An integrated rainfall-runoff water quality model in a mine impacted karst environment
De Klerk, Theunis Christiaan
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A fully distributed rainfall-runoff model was developed for the Wonderfonteinspruit catchment (± 1 600 km2), in order to accurately determine the rainfall-runoff and mass transport model’s hydrological components that may affect the natural water balance of the Wonderfonteinspruit catchment. The Wonderfonteinspruit catchment in the Gauteng and North-West provinces, is situated in a karst landscape with sinkholes and dolomitic aquifers. The catchment is also located in the mining region of the Witwatersrand, and extensive mining activities has taken place in the catchment for the past 120 years. These mining activities had a direct impact on the karstic landscape in the form of modifying the surface water and groundwater systems in the catchment. These modifications include: dewatering of the dolomites, drying up of the springs, changing the flow regime of the Wonderfonteinspruit, accelerating sinkhole occurrences, mining through impermeable dykes and water pollution. The EPA SWMM rainfall-runoff model was selected for this study, because of its ability to simulate pipelines and canals. The study area was divided into 7 092 subcatchments with 7 317 conduits, 7 089 junctions, 4 storage units and 312 dividers. To ensure that the wetlands were more accurately modelled in the riverbeds, a flow accumulation raster was used to determine two categories of wetland response. This made it possible to change the wetlands individually for the different categories. To incorporate the sinkholes into the model, sinkhole area was calculated for each subcatchment. A Sinkhole Loss Modification Value was calculated from sinkhole area, which was used to determine the sinkhole infiltration rate. The diverter object in SWMM was used to model the sinkholes. Observed data, i.e. rainfall, flow and quality data were collected for 41 months (1989 – 1992), which was the time period when most flow and rainfall gauges had data available. The Wonderfonteinspruit catchment include a range of imperviousness areas (0% - 100%) and several land uses (e.g. urban, agricultural and mining). To manage the calibration process more efficiently an Integrated Model Controller was developed. This made relative parameter value adjustment across all 7 092 subcatchments an automated task. In calibrating the SWMM rainfall-runoff model, the SCS curve number, wetland Manning’s n, sinkhole infiltration rate, Manning’s overland flow and seepage were the key parameters. Four flow gauges were used in the calibration process and their Nash-Sutcliffe Coefficient values ranged from good (0.75) – very good (0.95) and the Pearson correlation coefficient values (0.87 – 1.00), indicate a strong correlation between simulated and observed values. Building on the response of the SWMM rainfall-runoff model, a water quality model was developed to simulate the sulphate (SO4) concentrations in the Wonderfonteinspruit catchment. Available water quality data was collected for the same period as the rainfall and flow data (1989 – 1992). Three flow gauges were used in the calibration process. Two were adequately simulated, their Nash-Sutcliffe Coefficient values ranged from satisfactory (0.61) to very good (0.80) and their Pearson correlation coefficient values were 0.81 and 0.90 respectively. The last flow gauge in the hydrological network had an unsatisfactory Nash-Sutcliffe coefficient of -4.48 and a Pearson correlation value of -0.01. To demonstrate the applicability of the rainfall-runoff model, two possible future scenarios proposed by various authors for the Wonderfonteinspruit catchment, were simulated. The first scenario is the proposed Mega-compartment scenario, which assume that instead of having multiple aquifer compartments created by the dykes, there will be only one mega compartment. The simulation showed that for a dry, wet and normal rainfall scenario, the Wonderfonteinspruit in the Mega-compartment scenario would be mostly dry. The second scenario is the Rewatered compartment scenario, which suggest the springs start flowing again, the simulation showed that for the dry, wet and normal rainfall scenario the Wonderfonteinspruit will continue to have a base flow after all mine discharges have ceased. The aim of this research to accurately determine the relevant surface hydrological network consisting of appropriate hydrological components that accurately describe the hydrology of the Wonderfonteinspruit catchment was achieved.