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The influence of advection on water quality variation in a deep Australian impoundment

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Ferris, J M(John Murray) (1985) The influence of advection on water quality variation in a deep Australian impoundment. PhD thesis, University of Tasmania.

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Abstract

Data spanning twenty years (1961-1980), for a site adjacent to the dam
wall in Lake Burragorang (34° 55' S; New South Wales, Australia), is analysed
with the emphasis placed on physical and chemical (water quality) records.
Lake Burragorang is a deep (105 m), dendritic (shoreline development 11.53),
essentially warm-monomictic lake with a median bulk retention time of 1.8
years. Inflows are unpredictable seasonally and annually, however, and the
study included years with retention times ranging from 0.5 - > 22.0 years.
The aim of the investigation was to assess the long-term behavioural
variation and to explain the major factors contributing to that variation.
The cycles of thermal and oxygen stratification are profoundly influenced
by inflow/outflow relations. The monomictic cycle is interrupted in c. 50% of
years, by cold underflows which help to prevent the short period of
circulation (1 - 2 months) which occurs in other years. A significant role is
almost certainly played by the mid-depth outflow current, which is presumed
to interfere with convective mixing deeper than c. 44.5 m (below Full Supply
Level). The sub-surface off take is not used in dry years.
A seasonal cycle of inflow is found for the lake, and the interchange
between interflows and under low is predicted with about 80% success on the
basis of 4 temperature measurements, from the two major inflowing rivers
and the surface and bottom adjacent to the dam. A comparison of monthly mean profiles of temperature and dissolved
oxygen from wet and dry year groups (differing in total annual inflow by
approximately one order of magnitude) reveals that advective processes
increase the summer heat income and help to distribute heat more deeply into
the water column. Subsequently, these processes increase the rate of
autumnal heat loss, yielding an almost equal winter heat content and
effectively increasing the annual heat budget by 22% of the dry year budget.
The downward movement of heat is found to be most closely related to the
volume of water subtracted through the sub-surface offtake, rather than to the volume of influent water. Within the accuracy of profiling (6 m intervals)
the wet and dry years appear not to differ with respect to the depth of the
convectively mixed layer during the cooling phase (March - June). The summer
maximum of Schmidt stability is lowered by only 1% in wet years, but
autumnal stability is lowered by up to 33% while, in winter, stability is
enhanced by > 100% of the dry year stability. The maximum Birgean wind-work
increases by 100% in wet years as a result of advective effects which are
outside its conceptual framework. The volumetric hypolimnetic oxygen
depletion rate is doubled in wet years.
Turbidity is linearly related to the volume of underflows, but the
relationship is much weaker with respect to interflows which generally
occasion lower levels of turbidity (for a given inflow volume) at the outflow
site.
Lake Burragorang is phosphorus limited and a close relationship is found
between total phosphorus and chlorophyll concentrations.
Inflow and outflow are considered the major contributors to the variation
In Lake Burragorang's behaviour for the period 1961 - 1980, altering thermal
and oxygen stratification behaviour, markedly affecting water quality, and
Introducing the nutrients required to support algal growth above the usually
low levels of dry years (1 - 5 mg m-3 of Chlorophyll-a). No obvious period of
trophic upsurge is found for the lake, and the data presented does not indicate
any significant trend towards eutrophication, though such trends will be
difficult to determine against the advection induced variability of the system.

Item Type: Thesis (PhD)
Keywords: Water quality, Lakes, Water temperature
Copyright Holders: The Author
Copyright Information:

Copyright 1985 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:

Thesis (Ph.D.)--University of Tasmania, 1986. 11 transparencies in back pocket. Includes bibliography

Date Deposited: 09 Dec 2014 00:15
Last Modified: 16 Aug 2016 01:25
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