SPECIAL EDITION NEWS RELEASE
One mouth for Lake St Lucia system
Background Information Document BID 2012/09
iSIMANGALISO WETLAND PARK AUTHORITY
The strategy to allow the uMfolozi and Lake St Lucia estuary mouths to join to form a combined mouth is progressing well. The linking of the two systems via the beach spillway resulted in approximately 16.4 billion litres of fresh water reaching the Lake St Lucia estuary and lifting water levels at the end of the dry winter period. The early spring rains and natural breaching of the uMfolozi mouth, which resulted from the one-in-five-year flood, have created a marine connection for the system. The recent events are both natural and positive and are part of the much broader long term strategy to restore estuarine function to this important nursery for fish and invertebrates. Key to this is to allow these systems to function as naturally as possible. With the Lake St Lucia and uMfolozi systems joined, modelling shows that their combined mouth will be open more often than it is closed.
However, it is unlikely that there will be a rapid recovery of the Lake St Lucia system - one of South Africa’s most important estuarine systems and Africa’s largest estuarine lake (approximately 32 000 ha). Together with reduced water inflow from nine years of below average rainfall, the lake has had little or no water from the uMfolozi river for the past 60 years. The Lake St Lucia system requires large volumes of water before it is able to function within a range considered to be natural and indicative of a healthy system. Over the next two to three years the level of the water in Lake St Lucia will be highly dependent on rainfall, the amount of which will determine how quickly the level rises. This means that a large flood could fill the system quickly or that under average rainfall conditions, it could take a number of years.
Underpinning this management strategy implemented by iSimangaliso in partnership with Ezemvelo KZN Wildlife, is the work undertaken through the iSimangaliso GEF project. In its final stage, the Project is investigating and formulating a long term solution to restore, as far as possible, the natural hydrological and ecological functioning of this important system. The aim is the ecological restoration of Lake St Lucia and the protection of iSimangaliso’s World Heritage Site values such that ongoing human intervention is no longer needed or minimized. Phase 1 of the GEF project included a scientific review workshop attended by 41 scientists including hydrologists, estuarine ecologists, estuarine hydrodynamic specialists, social scientists and geologists, and stakeholder and public meetings. This research collaboration has already paid off with the new science guiding the ‘combined mouth’ strategy for Lake St Lucia that is currently being implemented.
iSimangaliso will continue to interact with and keep stakeholders and interested parties informed of the state of Lake St Lucia and progress made with the GEF Project. (For more information visit www.iSimangaliso.com and download ‘Lake St Lucia: understanding the problem and finding the solution’.)
Andrew Zaloumis, Chief Executive Officer, iSimangaliso Wetland Park
Beach spillway transfers water
The beach spillway, created to expedite the reconnection of the uMfolozi River with the Lake St Lucia estuary, was opened on 6 July 2012. (For more information on iSimangaliso’s strategy to let the uMfolozi River and Lake St Lucia rejoin refer to the textbox at the end of the document: ‘2011 New lake St Lucia Estuary Management Strategy’).
Between 6 July and 7 September 2012, the water flow through the spillway, as well as the water levels and salinity within the Lake St Lucia estuarine system have been monitored closely by park staff. The monitoring has indicated that the spillway was extremely successful in transferring a large volume of water from the uMfolozi catchment to Lake St Lucia while the mouth was closed.
Plate 1: The beach spillway transferred an estimated 16.4 billion litres of water from the uMfolozi River to Lake St Lucia over 8 weeks.
The amount of water transferred by the spillway during this period was dependent on rainfall in the uMfolozi catchment and by the end of winter the volume of water flowing down the uMfolozi river was significantly reduced. Analysis of the flow data has enabled us to estimate that the total volume of water transported by the beach spillway was approximately 16.4 billion litres while the uMfolozi river mouth was closed.
