The Meander Mechanism
by Lloyd Ramsey
who says:"Common notions about streams range only from wrong, to backwards."
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PREFACE The original hope of this study was that from a large amount and range of observations, some general notions of water behavior would emerge. This webpage presents current results as The Meander Mechanism. The theory derives only from direct observations of real stream flow, and is about only real stream flow as observed. No other theory is supported. The entirety of this theory and all parts of this presentation and all modifications are continually protected from all other claim and copy and distribution under Copyright and Intellectual Property laws which you agree to follow by proceeding. |
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ABSTRACT The general theory is that streams develop channel forms only as temporary states, as water continually responds to 1) gravity, 2) internal cohesion, 3) obstructions not yet moved, and 4) material currently moved. The key finding is that storm flows develop an accumulation of movable materials on the bed forming a central and generally straight ridge. Where the central ridge approaches a channel bank, lower flows are impounded so that the ridge is breached by flow across the ridge. The crossing flow erodes a ramp and trench that will increasingly direct general flows so that the streambed and receiving bank erode to the next stage of forms characterized as "meanders". EXPLANATION Gravity The force driving water flow is gravity, so that water will move until there is no direction that is down. Flowing over a bed, however, is not the same as falling directly over an edge as at a log or dam. Falling water is continually accelerating, while water flowing on a bed is restrained to a velocity. The direction of falling water provides extreme erosion that has no logical limit, nor any limit suggested by this study. The Pit that develops varies in form from nearly circular to elliptical, and will excavate under any upstream support. The pit is continually being flushed, with material generally deposited as a raised rim. This Rim Deposit will be breached downstream, commonly to both sides of the channel leaving a tongue-shaped deposit. A dominant flow may develop to one side in the manner of a meander, leaving the rim to develop as a minor bar. Flowing water does the same, with less force and less distinct forms, spreading material as wide shallows or Shoals. Deeper water is pressed by water above, so that the force available for erosion is increased by higher flows and in deeper parts of a bed. The strangest gravity effect is that a higher water level in a given channel will therefore go faster, as if falling off itself. Having only other water for a "bed", the tendency is to flow straight, generating the ridge/ramp process described following as meanders. It should be remembered that, except for bedrock or other solid conditions, what seems to be the bed of a stream is really a totally saturated accumulation of previously moved material, which becomes effectively fluid itself with stronger and faster flows. Internal cohesion The internal cohesion of water, one of its properties as a fluid, is how distributed precipitation gathers into distinct flows to begin with, and how it then offers buoyancy. After that, cohesion is not easily seen in stream flow, and seems to be contradictory. Within any channel, there will be various distinct flows. Where two streams meet, they will typically remain intact for considerable distance. Deep flows will have layers of behavior, so that bottom flow that falls over a bed feature will then form a different "bed" so that water above does not respond to the actual bed feature. Similarly, wide flows are many adjacent flows responding to different features and influences. Water fills a discontinuous channel by both diverging from a dominant flow direction, and by carrying more water with it as it seeks a retaining boundary. The role of friction seems to be zero. Obstructions At an Obstruction, water: a) does go around, b) can go under, eroding the bed in both cases, and c) may go over, and then resumes flowing through the available channel. At an instream rock there will commonly be a trench eroded upstream and beside. Sediment settles downstream as flow continues past, unless water falls over and so erodes as a pit in the general case. If water cannot flow around both sides of an obstruction, it will, after a way is eroded; and meanwhile will flow to refill the channel. The behavior at any object, in any location within any flow, will produce the same sort of results, continually (see examples at "Obstructions" at Illustrations; see also "Natural Spurs" and "Trees and Banks"). Sediment Streams do not "transport" sediment like some freight service. Water can neither support particles like a truck nor push them like a bulldozer. Water does have mass and does move, and so can exert force; but what is required for water to move a particle is Turbulence, to counteract gravity. Once in motion, sediment behaves similarly to sand dunes and snow drifts. Bed ripples