1,721,016 research outputs found
River channel planform changes in upland Scotland : with specific reference to climate fluctuation and landuse changes over the last 250 years
No full textRates of river channel change in three contrasting Scottish upland
environments have been studied within the context of Climatic
fluctuation and landuse changes over the last 250 years. The object of
the research was to assess the spatial and temporal variation in channel
types, the main controls on channel pattern and the dominant modes of
channel adjustment. This was undertaken in a hierarchic framework with
sites being investigated at three spatial scales.
At a macro-scale, the spatial and temporal variation in channel
pattern was evaluated through a random sample of river channel segments
for each study area, derived from the first and second editions of the
1:10,560 0.S. maps plus the 1:10,000 third edition. Each channel
segment was classified within a map-based channel system typology,
specifically constructed for upland Scotland. Measures of activity
collected for each sample incorporated sinuosity, braiding and lateral
shift indices. Flood histories were reconstructed for each study area
on the basis of discharge records, long rainfall records and
contemporary accounts, to assess if there was any evidence for climatic
change, fluctuation or periodicities. Estimates of the recurrence
interval of rainfall and runoff events of differing magnitude, frequency
and duration were assessed. Data, mainly of a qualitative nature, were
derived from contemporary sources and estate plans to evaluate whether
any landuse changes could have changed the runoff regime and sediment
mobility within each catchment.
At a meso-scale, 7 to 9 channel segments (already identified as
"active" within the macro-scale study) were subject to a more detailed
process-response analysis, using sequential aerial photographs. Finally
at a micro-scale, the unit stream powers at these sites were studied in
relation to specific runoff rates thereby relating channel process to
channel form.
The strength of the controls on channel planform type varied in
degree with the area studied. The glacial legacy, the positioning of
local baselevels and sediment size were found to be dominant controls.
In terms of channel dynamics, the position of the Channel planform in
relation to process thresholds and the existence of a quasi-equilibrium
condition were both very important. In terms of process-response, the
following general observations hold true. An extreme event of high RI
(>100 years) will have a major disruptive impact if there is room for
expansion of the channel system and providing thresholds for sediment
transport are exceeded. If these thresholds are high, the fact that the
channel has not recently been disrupted may also be important. The
modes of expansion across the active area depend on the type of channel
involved. Different study areas have different types of Channel
pattern present and thus a greater likelihood of certain types of
planform adjustment. The role of more moderate events (10-50 years)
varies principally with sediment size and Channel slope. Small-scale
modification may take place where stream powers associated with more
moderate events exceed competence thresholds.
It was found that process rates were highly variable in both time
and space and that present rates were not necessarily representative of
the past 250 years. Even within this timespan, there have been periods
of increased activity in response to increased discharges of moderate
magnitude (eg. 1870s-1880s within the Dee study area) and random
extreme magnitude floods (eg. between 1948-1956 in the Tweed study
area). The impact of landuse change, especially in relation to sediment
mobilisation (Dee and Spey study areas), and speed of runoff (Tweed study
area) also appeared to be important
Bedload transport, vertical exchange and sediment storage in two Scottish Highland gravel bed streams
No full textHigh recovery magnetic tracing techniques have been utilised to record the 3-
dimensional downstream transport of individual clasts within two Scottish Highland
gravel-bed streams over a period of 16 months. The results provide the basis for a
conceptual model of bedload transport and storage involving the interaction of multiple
variables.
Above a threshold level of discharge, determined by local morphology and
sedimentology, flow (in terms of magnitude and duration) remains the principal control of
transport parameters (occurrence, vertical exchange, depth of scour, and transport
distance). Three phases of transport based on increasing discharge are identified; discrete
surficial transport, shallow exchange transport and deep exchange transport. Variations in
apparent flow and transport relationships are explored in relation to individual clast
characteristics and contrasts in bed sedimentology between streams.
The results suggest a large degree of equal mobility in terms of clast size. However,
a tendency exists for material approximating the median bed surface grain size to show
maximum mobility, indicating the importance of hiding effects on gravel entrainment. In
terms of clast form, flattened shapes are the least mobile. This reflects a greater stability at
entrainment, and increased occurrence of capping, for clasts with small c/b axial ratios. A
high percentage of angular material leads to bed strengthening through interlock.
Two clast location variables, morphological position (bar, riffle, pool or
undifferentiated channel), and burial state (top, within or buried), are considered in relation
to the occurrence and distance of transport. The importance of temporary bar and subsurface
storage is highlighted. Residence periods for bed material in both such locations are
determined principally by flow conditions, particularly peak discharge. Powerful floods are
associated with increased submergence of high bars, and erosion to great depth. Material
deposited in high bar positions, or at the base of deep scour zones, is effectively locked in
storage until the next flow of equal or greater discharge.
