A hydrological and hydraulic model was built representing two of the river systems that flow into the Zandvlei estuary - the Keysers and Westlake River sub-catchments. 6-7 6-7 Configuration of the modeled Dreyersdal wetland upgrade 6-8 6-8 Turbid water flowing in the stream behind Blue Route Mall 6-9 6-9 Plan view of the Blue Route stream with surrounding views (Plan view: . Bing Maps, 2021).
List of Tables
Acronyms, abbreviations and chemical formulae
Acknowledgements
Introduction
- Background
- Research problem
- Objectives
- Method overview
- Dissertation structure
The mouth of the Zandvlei – a shallow body of water on the edge of the Muizenberg coastline in the city of Cape Town – is no exception. Among the many stormwater pollutants entering the estuary, elevated nutrient levels (particularly phosphates and inorganic nitrogen) in the rivers are a major problem as they can lead to eutrophication (CCT, 2010).
Literature Review
- The impact of development on catchments
- The effect of urbanisation on runoff quantity
- Pollutants in urban stormwater
- The threat of eutrophication
- South African water quality standards
- Managing nutrient levels: Sustainable Drainage Systems
- Introduction
- Classification of SuDS measures
- Local controls
- Regional controls
- Treatment capacities
- SuDS selection
- Summary
Infiltration trenches are used in a variety of environments; they are most effective when placed along roads, sidewalks, parking lots and other impenetrable areas (Woods Ballard et al., 2015). The runoff is then collected in an underdrain or infiltrated (Woods Ballard et al., 2015).
The Zandvlei Catchment
- Overview
- River systems feeding into the Zandvlei estuary .1 The Keysers River system
- The Westlake River system
- The Diep/Sand River system
- The Zandvlei Estuary .1 Overview
- Brief history
- Some brief catchment challenges
- Salinity
- Sedimentation
- Litter and debris
- Nutrient content in rivers
- Pondweed management
- Sewer spills
- Urban runoff
- Summary
The Cape Granites intrude the Malmesbury Group and occur in the western half of the catchment. The Zandvlei watershed and estuary experience many challenges; some of the key watershed management issues are described in the following sections.
Methods
- Study overview
- Water quality sampling and testing
- Limitations
- Sampling programme
- Testing procedures
- Data collection and processing
- Topography
- Rivers and waterbodies
- Stormwater network data
- Land use data
- Geological and soil data
- Pollutant wash-off data
- Stream level data
- Long term water quality data
- Rainfall data
- Temperature data
- Hydrological model construction
- Sub-catchment delineation
- Sub-catchment parameters
- Conduits, nodes and storages
- Calibration and verification
- Input of EMC data
- Development of SuDS scenarios
- SuDS selection and placement
- Sizing of SuDS
- SuDS treatment objectives
- SuDS simulation
- Modelling methods in PCSWMM
- SuDS simulation in the model
- Summary
This test phase used OHAUS 'starter pen gauges'; the ST20 pH Pen Meter and the ST20C-B Conductivity Pen Meter were used to measure pH and EC, respectively. Security - the set up had to be small or unobtrusive enough to be easily hidden or disguised to reduce the risk of theft. Cost – the limited project budget necessitated a low-cost solution, which precluded the selection of expensive components.
The linked hydrological, hydraulic and water quality models are simply referred to as 'the model' for the sake of readability. The two main components driven by the engine are the runoff and routing components - the former operates on sub-basins each collecting rainfall and (in the model) producing hydrographs and pollutographs, based on input rainfall data and concentrations of pollutants. There was a major difference in the shape of the hydrograph: the modeled hydrograph showed steep recession curves, while the observed graph showed shallower recession curves and lower peaks.
Water quality in the study area
- South African water quality standards
- Water quality sampling and testing
- Sampling locations
- Sampling results
- CCT historical data
- Monitoring locations
- Historical nutrient trends
- Areas of concern
- Calculating EMC values from site measurements
- Deriving SRP and TIN EMC values from continuous sampling
- Comparing TP and TSS EMC values to monthly grab samples
- Summary
3 SRP could not be tested in the second week of summer sampling due to a shortage of the required reagents. The elevated TIN levels may be the result of the golf course's use of treated wastewater for irrigation in the summer months. CR06 is located in the middle reaches of the Keysers system on the Spaanschemat River which drains the northern portion of the Keysers River sub-catchment.