Table 1: Total volume of water flowing into Lake St Lucia through the spillway during the closure of the uMfolozi mouth
The volume of water transferred through the spillway raised the water level in the Lake St Lucia Narrows by 33 cm. Once levels were raised, the water began to flow northwards from the 20 km long Narrows into Makakatana Bay and the southern portion of the lake.
Spring rain
Early spring rains began with an intense and rapid-forming cut-off low pressure system centred over the eastern part of the country on 5 September 2012 (see weather map below).
Figure 1: Rainfall forecast for 5 September 2012 from SAWS
This weather system resulted in significant amounts of rain falling throughout the park during 4 – 7 September 2012 (see table 2 below).
The uMfolozi River water level rose rapidly and brought increased volumes of water through the spillway for a period of 24 hours before breaching directly into the sea. The spillway was substantially widened and deep water flowed bank-to-bank into Lake St Lucia Estuary (see plate 3 below). By uMfolozi standards this event was a relatively small flood (a so-called one-in-five year flood). The steady rain falling over the catchment lifted the uMfolozi water levels over a 24 – 36 hour period. Rather than bursting out to sea along the most direct route, the rising water levels overtopped the beach sand barrier at a low point and the river opened to the sea in the early hours of 8 September 2012 (see plate 4 below).
Plate 2: The beach spillway was substantially widened by the early spring rains. It connects the uMfolozi, Lake St Lucia and the sea. This photograph was taken during the spring high tide on 17 September 2012
Once the uMfolozi River breached, water flowed southwards from the Estuary through the beach spillway and out through the open mouth. This occurred until the water level in the Estuary dropped to levels below the base of the beach spillway. In the final account, the water level in the Narrows has risen by a total of 25cm, with a corresponding increase in the rest of the Lake, and levels are the highest they have been since November 2011. In this configuration it is likely that the beach spillway will continue to intermittently connect the uMfolozi, Lake St Lucia and the sea. The extent to which this will occur is dependent on sea condition and wave action, the water levels in the Lake and the uMfolozi river and tidal influence.
Plate 3: The now-open uMfolozi River mouth has linked Lake St Lucia to the sea via the beach spillway.
Since these first spring rains broke the winter dry period further rains have been received over the weekend of 14–16 September 2012. These have resulted in good rainfall being received across iSimangaliso (see table 2 below).
Table 2: Early spring rainfall figures in iSimangaliso
Salinity values of Lake St Lucia
These two rainfall events have brought much needed water to fill wetlands and pans, and to recharge groundwater levels. Rivers like the uMkhuze, uMphathe and Nyalazi that flow directly into the Lake are also flowing strongly. Combined with the effect of direct rainfall on the Lake, salinities throughout the system have decreased substantially with the highest salinity values in the north of the system at 53 parts per thousand (Listers Point) and 2 parts per thousand in the Narrows.
Figure 3: Salinity values of the Lake St Lucia system from December 2011 to 14 September 2012
Estuarine functioning
The current configuration of the uMfolozi and Lake St Lucia mouths, connected by the beach spillway, marks the beginning of a period of marine connectivity. Sampling carried out on either end of the spillway by researchers from the University of Zululand in August 2012, confirm this. Juvenile fish that entered the uMfolozi River through its open mouth last year were found to have moved through the beach spillway into the Lake St Lucia system. These fish species are estuarine dependent species that need estuaries to complete their life cycle. Prawns and estuarine dependent fish species like grunter, stumpnose and cob spawn at sea and come into the estuary when they are small juveniles (approximately half a centimetre in size). They leave the estuary as sub-adults. Estuarine residents like goby are spawned and remain in the estuary for their whole life cycle.
Park staff will continue to monitor the fishing effort in the mouth area and members of the public are requested to respect conservation measures implemented to assist the fish and invertebrate stocks to recover.
Birds
Biannual monitoring that takes place as part of the national Coordinated Water Bird Counts (CWAC) project shows that healthy numbers of many species of birds currently use the Lake St Lucia system, with 35 000 water birds counted during the last monitoring trip undertaken by Park staff.
