Wind induced sediment re-suspension in a shallow lake.
Loading...
Date
2011
Authors
Pringle, Justin James.
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
Wind induced turbidity within shallow lakes can greatly affect the biological functioning
of a system in either a positive or negative manner. This research aims to understand
and model the physical processes that cause sediment re-suspension. Lake St Lucia
on the east coast of South Africa, a UNESCO World heritage site was used as a case
study. Lake St. Lucia is a shallow water system which commonly experiences high
levels of turbidity. Coupled with the naturally shallow depth of the lake, it is currently
drought stricken, resulting in abnormally low water levels. A simple model has been
developed which accounts for sediment re-suspension due to wind-driven waves and
their associated bed shear stresses. The wave heights within a shallow lake such as St
Lucia are controlled either by the fetch (for a large water depth), or the water depth (for
a large fetch). When the wind is strong enough, the wind-driven turbulent mixing
causes the water column to become fully mixed. When the wave-driven boundary layer
becomes turbulent, sediment, being entrained within the water column increases
significantly. The model also accounts for the effects of temporal consolidation on the
re-suspension of sediments by setting a time scale for the erosion processes. It was
found that the median of the monthly turbidity levels over the past ten years exceeded
the average turbidity levels over the past 92 years. In all cases it was shown that mouth
linkage with the uMfolozi resulted in lower turbidity levels than without any linkage due
to the higher average water levels.
The model was then developed to predict the spatial variation in turbidity within the
Southern Lake. This was achieved through the use of existing bathymetric data for the
Lake. This spatial model was then used to show how the turbidity varied for different
wind and water depth conditions. Two conditions were considered, a NE and SW wind
blowing at 8m/s for water levels of 0 EMSL and -0.5 EMSL. The spatial model showed
that a decrease in water level increases the turbidity within the lake significantly. The
wind directions appeared to yield similar results of sediment re-suspension. It was also
shown that the high turbidity values were situated in the shallow depths even though
the wave heights were small in comparison to those in deeper water.
Description
Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2011.
Keywords
Turbidity--KwaZulu-Natal--Saint Lucia, Lake., Sedimentation and deposition--KwaZulu-Natal--Saint Lucia, Lake., Wind waves--KwaZulu-Natal--Saint Lucia, Lake., Theses--Civil engineering.