The site is located on the grounds of Pollsmoor Prison and drains part of the grounds, the Westlake Village low-cost housing area and the residential and commercial areas of Westlake. CR04 is located on the grounds of Pollsmoor Prison, which has historically experienced several sewage spills (DEA&DP, 2018). SRP and TIN – to compare modeled pollutant concentrations with eutrophic concentration ranges listed in SA Water Guidelines (DWAF, 1996) (Section 2.2).
Model scenarios
- Current Scenario
- Rivers, streams and ponds
- Existing SuDS practices
- Pre-development Scenario
- Modelled SuDS interventions
- SuDS in the Keysers system
- SuDS in the Westlake system
- SuDS scenarios
- Summary
A plan view of the stream with some images of the surrounding area is shown in Figures 6-9. Three sections of the grassland along the creek were modeled as bioretention areas, as shown in Figures 6-10. Two swales were modeled in the golf course along the eastern and southwestern edges of the golf course, respectively (Figure 6-12).
This part of the golf course currently flows into the canal that runs under Catharina Avenue. This bioretention system was modeled with more vegetation than its Steenberg counterpart due to higher pollutant concentrations in the golf course stream. To address these water quality concerns using SuDS, a bioretention system was modeled on the Pollsmoor property just upstream of an irrigation pond (Figure 6-14) that discharges to a river just upstream of the pond.
Simulation results
- Current scenario model
- Pre-development and Current Scenario comparison
- SuDS scenarios
ZA Ghoor: Managing Nutrient Flows in the Zandvlei Estuary, Cape Town using Sustainable Drainage Systems (SuDS). It is recommended that future studies focus on testing for TSS in catchments so that more reliable EMC values can be calculated. The pre-development scenario model attempted to predict the flow characteristics of the area if it were in its natural, undeveloped state.
Since no pre-development data are available for this watershed, the simulation results are a rough estimate. A comparison of runoff flows between the current scenario and the pre-development scenario is presented in Figure 7-1; this figure shows the higher, steeper flow profile characteristic of. ZA Ghoor: Managing Nutrient Flows to the Zandvlei Estuary, Cape Town Using Sustainable Drainage Systems (SuDS).
7.3.1 24-hour simulation results
Pollutant removal of each SuDS component
The CCT policy (Section 4.5.3) stipulates that for new developments, SuDS must remove 45% of TP and 80% of TSS. The modeled SuDS showed removal percentages that mostly adhere to those determinations, except for a few instances of TSS removal. Where TSS removal is insufficient, additional components will be required to assist in sediment control.
The following section discusses SRP and TIN concentrations in relation to eutrophic limits based on South African guidelines (DWAF, 1996).
Pollutant removal of scenarios
The equations used to model the treatment of wetlands were taken from international literature, not local South African research. Based on these factors, it is difficult to say how realistic these removal percentages are.
Volume and peak flow rate reduction
As expected, the combined SuDS scenario (scenario 3) yields the lowest peak discharge flow rate of 22.4 m3/s, while the peak discharge flow rate of scenario 2 is slightly higher at 23.7 m3/s. The introduction of SuDS measures should ideally result in hydrographs comparable to the pre-development hydrograph; this is not the case here as the modeled SuDS scenarios have steep recession curves. This is likely due to the continued presence of connected impermeable areas, as the inclusion of SuDS interventions could not address all of the impermeable runoff in the study area.
7.3.2 8-year simulation results
- Pollutant removal
- Volume and peak flow rate reduction
- Summary
- Conclusions and recommendations
In the current scenario model, the annual SRP concentrations at the outfall were above the eutrophic threshold every year, as shown in Figure 7-5. For each scenario, the upper treatment limit shows concentrations below the eutrophic threshold each year in the simulation. This is consistent with the CCT historical data that rarely showed TIN concentrations above the limit in the lower reaches of each river system.
