Hydraulics of Estuarine Sediment
Dynamics in South Africa. Implications for Estuarine Reserve
Determination and the Development of Management Guidelines
Report No 1257/1/04
The South African climate oscillates between drought and flood. This
leads to extremes in river flow. While storms last for minutes to days,
the hydrological critical low flows can last for years during droughts.
The strong variation in fresh water flow to estuaries has the following
Estuaries are complex water bodies and differ considerably from fluvial
systems. In estuaries the flow reverses due to the tidal currents and
depth depends primarily on the tides and not the flow. An estuary has
two sources of sediment: the river during floods and the ocean that
supplies marine sediment through littoral drift which is transported by
tidal currents into the estuary. Water quality charges in an estuary
are also complex due to both upstream and downstream sources.
Oversimplified models cannot be used to investigate the hydrodynamics
and geomorphology of an estuary due to its complexity. Findings of a
study at one estuary can also not be easily transferred to another due
to the unique nature of every estuary.
- the methodology used for
the assessment of the reserve for each discipline must allow for such
- the proposed flow regimes
must mimic natural or present day flows from low to high flows
- models must incorporate
the variability and non stationarity of sediment transport data
- most of the sediment
transport and channel forming process occur during medium to large
In the past it was often believed that floods larger than the 1:2 year
flood will not be impacted on by dams in the catchment. This is however
not true and in many cases flood attenuation will occur due to the
relatively large storage capacity: mean annual runoff (MAR) ratios of
many of the South African dams.
Simulated monthly flows cannot be used to interpret the effect of a dam
on floods (high flows). Even if the monthly data indicate little change
between natural and post-dam conditions, flood attenuation can still
play a major role in reducing flood peaks during storms and thereby
changing the sediment transport capacity of the flood flow.
Flood peak attenuation is only one of the impacts of a dam on the
downstream river and estuary system. Sediment trapping in a reservoir
upstream of an estuary has often not been considered in the sediment
transport calculations, but is as important as the flood attenuation
effect of a dam.
Sedimentation of South African estuaries has created several
environmental and social problems. Sediment transport imbalances have
been caused by changes in the river catchments such as increased
sediment yields and flood peak attenuation due to dam construction.
Historically floods used to flush estuaries to maintain the long-term
sediment balance in the river-estuary system, but with reduced flood
peaks, sediment transport capacities at the estuaries are reduced and
flushing efficiency decreased, resulting in the marine dominance in
many estuaries. In the long-term this may lead to the complete closure
of some estuaries.
The main objectives of this study were:
The study incorporated the following components:
- Hydraulic description of
sediment transport processes through the estuary during tidal cycles as
well as during floods (Chapter 4 and 5).
- Hydraulic description of
flushing efficiency of estuaries with or without mechanical breaching
- Development of guidelines to
determine and manage the estuarine reserve to ensure a long-term
equilibrium morphology (Chapter 8).
The key conclusions and recommendations are:
- Literature survey of
sedimentation problems experienced at estuaries in South Africa and
elsewhere, with quantification in hydraulic terms of possible causes.
- Hydraulic description of
sediment transport processes involved, such as sediment transport
capacity during tidal cycles and floods, critical conditions for
re-entrainment of sediment, bed roughness, flushing morphological bed
changes during floods and sediment deposition.
- Quantification of marine
- Field work at three South
estuaries (Goukou, Klein and Groot Brak) involving the measurement of
sediment transport, flow velocities and water levels at various
locations in the estuary during the tidal cycle to investigate the
movement of marine sediment in the estuary. Sediment characteristics
were analysed. In the case of mechanical breaching of the river mouth,
the flushing efficiency and extent were monitored by taking suspended
sediment samples with water level measurements. Estuary cross-sections
were surveyed before and after breaching. The data were used to
calibrate, verify and refine the hydraulic modelling techniques
described in (b) above.
