The viability of rainwater and stormwater harvesting in the residential areas of the Liesbeek River Catchment, Cape Town Lloyd Fisher-Jeffes 11 November 2015 Urban Water Management research unit: Team meeting www.wsud.co.za
Hypothesis 2 Stormwater harvesting is a viable water resource that offers the potential to improve water security in the residential areas of the Liesbeek River Catchment, Cape Town.
Liesbeek River Catchment 3 2,600 hectares 1,300 hectares urbanise 7 suburbs 30,000 people 15,000 people in single residential dwellings 6,200 single residential dwellings 40 % of single residential dwellings have swimming pools Precipitation varies from 600-1500mm/yr. Evaporation varies between 1300-1550mm/yr.
Method 4 Property data Demand data Precipitation data Evaporation data Population data
Climate Change 5
Scenarios 6 Rainwater harvesting considered 20 scenarios 10 considering entire roof area 10 considering 50% or 100m 2 Stormwater Harvesting considered 6 scenarios + + Combined rainwater and stormwater harvesting considered 4 scenarios
Rainwater Vs Stormwater harvesting 7 Rainwater Harvesting (RWH) is the collection and storage of runoff from the roof/s present on an individual property and the subsequent use within that property Stormwater Harvesting (SWH) is the collection and storage of runoff from an urban area, and the subsequent redistribution for use by one or more independent users for any appropriate purpose.
Potential water demand met - RWH 8
Financial considerations - SWH 9 Scenario Scenario 25 (with each drainage subcatchment treated as a SWH scheme) Scenario 25 (With 30 SWH schemes the result of combining smaller drainage subcatchments) Average SWH catchment area (ha) Total number of SWH catchments Average cost per kilolitre (2013ZAR) 10 130 30.00 45 30 16.60 Scenario 26 1300 1 12.70
Potential water demand met - SWH 10
Climate change RWH & SWH 11
Stormwater attenuation - RWH 12
Stormwater attenuation - SWH 13
Results - SWH 14 Stormwater Harvesting land take (0,75-3,33%) Local scale (2,5-3,33%) Regional scale (0,75%-2,5%) External benefits 2013ZAR 2 7.2 million/yr Vs. 2013ZAR12-42 million/yr
Results - SWH 15 No. Description Scenario 26 - centralised Scenario 23 - decentralised (2013ZAR/kl) (2013ZAR/kl) 1 Benefits (Table 5-5) 2.27 5.16 2 Land costs 14.01 31.85 3 Net benefits (1-2) -11.74-26.69 4 Reduced flood costs 0.00 3.74 5 Cost of SWH, excluding benefits and land costs 12.85 16.38 6 Cost, including benefits and land costs (5-1+2) 24.59 43.07 7 Cost, including only benefits (Table 5-5) (5-1) 10.58 11.22
Conclusions - RWH 16 In general RWH: offers a means of reducing municipal water demand; Is only economically viable for the minority of property owners; RWH is only a viable option economically when Used for as many end uses as possible The largest possible catchment area Climate change may have an extreme impact on RWH depending on which climate change model is used in some cases, positive, and in others, negative. RWH was an unreliable means of attenuating peak flows, even for small storm events.
Conclusions SWH 17 In general, SWH: 20 % reduction in municipal water demand Potentially offers other benefits Real Time Control could enhance the SWH system's ability to attenuate peak flows during both small and extreme events. Reduction in potable water demand would be significant for the CoCT, but would require that all residents and businesses make use of harvested stormwater. Non-financial benefits should be recognised and may act to partially offset the costs of operating the SWH system. Some climate change scenarios indicated significant decreases in runoff volumes, others showed limited change. Overall this investigation found that SWH may, under certain conditions, be a viable alternative to potable water in the Liesbeek River Catchment.
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