The total and maximum runoff for each year in the simulated time period is shown in Figures 7-9. In terms of pollutant removal, scenario 3—the scenario that combined local controls and wetlands—showed the greatest pollutant load reduction and showed the lowest pollutant concentrations at the modeled outlet. TSS load reduction is only possible in the model when the wetlands are most effective.
Retrieved October 14, 2020 from OpenSWMM: https://www.openswmm.org/Topic/4586/applications-of-swmm-in-rural-undevelopment-areas. Assessment of the effects of land use on surface water quality in the Lower uMfolozi Floodplain System, South Africa. Impact of waste water on surface water quality in developing countries: a case study of South Africa.
In DEFRA, Modeling the impact of sediment and phosphorus loss control on catchment water quality (SCAMPER): Final Report. Storm Runoff Water Quality and Comparison of Procedures for Estimating Storm Runoff Loads, Volume, Event-Average Concentrations, and the Average Load for a Storm for Selected Properties and Constituents for Colorado Springs, Southeast Colorado, 1992. Retrieved December 3, 2020, from Susdrain: https://www.susdrain.org/delivering-suds/using-suds/suds-components/inlets- outlets-and-control-structures/component-Inlets-outlets-and-controls.
Appendix A: List of rain gauge data collected
Appendix B: Preliminary water quality data
Appendix C: Published EMC data
Appendix D: Model input parameters
Appendix E: SuDS design standards and assumptions
E-1: Water Quality Volume (WQV)
E-2: Swale design
The staff roughness coefficient (n) was assumed to be 0.25 for all fungi, except for fungi with additional vegetation near streams where n = 0.35 (based on the recommendations of Woods Ballard et al.
E-3: Bioretention system design
Infiltration coefficient (q): Woods Ballard et al. 2015) suggest that infiltration tests be performed on site to determine the infiltration capacity of the filter media. This was not possible for this study; instead, the infiltration coefficient specified by the Minnesota Stormwater Manual for sand/clayey sand was used: 0.02 m/h (MPCA, 2020).
E-4: Wetland design
Appendix F: Inlet and outlet structure examples
Appendix G: Local & regional SuDS controls: some advantages and disadvantages
G1. Local controls
Infiltration trenches are shallow excavations lined with a geotextile layer and filled with coarse-grained material, such as rock or a synthetic cavitating material. The configuration of infiltration trenches offers several advantages: their narrow cross-section makes them suitable for deployment in urban areas, including brownfield and retrofit sites. Compared to other infiltration-based SuDS measures, the construction costs of infiltration trenches are relatively low.
Clogging can also be avoided by including a vegetated swale or buffer in front of the infiltration ditches in the treatment array, which would reduce sediment flow into the ditch. While the cost of constructing infiltration ditches is relatively low, the cost of maintaining them is usually relatively high, especially in areas with fine-grained soils (Taylor, 2003). Bioretention areas are versatile in their use, offering many advantages: they can be incorporated into many other SuDS measures, such as breakwaters and retention ponds, and fit easily into a wide range of landscapes.
G2. Regional controls
The ability to clean retention basins is one of their greatest strengths; they can be a source of non-potable water for various irrigation and domestic non-potable purposes. Public health and safety are especially important with stocked ponds – simple access to the pond can be dangerous, and the potential for contaminated water to enter the pond poses a health risk. Certain wildlife attracted to the pond can increase phosphorus levels in the water through their droppings.
Wetlands offer the advantage of greater pollutant removal capability compared to detention and retention ponds of the same volume (Armitage et al., 2013). Additionally, wetlands attract undesirable insects such as mosquitoes and birds, whose droppings add to the nutrient load in the water; biological controls such as dragonflies can be introduced into wetlands to manage the mosquito population (Armitage et al., 2013). Adequate maintenance is necessary to avoid the growth of invasive plant species that threaten indigenous wetland vegetation and, in turn, the sustainability of the system (Woods Ballard et al., 2015).
Appendix H: Photographs of sampling points
The river is shallow at this point and flows in the dense reeds below Reddam Avenue.