- Mathematical modelling to
long-term and short-term estuarine hydrodynamic and sediment transport
- Guidelines for the
the estuarine reserve to maintain a long-term morphological equilibrium
a) Net imports of marine sediments under
Approach: by measurements of sediment transport through cross-section
during tidal cycle
Result: The results did indicate that the net upstream sediment
transport is small, but they also showed that sediment movements
through a cross-section during a tidal cycle are extremely complex and
it is impossible to achieve accurate results by direct measurements.
Such measurements are therefore, with present day instrumentation and
measuring techniques, not effective in quantifying the net imports of
sediments into an estuary with reasonable accuracy.
Approach: by modelling of the sediment transport in an estuary under
Result: It is concluded that the Delft3D-MOR and Mike 21C, both 2D
numerical models are appropriate tools for studying hydro- and sediment
dynamics in SA estuaries, as the models correctly reproduces water
levels, achieves good agreement with local water velocities and at this
interim stage appears to simulate sediment dynamics sufficiently well.
The modelling shows that the sediment balance in the estuary relies on
a subtle balance between dominant flood and ebb tide flows. It is
therefore not correct to simply conclude that sedimentation occurs
upstream due to the stronger flood tide since the cross-sections and
durations of the flow differ during the two tidal phases. Wave action
is an important stirring mechanism in the mouth and it is important to
include suspended sediment transport in the modelling. Although the
models performed very well, there are still additional processes to
include such as time varying roughness changes and cohesive sediments.
For long-term and long reach simulations, one-dimensional (or
quasi-two-dimensional) models will also be required in future.
- Net imports of marine
sediments under tidal conditions seem to be small, but they cannot,
with present day techniques, be quantified with sufficient accuracy by
direct measurements or by simulations with mathematical models.
- Further developments of
measuring and modelling techniques should be monitored to establish
whether sufficient accuracy can be achieved in the future.
- One indirect method to
quantify the upstream movement of marine sediment is by the undertaking
of bathymetric surveys at regular intervals. The undertaking of such
surveys is therefore recommended for important estuaries.
- Another indirect method to
establish whether shallowing is taking place because of ongoing
sedimentation is by continuous recording of water level variations. The
installation of water level recorders in important estuaries is
b) Exports of sediments during floods
Field measurements during floods: Opportunities to measure sediment
transport through crosssections directly during floods did not arise
during the study period. However, the results from the fieldwork
undertaken under tidal conditions indicate that it will be difficult to
measure sediment transport during a flood reasonably accurately.
Mathematical modelling: Results were obtained from simulations of
floods with 1-dimensional and 2-dimensional models. These
models were not calibrated for flood conditions, because of lack of
field data, but the results seem to be reasonable.
- Mathematical modeling can be
used to simulate the flushing of sediments during floods, but attempts
should be made to calibrate these models when adequate field data
become available in the future.
- The one- and two-dimensional
modelling has shown that floods play a very important part in estuarine
sediment transport processes. Large magnitude floods are capable of
removing vast amounts of sediment from an estuary and are necessary to
keep the upstream ingress of marine sediment and the accumulation of
catchment sediment in check. However, these floods have to occur at a
regular basis, or at least smaller floods occurring between the large
magnitude flood events, otherwise the sediment, especially cohesive,
will have time to consolidate. This means that it will be difficult to
remove these sediments. It was also found that during the falling stage
of a large flood, sediment is deposited in the estuary which should be
removed by following smaller floods.
- An indirect method to
quantify the flushing of marine sediment during floods is by the
undertaking of bathymetric surveys before and after floods. The
undertaking of such surveys is therefore recommended for important
- Another indirect method to
determine the flushing of sediments during floods is by the recording
of water level variations before and after floods. The installation of
water level recorders in important estuaries is therefore recommended.
