Hydrological And Sedimentological (Dis)Connectivity In An Urban Suds System Based On High Resolution, Multi-Event Monitoring

Sustainable Urban Drainage Systems (SUDS) are an integral part of the design in new housing developments. They are designed to manage surface water flooding and provide temporary storage for urban water runoff as well as allowing for the improvement of water quality through the natural operation of physical, chemical, and biological processes and through the trapping of sediments that often carry undesirable contaminants at above background concentrations. The study site was based in the Upton housing development, on the western side of Northampton, UK, which was designed in the 1990s based on key principles to promote sustainable urbanism. The SUDS formed an integral part of the development and comprises of a number of swales that discharge into a series of ponds linked together with overspill-weirs. The ponds also receive inputs from a car park, local roads and a major road on the northern edge of the new housing development. It was one of the first developments in the UK to provide a “roof to river” surface water management strategy, with the SUDS designed to provide a “treatment train” before surface run off eventually discharges into the River Nene. The efficiency of SUDS has been reported in the literature in terms of sediment and contaminant retention, but such studies do not always address the long-term source/ pathway/ sink receptor relationships. Little is known about the effects of how rainfall driven transient events (in terms of water levels) affect the performance of such systems. The aim of this research was to assess the hydrological and sedimentological (dis)connectivity within the system based on high resolution, multi-event monitoring. Rainfall was measured at 5-minute intervals local to the SUDS and pressure transducers were also installed in order to capture 5-minute water level data in up to 3 locations simultaneously. Data were collected over a four-year period from 2014 to 2018. The high-resolution water level data were used to produce hydrographs and the water level rise and fall dynamics were analysed for a series of individual events to illustrate performance over a range of rainfall intensity / durations. The impact of rainfall on water levels within the system varied over the study period and highlighted the complex response of SUDS to rainfall input. It also highlighted the lack of connectivity within the system unless significant rainfall (storm) events occurred. Time-integrated sediment samplers allowed the collection of sediment throughout the study period at various points in the SUDS. The sediments were analysed for heavy metals (Cd, Cr, Cu, Ni, Pb and Zn), gamma emitting radionuclides and mineral magnetic parameters (to establish if these could be used as a rapid inexpensive alternative to heavy metal analysis). A variety of statistical methods were used to establish the similarity between the sediments trapped in the samplers in terms of their contaminant concentration and radionuclide activities as well as aiding the identification of potential sediment sources. There was little evidence to suggest that environmental magnetism could be used as a surrogate for heavy metal concentrations in this particular SUDS. The results from both the hydrological and sedimentological data suggested that little connectivity exists between all of the SUDS components and instead of acting as a “treatment train” most of the sediment is deposited in only one of the ponds, potentially creating a “sink” for pollutants. Sedimentation within this pond was also complex with high rates of accumulation at points close to drain inflows suggesting that there was insufficient energy most of the time to suspend and redistribute the sediment across the bed of the pond. A series of summary models were created based on 3 different rainfall scenarios and demonstrated that equal consideration should be given to both the hydrological and sedimentological response for such a system at the initial design stages. In addition, the siting of SUDS should consider the nature of the surrounding landscape and land use and ascertain whether potential local inputs could further contribute to the sediment and pollutant loads at different points in the system. The metal concentrations of the sediment within this system were found to significantly exceed background levels. Rates of accumulation and the potential for exceedance of sediment quality guideline levels for soils need to be further investigated as well as the potential for pollutant linkages.