Abstract
Gully pots actively trap sediments transported by urban runoff to prevent in-pipe blockages and surface flooding. However, due to poor maintenance (resulting in sediment build-up) and increasingly extreme wet weather events, the scour of previously-deposited sediments from gully pots is identified as a potential contributor to EU Water Framework Directive failure. While basal sediment scour deterministic models have been developed and validated using laboratory and field gully pot data sets, the ability of these models to predict behaviour at sites other than those for which they were established has not been addressed. Nor has the impact of future rainfall predictions on the role of gully pots as sediment sources been systematically examined. As a contribution to addressing these knowledge gaps, the performance of two gully pot basal sediment scour models of distinct complexity levels are evaluated under current and future rainfall conditions. The output from Model One suggests that the scour-induced total suspended solids in gully pot discharge can be kept well below 25 mg/L if the gully pot fullness level is maintained at under 60%. Results identify the opportunity to incorporate the actual/targeted ecological status of recipients in scheduling gully pot maintenance operations and that proactive gully pots maintenance will reduce the impacts of increased rainfall intensity/duration on the magnitude of sediment scour. Results from Model Two suggest that fine sediments are particularly susceptible to in-pot scour. For example, sediment with a specific gravity of 1.1 and diameter of >63 μm accounts for 50% of scour-induced total suspended solids in gully pot discharge. The effluent suspended solids concentrations predicted by the two models differ by up to two orders of magnitude. However, without further empirical field data pertaining to their respective competences/applications, neither model could be discounted at this stage. For example, the use of Model One is more appropriate in the establishment of gully pot maintenance schedules, with Model Two more suited to the dimensioning of gully pots based on performance requirements. This application, however, relies on the development and adoption of a more stringent regulation on gully pots discharge.
Highlights
In contrast to point source pollution, which has been the subject of legislation for several decades, the need to address diffuse pollution was not widely recognised until the 1970s (Campbell et al, 2005)
Declines of a similar magnitude can be found as the fullness level falls from 90% to 80% and from 80% to 70%, indicating that emptying gully pots when
Whilst the role of models is valuable for understanding and quanti fying gully pot sediment scour, this study shows limitations for their application by practitioners
Summary
Its associated negative impacts such as nutrient enrichment and sediment contamination are acknowledged as major threats to the receiving water ecosystem health. Annex II 1.4 identifies urban areas as a major source of diffuse pollution, which is mobilised by rainfall and transported to receiving water bodies as urban runoff. Total particulate matter is a key contaminant in urban runoff. The term total suspended solids (TSS) is used within this study with the filter pore size included in brackets (where reported). Several studies report that the majority of diffuse urban pollutants are associated with the particulate phase Adsorbed pollutants may subsequently be released from sediments and re-enter overlying waters as a result of remobilisation during extreme rainfall events which may, for example, alter redox conditions (Lundy et al, 2017). Increases in precipitation extremes predicted as a result of climate change (IPCC, 2013), brings a higher probability of sediments scour events, in terms of both scour frequency and mass discharged
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