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 wellunderstood 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 ASlevel 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 Alevel 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?
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