are transverse dune forms, with the same slope and slip face aspects and movement. The longitudinal central ridge deposits of storm flow proposed here, and the deposits downstream of a regular obstruction, are seif or sword dune forms. Presumably because water can move all sizes of particles, so that conditions needed do not occur, barchans have not been seen in streams by this study. This study has found no differences in forms of deposits by particle size or size distribution, and considers all studies of those details to be a waste and diversion of resources. This study has found the association of plants with deposits to be a matter of increasing or reducing erosive turbulence, showing nothing of "trapping" or "slowing the flow" as commonly claimed by others. This study has noted Berm Deposits at all longitudinal edges, not just the "riparian levee" and "inner berm" studied by others. This study has seen nothing of a channel "aggrading" as a general or common condition, with bed erosion taken as a given of channel development. The confusion of the term "degrade" as both erosion of the stream bed and a judgement of a stream's condition generally, is deliberate, manipulated to justify endless stream projects. For this study, delta outwash and braiding are special cases of the flushing mentioned above with pit development. As the stuff of the earth for all of time and evolution, sediment cannot be reasonably considered a "pollutant". Calling it that allows endless make-work projects that never have stopped the natural behaviors of streams, but do disrupt the natural systems that have evolved dependent on truly natural conditions (see "Bank Habitat" at main index Topics section). The Meander Mechanism The center of a storm flow has been shown to flow fastest, and it carries the bulk of the sediment during the straight flow. This study finds that sediment is then deposited as a central ridge that is similarly straight. Because the sides of a channel are not going to be so straight, the Central Ridge Deposit will impound water where a bank may be nearest. During a flow higher than the ridge, there will be a breach that will wash out a flat ramp. The Breach Ramp then drains the Impoundment at lower flows. As that flow falls to the other side, it will erode something of a pit that will then develop to a trench along the base or "toe" into which that bank may collapse. As gravity deepens the trench and the increasing water lengthens the trench, the Trench Flow becomes a more general flow direction and channel feature. More of higher flows will be affected by this shift, driving further redirection and bank collapse in the manner called Meander. The breaching of a ridge is essentially random in development. The dominant crossings will typically develop, however, as an oscillation of flow direction, with "alternating point bars" as remnants of the central ridge, continually modified by further deposits and erosion. The general storm channel and previous central ridge deposits are shown by "connecting the dots" of the later alternating point bars. The older a given channel, the more erosion and deposit and bank collapse are overlaid, with point bars rising and vegetating to blend with the flood terrace, and crossings becoming sub-channels with the same developments at that smaller scale. The process of development is thereby increasingly hidden. This study has therefore included newer channels as being more revealing of the basic mechanisms for channel development (see first four at Illustrations), and so producing unusual perceptions and explanations of streams generally. This study has found no evidence of, nor any reason to believe that there would be, any limit to the meander process. Meanders commonly extend at a rate and to a degree regulated only by the frequency and force of flows, and the resistance of materials encountered (see "Meanders" at Illustrations). This study has found almost no occurrence, on streams of the scale accessible to this study, of an "oxbow pond" form resulting from excessive meandering. This study has not found any consistency in how the material of a bank falls, finding such a range of block and rotation and shatter and ravel, as to call all of it Slump. What happens to what may fall is also variable, with a Slump Slope commonly forming at the base and quickly growing wild plants starting with sod from the terrace or previous growth on the slope. The angle of such slopes is remarkably uniform across occurrences, about 1.5 : 1 as common to previous engineering, while completely variable per occurrence so that where and how it is measured would predetermine the result making all such measurements meaningless. The uniform flat banks commonly imposed by stream reconstructions have not been found as a natural result of any stream development in any valley material. While vegetated slump slopes are often as "stable" as stream banks can be, this study has found no convincing evidence that trees or any other roots "hold banks". The real question would be whether a bank will hold the roots (see "Trees and Banks" at Illustrations). Entrenchment It is tempting to include entrenchment as