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The influence of wind flow over a long-walled asymmetric coastal parabolic dune : morphodynamic feedback, evolution and migration

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Mowling, FA (2006) The influence of wind flow over a long-walled asymmetric coastal parabolic dune : morphodynamic feedback, evolution and migration. PhD thesis, University of Tasmania.

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Abstract

Mobile transgressive dune systems are often overlooked in the study of coastal
morphodynamics and sediment budgets. Inundation bK sand of abutting agricultural
land and infrastructure has recurred throughout the 2ot 1 century, frequently
accompanied by requests by farming communities to the government to stabilise the
dunes. In addition, the association of sea-beach-coastal dunes provides an attractive
setting for residential I holiday developments on dune systems. Yet little is known
about the pattern of dune mobility in response to changing atmospheric conditions
(rainfall, air temperature and wind speed) and their effect on dune mobility and the
rate of actual and predicted dune migration. The work presented in this thesis
addresses the evolution of a parabolic dune, dune mobility and migration.
Quantitative data were collected to determine process-response in an actively mobile
long-walled asymmetric parabolic dune located on the western coastline of northeast
Tasmania, Australia. The data were acquired over a period of four years from a
composite of geomorphic survey techniques that incorporated repeated surveys of the
ground surface topography using kinematic GPS and arrays of erosion pins, which
were linked to analysis of wind flow patterns acquired from an on-site 2 m high
meteorology station, and analysis of sediment that forms the dune. Wind shear
velocity (u*) and roughness length (zo) were measured close to the ground on
windward slopes, and speed-up ratios were determined. Analysis oftemporallyspaced
ortho-rectified and georeferenced aerial photography extended the duration of
the survey by fifty years to establish the longer term evolution of the parabolic dune,
including the downwind and lateral migration rates of the dune system.
The level of accuracy (planimetric accuracies typically in the range of 2-5 cm and
vertical accuracies of 3-10 cm) achieved in the six epochs of kinematic GPS surveys
provided reliable GPS data that were used to construct three-dimensional
topographic models of the dune system on 5 m x 5 m grids using kriging. The
topographic models displayed the general patterns of displacement and accretion of
sediment that maintain dune form. Using cross-sectional profiles, at representative
locations across the dune, the pattern of process-response between wind speed I wind
direction on the direction of sand movement and on morphodynamics was clearly
evident. A series of morphological parameters were extracted from the kinematic
GPS data to quantify dune migration rates, dune volumes, and dune reconstitution
time.
The measured migration rate of the parabolic dune head was 21.5 m/yr (mean value)
measured over 4 years, and 27.3 m/yr measured from a fifty-year sequence of orthorectified,
georeferenced, digitised aerial photography. The head of the dune
reconstitutes at the rate of 8.9 years, with a volume of 100,800 m3.
Volumetric change for the study site over an interval of thirty months was in the range of
656,400 m3. The majority of sand transport is generated by moderate velocity wind
flows which have a higher frequency of occurrence. Forty one percent of wind events
occur in the moderate velocity category (4 to< 12 m/s), twenty seven percent occur
in the high velocity category of> 12 to> 20 m/s. Measurement of flow close to the
ground on windward slopes indicated that shear stress does not progressively
increase from toe to crest of slope due to interaction between flow and bed form.
Some seventy percent of wind events are topographically aligned; across dune flow
from the SW sector results in lateral migration of the trailing arms, with the smaller
south trailing arm migrating ENE at more than twice the rate of the north trailing
arm. This rate of lateral displacement maintains the 3: 1 length to width ratio of the
asymmetric parabolic dune by maintaining the dimensions of the deflation plain
which in turn maintains the dynamic equilibrium of the aerodynamic envelope of the
dune form.
This integrated approach has provided quantitative evidence that demonstrates the
dynamic feedback between the prevailing wind regime and seasonal and discrete
wind flow events on the dune bed, and the influence of atmospheric conditions on
dune mobility.

Item Type: Thesis (PhD)
Keywords: Sedimentology, Coast changes, Sand dune conservation, Wind erosion
Copyright Holders: The Author
Copyright Information:

Copyright 2006 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Available for library use only but NOT for copying until 13 September 2007. After that date, available for use in the Library and copying in accordance with the Copyright Act 1968, as amended. Thesis (PhD)--University of Tasmania, 2006. Includes bibliographical references. Ch. 1. Introduction -- Ch. 2. Theoretical context -- Ch. 3. Grain-size analysis and composition of parabolic dune sediments -- Ch. 4. Wind regime, atmospheric parameters and wind flow pattern over a long-walled parabolic dune -- Ch. 5. Flow and morphodynamics -- Ch. 6. Conclusions

Date Deposited: 03 Feb 2015 03:05
Last Modified: 10 Apr 2016 23:44
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