Zeekoevlei / Rondevlei Rehabilitation Study ... - Southern Waters

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Southern Waters Ecological Research and Consulting This report considers the removal of sediments from Zeekoevlei, and curtailment of the seepages to be achievable short term objectives. It is predicted that the successful mitigation of nutrient loading from these two sources will result in a reduction in algal growth of between 2.5 and 5x the present level (as measured using chlorophyll-a). Any further reductions in nutrient attenuation achieved in the catchment (a medium to long term process), will further enhance the level of water quality improvement. Recommendation 2: 2.1 Remove the estimated 1.1 million m 3 of sediment from Zeekoevlei by dredging. This process will amount to an annual load reduction of 25% of the present total. The estimated cost of the dredging procedure is 1 million US dollars, plus a further 0.5 million USD for the processing of the dredge spoils (the cost of the latter operation may be offset against resale of the product). Between nine and fifteen months will be required to complete the operation. 2.2 Concurrent with the dredging operation, implement those measures deemed most appropriate for the attenuation of the phosphorus-rich seepage flows from the Strandfontein WWTP to Zeekoevlei. This will reduce the annual phosphorus loading of the vlei by up to 35%. The cost of this mitigation is estimated at between ZAR 2.5 and 5 million (2000 prices). 2.3 That watershed-directed Best Management Practices be identified and implemented as part of the envisaged Catchment Management Plans for the Big and Little Lotus River catchments. 16 Zeekoevlei / Rondevlei Rehabilitation Study: Action Plan
Southern Waters Ecological Research and Consulting 4.3 Sources of phosphorus loading in Rondevlei Phosphorus loading to Rondevlei stems from diffuse catchment sources, and from sediment loading of the upstream Princess Vlei (see diagram). In contrast to Zeekoevlei, internal sediment loading in Rondevlei was not deemed to be a significant (13%) source of nutrients. Reed beds 5.4% Internal loading 13.2% Local catchment 1.1% Groundwater 0.7% Lake surface 0.3% Rondevlei catchment 23.0% Princess Vlei catchment 56.3% Diagrammatic representation of the identified annual phosphorus loads, by source and percentage of the total, for Rondevlei. A total annual load of some 2300 kg of phosphorus per annum was estimated for Rondevlei. The bulk (56%) of this loading stems from the upstream Princess Vlei (a vlei for which a separate rehabilitation study is currently underway), with the urban catchment draining directly to Rondevlei contributing a further 23% of the estimated total. Given the surface area to volume proportions of Rondevlei, this amounts to a loading of approximately 40x the guideline value for a shallow lake. Notwithstanding the level of phosphorus loading, the ecological functioning of Rondevlei is impaired to a much lesser degree compared with Zeekoevlei. This is probably because the vlei has been impacted at a lower level and for a shorter period of time; has a lower dominance by bloom-forming algae; a greater degree of natural depth and water level Zeekoevlei / Rondevlei Rehabilitation Study: Action Plan 17
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Zeekoevlei / Rondevlei Rehabilitation Study ... - Southern Waters

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