Evaluating the transport and composition of gully pot sediments : Under changing anthropogenic activities and rainfall regimes

Gully pots are a fundamental runoff quality and quantity control measure installed in urban catchments around the world. However, their ubiquitous use in urban catchments also incurs a substantial resource demand in terms of routine maintenance if their continuous performance is to be assured. The studies included in this licentiate thesis advance knowledge on the potential risks and benefits associated with gully pots systems with regard to their role as a component of the pathway of surface runoff from urban catchments to receiving waters. The thesis interrogates the risks of previously deposited sediments being scoured under current and potential future rainfall conditions and the benefits of gully pots functioning as a temporal repository for pollutants as well as their potential role as a tool for tracking diffuse pollution changes at a local catchment level. Pollutant mobilisation during artificial gully pot emptying processes, and the implications of this for their subsequent management, are also discussed. The results provide insights into additional factors that need to be taken into account when developing gully pot management regimes. For the above purposes, the materials presented in this thesis include modelling, field and laboratory studies. To estimate the impact associated with scouring of deposited solids from gully pots in the Swedish context (i.e. rainfall conditions and catchment characteristics), two storm-event based condition matrices were developed based on the output derived using two models from the literature. Model one outputs indicate that the scour-induced total suspended solids discharged from the gully pot can be kept below 25 mg/L if the gully pot fullness level is maintained below 60 % fullness level. Model two highlighted the higher susceptibility to scour and remobilisation of finer and lighter solids. The potential of this model to inform the dimensioning of gully pots to reduce the scouring of solids of a certain size range under targeted rainfall events is identified. In the field study, seven gully pots in two catchment types (trafficked road and housing land uses) in Luleå, Sweden, were fully emptied and the physicochemical properties of sediments and dredged waters characterised. By comparing data with the results of an earlier sampling of the same seven gully pots, a temporal increase in the normalised solids accumulation rate in the road catchment was highlighted, reaching 0.176 – 0.819 kg·m-2·year-1. In terms of pollutants, a significant temporal decrease in median As and Pb concentrations and increases in median Cd, Cr, Zn, Ni and Co concentrations in sediments of certain size ranges were reported. In terms of polycyclic aromatic hydrocarbons (PAHs), a major temporal shift from a light weight PAH dominated profile to a heavy weight PAH dominated profile was identified. Discussions consider if, and if so how, the characterised temporal changes may relate to changes in catchment practices which have occurred during the time interval between sampling periods. With the exception of Cd, total pollutant loadings per gully pot were 1.5 – 8 times higher in road catchment gully pots than housing catchment gully pots. The hydraulic dredging approach to gully pot emptying used in this work remobilised < 20 % of pollutants from the sediments phase into dredged waters, with the majority of these pollutants associated with suspended materials. A comparison of the pollutant concentrations in dredged waters with available quality standards/guidelines identifies the need for further treatment to minimise their potential ecological impacts on receiving water recipients.