration that the role played by ing streams in nature. Pools apparently are sediment during erosion, transportation, and analogous to deeps, and riffles to crossings. deposition during development and migration Introduced silicon carbide granules form point of meanders is essentially collateral (Leopold bars spaced approximately two to three times and others, 1964). To reinforce this concept, an the stream width in the straight stream and five examination of Tanner's (1960, 1962) labora- to seven times the stream width in the me- tory stream apparatus was made which demon- andering stream in accordance with field obser- strates the meandering nature, in the absence vations. of sediment, of a nondissipating water current. Reversing helical flow can be demonstrated in the meander bends but this is an exaggeration of sinuous water motion or hydraulic drag MATERIALS AND METHODS already present in the straight stream. A The general form of the stream plate is simi- physical model demonstrating the initiation of lar to the "model" designed by Tanner (1960, meandering is also described which apparently 1962) but differs in regard to the type of is, in essence, similar to the initiating mecha- surface and the use of an ink injection system nism responsible for production of beach cusps and dissecting microscope to study and describe and other evenly spaced longitudinal current- water motion within straight, sinuous, and formed structures. meandering streams. The stream plate is basi- Geological Society of America Bulletin, v. 84, p. 175-186, 16 figs., January 1973 175 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/1/175/3443303/i0016-7606-84-1-175.pdf by guest on 26 September 2021 176 M. A. GORYCKI cally a smooth plane surface (Masonite) about 22 X 40 cm that has been rendered hydropho- bic to a degree by giving it a fresh coat of white high gloss enamel paint and supporting it on a frame so that its long dimension makes an angle of about 25° with the horizontal. An adjustable but stable water supply in a draina- ble 2-1 container is supported above the upper edge of the plate. Water is conveyed from the container through a 20-cm length of rubber hose terminating in a thin-walled cylinder or nozzle about 5 cm long and 3 mm in diameter. The nozzle rests tangentially on the plate to which it is fixed near the plate's upper edge and the axis of the nozzle points directly down- slope. A suitable water collection and disposal trough is also installed along the lower edge of the plate. An ink injection system was used to study water motion within the experimental streams. Figure 1. General view of stream plate apparatus The system consists of a polyethylene squeeze consisting of a black-painted wooden frame, supporting bottle filled with washable ink and connected a 40-cm-long piece of Masc nite, which has a fresh coat of white high gloss enamel paint. Water, colored for by polyethylene microtubing to a pipette visibility, draining from container meanders down drawn by flame from glass microtubing. The stream plate, inclined 25° from the horizontal. Col- pipette is held in a suitable mount so that its lecting trough and drain ;tt the lower edge of plate. orifice can be held either against or slightly Ink injection system, composed of ink-filled squeeze above the surface of the plate and accurately bottle connected by polyethylene microtubing to glass positioned anywhere on it but with enough micropipette held in support, rests on apparatus. flexibility in the mount so that the pipette Dissecting microscope rests on platform attached to does not break if in contact with the plate. underside of wooden frame. Ink can be injected in the form of numerous filaments into the stream flowing on the plate therefore, increases from 1 to a maximum of surface. Single filaments can also be produced nearly 1.8 (see Fig. 7) (Leopold and others, by gentle pressure on the ink bottle. Paste 1964) and the wave length of the curves as well composed of water and silicon carbide granules as the channel length also increases. Close in- (2F grade) and added to the upper end of the spection shows that as the meanders develop stream allows observations to be made of sedi- and the stream deviates from the original ment motion and deposition within the stream. straight course the wettsd surface of the plate A frame for attaching a dissecting microscope abandoned by the migrating current becomes can also be constructed on the underside of the dry because of surface tension effects of the apparatus if close observation of the ink fila- water and the hydrophobic property of the ments or carbide granules is desired (Fig. 1). plate. As a result, the si:ream maintains a dis- creet course of fairly constant width. It is im- Experimentation portant to stress that the condition of the A gentle stream of water is allowed to flow stream plate surface is critical to meander down the plate. Initially, the stream has essen- formation. It must be clean and dry and the tially a straight course (Fig. 2). As the flow of water used should be free of wetting agents such water is slowly increased, however, a series of as soaps and detergents. A freshly painted symmetrical curves slowly but simultaneously smooth surface is best but polyethylene and develops along the length of the stream (Figs. other plastic or treated surfaces may be suitable. 3, 4). With increased flow one or more of these Tanner (I960, p. 993) suggests that ". glass curves develop into meanders which tend to which had been collecting dust lightly for one migrate downstream and from which new me- or two years" was necessary to the production anders appear to be generated farther down- of meanders on his apparatus, and that ". stream (Fig. 5). The sinuosity of the stream, After many experiments had been run on a Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/1/175/3443303/i0016-7606-84-1-175.pdf by guest on 26 September 2021 HYDRAULIC DRAG: A MEANDER-INITIATING MECHANISM 177 Figure 2. A gentle flow of water issuing from noz- Figure 3. An increase in water flow produces sinu- zle on left produces straight stream on the stream plate. ous stream. Each curve seen was present as sinuous Ink filaments {see Fig. 8) reveal that lower water levels water motion within the straight stream (see Figs. 2, in this stream have sinuous course (meandering 8, 9, 10, 12). Stream width, 3.5 mm. thalweg), the result of hydraulic drag. Stream width, 3 mm. (In this and all other photographs and diagrams except Figs. 1, 6, 12, and 14, water motion is from left to right.) Figure 5. Meanders produced by further increase of water flow. Sinuosity is only 1.43, but becomes greater with increased flow (SM Fig. 7). Note curvature variation within meander bends (see Figs. 7, 15, 16). Stream width, 4 mm; average meander length, 50 mm. Figure 4. A more sinuous stream than in Figure 3. As water flow and sinuosity increase, overrolling of developed in the hose or exit nozzle. Tanner water filaments (reversing helical flow as seen in Fig. (1962) indicates this by allowing water to flow 16) occurs at bends.
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