Hydraulics of Estuarine Sediment Dynamics in South Africa. Implications for Estuarine Reserve Determination and the Development of Management Guidelines
Report No 1257/1/04
Dec 2004

Executive Summary

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 implications:
  1. the methodology used for the assessment of the reserve for each discipline must allow for such behaviour
  2. the proposed flow regimes must mimic natural or present day flows from low to high flows
  3. models must incorporate the variability and non stationarity of sediment transport data
  4. most of the sediment transport and channel forming process occur during medium to large floods.
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.

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:
  1. Literature survey of historical sedimentation problems experienced at estuaries in South Africa and elsewhere, with quantification in hydraulic terms of possible causes.
  2. Hydraulic description of estuarine 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.
  3. Quantification of marine and catchment sediment yield.
  4. Field work at three South African 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.
  5. Mathematical modelling to simulate long-term and short-term estuarine hydrodynamic and sediment transport processes.
  6. Guidelines for the determination of the estuarine reserve to maintain a long-term morphological equilibrium
The key conclusions and recommendations are:

a)    Net imports of marine sediments under tidal influence

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 tidal influence.

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.

Conclusions:
Recommendations:

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.

Conclusions:
Recommendations:

c)    Exports of sediments during mouth breachings

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: 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 estuary.

It is recommended that the following aspects are included/considered in the protocol:
f)    Recommendations for future research

The following are considered important for future research related to sediment dynamics: