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Controls on the Morphology of Fluvial and Tidal Influenced Channels in the Gulf of Carpentaria, Australia

Rosemary N. Okafor

Master of Science (Petroleum Geoscience)

Australian School of Petroleum

The University of Adelaide

Abstract

The influence of wave, tide and fluvial processes interact to control sediment erosion, transport and
deposition in clastic coastal environments. This results in mixed-process coastal systems, which can
form good quality, yet geometrically complex reservoir deposits. Channels in modern analogue
mixed process environments provide useful insights into how wave, tide and fluvial processes affect
paleochannels and can aid subsurface stratigraphic correlation and palaeoenvironmental
reconstruction.

The Gulf of Carpentaria is an epicontinental sea which formed as a result of sea level rise during the
last marine transgression. Holocene deposits have prograded over low gradient bathymetry and
display a range of channel forms. The channels that traverse these coastal plains cannot be
satisfactorily characterised by simple morphological classification. They have morphologically
complex patterns which have developed in response to the seasonal variations in fluvial, tide and
wave energy.
A numerical examination of the effects of fluvial, tidal and wave energy on the geometry and
morphology of 70 single and distributary channels in the Gulf Carpentaria was undertaken using
desktop based statistical analyses. Catchment area was used as a proxy for fluvial discharge in these
largely ungagged catchments while wave height and tidal range was applied directly from available
gauged data. These channels were divided into two subsets based on whether their channel mouths
were best described by linear or exponential trends and by the channels being either distributary or
single channels. These four groups were then tested separately to assess their geometrical
characteristics, including the rate of decrease in channel width downstream of equi-width (fluvial)
reaches, and the wave, tide and fluvial ratios.

Of the analysed channels 65% where found to have a linear width profile, and their parallel banked
profile was further enhanced by a strong positive correlation with catchment which exhibited very
low slope of distance to equi-width. These parallel banked (linear) channels were found to be fluvial
dominated while the other 35% had an exponential width profile with their entrance width visually
exhibiting a strong funnelling characteristic, and the channels having a wider entrance width and a
longer distance to equi-width. These exponential channels are more tidally dominated. There was a
general decrease in tidal energy and wave influence with increase in distance to equivalent width in
the channels, this relationship implies that a channel's distance to equivalent width can be used to
identify where the fluvial energy is least effective moving downstream indicating that another
process is dominant which is usually tidal or in some cases wave energy.

It is also shown that there are characteristic relationships between geometric variables like entrance width at the channel mouth and the rate of decrease in channel width downstream of equivalent -width (slope) with proxies of fluvial influences and tidal range. These relationships may overlap as a result of the complex morphodynamic feedbacks between fluvial discharge, tidal propagation and wave height in this tropical monsoonal environment.

Australian School of Petroleum
THE UNIVERSITY OF ADELAIDE

SA 5005 AUSTRALIA

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