an important part or even as the major factor in channel development. There does seem to be a rate relationship, so that more entrenchment means more channel erosion. However, that is circular: the more entrenchment, the more water is inside the channel doing what water does. The stream in a canyon in fact shows the same general characteristics as other streams. At present, entrenchment is left as an interesting variance, rather than a variable, in the general process of channel formation. Sinuosity Water flow does not produce curves. The curves we think we see are perceptual simplifications of the sort called "gestalt" in psychology. Any closer examination of any apparently regular form at any starting scale will reveal perturbations that increase with closer examination; yet at each scale, one will continue to simplify the conditions to averaged directions and boundaries to again "see" curves. There is a math for producing such curves from such scattered actual locations, proving only the power of math to distract, and it has been deliberately ignored by this study of real water flow. Meanders have no necessary result or direction, as commonly running straight as curved where active, and as often proceeding upstream as downstream. It is only the continuing tendency of storm flow to go straight that drives erosion farther to a given "side" of a previous or general direction, from which it would return by the same tendency, moving always and only however down could be. OCCURRENCE The Meander Mechanism process and other aspects of the theory have been found to occur at all scales in all movable materials. No fundamental differences in the nature of stream forms have been found to result from valley or bed slope or proportions or constituent materials. Forms are found to be so similar across such a range of conditions as to demand a theory of general mechanisms, rather than a theory based on "classification". The hundreds of photos presented at this website over five years have already shown both much of the development of the theory and various aspects of the theory, and together serve as substantiation of the theory by offering real examples. FURTHER DISCUSSION The theory presented here not only does not support other theories; it could be argued to refute the bulk of "fluvial geomorphology" and the studies and practices deriving from it, particularly those of Rosgen and thereby the bulk of stream projects worldwide. However, it is the purpose of this website and this treatise to report, not to argue, with the hope of teaching at least something of observation skills. Luna Leopold has imagined and propounded a mysterious force arising from the flow of water that produces a mathematically expressible curvature and oscillation of direction that will tend toward a stable channel form. He has argued from "dimensionless graphs" that are therefore about nothing. This study has not found, on the ground or on maps and aerial photos, any such sequence of even two consecutive turns, or even such turns. His romantic vision and mathematicalizations are supplanted here simply by showing the actual, practical processes. Luna himself has denied ever intending that his theory be used to control streams, and doubted that it would do so. David Rosgen's derivative proposal of a calculated and artificially produced "natural stream channel form" that would therefore be "stable" has nowhere been found by this study to be supported by his own and his followers' stream projects. As mentioned, the curves and banks produced do not naturally occur; and no such forms as the "structures" are ever produced by streams. Rather, all such stream reconstructions studied exhibit the Meander Mechanism and other aspects of this theory in definite and even rapid developments tending to obliterate those unnatural projects, as presented elsewhere at this website (see main index). This study has found no such continual line of flow as the fanciful "thalweg" in nature or in what remains of such designed projects. Please note that the so-called "riffle-pool sequence" is not the same as this theory. Those commonly recognized, but obviously not well observed, forms are taught as objects that can be installed at will and by means of machinery. It is not possible for machines to do, or make, anything natural. A true riffle is a longitudinal bed form that sometimes occurs with higher rock content (see "Straights" at Illustrations). They are uncommon, because that is not what water flow commonly does with movable material. Similarly, a true instream pool is a continually produced pit or trench that is a "pool" only when water is in it, and is continually producing deposits downstream as described above. The breach ramps and shoals of this theory occur with widely varying location and character, with associated erosion also widely variable. For the Meander Mechanism theory "crossing" does not refer to the long straight between turns, which is only a possible development of the channel after a crossing as described here. The Meander Mechanism theory proposes general processes that contradict notions of "stream classification". The extensive and intensive