Influence of seasonal activities and traffic conditions on the accumulation and particle size distribution of gully pot sediments.

Gully pots (GPs) are ubiquitously used in urban catchments, to direct surface runoff into piped sewer systems, reduce the risk of sediment-induced pipe blockages and the loading of solids and associated contaminants on the receiving waters. Over time, the build-up of sediments in GPs will lead to impaired hydraulic performance, putting the urban catchment at risk of flooding. However, GP maintenance strategies lack both a robust evidence base and clear aims against which their effectiveness can be benchmarked. An improved understanding of the factors influencing long-term in-situ solids accumulation in GPs is required for further assessment and optimisation of the GP maintenance scheme. As a contribution to addressing these knowledge gaps, sediments from 27 GPs located in urban catchments associated with three street feature types (roundabout; crossing; straight road) and varied traffic intensities were collected on two occasions, representing two distinct seasonal accumulation periods. Parameters including GP fullness levels, particle size distribution (PSD), and solids mass accumulation rates were evaluated with regards to the impacts of seasonal activities, traffic conditions as well as configuration and dimension of GPs. The results indicate impacts of seasonal activities on solids accumulation rates were not consistent across all GPs, with the magnitude of the difference larger following winter-spring catchment practices. Additionally, synergistic impacts of seasonal activities and traffic conditions on PSD and solids accumulation rates were observed. For example, the winter road safety measures may enhance the impacts of the characteristic vehicle turning motions at roundabouts, contributing to the negative correlation identified between traffic intensities and solids accumulation rates. The overall exhibited large variations in sump fullness levels (4 %-73 %) and solids dry mass (0.5-51 kg) directly challenged the commonly adopted generalised GP maintenance frequency without taking the catchment-specific activities into consideration.