Modelling the physical dynamics of estuaries for management purposes.
Date
1996
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
South African estuaries are characterised by highly variable inflows owing to the semi-arid nature
of the land mass which they drain. The interaction of this variability with that of the marine
environment (seasonality, high wave events, synoptic effects) gives rise to the distinctive
character of South African estuaries. In general, they are small, micro-tidal, bar-built systems
with strong flood tidal dominance. Approximately half of the 273 systems along the coast
exhibit intermittent closure of the mouth, while a number can become hypersaline during dry
periods. In view of the increasing development pressures on the rivers and estuaries of South
Africa and their strong dependence on freshwater flow for the maintenance of their character and
functioning, and the need for justifiable, scientifically-based decision making regarding the
freshwater requirements of estuaries is evident.
This study was initiated to address this issue by first developing a model to simulate the physical
dynamics of South African estuaries over time scales from months to years, so enabling
prediction of the medium to long term consequences of alterations in the freshwater inflow on
the abiotic components of an estuary. Thereafter, the efficacy of management policies involving
water releases and mouth breachings could be evaluated in terms of their success in maintaining
the character and functioning of an estuary.
A semi-empirical estuarine systems model incorporating seven state variables, namely water
volume, salt content, stratification, circulation, tidal flushing, freshwater flushing and the height
of the sill at the mouth, was formulated and implemented on two case studies. Estuarine physics
concepts were incorporated dynamically in the model in a novel manner. For instance, the bulk
densimetric Froude number and the Estuarine Richardson number are used in the simulation of
the stratification-circulation states, while the Ackers and White sediment transport formula was
modified to yield results which agreed with field observations of the closure and breaching of the
mouth of the Great Brak Estuary. Additionally, tidal exchange through the mouth was modelled
phenomenologically and successfully calibrated against observations for both case studies.
Model results were found to be fairly robust to uncertainties in parameter values. However, most
encouraging of all is that behaviour known to occur in shallow estuaries, such as modulation of
the n11.:.m water level by low frequency forcing and the generation of overtides, was reproduced by the estuarine systems model although it was not specifically included in the model
formulation. The model is thus considered to reliably predict the physical dynamics of South
African estuaries over time scales of months to years.
A number of management policies involving freshwater allocations, water releases and
breachings of the mouth (where appropriate) were tested on the two case studies, namely the
Great Brak Estuary, a small, temporarily open system, and the permanently open Kromme
Estuary. The results indicate an increase in marine dominance as freshwater flow to the estuaries
decreases. The variability in the estuarine environment declines and the systems become more
inert to freshwater flooding and more sensitive to marine forcing. By applying the estuarine
systems modelling approach, the performance of different management policies could be
evaluated in comparison with reference policies. Accordingly, for both case studies, preferred
management policies which utilize the present total annual allocations to the estuaries more
beneficially could be indicated. Further management applications included the use of the
estuarine systems model in a linked system of abiotic and biotic models to facilitate more
comprehensive prediction of the consequences of freshwater abstraction and so more informed
assessment of estuarine freshwater requirements. The estuarine systems model results were
critical in enabling the prediction of the faunal and floral responses in the intermittently closed
Great Brak Estuary as it is presently the only abiotic model capable of simulating the closure and
breaching of the estuary mouth over a number of years. It is anticipated that further
developments will occur in biological prediction in the near future and that this could require
developments or adaptations to the estuarine systems model, particularly when details of the type
of information required for biological prediction becomes known. Additionally, the use of the
estuarine systems model in a strategic management sense is suggested. It could play a role as a
screening tool for regional water resource planning, while the preliminary quantification of the
extent of anthropogenic influence in expediting the movement of estuaries towards the later
successionary stage of a coastal lagoon is a powerful indication of the level of prediction which
could become possible in the future. Thus enhanced management decision making is now
possible on a site specific basis and at a more strategic water resources planning level.
Description
Doctoral Degree. University of KwaZulu-Natal, Durban.