The performance of SUDS ponds in Eastern Scotland
The study reported in this thesis furthers knowledge on the performance of SUDS ponds in Scotland and provides an enhanced method for analysing and reporting SUDS water quality performance. The research is based on information gained from intensive flows and water quality studies at two SUDS ponds, and additional water quality data obtained at a third pond, over a period of three years. The ponds are located in a new development area, Duloch Park, in Dunfermline in eastern Scotland, commonly known as DEX. Hydraulic monitoring of two ponds clearly showed that both performed well in attenuating flow rates and it was also evident that one of the two ponds, in a catchment with soil class 4, significantly reduced runoff volumes. This demonstrated that even in soil class 4, SUDS can reduce runoff volumes under certain conditions. The other pond was subject to groundwater inflow and did not achieve reduction in runoff volumes. Monitoring results also suggested that detention basins can significantly reduce peak flows of runoff events that have flow rates below the activation level of the outflow control. Catchment runoff results suggested that the standard method used by Scottish Water for calculating greenfield runoff, the IH 124 method, may not be best suited for determining outflow rates from SUDS. For two adjacent catchments at Duloch Park, this method under estimated all but one of the predicted outflow rates for one of catchment, and over estimated all but one of the predicted outflow rates for the other. It was found that the ponds performed well in removing certain pollutants from urban runoff and in attenuating peak concentrations of others. One of the three ponds performed consistently better in attenuating pollutants than the others, most likely due to calmer flow conditions and water seepage that often resulted in partial or total containment of runoff events. Results from sediment data suggest that a SUDS pond’s required sediment removal cycle is strongly depending on its flow regime. Runoff quality data highlighted that the largest amounts of pollutants were generated in the residential housing catchment. The effectiveness of the three ponds in terms of nutrient removal was not established with absolute robustness, as the collected data did not cover a complete seasonal cycle. The water quality data was interpreted by means of a water quality index which was modified from an existing index for Scottish watercourses and is parameter weighted. Parameters that were deemed irrelevant for the analysis of SUDS performance were deleted, while the weighting for the remaining parameters was changed. On a scale from zero to 100, the index can typify the performance of single water quality parameters and also represent the overall water quality when all parameters are combined. It was found that the index significantly enhanced data analysis and performance reporting of SUDS facilities. The index is a versatile tool that can be used in both spreadsheet data analysis and modelling software. Further amendment and calibration of the parameter weighting of this index is required to make it suitable for water quality analysis and performance reporting of SUDS. Two of the monitored catchments were modelled in PCSWMM 2002 (Personal Computer Storm Water Management Model), and the existing SUDS layouts were compared to alternatives. The performance of each layout was evaluated for its predicted performance in peak flow and volume reduction. For this, a method that rated the predicted SUDS performances against a conventional pipe system on a scale from zero to one was used. The modelling study highlighted several shortcomings of SWMM and other currently available drainage models in the analysis of SUDS. The study concludes that the SUDS ponds investigated are performing well for their purpose, but that alternatives to the current ponds and treatment train layouts would achieve similar results while being more space efficient. It is also concluded that the water quality index proposed in this study is an excellent method for analysing and reporting of SUDS performance, but further amendments and calibration is needed to make it fit for purpose. It is proposed that this is undertaken by a panel of experts that covers all fields related to urban runoff quality and SUDS issues. When fully developed, the proposed water quality index could serve as the base for the first standardised SUDS efficiency analysis and reporting tool world wide.
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