observations of this study would not have suggested and do not support Rosgen's proposed discontinuity of forms at some abstract percent of bed slope. Channel forms such as Rosgen's E have been seen adjacent to A, with a piece of D in an otherwise B, with the general mechanisms summarized here occurring throughout. Easily "communicating" what is false, is not helpful. The insignificance of particle size has been mentioned above. This study does not find "width-depth ratio" to be a real or useful factor. Rather, it contradicts the notion of "U-shaped" or "concave cross section" by suggesting an optimal depth for all waterways across various widths. Neither has any sense to "bankfull" been found in the highly varying channels and flows seen everywhere outside a few meadows. Any number of flow channels can be found nested within a general channel, increasing with entrenchment and also with total width, with each such flow following the general factors of this theory. How some magic could be found in one such channel to "stabilize" the results of all other flows in the general channel, has not been found by this study. The notion of a "reference reach" to be copied is as absurd as recurring "rain events". The notion of "roughness", used to justify any sort of imposition on a channel, is basically backwards. Any source of turbulence, as all roughness will make in some flow, produces the erosion that people do not want. Neither has it been seen possible to simply "slow the flow" without therefore impounding that flow with further, and generally undesirable, consequences. There must be some material that can be moved by available flows for there to be erosion; and any material being moved that cannot be moved in another circumstance will then be deposited. But this study has found no connection between these two typical conditions. There is no "balance" necessary within a given channel. The only natural places that have revealed any longevity to this study are bedrock with extensive plants. If roots did "hold the banks", then there would be only burbling brooks shaded by those plants (see "Trees and Banks" at Illustrations). This study finds that any upright or resisting element, plant or not, poses an obstruction and such developments as noted above. Where plants offer a smooth surface to storm flow, particularly grass and forbs laid flat and parallel to the flow, the resulting lack of turbulence is all the "protection" that has been observed. Deposit does not rely on this condition; but any that occurs is also so protected, and so can increase, giving the false impression of causing deposits. This study has found no constructed banks to be more stable than rock revetment allowed (which is rare) to vegetate and silt in and so becoming smooth; and plain grassed slopes have been seen as stable as those with concrete blocks in them. Of the various structures placed in beds with claims of "redirecting the flow", none have been seen to convincingly do so as claimed, as shown elsewhere at this website (see main index). Water flow does not bounce, and so cannot be "deflected"; and the redirection that does occur, as described above, usually erodes the structure. The only direction water has when falling over anything, is down; and what it does at whatever bottom, is erode that bottom. After that, water can only find a way through the resulting turbulence to again flow along the channel. It is not possible to compress water nor keep it from filling the available channel; so it is not likely that water could be "redirected to the center of the channel" as claimed for various disrupting structures. Any increase in rate of flow by constriction at a structure has been seen to produce greater bed erosion, generating the deposit scenarios as above, as well as wrecking the structure, as shown elsewhere at this website. This study has found no evidence of anything happening in streams as if by the action of scissors, indicating that "shear forces" are sheer fantasy. The practice of "sizing the rock" is based only on repeating that practice, not on any action of water observed by this study. Nor has any sign of a helical flow at turns been found by this study, though upwelling has been seen during storm flows. For this study, the term "water column" would mean something vertical and realistically apply only to recurring local upwelling, usually at a deep bed obstruction; so any use of that term about streams is an alarm for ignorance and confused thinking. CONCLUSION The Meander Mechanism theory holds that it is the bed, not the banks, that generates stream channels, so that what has been done to banks has been at best irrelevant to any purposes of control, and what is done to beds is only, and increasingly violently per Rosgen, disruptive. This study does not intend to change anyone's mind, or tell anyone what to go out and do, or "prove" anything. The general summary of this study is: What does not occur in natural streams, will not occur in any stream. And the only proposal is: |
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Lloyd Ramsey, Alternative Stream Research4948 Old Baux Mtn. Rd., Winston-Salem NC 27105