Acknowledgements
3. The Zandvlei Catchment
3.4 Some brief catchment challenges
The Zandvlei catchment and estuary experience many challenges; some of the main catchment management issues are described in the following sections.
3.4.1 Salinity
Maintaining salinity levels is important for the role the estuary serves as a fish nursery. The history of mouth manipulation has resulted in the estuary being unable to naturally achieve the salinity levels required for fledgling fish species to grow (Quick & Harding, 1994). Historically, salinity has continuously fluctuated from the 1980s, with changes often linked to adjustments of the weir level (Hutchings et al., 2016).
3.4.2 Sedimentation
Sedimentation, largely from bank and bed erosion in response to urbanisation in the upstream catchments, is a long-established problem in the estuary; Decades ago, Morant & Grindley’s (1982) study pointed to the loose material from non-stabilised areas, such as building sites, infiltrating rivers and streams as the cause of sedimentation in the estuary. They predicted that continuing urbanisation in the catchment would only intensify the silt levels in the rivers – a prophetic statement considering Ninham Shand’s (2000) report (as cited in DEA&DP, 2018) that indicated that between 1988 and 2000, 44 000 m3 of sediment had been deposited in the estuary.
For that 12-year period, this equated to 77 mm across the entire estuary area – higher than the 50 mm for the 12-year period between 1958-1970.
Besides upstream sediments, tidal inflows carrying sand from the sea increases the sediment load in the estuary’s outlet channel, where it gets trapped by the weir. Even though various interventions exist to mitigate the sedimentation from both upstream and downstream of the estuary – such as the extensive reed bed at the confluence of the Keysers and Westlake Rivers and a light dredging programme at the outlet channel – reducing sedimentation continues to be a key priority for the catchment (CCT, 2010; DEA&DP, 2011; Hutchings et al., 2016; DEA&DP, 2018).
3.4.3 Litter and debris
A problem that has continued to aesthetically plague the Zandvlei estuary is the stream of litter and debris that flows into the estuary, particularly in the rainy winter months. Litter enters the estuary from both the upstream rivers, particularly those draining low-income or informal residential areas, and along the banks of the estuary through recreational use (CCT, 2010). The prevalence of debris in the waterbody presents a danger to local flora and fauna in the form of entanglement and ingestion.
ZA Ghoor: Managing nutrient flows into the Zandvlei Estuary, Cape Town using Sustainable Drainage Systems (SuDS)
Chapter 3: The Zandvlei Catchment
Various measures have been instituted to control litter flow into the estuary, such as the littler trap on the Sand River Canal (Figure 3-4). This litter trap frequently gets blocked during a storm event, causing the canal to flood and the litter to overflow and thus flow into the estuary.
The ZPAAC recently commissioned the design of an improved litter trap, which has yet to be constructed (ZPAAC, 2018).
Figure 3-4: Blocked litter trap on the Sand River Canal during a storm
3.4.4 Nutrient content in rivers
With such a spectrum of land uses in the catchment, it is no wonder that elevated nutrient content is one of the more serious issues afflicting the rivers and the estuary.
Concerns arose as early as the 1970s when a monitoring of nitrogen and phosphorus in the estuary indicated that it was in a eutrophic state. More recent monitoring between 2000-2009 found a similar, though slightly improved, situation (CCT, 2010). Eutrophication in any waterbody is undesirable, but in one with the biodiversity and recreational value of the Zandvlei estuary, it could be catastrophic. The excessive plant growth caused by eutrophication blocks sunlight from penetrating the water surface, depletes oxygen levels in the water – affecting both aquatic plant and animal and life – and thus allows the plants to rapidly grow. In extreme cases
ZA Ghoor: Managing nutrient flows into the Zandvlei Estuary, Cape Town using Sustainable Drainage Systems (SuDS)
Chapter 3: The Zandvlei Catchment
toxins may be released, exacerbating this vicious cycle. Such was the case in 2012 when a
‘golden’ algal bloom (specifically Prymnesium parvum) left hundreds of dead fish washed up on the estuary shore, birds floating lifelessly on the water surface and the estuary itself in an inaccessible state (Isaacs, 2012). A similar bloom, resulting in yellow surface waters and gasping fish, occurred in 2017 (Lwandle, 2017; ZPAAC, 2017).
Increased nutrient levels produce other blooms in the catchment that, while less destructive than golden algae blooms, are still undesirable. The Westlake River experiences frequent hyacinth blooms (Figure 3-5), while the estuary and Marina da Gama canals often suffer pondweed overgrowths (Stuckenia pectinata, previously known as Potamogeton pectinatus) (DEA&DP, 2018). However, pondweed occurs naturally in the estuary, so its existence itself is not due to nutrient-enriched waters – rather, it is a vital component of the estuary’s ecosystem (Quick & Harding, 1994). This is explored further in the following section.
The possible sources of raised nutrients in the catchment are discussed in Chapter 5.
Figure 3-5: Water hyacinth bloom in the Westlake River at Kerner Close, facing Main Road (downstream)
ZA Ghoor: Managing nutrient flows into the Zandvlei Estuary, Cape Town using Sustainable Drainage Systems (SuDS)
Chapter 3: The Zandvlei Catchment
3.4.5 Pondweed management
Although managing the pondweed population is a continuous challenge in the Zandvlei estuary, it is an essential biotic component of its ecosystem responsible for maintaining good water quality. Besides providing feeding areas for juvenile fish and a variety of aquatic fauna, it oxygenates the water while reducing nutrient loads. Furthermore, it increases water clarity by reducing sediment resuspension caused by wind or recreational activities (Quick & Harding, 1994). However, these benefits only occur when the pondweed population is at an optimum level.
When there is an overgrowth of pondweed, dense plant mats form that have typical eutrophication effects: limited sunlight penetration, unpleasant odours and raised nutrient levels.
In the Zandvlei estuary, excessive pondweed growth also hinders boating activities – one of the more popular recreational uses of the estuary – as well as restricting the flow of current, which adds to the existing problem of circulation in the Marina canals.
Currently the pondweed is managed by mechanical harvesting; but occasionally too much pondweed is removed causing an imbalance in the estuary’s ecosystem (CCT, 2010).
3.4.6 Sewer spills
The estuary repeatedly undergoes closure on account of sewer spills, largely caused by faulty or vandalised pump stations and/or power failures. There are 14 pump stations in the areas around the estuary which regularly overflow. The pollution from overflowing pump stations is exacerbated by overflowing manholes caused by sewer blockages as a result of incorrect disposal of solid objects (DEA&DP, 2018). These spills flood the estuary E. coli and other pathogens that are harmful to human health – at times posing a risk considering the frequent recreational use of the estuary. One site that frequently experiences sewage overflows is Pollsmoor Prison, which drains into the Westlake River (Source-to-Sea, 2016).
Currently these spills are managed by estuary closure and regular monitoring, as well as pump station maintenance.
3.4.7 Urban runoff
Stormwater drains carry urban runoff directly into the Zandvlei estuary, carrying pollutants collected all over the catchment into the estuary. Runoff from the various land uses in the catchment are highlighted by many studies as sources of raised nutrients in the estuary. Particular regions of concern are the agricultural areas on the western slopes of the catchment, the industrial area in the lower reaches of the Keysers sub-catchment, the two golf courses in the study area and the low-cost housing area in Westlake, which the Westlake River flows alongside (CCT, 2010; Source-to-Sea, 2016; DEA&DP, 2018).
ZA Ghoor: Managing nutrient flows into the Zandvlei Estuary, Cape Town using Sustainable Drainage Systems (SuDS)
Chapter 3: The Zandvlei Catchment