Figure 4: Bird numbers across the St Lucia system from the July 2012 CWAC count
For example, flamingos are currently found in the northern parts of the lake in large numbers indicating the presence of shallow brackish water that supports the type of food they eat. White pelicans are also present in large numbers and have bred successfully with more than 700 large chicks found in this sheltered breeding colony.
Plate 4: Pelicans at Charters Creek
There is also a large number of bird species using the intertidal areas at the open mouth for feeding and resting. Migratory waders such as curlew sandpipers, whimbrels and ringed plovers have recently arrived from the Palaearctic for the summer. They are feeding on invertebrates living in the sand and mudflats, such as worms and small crustaceans. Pelicans and herons are feeding in the shallow water on fish and prawns. Large flocks of terns, predominantly swift terns but also including little terns, Caspian terns and the rarer vagrant Sooty terns are also found in large numbers. The sooty tern has drawn a lot of interest from bird-watching enthusiasts and has been reported on the South African Rare Bird report over the past few months.
Plate 5: Pelicans and terns at the mouth
Where to now?
The strategy to allow the uMfolozi and Lake St Lucia estuary mouths to join to form a combined mouth is progressing well. The linking of the two systems via the beach spillway resulted in approximately 16.4 billion litres of fresh water reaching the Lake St Lucia Estuary and lifting water levels at the end of the dry winter period. The early spring rains and natural breaching of the uMfolozi mouth, which resulted from the one-in-five-year flood, have created a marine connection for the system. The recent events are both natural and positive and are part of the much broader long term strategy to restore estuarine function to this important nursery for fish and invertebrates. Key to this is to allow these systems to function as naturally as possible. With the Lake St Lucia and uMfolozi systems joined, modelling shows that their combined mouth will be open more often than it is closed.
However, it is unlikely that there will be a rapid recovery of the Lake St Lucia system - one of South Africa’s most important estuarine systems and Africa’s largest estuarine lake (approximately 32 000 ha). Together with reduced water inflow from nine years of below average rainfall, the lake has had little or no water from the uMfolozi River for the past 60 years. The Lake St Lucia system requires large volumes of water before it is able to function within a range considered to be natural and indicative of a healthy system. Over the next two to three years the level of the water in Lake St Lucia will be highly dependent on rainfall, the amount of which will determine how quickly the level rises. This means that a large flood could fill the system quickly or that under average rainfall conditions, it could take a number of years.
Underpinning this management strategy implemented by iSimangaliso in partnership with Ezemvelo KZN Wildlife, is the work undertaken through the iSimangaliso GEF project that is investigating and formulating a long term solution for the hydrological and ecological functioning of the Lake St Lucia system. The aim is the ecological restoration of Lake St Lucia and the protection of iSimangaliso’s World Heritage Site values such that ongoing human intervention is no longer needed or minimised.
The first phase of this component is complete and involved the analysis of the possible management options that will promote restoration of the system. These options have been presented for comment and input to scientists and other stakeholders, such as local communities and ratepayers, the floodplain sugar farmers, the tourism operators, government, NGOs and other interested parties. The stakeholder events included a scientific review workshop attended by 41 scientists including hydrologists, estuarine ecologists, estuarine hydrodynamic specialists, social scientists and geologists.
The project is in its final phase, which includes the appointment of the team of specialists to provide detailed analysis of the selected options. This process will take ecological, social, financial, political and economic considerations into account, and will include environmental authorisation. The consultants’ work is expected to take 12 months. A portion of the GEF budget has also been set aside to finance the implementation of the preferred option. iSimangaliso Wetland Park staff and scientists will continue to monitor the status of the system and its response to the current management intervention
iSimangaliso will continue to interact with and keep stakeholders and interested parties informed of the state of Lake St Lucia and progress made. (For more information www.iSimangaliso.com and download ‘Lake St Lucia: understanding the problem and finding the solution’.)
No comments:
Post a Comment