- The effect of the
consolidation of cohesive sediments on the flushing efficiency of large
floods should be investigated.
c) Exports of sediments during mouth
The work undertaken to quantify the export of sediment from an estuary
during a mouth breaching included:
The main purpose was to quantify the export of sediments from the
estuary upstream from the mouth. Additionally, attempts were made to
show the benefits of breachings at higher water levels to flush more
sediment from the estuary.
- Field measurements
- Physical modelling
- Mathematical modeling
Field measurements: The field measurements included the measurements of
sediment transport through a cross-section upstream of the mouth and
the measurements of the scouring of the mouth itself. The
results from the measurements at the cross-section upstream of the
mouth indicated that it is very difficult to measure sediment transport
through a cross-section upstream of the mouth reasonably accurately.
The results obtained of the scouring of the mouth itself were more
accurate and indicated the importance of breaching at higher water
levels to scour more sediments from an estuary.
Physical modelling: Physical modelling was undertaken of the breaching
of an estuary mouth, which had similarities of the mouth of the Klein
River near'Hermanus. The scale of the model was 1:50. The main aim was
to illustrate the merits of the breaching at higher water levels. The
physical model data were also used to calibrate a mathematical model in
order to simulate actual field conditions with more reliability.
The physical model experiments have shown that the equilibrium mouth
width and depth are determined mainly by the maximum discharge during
breaching. The discharge on the other hand is determined
mainly by the water level in the estuary when breaching. Therefore, the
higher the water level in the estuary at the start of breaching, the
more efficient the breaching process will be.
Mathematical modelling: The data obtained from the experiments was used
to calibrate and verify a mathematical model Mike 21C. The mathematical
modelling of the breaching process at the Klein River estuary indicates
much the same as has been observed during numerous breachings in the
field, i.e. that breaching at higher water levels and towards the east
side are more effective.
d) Imports and exports of catchment
sediments over longer periods, including floods
At many South African estuaries the catchment sediment yield is much
higher than the marine sediment supply and dominates the fluvial
processes. It is therefore recommended that suspended sediment sampling
stations are established upstream of important estuaries. Small to
large floods are important in transport of sediment through an estuary.
e) Recommendations for inclusion in the
Reserve Determination protocols
The protocols for the undertaking of projects to determine the Resource
Direct Measures (RDM), normally called The Reserve, in terms of the
National Water Act (No 36 of 1998) currently include only limited
details on a methodology to investigate the sediment dynamics of an
It is recommended that the following aspects are included/considered in
f) Recommendations for future research
- The geomorphology of an
estuary forms the basis of the physical characteristics of the estuary
to which the biological processes are related. It is therefore
important that the sediment dynamics is evaluated with suitable 1 D and
2 D hydrodynamic mathematical models.
- The methodology as proposed in
Chapter 6 should be incorporated. Calibration should be based on the
- Monthly or daily flow data are
not suitable to evaluate the sediment dynamics, because of the
importance of floods. Runoff scenarios should therefore be based on
observed data and adjusted records for future scenarios. Sediment yield
changes due to
dams with more water abstraction should be considered in future
- The rapid assessment should
include a specialist on geomorphology so that the
physic characteristics can be well defined.
- The geo-morphological
boundaries, specifically the lateral boundary considered in the
protocol to be the 5 m (MSL) contour along each bank, should rather be
based on a floodline such as the 1:100 year flood.
- The runoff from a catchment
and classification of the sensitivity to reduce river flows such ;
mentioned in the protocol is too general and should not be used without
more details consideration of the sediment transport processes.
The following are considered important for future research related to
- The role and modelling of
cohesive sediment, including floods and tidal currents
- Effects of man-made
obstructions to the flow in an estuary such as causeways, bridges,
marina weirs, etc.
- Interrelationship between
vegetation, hydrodynamics and sediment transport
- Interrelationship between
abiotic and biotic components in an estuary
- Off-shore fresh water and
sediment requirements for prawn banks and fishing industry, related
- Development of management
guidelines for dredging in estuaries