Although the model presented is based on work at just two sites, it is suggested that
the range of morphological and sedimentological conditions encountered make the general
principles applicable across a broad range of similar environments. Given consideration of
the variables discussed, a specific variation of the model should apply in most river
reaches
Estimating flood statistics from basin characteristics in Scotland
No full textEstimation of the probability of occurrence of future flood events at
a site is frequently required for the design of bridges, culverts,
dams and other river engineering works. This study considers a method
for estimating the flood frequency distribution from the physical
characteristics of the drainage basin for use in Scotland when
adequate records of river discharge are not available. The data base
collated includes 3071 station years of annual maximum flood peaks for
168 high quality gauging stations and 12 physical characteristics for
each drainage basin. A linear regression model is derived which
explains 91% of the variation in the average magnitude of floods using
five physical characteristics indexing drainage area, rainfall, stream
density, soil type and lake storage. This model appears robust over
the range of basin types and shows no improvement when shrinkage or
ridge regression is employed. Five physically homogeneous subsets of
basins are derived using a clustering algorithm (NORMIX) and the same
five characteristics, with the addition of an index of channel slope.
For each of subsets 1, 3, 4 and 5, the individual dimensionless flood
frequency distributions for each station are not significantly
different from a single GEV distribution derived for that subset.
Consequently these subsets are considered to be hydrologically
homogeneous in addition to their physical homogeneity. Dimensionless
regional flood frequency distributions are produced for each subset
which allow the estimated average flood magnitude to be scaled to
estimate floods of less frequent occurrence. These regional 'growth
curves' imply a larger return period for a given magnitude flood than
those from the Natural Environment Research Council Flood Studies
Report (NERC, 1975). When the floods are described by a lognormal
model which allows for cross-correlation between stations the
respective return periods are smaller
Late Quaternary alluvial fans, debris cones and talus cones in the Grampian Highlands, Scotland
No full textAlluvial fans, debris cones and rockfall talus cones
are widespread in upland Britain, but remarkably little is
known about their characteristics, development and
significance. This research project has three main
objectives:
1. to establish the morphological and surface sedimentary
characteristics of alluvial fans, debris cones and talus
cones in the Grampian Highlands of Scotland;
2. to identify the factors that have controlled their
formation and distribution; and
3. to determine the timing, nature and rate of fan- and
cone-forming processes.
On the basis of previous literature, an Orioni model
that describes a continuum of fan and cone morphological and
surface sedimentary properties was devised. The
applicability of this model was tested using data for six
variables (long profile gradient, slope form, downslope
changes in clast size, roundness and form, and a scale ratio
of maximum clast size to total fan or cone length) obtained
for fans and cones in the Grampian Highlands and the Lyngen
Peninsula in Northern Norway. The results of these tests
were then used to produce a modified model appropriate to
fans and cones in upland Britain.
Using a combination of map, field and aerial photograph
data, several environmental and morphometric controls on the
distribution and type of fan and cone development were
investigated. The dimensions of different types of fan and
cone are shown to be determined by basin morphometry,
lithology and glacial history. Discriminant analysis
identified basin gradient, basin width and basin height as
the principal catchment properties that influence the
dominant type of fan- or cone-forming process.
Stratigraphic and radiocarbon evidence suggests that
many debris cones are essentially paraglacial landforms that
formed in the earlier part of the Flandrian. Many of these
cones have subsequently been modified in the late Flandrian
by fluvial processes, in some cases in response to
anthropogenic interference. However, evidence from one site
has also revealed that substantial debris cone aggradation
has occurred since c 300 BP, implying high rates of
denudation in the recent past at this site. The volumes of
other debris cones imply that as much as 1-3m of surface
lowering has occurred in gullies upslope since deglaciation.
Much lower values of surface lowering are associated with
alluvial fan development, suggesting that, locally at least,
denudation by fluvial processes has been less significant
than denudation resulting from debris flow
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
The effect of storage upon sediment transfer processes in a small Scottish gravel-bed river
No full textA detailed morphological approach is used to determine the effect of storage location upon
sediment mobility, providing partial explanation for previously reported non systematic characteristics of sediment transfer. Data were collected over a period of 2 years from a small river in the Highlands of Scotland. Sediment transfer fluxes and volumetric storage were
measured using an integrated data collection programme utilising 990 magnetic tracers and 225 cross sections spaced at c1 m intervals over two contrasting reaches (A and B).
Scaled tracer fluxes were monitored between 6 numerically defined storage types (very active, active, semi active, stable, inactive and dormant). Storage characteristics were determined using
response time, defined as the time at which cumulative output from a store exceeds sediment in storage. Activity progressively declined from very active to inactive stores, the exact magnitude being a function of local morphology, particularly the presence of fixed bars.
Inter store exchanges of sediment were assessed using descriptive matrices categorised into individual types according to transfer and storage proportions. A relative dimensionless shear stress scale used to differentiate the storage conditions responsible for the occurrence of each
matrix type indicated that transfer in reach A is a function of hydraulic conditions, grain size and storage, in reach B grain size is the only dominant factor. These controlling factors were subdivided into peak stage, duration, relative and absolute grain size, morphology and burial and
assessed with reference to fractional transfer distances. The relative importance of each factor to transfer depends upon storage location and the incidence of morphological change. Comparison between tracer and volumetric fluxes provides explanation for sediment transfer distributions and allows assessment of morphological controls upon channel pattern maintenance. The results from this study were summarised in two conceptual models describing downstream and within reach
sediment transfer
‘How can we learn to live with floods? Challenges for science and management’:guest editorial introduction
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