Rainfall
Plate 2-8: Depletion and accretion as observed on Mhlanga beach
2.10. Studies relating flow and mouth state
To ensure the demands on South African water resources are satisfied, both in terms of quality and quantity, for human requirements and ecological sustainability the National Water Act (Act 36, 1998) was introduced. Jezewski, Pyke and Roberts (1984) reported one of the first attempts to quantify the freshwater requirements of estuaries in South Africa. Jezewski used the evaporative requirement and the flooding requirement of each estuary as a basis for predicting the freshwater required for the abiotic functioning of each estuary. The evaporative requirement was defined as the volume of water required to replace the water lost by the estuary through evaporation and thus prevents the occurrence of hypersaline conditions. The flooding requirement was defined as the volume of water required to periodically breach the estuary, flush accumulated sediments from the system, and flood the wetlands around the margins of the system. It was argued that this could be estimated using the 2-year return period flood hydrograph for the system. The calculated freshwater requirements were generally less than 6%
of MAR, with details of individual estuaries given in appendix B. These estimates are significantly lower than the requirements inferred using RDM procedures, that take a holistic approach incorporating both the abiotic and biotic functions of estuaries. This recent approach is outlined in CSIR, (2002). The RDM approach views a reduction of freshwater inflow as a threat to the natural functioning of the estuarine system, ultimately affecting the biodiversity of an estuarine system.
During the past two years, the RDM methodology has been applied to Mdloti and Mhlanga Estuaries (CSIR, 2002,2003). Some of the data used in the RDM study of Mhlanga Estuary included data from this current study.
The purpose of the RDM methodology is to determine the impact of changes in flow on the estuarine ecosystem and provide an indication of the health of the estuary in its present state relative a natural (reference) state.
2.10.1. RDM process
There are four levels of RDM studies, namely desktop, rapid, intermediate and comprehensive.
Table 2-6 presents the use of the various levels, with the different levels requiring different degrees of detail.
Table 2-6: The potential uses of the various levels of RDM (DWAF, 2003)
Level Use
Desktop estimate For use in Water Situation Assessment Model (WSAM) as part of the planning process only
Rapid determination Individual licensing for small impacts 10 unstressed catchments of low importance & sensitivity; compulsory licensing "holding action"
Intermediate determination Individual licensing in relatively unstressed catchments Comprehensive determination All compulsory licensing. In individual licensing, for large
impacts in any catchment. Small or large impacts in very important and/or sensitive catchments.
In tenns of flow and mouth state the RDM studies have def10ed four typical abiotic states. Each RDM study aims to provide flow ranges, for a particular estuary, for each state. Each state is then described in terms of the abiotic characteristics and processes and the occurrence of each state estimated using simulated monthly flows for the present state of the estuary. Table 2-7 defines the four abiotic states.
Table 2-7: Definition of the different estuary states (after CSIR, 2002)
State Name
Closed, where the estuary is cut-off from the sea with I
no water exchange.
Semi-closed, with a perched outlet channel but no 2 seawater intrusion (except occasional over-wash), and
with water still flowing out to sea 3 Open, with seawater intrusion
Open, with no seawater intrusion under high flow 4
conditions (river dominated), Le. completely fresh
The RDM also determines the reference or natural condition of an estuary using simulated data.
Comparing the present state of the estuary to the reference condition provides a method of determining health of the estuary relative to its natural state. This methodology is also used in the analysis of the effects changes in flow will have on the natural functioning of the estuary in question.
2.10.2. Mblanga Estuary
The Mhlanga Estuary was reviewed using the rapid RDM process (CSIR, 2003). AB part of this study the present and reference (or natural) conditions of the estuary were evaluated using flow duration curves from 75 years of simulated monthly flows. The occurrence of the four estuary states are tabulated in tables 2-8 and 2-9 for the present and reference conditions respectively, with the estimated threshold flows as presented in CSIR (2003). CSIR (2003) determined the percentage of time the estuary was in each state by combining the threshold flows and the flow duration curves. Note that data collected as part of this current study contributed towards the RDM study.
Table 2-8: Mouth state proportions under present state conditions (after CSIR, 2003) Threshold
State Percentage
Flows(m3/s)
Closed <004 49
Semi-closed 004-0.5 26
25 Open - estuarine 0.5-5.0
Open - fluvial >5.0 < 1
Table 2-9: Mouth state proportions under reference state conditions (after CSIR, 2003) Threshold
State Percentage
Flows(m3/s)
Closed <004 82
Semi-closed 004-0.5 3
Open - estuarine 0.5-5.0 15 Open - fluvial >5.0 <1
The results from CSIR (2002) reproduced in tables 2-8 and 2-9 indicate that the Mhlanga Estuary was naturally closed for approximately 82% of .the time however under present conditions it is only closed approximately 49% on average. The reduction in the percentage of time the estuary is closed on average can be attributed to the increase in flow due to the addition of treated effluent into the estuarine system.
2.10.3. Mdloti Estuary
The rapid RDM method was used to assess the Mdloti Estuary by CSIR, 2002. The estimated threshold flows used to defme the four biotic states are tabulated for present conditions and reference conditions in tables 2-10 and 2-11 respectively. The percentages given for each state were determined by CSIR (2003) by combining the threshold flows and flow duration analysis based on 75 years of simulated monthly flows.
Table 2-10: Mouth state proportions under present state conditions (after CSIR, 2002) Threshold
State
Flows (m3/s) Percentage
Closed <0.3 45
Semi-closed 0.3-2.0 28
Open - estuarine 2.0-5.0 13
Open - fluvial >5.0 13
Table 2-11: Mouth state proportions under reference state conditions (after CSIR, 2002) Threshold
State
Flows (m3/s) Percentage
Closed <0.3 2
Semi-closed 0.3-2.0 61
Open - estuarine 2.0-5.0 20
Open - fluvial >5.0 17
CSIR (2003) indicate that the flows at Mdloti Estuary have been reduced between reference and present conditions from 98.7 to 72.3 million m3 per year. This lowered flow rate is a combination of the effect of the construction of Hazelmere Dam and the abstractions for irrigation. From tables 2-10 and 2-11 it can be deduced that with the reduction in flow there was a large increase in percentage of time the estuary is closed.
2.10.4. The Great Brak
Another example of a study conducted on an estuary, was Huizinga (1995) on the Great Brak Estuary. The need to study and develop a management strategy for the Great Brak Estuary arose with the construction of the Wolwedans Dam 2km upstream of the estuary. Huizinga (1995) identified that the mouth state would be greatly impacted by the reduction of streamflow resulting from the dam. The Great Brak was expected to remain open during summer as the prevailing wave conditions in the area are low whilst in winter high waves close the estuary.
Typical prolonged periods of open conditions were expected at the Great Brak Estuary during summer, therefore management of the flows in summer is important in order to imitate the natural behaviour of the estuary. Should the flows be greatly reduced the estuary will not open as regularly causing problems for fish migration, poor water quality and is unattractive for tourists.