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Big Floods and Fluvial Bedforms

I have chosen to formulate the suggested response for this activity in the form of an essay using a title that is based on the Activity Description of the activity file. I had no desire to prescribe this form of response, or an exact question in the activity, preferring to give you latitude to focus on the aspects of the topic that you found most interesting. However, if you have completed the activity and produced an output in an alternative form, you will hopefully find that this response is sufficiently broad to be relevant to your own work. Note how in this essay, after some contextual material, I attempt to get to the heart of an argument in the introduction and then try to sustain it throughout, rather than just hinting at it in passing, as commonly occurs in undergraduate work. By setting out your stall with a strong argument, you grab the examiner’s attention. However, please don’t think you should agree with the argument I have outlined here – that isn’t the point. At university you are marked on your ability to construct a novel argument based on the best use of scientific evidence. Consequently, one would hope for a range of positions to be articulated, although in practice, most essays in years 1 and 2 are a bland list of case-studies top-and-tailed by an introduction and conclusion. Note also that the introduction helps the reader through the material by describing the structure of the essay and is a significant part of the essay, rather than a throwaway sentence or two. In answering the question posed I have drawn upon the resources listed on the activity sheet and supplemented them with some general knowledge of the landforms studied at AS level (plus a couple of bonus facts!). Excluding the question, this response is under 1600 words, which is a not uncommon word limit at university for shorter assignments.

To what extent are the landforms studied in the rivers module at AS level the most significant for understanding how flow and landforms are coupled? What does the focus of the AS level syllabus say about the nature of the Geography syllabus? Introduction As the Laurentide ice sheet receded at the end of the last Ice Age, a series of floods were released from glacial Lake Missoula across a large swathe of the north western United States. The largest flood had an estimated discharge of 50 km3 per hour and these cataclysmic events have had a lasting impact upon the landscape of the region, even explaining today’s name of “channelled scablands” for much of this area. Such floods leave relict features that can help us to estimate their size (and consequently, couple our understanding of landforms to flow processes). Some of these, such as canyons, dry waterfalls, relict channels and 3 m high bedload boulders are the types of landforms that are commonly investigated at AS level. However, observed deposits also include giant bed ripples 10 m high, and sedimentological sequences that in general, are not studied at AS level. If bedforms are considered to be fluvial landforms then it is curious that such features are excluded from the syllabus at AS level. A case could be made that this is justifiable if an analysis of such features provides little insight into the nature of the formative flow processes. However, this essay will demonstrate that the converse is actually more generally the case and will suggest that the choice is more to do with an attempt to provide a clear demarcation between ‘Geology’ and ‘Geography’ than to consider the most relevant landscape phenomena. Hence, this boundary is felt to be artificial

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and dictated by tradition rather than science; it is not conducive to providing the best scientific explanation of the landscape given the flow or vice versa. In the first section of this essay I will consider the extent to which bedforms and landforms can be treated as separate categories, an issue that is critical to accepting or rejecting the view of the syllabus regarding its coverage of landscape features. Having concluded that it is problematic to assert a clear division between these terms, I will then argue that bedforms provide clear evidence of the nature of the formative flow processes, using dunes and antidunes as specific examples. I conclude by reflecting on the cultural and sociological factors that have resulted in the exclusion of a whole suite of sedimentary processes from the AS level syllabus. Bedforms and landforms At a first examination, the distinction between bedforms and landforms should be clear. Bedforms are features that develop on the bed of a river channel and are formed by the flow (bars, ripples, dunes, antidunes). In contrast, fluvial landforms could be defined as those features formed by the migration, erosion and deposition of the river channel (oxbow lakes, meanders, braidplains), and are implicitly considered to be larger scale phenomena. However, the Lake Missoula floods demonstrate that the idea of scale is problematic. Bed ripples from this event were as long (50 m) as the width of all but the largest British rivers and, consequently, were on a similar scale to many meander bends or similar features in the U.K. Of course, it could be argued that scale is relative to the formative flow event, but while this might be scientifically correct, it contradicts the nomenclatures commonly employed in physical Geography, where a small rill incised onto a hillslope is not termed a river. Furthermore, a succession of dunes on the bed of a channel may extend at least as far as a meander bend or similar landscape feature. Thus, there would appear to be no clear basis for separating these terms on the basis of scale. Based on the definitions provided above, an alternative way to make the distinction would be to state that landforms are created by the action of the channel as a whole, while bedforms are created within the channel. However, a meandering thalweg generally results from the deposition of point bars on alternate banks of the channel and such deposits are bedforms, while a braidplain may be completely submerged during high flows, meaning that the braided pattern seen at normal flow is replaced by a single channel and a suite of complex bedforms at high flow. Thus, this type of distinction also lacks universality. One could argue that the role of erosion plays a greater role in the development of landforms,

while bedforms are depositional. Hence, a meander incises on its outer bank (although deposition occurs on the inner bank) and a waterfalls and kolks are erosive features. However, both erosion and deposition occur in bedform formation and migration, and this can be seen clearly in the bedform phase diagram where as bed shear stresses increase, dunes are removed to be replaced by upper stage plane beds (Fig. 1). Figure 1. A bedform phase diagram. Taken from http://www.geog.leeds.ac.uk/people/c.keylock/ygt/as_act1.html From these arguments there would appear to be no simple way to distinguish between ‘landforms’ and ‘bedforms’ and we argue that using these terms to

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delimit different types of fluvial features is problematic, even if they are useful informally. Consequently, the only way in which it would be justifiable to exclude bedforms from consideration at AS level would be if such phenomena did not demonstrate any clear linkages to the formative flow processes compared to the landforms studied within the syllabus. However, in the next section it is suggested that the converse is actually the case. Linking landforms and bedforms to flow processes The complexity of physical geographic environments means that inferring the flow discharges, velocities and shear stresses that produced particular landforms is difficult. Braided rivers occur in high energy environments such as the steep rivers in the South Island of New Zealand, but the actual pattern of braiding is perhaps too complex to permit predictions of the type of braidplain that will develop for specific flows. Erosive features such as canyons may be easier to understand, but the rate of erosion of the canyon will also depend upon bedload transport and depositional processes if there is sufficient development of a river bed. The rate of erosion of meander bends relates to both the geometry of the bend, the flow in the channel and the resistivity of the sediment in the banks. In contrast, bedforms are relatively simple features for which there are quite precise and well-understood relationships between the flow, sediment, and nature of the bedform. The bedform phase diagram in Figure 1 illustrates that if a particular bedform is observed in the field (e.g. a ripple) and the grain size making up this feature is determined, then the relevant flow conditions can be determined. In the case of dunes, additional relationships are known regarding the wavelength of the dune (λ) and the average velocity acting upon it (U), permitting flow velocities to be determined from field measurements of dune wavelength:

where g is gravitational acceleration. In the case of antidunes (dunes which form in supercritical flow conditions and which migrate upstream rather than downstream), the Froude number (Fr) may be used to establish a minimum flow velocity for their formation, if the flow depth (h) can be inferred, because antidunes only develop when Fr exceeds a value of about 0.84:

Because these features are preserved in the sedimentary record more effectively than the landforms studied at AS level, they can be used to study both contemporary and palaeoenvironmental systems, making them extremely valuable to fluvial geomorphologists, and arguably more useful for interpreting palaeoflood deposits than the traditional landforms of the Geography syllabus.

Figure 2. Turbidite deposit in the Tertiary Blakely formation illustrating the Bouma sequence. Image copyright Marli Miller, University of Oregon. Sociological and cultural factors influencing the formulation of the AS-level syllabus In light of the difficulty of providing definitions that clearly discriminate between bedforms and the landforms studied at AS-level and the much more clearly expressed relationships between bedform type and geometry and flow processes, it is

surprising that bedforms are neglected at AS level. Why might this be? Although physical

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Geography has always been interested in rivers, Geology has traditionally been interested in the rock record. However, as Geology matured as a science, it began to try to infer why certain rocks formed in particular ways. The clear preservation of ripples and similar features in sedimentary rocks (Fig. 2) naturally led geologists to investigate these features. In contrast, geographers were cataloguing river channel types and attempting to understand phenomena such as bank erosion and meander initiation. Thus, there are clear historical reasons for the emphasis of the AS-level syllabus, but this distinction appears to have become entrenched. If the aim of the AS-level syllabus is to help students understand how flow processes and landforms are coupled, then surely bedforms should be studied? Indeed, at the research level, there is no necessary difference between the work undertaken in Geography and Geology departments in this field. The fact that the journal Sedimentology, the leading forum for this type of research, is edited by Prof. Paul Carling from the Southampton Geography Department, is testament to this. On the other hand, it is perhaps much simpler to only slightly modify components of the A-level syllabus to ensure that all teachers are familiar with the material and to guarantee that the syllabus is delivered effectively. Thus, there is a certain sociological and cultural inertia to the syllabus. However, if such barriers between Geography and Geology have been broken down at the research level, perhaps it is time to reconsider the nature of the fluvial landform at AS-level?