Aurecon Water Resources Management experts Nico Rossouw and André Görgens discuss the mechanisms that lead to the switch and how it can be prevented by developing an understanding of the hydrodynamic behaviour of shallow reservoirs through hydrodynamic water quality modelling.
Shallow lakes or reservoirs generally exist in either of two stable states; a clear water state dominated by rooted water plants, or a turbid state dominated by free floating algae. A dramatic event can switch a shallow reservoir from one state to another. Voëlvlei Dam, a relatively shallow off-channel storage dam in the Berg River catchment, supplies water to Cape Town and towns in the Swartland and West Coast area of the Western Cape, South Africa. The dam switched from a stable, clear water system to a turbid, algal dominated system when it was severely drawn down during the drought in the mid-2000s.
Voëlvlei Dam was the first large water supply scheme that was developed in the Berg River. The first Voëlvlei scheme was completed in 1953 when the natural Vogelvlei Lake was impounded by building a small wall structure. The natural wetland had a very small catchment of only 40 km2 and additional water was therefore diverted from the Klein Berg River, where a small weir was built, into a canal to the dam.
In 1971, the dam was raised to its present full supply capacity of 172 million m3 (DWAF, 1994). Voëlvlei Dam is currently supplied by runoff diverted through a system of canals from the Klein Berg River, Twenty-four Rivers and Leeu River catchments, (Figure 1).
Figure 1: Schematic representation of Voëlvlei Dam and the water transfers into the dam.
The dam supplies water to the Cape Town metropolitan area, to towns in the Swartland district, and some irrigation water for downstream users. The water for the Swartland Scheme supplies Riebeeck Kasteel, Riebeeck Wes and Malmesbury, while the Voëlvlei Water Treatment Works supplies the City of Cape Town (DWAF, 1992). Irrigation water is released into the Berg River along with water for the Withoogte Scheme which is then abstracted further downstream at Misverstand Weir (DWAF, 1994) for supply to towns of the West Coast district.
It appears that the low water levels during the drought of 2004/2005, and high wind mixing and re-suspension of bottom sediments, resulted in a substantial increase in turbidity in the dam.
The increase in turbidity created unfavourable underwater light conditions for rooted water plants and their numbers started to decrease. The turbid conditions also created unfavourable conditions for predatory smallmouth bass in the dam because they depend on clear water to see their prey. At the same time the elevated turbidity favoured the increase in the number of bottom feeding fish such as carp and catfish. Bottom feeders churn up the bottom sediments when they forage.
Wind mixing during the summer months is then sufficiently strong to transport the re-suspended sediment back into the water column and to reduce the rate at which sediments settle out, thereby maintaining the dam in a turbid state. Re-suspended sediment also provides a mechanism to return nutrients that have settled to the bottom, back into the water column. In the surface layers of the dam phytoplankton, rather than rooted water plants, now utilise the high nutrients leading to elevated algal concentrations in the surface waters.
A return to water levels that were common prior to the 2004/5 drought has not resulted in a return to turbidity levels that were observed prior to the drought. The major change that took place after the drought was the shift in fish species to bottom feeders and the reduction in rooted water plants. The presence of large numbers of bottom feeding fish (carp and catfish), the absence of rooted water plants, and wind mixing now appears to maintain the dam in a stable turbid state.
One of the strategies to return Voëlvlei Dam to a stable, clear water state is biomanipulation. In this case it might require the removal or aggressive harvesting of bottom feeding fish to break the first link in the sediment re-suspension chain. At the same time, water levels should be maintained above 60 per cent of the full supply level or at least not be allowed to drop to levels that were experienced during the 2004/5 drought.
Restoring water clarity would stimulate rooted water plants as sunlight would start to penetrate to the bottom of shallower areas. The rooted water plants would start to compete with free-floating algae for the available nutrients. It would also provide protection for juvenile bass and may even restore bass numbers to levels where they can start to control carp and catfish numbers. Rooted water plants would also mitigate against wind re-suspension of sediments in the shallow areas of the dam.
It appears that there is tipping point beyond which a shallow reservoir can switch from one stable state to another and that there are buffers that maintain it in a specific state. Voëlvlei Dam is a good example of what such a switch might be (low water levels and high wind mixing) and what buffers (change to bottom-feeding fish species) may maintain it in the new state.
It is only by understanding the hydrodynamic behaviour of a shallow reservoir that one can predict what these switches and buffers are. Complex hydrodynamic modelling (Kamish et al., 2007) is one of the tools that water resource managers can employ to develop such an understanding and to prevent permanent changes in the quality of a dam that can have high cost implications to bulk water suppliers.